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Sports Med 2007; 37 (9): 807-826 0112-1642/07/0009-0807/$44.95/0 REVIEW ARTICLE © 2007 Adis Data Information BV. All rights reserved. Altering the Length-Tension Relationship with Eccentric Exercise Implications for Performance and Injury Matt Brughelli and John Cronin School of Exercise, Biomedical and Health Sciences, Edith Cowan University, Joondalup, Western Australia, Australia Contents Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 807 1. Eccentric Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 808 2. The Length-Tension Relationship . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 808 2.1 Single Fibre (Sarcomere) Force-Length Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 809 2.2 Whole Muscle Force-Length Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 810 2.3 Single Joint Torque-Angle Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 810 3. Studies Reporting a Shift in Optimum Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 811 3.1 Dynamometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 812 3.2 Curve Fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813 3.3 Equipment and Testing Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813 3.4 Exercise Methodology and Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814 4. Mechanisms for the Shift in Optimum Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816 4.1 First and Second Shift in Optimum Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816 4.2 Theory of Sarcomereogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 817 4.3 Theory of Passive Tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 818 5. Implications for Athletic Performance and Injury Prevention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 819 5.1 Muscle Injury Prevention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 819 5.2 Eccentric Exercise and Athletic Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 820 6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 823 Abstract The effects of eccentric exercise on muscle injury prevention and athletic performance are emerging areas of interest to researchers. Of particular interest are the adaptations that occur after a single bout, or multiple bouts of eccentric exercise. It has been established that after certain types of eccentric exercise, the optimum length of tension development in muscle can be shifted to longer muscle lengths. Altering the length-tension relationship can have a profound influence on human movements. It is thought that the length-tension relationship is influenced by the structural makeup of muscle. However, the mechanism responsible for the shift in optimum length is not readily agreed upon. Despite the conflict, several 808 Brughelli & Cronin studies have reported a shift in optimum length after eccentric exercise. Unfortunately, very few of these studies have been randomised, controlled training studies, and the duration of the shift has not yet been established. Nonetheless, this adaptation may result in greater structural stability at longer muscle lengths and consequently may have interesting implications for injury prevention and athletic performance. Both contentions remain relatively unexplored and provide the focus of this review. 1. Eccentric Exercise Pre Post 160 © 2007 Adis Data Information BV. All rights reserved. Torque (Nm) 140 Over the last 10 years, interest in eccentric exercise and its adaptations has grown substantially. There are a number of reviews dedicated to the effects of eccentric exercise on muscle damage,[1-4] delayed onset of muscle soreness (DOMS),[5,6] the repeated bout effect,[6-8] injury prevention,[9,10] athletic performance[11,12] and rehabilitation.[13] The interest can be attributed to the unique physiological and mechanical properties of eccentric muscle contractions. For example, skeletal muscles are capable of producing the greatest magnitudes of force,[14] with little metabolic effort[15,16] when contracting eccentrically. The high force levels and/or longer muscle lengths[17] are thought to be responsible for the damage caused to the contractive,[3,6] connective[18,19] and cytoskeletal elements of muscle.[20-22] Muscle damage is also associated with decreased force capabilities,[23] inflammation[24] and impairment of the excitation-contraction coupling process.[25-27] All forms of exercise can cause muscle damage; however, only eccentric exercise has been found to induce severe stiffness and soreness in the days following the eccentric exercise bout. An interesting adaptation occurs after certain types of eccentric exercise that affects the mechanical properties of muscle. After a single bout of eccentric exercise, the length-tension relationship of muscle can be altered. The optimum length of peak tension will occur at longer lengths, thus shifting the length-tension curve to the right (see figure 1). This phenomenon may have interesting implications for 120 100 80 60 40 20 0 0˚ 10˚ 20˚ 30˚ 40˚ 50˚ Angle 60˚ 70˚ 80˚ 90˚ Fig. 1. Schematic representation of a shift in the optimum lengthtension relationship. Pre = pre-exercise; Post = post-exercise. injury and performance, the discussion of which provides the focus of this article. First, the lengthtension relationship at a single fibre, whole muscle and single joint levels are briefly discussed. Secondly, the literature that has reported a shift in optimum length after eccentric exercise is critiqued. Thirdly, the proposed mechanisms responsible for the shift are described. Finally, the implications of this rightward shift with regards to athletic performance and injury prevention is elaborated upon. 2. The Length-Tension Relationship The length-tension relationship plays a very important role in the function of skeletal muscle. The magnitude of force a muscle can generate depends on its length, velocity and stimulation.[28,29] When creating a length-tension curve, muscle velocity and stimulation are held constant. Force levels are then plotted against each muscle length, thus creating the length-tension curve. The length-tension curve Sports Med 2007; 37 (9) Length-Tension Relationship with Eccentric Exercise 2.1 Single Fibre (Sarcomere) Force-Length Curve The isometric force-length curve is generated by maximally stimulating a single muscle fibre (sarcomere) over a range of lengths and measuring force outputs. In this situation, velocity is held constant at zero and stimulation is held constant at maximum levels. Maximum force levels are plotted against each starting length to generate the forcelength curve. The active force levels produced and the shape of the curve are a direct result of the © 2007 Adis Data Information BV. All rights reserved. Plateau region a Force (%) 100 Ascending limb Descending limb 2.0 2.2 Passive force 1.7 50 0 1.27 1.7 2.0 2.2 Sarcomere length (µm) 3.6 b 80 Combined tension Passive tension Force (N) 60 40 Active tension 20 0 0.5 0 1.0 1.5 Length (cm) 2.0 2.5 Optimum length c 150 Torque (Nm) gives valuable information on the lengths that can produce the greatest or least amount of tension. From this information, the regions of the ascending limb, descending limb and optimum length can be determined (see figure 2a). Optimum length is often used to describe sarcomere length (see section 2.1), muscle length (see section 2.2), or joint angle (see section 2.3), and tension is often used to describe force (see sections 2.1 and 2.2), or torque (see section 2.3). Note that the length-tension curves for the single fibre (sarcomere), whole muscle and single joint all have different shapes. At the single fibre level (see figure 2a), only the cross-bridge interaction between myosin and actin contributes to the shape of the curve and the development of active force within the sarcomere. At a whole muscle (see figure 2b) and single joint level (see figure 2c), the curve has more of a smooth shape because of the variations in muscle, tendon and joint designs, and the contribution of all these factors to the length-tension curve. There are many inconsistencies in the literature regarding the length-tension relationship. Most of the confusion revolves around the reporting of force and torque interchangeably. A brief treatise of forcelength curves (see sections 2.1 and 2.2) and torqueangle curves (see section 2.3), and how they are measured should clarify some of the conjecture in the area. 809 Active and passive tension 100 50 0 0˚ 20˚ 40˚ Angle 60˚ 80˚ Fig. 2. Schematic representations of (a) single fibre force-length curves; (b) whole muscle force-length curve; and (c) torque-angle curve. Active and passive tension curves as well as the ascending, plateau and descending regions are indicated on the single fibre and whole muscle curves. magnitude of overlap between active and myosin filaments (see figure 2a). Larger magnitudes of Sports Med 2007; 37 (9) 810 force can be produced with greater numbers of cross-bridge attachments. Altering the length of the fibre affects the number of attached actin and myosin cross bridges[22,29] and subsequent force development. Gordon et al.[30] was the first to use isolated, intact single skeletal muscle fibres to generate the force-length curve. As detailed in figure 2a, the maximum percentage of tension is plotted against sarcomere length. At short sarcomere lengths (<1.7µm), the double overlap between the actin and myosin filaments results in depressed force levels. As the length of the sarcomere increases to the steep portion (1.7–2.0µm), the actin filaments from one side of the sarcomere no longer interfere with the cross-bridge formations on the other side of the sarcomere, and force increases. The region of the curve where active force is first measured (1.27µm), until maximum force levels (2.0µm) is called the ascending limb of the force-length curve. The plateau region (2.0–2.2µm) of the curve is where maximum force levels are attained and held constant. At this range, the greatest amount of actin and myosin overlap occurs. Although there is greater overlap from 2.2 to 2.0µm, no additional cross-bridges are formed. This is due to a bare region (0.2µm in length) at the centre of the myosin molecule that is devoid of cross-bridges.[29] The plateau region is also known as the ‘optimum length’ of the forcelength curve. The descending limb region of the force-length curve occurs from the sarcomere lengths of 2.2–3.65µm. At sarcomere lengths of >3.65µm, active force levels drops to zero as there is no overlap between actin and myosin. Notice in figure 2a that there is also a passive tension curve. As the muscle is stretched to greater lengths, passive tension increases dramatically. The elements responsible for passive tension lie outside of the crossbridges and do no need active stimulation. © 2007 Adis Data Information BV. All rights reserved. Brughelli & Cronin 2.2 Whole Muscle Force-Length Curve The force-length curve of whole muscle is generated similar to that of the single fibre. The whole muscle is maximally stimulated over a range of discrete lengths. Muscle velocity is again held constant at zero, i.e. isometric contraction. Maximum isometric force is plotted against muscle length, thus creating the whole muscle force-length curve. Maximum force at each muscle length is due to both active and passive forces. At shorter lengths, all the force is due to active components (cross-bridges), and at longer lengths most of the force is due to the passive components.[31] The whole muscle force-length curve has a different shape to the single fibre force-length curve. The magnitude of active force generated is not due solely to the interaction of cross-bridges as is the case for the single fibre force-length curve. The different shape is due to the contractile properties and design of whole muscles and tendons.[29] Whole muscles are made up of a variety of fibre types and sarcomere lengths. This variety means that the different fibres will have different optimum lengths of force development, which smoothes the curve and broadens the plateau region.[29,31] Architecture also plays a role as the number of sarcomeres in series, parallel, or angle of pinnation contribute to force development and help to shape the force-length curve. Tendon fibres in series with muscle fibres also contribute to the shape of the curve. Tendon compliance can affect force development by allowing simultaneous muscle shortening and tendon lengthening. All of these factors lead to a smoother force-length curve with an extended peak region.[29,32] 2.3 Single Joint Torque-Angle Curves At the single fibre level, sarcomere force is plotted against sarcomere length, whereas at the whole muscle level, muscle force is plotted against muscle Sports Med 2007; 37 (9) © 2007 Adis Data Information BV. All rights reserved. 15.4 (IMM), 10.5 (4d post), quadriceps Prasartwuth et MI, eccentric elbow extension LL, 60° to full elbow extension 16.7 (2h post), 14 (1d post), al.[46] biceps brachii Philippou et al.[45] HI, maximum eccentric elbow MV, 2 sets of 25 reps = 50 total LL, 50–170° of elbow extension 16 (1d post), 18 (2d post), extension reps biceps brachii HI = high intensity; HV = high volume; IMM = immediately post-exercise; LI = low intensity; LL = long muscle length; LV = low volume; max = maximum; MI = moderate intensity; ML = moderate muscle length; MV = moderate volume; post = post-exercise; reps = repetitions. LL, platform height set at the patella ML, 80–130° of elbow extension 7.7 (IMM), 8.5 (4d post), hamstrings 10.4 (IMM), triceps brachii ML, 90 to <30° of knee extension HV, 12 sets of 6 reps = 76 total reps MV, 3 sets of 25 reps = 75 total reps HV, 12 sets of 20 reps = 240 total reps HV, 40–160 total reps LV, 2–3 sets of 5–8 reps ML, 90 to <30° of knee extension 6 (IMM), 7.6 (2h post), triceps surae 6.5 (4wk post), hamstrings ML, toe-to-heel action ML, 1h duration 4.4 (IMM), triceps surae ML, toe-to-heel action ML, 1h duration Muscle length ML, toe-to-heel action Exercise volume ML, 2h duration Bowers et al.[42] Pettitt et al.[44] Brockett et al.[47] Clark et al.[48] Whitehead et al.[40] Whitehead et al.[39] Exercise intensity LI, walking backwards on treadmill, 1.3 km/h LI, walking backwards on treadmill, 3.5 km/h MI, walking backwards on treadmill, >3.5km/h with 5–10kg weight belt MI, bodyweight exercise to failure, Nordic hamstring with partner MI, bodyweight exercise to failure, Nordic hamstring HI, max eccentric elbow contractions, isokinetic dynamometer MI, step-down exercise Several studies have reported a shift in optimum length after eccentric exercise (table I). This phenomenon has been observed in animal and human studies. Animal studies have reported shifts in single toad fibres,[33,34] single frog fibres,[14,35] rat fibres[36] and individual motor units in cats.[37] Acute shifts have also been reported in human plantar flexors,[38-40] forearms,[41] quadriceps,[42,43] triceps brachii,[44] biceps brachii[45,46] and hamstrings.[47,48] Muscle architecture, in terms of pinnation angle and fibre type, does not appear to have an affect on the shift in optimum length after eccentric exercise. The greatest shifts have occurred in the human quadriceps[42] and biceps brachii.[45] Study Jones et al.[38] 3. Studies Reporting a Shift in Optimum Length Table I. Influence of eccentric exercise intensity, volume and muscle length on the magnitude of the shift in optimum length length. Conversely, at the single joint level, joint torque is plotted against joint angle. Torque is determined by the magnitude of muscle force and moment arm relative to the joint.[29,31] Both muscle force and the length of the moment arm change throughout a joint movement and affect torque development. They can both be graphed against joint angle. Joint torque is the product of these two measurements. Peak torque and peak force can actually occur at different joint angles.[29] Muscle force can only be measured at this level if the moment arm was determined at each discrete joint angle. The shape of the single joint torque-angle curve is unique to that of the single fibre or whole muscle force-length curves. Joint movements typically involve multiple muscle groups crossing the joint and producing torque, thus affecting the shape of the curve. Other factors that affect the shape of the curve include the constraints of the joint itself, and range of motion. These differences lead to a different shaped curve. The torque-angle curve is smoother and flatter than the muscle-force curves. Notice also that the peak of the curve is extended and occurs at longer lengths (see figure 2c). 811 Shift magnitude (°) 3.9 (IMM), triceps surae Length-Tension Relationship with Eccentric Exercise Sports Med 2007; 37 (9) 812 Brughelli & Cronin A pilot study by Clark et al.[48] reported a sustained shift in optimum length after 4 weeks of eccentric exercise. This is the only training study, to our knowledge, that has reported a sustained shift in optimum length. Brockett et al.[47] and Bowers et al.[42] both observed a sustained shift after two separate eccentric exercise sessions, which were separated by 8 days. The shifts lasted for 18[47] and 24[42] days, respectively. Although many authors have argued that eccentric exercise can lead to a sustained shift in optimum length,[9,47,49,50] there are currently no randomised/controlled training studies that have reported a sustained shift in optimum length with eccentric exercise. The reader should be aware of this major limitation in the literature. The following sections will review the human studies that have directly measured a shift in optimum length. Both men and women were used as subjects in these nine studies.[38-40,42,44-48] Overall, 60 men and 40 women participated. All subjects were between 18 and 37 years of age. With the exception of Clark et al.,[48] who used amateur Australian Rules football players, all of the subjects were classified as being recreationally active. The reader should be aware of these limitations and delimitations when interpreting the results and developing conclusions regarding shift in optimum length. The following sections of this discussion will focus on dynamometry, subjects, muscle groups, eccentric exercise protocols, measurement protocols, magnitude of the shift and duration of the shift. Such an approach will expose some of the delimitations and limitations associated with the literature in this area. 3.1 Dynamometry All the human studies that have directly measured a shift in optimum length after eccentric exercise measured tension at the single joint level, thus joint torque was measured and plotted against joint angle. Isokinetic dynamometers, such as the Bi© 2007 Adis Data Information BV. All rights reserved. odex® III (Biodex Medical, Inc., Shirley, NY, USA)[42,47,48] or Kin-Com (Chattecx Corp., Inc., Hixson, TN, USA),[44,45] were the most frequently used to measure joint torques and angles. These machines have an attached lever arm that is controlled by a motor. The angular velocity of the lever arm is controlled by the machine. The subjects can either push or pull against the lever arm to measure torque capabilities. Isokinetic dynamometers also typically have an isometric function that allows for isometric torque measurements. Regardless of whether the muscle contraction is isometric or isokinetic, torque levels can be measured over a range of discrete joint angles. Peak torque and joint angle can then be plotted against each other to generate the torque-angle curve. There are two methods for measuring the range of peak torque levels. The first (and more time consuming) method involves measuring maximum isometric contractions over a range of discrete joint angles. The second involves measuring peak torque during a continuous concentric contraction, which is an easier and faster way of generating torque-angle curves (a maximum isokinetic muscle contraction can be completed in only a few seconds). This second method was first utilised only recently.[47] More studies have since utilised isokinetic dynamometers to generate in vivo torque-angle curves.[42,44,48] In all these studies, peak torque was measured for each contraction and plotted against ankle angle; the curves were then used to determine the optimum angle of peak torque. The shift in optimum length can be measured directly or indirectly with this equipment. Three studies reported a decrease in isometric peak torque at short lengths, and an increase in isometric peak torque at longer lengths after eccentric exercise.[33,43,51] This would be considered an indirect measure of a shift in the optimum length since the optimum length was not directly measured. Sports Med 2007; 37 (9) Length-Tension Relationship with Eccentric Exercise 3.2 Curve Fitting Direct measures of optimum length shifts can be determined by utilising curve fitting procedures. By fitting a curve to a range of peak torque values at discrete joint angles, a torque-angle curve can be generated. Curve fitting is a convenient way of identifying the angle of peak torque. The most frequently used curve fitting procedures are the Gaussian and quadratic (fourth order) polynomial. In order to use a Gaussian fitting curve, the data must be normally distributed. However, the shape of the single joint torque-angle curve is not symmetrical, thus the data are not normally distributed. Only datapoints >75–90% peak torque can be used to generate the curve. Datapoints above these values have more of a normal distribution. Thus, only the top of the torque-angle curve is used to identify the angle of peak torque. The Gaussian fitting curve is the most commonly used length-tension fitting curve in the literature for both human and animal studies. Jones et al.[38] and Whitehead et al.[39,40] fitted their datapoints above 75% peak torque. Several isometric contractions were measured over an ankle range of 50–90°. Others have used Gaussian fitting curves for datapoints >90% peak torque during isokinetic concentric contractions.[42,47] The greatest concern with the Gaussian curve fitting is that it does not describe changes in the entire torque-angle curve, since only the peak is considered. For this reason, some researchers use quadratic polynomial curve fitting for their data collection.[45,46] Like the Gaussian curve, the angle of peak torque is easily identified. However, with the polynomial fitting curve, changes can be described across the entire range of motion. Another advantage of this procedure is that the data do not need to be normally distributed. © 2007 Adis Data Information BV. All rights reserved. 813 3.3 Equipment and Testing Protocol Brockett et al.,[47] Clark et al.,[48] and Bowers et al. utilised a Biodex® III isokinetic dynanometer to measure knee joint angle and peak torque values during maximum concentric contraction. They all used a constant velocity of 60° per second during knee extension and knee flexion. The ‘zero angle’ was set at 90° of leg flexion for Brockett et al.,[47] thus an increase in degrees meant an increase in length (joint angle). Conversely, Clark et al.[48] and Bowers et al.[42] set their zero angles at full leg extension. Either five[48] or seven[47] maximum concentric contractions were averaged and used as data in these studies. [42] Pettitt et al.[44] and Philippou et al.[45] used a KinCom isokinetic dynamometer to measure torque and corresponding joint angles. Philippou et al.[45] had their subjects perform maximum isometric contractions of the elbow flexors (biceps brachii). The contractions were measured over a range of five joint angles (50, 70, 90, 140 and 160°) in random order. The subjects performed two maximum voluntary isometric contractions of 3-second durations at each joint angle. The better of the two trials was used for the data collection. The subjects in the study of Pettitt et al.[44] performed three maximum concentric contractions of elbow extension (triceps brachii). The repetition with the highest torque value was used to determine optimum length. Prasartwuth et al.[46] had their subjects perform maximum isometric contractions on an isometric myograph, which measured peak torque and joint angle. The average of two maximum isometric contractions (biceps brachii) was measured at each discrete joint angle. Joint angle measurements started at 60° and progressed in 10° increments to 150°. In addition, during each contraction, paired-pulse stimulation was used to evoke a superimposed twitch. It was reported that when voluntary force is reduced, paired-pulse stimulation is an appropriate stimulus Sports Med 2007; 37 (9) 814 Brughelli & Cronin for estimations of voluntary activation. Peak torque was plotted against each discrete joint angle. Whitehead et al.[39,40] and Jones et al.[38] used a custom-made adjustable chair with a steel frame to measure torque and joint angle. The steel frame supported a footplate that was attached to a rotating axis. Torque was measured with four strain gauges attached to an axle supporting the footplate. Torque was measured isometrically from 90 to 50° in 5°[38] or 10°[39,40] increments. Ankle angle was defined as the angle between the footplate and the shins. All three studies determined the torque-angle curve by using double pulse stimulation of the tibial nerve, using 1ms pulses 20ms apart. It was reported that a torque-angle curve constructed this way was easier to control and similar in shape and optimum length to those taken from maximum voluntary contractions.[35] 3.4 Exercise Methodology and Results Up until this point there has been no discussion on the magnitude of the shift in optimum length after eccentric exercise. Table I lists the nine studies in order from the smallest to largest shifts in optimum length. There are three main factors that influence the magnitude of the shift: 1. eccentric exercise intensity;[40,44,45] 2. eccentric exercise volume;[42,46,47] 3. length of the muscle during eccentric exercise.[42,45,46] Jones et al.[38] and Whitehead et al.[39,40] used similar eccentric exercise protocols and thus reported similar shift magnitudes (3.9, 4.4 and 6°). Their subjects walked backwards on an inclined moving treadmill, which was meant to eccentrically strain the triceps surae. However, the exercise protocols were not exactly the same, and thus may explain the differences in their shifts. For the experimental legs, the subjects were instructed to step backwards with a toe-to-heel action. This technique ensured that the triceps surae was contracted eccentrically. The sub© 2007 Adis Data Information BV. All rights reserved. jects performed the toe-to-heel action in one leg (experimental), while the other leg (control) landed with the toe and heel simultaneously. When the toe and heel land together, eccentric contraction is limited. All three studies inclined their treadmills to 13°. Jones et al.[38] used a very low intensity protocol (1.3 km/h stepping rate), which was carried out for a long duration (2 hours). Whitehead et al.[39] had their subjects step at a higher rate (3.5 km/h), which increased the intensity of the eccentric contractions. Whitehead et al.[40] had their subjects step at a slightly higher stepping rate (>3.5 km/h). However, the main difference between the studies was that Whitehead et al.[40] had their subjects carry an additional 5–10kg weight belt. The additional load and step rate increased the intensity of their eccentric exercise protocol, which may explain why they had a greater magnitude of shift in optimum length. Brockett et al.[47] and Clark et al.[48] used the same exercise (Nordic hamstring) and reported similar shifts (7.7 and 6.3°). The Nordic hamstring exercise involves a subject kneeling on the ground (or a board), and while maintaining a constant and open hip angle, the subject slowly lowers their body towards a prone position. In this exercise, the hamstrings are isolated and control the lowering body while contracting eccentrically. Brockett et al.[47] used a custom-made 2m long wooden board with upholstered areas for the chest and knees. The ankles of the subjects were stabilised to the board with ankle straps. Clark et al.[48] used the same exercise, except the subjects were placed on the ground and had a partner. The subjects placed a towel below their knees and had a partner apply pressure to their heels to ensure that their feet stayed in contact with the ground throughout the movement. The Nordic hamstring exercise can be classified as a submaximal exercise intensity, but Brockett et al.[47] used a very high exercise volume (12 sets of 6 repetitions). This volume was meant to acutely damage the hamstrings, and then measurements would be taken to Sports Med 2007; 37 (9) Length-Tension Relationship with Eccentric Exercise determine how long the shift in optimum length would last. The subjects in the study by Clark et al.[48] performed 2–3 sets of 5–8 repetitions. This protocol was repeated 1–3 times per week for 4 weeks. This protocol started with a low volume that increased over a 4-week period. The objective was not to cause acute damage, but to slowly increase the intensity and volume of eccentric exercise. From these two studies, it appears that a high-volume eccentric protocol can cause an acute shift in optimum length. If volume is increased overtime, acute muscle damage can be avoided, while a shift in optimum length may occur over a 4-week period. Important technical notes were to slowly lower the body while maintaining contraction of the hamstrings for as long as possible. Pettitt et al.[44] and Philippou et al.[45] utilised an isokinetic dynamometer (Kin-Com) to perform maximum eccentric contractions for their exercise protocols. The elbow flexors (biceps brachii)[45] and the elbow extensors (triceps brachii)[44] were used to perform maximum eccentric contractions. The nondominant arms were utilised in both protocols. In the study of Pettitt et al.,[44] the subjects performed three sets of 25 maximum eccentric repetitions (50–170°) at a velocity of 60° per second. This study investigated the effects of eccentric exercise performed at short muscle lengths (0–80°) and long muscle lengths (80–130°). The short-length group did not show a shift in optimum length after eccentric exercise. The long-length group did show a significant shift in optimum length (10.4°). The eccentric exercise protocol used a much higher intensity than the previous studies. The eccentric contractions were performed with maximum voluntary effort. This could explain why the shift was higher than the previous studies. However, Philippou et al.[45] used a very similar exercise protocol to Pettitt et al.,[44] but reported a much greater shift in optimum length (16.1° vs 10.4°). In fact, the subjects performed 25 fewer repetitions (two sets of 25 repetitions). The © 2007 Adis Data Information BV. All rights reserved. 815 subjects in the study by Philippou et al.[45] performed maximum eccentric contractions of the elbow flexors to 170° elbow extension. Pettitt et al.,[44] on the other hand, had their subjects perform maximum eccentric contractions of the triceps to 130° elbow flexion. It should be noted that the range of motion of elbow flexion is much less than elbow extension. The design of the elbow joint will not allow the triceps brachii to be fully stretched. Thus, the differences in shifts may be due to the muscle length operating range during eccentric contraction. From these two studies, it appears that very little damage occurs at short lengths (0–80° elbow extension),[44] where damage is observed at longer lengths (80–130° elbow extension),[44] and damage is the greatest at the longest lengths (50–170° elbow flexion).[45] Bowers et al.[42] reported a 15.4° shift after a stepdown exercise that eccentrically contracted the quadriceps. The subjects stepped up to a platform (height set at the patella) with their left leg (control), followed by their right leg (experimental). Once both feet were placed on top of the platform, the subjects slowly stepped down with their left leg leading. The subjects were instructed to control the movement with their right leg, which remained on the platform during the step down. This allowed the quadriceps of the right leg to contract eccentrically. The subjects performed 12 sets of 20 step-downs for a total of 240 steps. This study used a very high volume of 240 total eccentric contractions. Also, the exercise was designed to have the eccentric contractions occur at long muscle lengths. The subjects stepped down from a platform while their experimental leg contracted eccentrically to control the movement. Prasartwuth et al.[46] also reported a large shift in optimum length (16.7°) after eccentrically training the elbow flexors (biceps brachii). A pulley wheel was designed for the eccentric exercise protocol. The axis of rotation of the pulley wheel was aligned Sports Med 2007; 37 (9) 816 Brughelli & Cronin with the subjects’ elbow joints. The subjects lowered a weight attached to the wheel from 60° to full elbow extension. Initially, five sets of ten repetitions were performed. The sets were continued until maximum voluntary torque fell by 40%. The total amount of repetitions varied from 50 to 160. The eccentric contractions were performed from 60° to full elbow extension. The combination of high volume and long muscle lengths in this and the previous study probably contributed to the large shifts in optimum length (15.4[42] and 16.7°[46]). From these nine studies, a few tentative conclusions can be made: • high-intensity eccentric exercise results in greater shifts in optimum length; • muscles that are eccentrically contracted at longer lengths result in greater shifts in optimum length; • high-volume eccentric exercise results in greater shifts in optimum length; • the combinations of high-intensity/long muscle length or high-volume/long muscle lengths produced the greatest shifts in optimum length; • muscle architecture does not seem to affect the shift in optimum length; • eccentric exercises that involve long muscle lengths and either high intensity or high volume should be developed to produce the greatest shifts in optimum length; • it may be possible to produce a sustained shift in optimum length after 4 weeks of eccentric exercise; • muscle damage may not need to be induced for this adaptation (shift in optimum length) to occur. Study investigators should keep these factors in mind when developing eccentric exercise protocols. © 2007 Adis Data Information BV. All rights reserved. 4. Mechanisms for the Shift in Optimum Length Several mechanisms have been proposed for the rightward shift after eccentric exercise, which include a partial transformation of active contractive elements into passive elastic elements;[14] damage to the myotendinous attachments;[41] and damage to the calcium handling structures.[27,52] It has further been proposed that two different shifts occur after eccentric exercise. The first shift is acute and appears after the muscle has been damaged, and the second shift is due to an adaptation that occurs over a period of time (10 days to 8 weeks).[2,8,42] The mechanisms proposed for this second shift include an increase in sarcomeres in series (sarcomereogenesis)[53] and an increase in passive tension at longer muscle lengths.[12] 4.1 First and Second Shift in Optimum Length Structural muscle damage has been observed immediately after eccentric exercise. Electron microscopic examinations have shown Z-line streaming (sarcomeres out of register with each other), regions of overstretched half sarcomeres and t-tubule damage.[4,54,55] The disrupted sarcomeres are thought to be responsible for the first acute shift in optimum length. The descending limb of the length-tension curve is thought to be a region of instability, where force levels decrease. It has been proposed by Morgan[53] that during active muscle lengthening when the myofilaments are stretched onto the descending limb, some of the weaker sarcomeres will be stretched more than others. These sarcomeres will become progressively weaker until there is no overlap between myofilaments. When the eccentric contractions are repeated, more sarcomeres will be overextended from weakest to strongest. At the end of each contraction, as the muscle relaxed, a number of sarcomeres may not reintegrate, thus becoming disrupted. The disrupted sarcomeres will be scattered at Sports Med 2007; 37 (9) Length-Tension Relationship with Eccentric Exercise random along muscle fibres, which increases series compliance. The acute shift in optimum length after eccentric exercise is thought to result from an increase in series compliance, due to disrupted sarcomeres.[2,53] It has also been proposed that a second shift occurs after eccentric exercise. This delayed shift is argued to be a result of either a mechanical[13,56] or cellular[10,53] adaptation in the muscle. Despite the arguments, there is a growing interest in the additional stability at longer muscle lengths due to the shift in optimum length. One of the proposed mechanisms for this second shift is an addition of sarcomeres in series (sarcomereogenesis) after eccentric exercise.[2,10,53] Many believe that a more compliant muscle will avoid the unstable regions (descending limb) of the length-tension curve during further eccentric exercise.[2,50] The other proposed mechanism for the second shift is an increase in passive tension at longer muscle lengths. This adaptation is thought to occur after a period of eccentric training (6–8 weeks). The belief is that eccentric exercise leads to greater passive stiffness, or spring-like qualities.[12,56] LaStayo et al.[13] argued that an increase in stiffness at longer lengths will increase force production before failure. Stability at longer muscle lengths may prevent active muscle strain injuries and possibly enhance athletic performance. 4.2 Theory of Sarcomereogenesis Sarcomereogenesis is the addition of sarcomeres in series within a muscle fibre and is thought to have an important role in maintaining the relationship between sarcomere length and joint angle. The number of sarcomeres in series is thought to be an adaptable property of muscle. After sarcomereogenesis has occurred, sarcomere length will be shorter for a given muscle length.[53] This adaptation is thought to keep the myofilaments off the descending limb of the length-tension curve during future © 2007 Adis Data Information BV. All rights reserved. 817 eccentric contractions. Thus, the muscle will maintain stability at longer muscle lengths. It has been argued that the number of sarcomeres in series is highly plastic and can be altered with training.[32,57,58] Herzog et al.[59] found that lengthtension properties of the rectus femoris were different between runners and cyclists. With running, the rectus femoris experiences a stretch-shortening cycle, where high force levels were required at long lengths during both eccentric and concentric contractions. Cycling requires high forces at shorter muscle lengths during concentric contractions. Consequently, it would be expected that the runners have more sarcomeres in series than the cyclists. The study reported that runners produce peak torque at long lengths, and cyclists produce peak torque at short muscle lengths.[59] Direct evidence for sarcomereogenesis has been observed in several animal and human studies. Most of the studies reported that sarcomereogensis occurred after a static stimulus (passive static stretching or shortening). However, there are a few studies that have shown an increase in serial sarcomere number after eccentric exercise. Lynn and Morgan[60] were the first to report direct evidence of an increase in the number of sarcomeres in series after eccentric exercise. Rats were trained to run uphill (concentric group) or downhill (eccentric group) on a treadmill for 5 days. Running downhill caused the muscles to contract eccentrically as they controlled the movement, while running uphill was predominantly a concentric exercise. The number of sarcomeres in series (vastus intermedius) increased in the downhill group. Since then, four more studies have reported an increase in serial sarcomere number after eccentric exercise.[61-64] Butterfield et al.[63] also reported an increase in sarcomeres in series (vastus intermedius) after trained rats ran downhill for 10 days. More surprisingly, the group of rats that ran uphill decreased the number of sarcomeres in series in the vastus intermedius muscle. Sports Med 2007; 37 (9) 818 Brughelli & Cronin Furthermore, Whitehead et al.[39] found that if eccentric exercise is followed by previous concentric exercise, muscle damage and the shift in optimum length was greater than if eccentric exercise was performed alone. The authors concluded that concentric-based training programmes produce changes in the muscles that make them more prone to injury caused by eccentric exercise. Athletes such as triathletes and marathon runners, who regularly perform both concentric and eccentric muscle contractions for a prolonged period of time may benefit from these findings. Programmes should be developed to help these athletes enhance their performance, while preventing muscle damage induced by eccentric exercise.[39] 4.3 Theory of Passive Tension Another proposed mechanism for the second shift in optimum length after a period of eccentric exercise (6–8 weeks) is a greater contribution of the passive elements at longer muscle lengths.[13,56] More passive tension at longer muscle lengths can also shift the length-tension curve to the right. Eccentric exercise can cause disruption to the muscles passive components. Cytoskeletal proteins, such as desmin and titin, play a significant role in the structure and function of the sarcomere.[29] After eccentric exercise, disruption and degradation occurs to the desmin and titin proteins.[22,29] Titin content has been reduced by up to 30% in human subjects (vastus lateralis) after a single bout of eccentric exercise.[65] A protective adaptation has been suggested to occur that strengthens the cytokeletal proteins, and prevents them from being damaged in the future. Barash et al.[66] reported an increase in desmin content 7 days after eccentric exercise-induced muscle damage in rats. Severe or continuous muscle strain is thought to be a stimulus for adaptations in titin expression.[67,68] Spiers et al.[68] reported that sarcomere strain amplitude and titin isoform size were correlated in fish (carp). Fibre types are sepa© 2007 Adis Data Information BV. All rights reserved. rated anatomically in fish, making them a good model for study. Sarcomeres that are exposed to greater strains may express titins with larger elastic segments. Differences in titin expression have also been reported in humans. Weight lifters, power lifters and sprinters expressed a greater percentage of titin-1 isoforms than non-athletes.[69] The subjects in the athletic groups would be expected to experience strain more often and to a greater degree. The titin filament has a major role in the development of passive tension. However, the role of the titin in muscular performance during explosive activities is unknown. It is possible that the titin is capable of storing and releasing elastic energy.[13,70] McBride et al.[69] reasoned that a more elastic muscle (due to altered titin isoforms) would lead to greater power production, and proposed that explosive training can alter titin expression. In this study, subjects performed explosive squat-jump training for 8 weeks with either 30% or 80% of their maximum back squat. There was no change in titin isoform expression after the training period. However, the training stimulus did not induce a great amount of muscle strain, which would be needed for a mechanical adaptation to occur. Despite the claims that eccentric training can alter titin expression, currently there are no training studies that have examined the effects of eccentric training on titin expression. There are, however, studies that have observed increases in passive stiffness at longer muscle lengths after eccentric exercise. Increases in passive stiffness have been reported in animal[56,71] and human training studies[71,72] after eccentric exercise. Labeit et al.[73] reported that passive force enhancement was caused by a stiffer molecular titin. It was also reported that the titin becomes more sensitive to calcium at longer lengths.[73,74] Since the titin filament is responsible for the majority of passive tension and stiffness at longer muscle lengths,[29,73] this could be an indirect indicator that an adaptation has occurred. Even Sports Med 2007; 37 (9) Length-Tension Relationship with Eccentric Exercise though the torque-angle curve is taken when a muscle is active and contracting maximally, both passive and active elements contribute to the curve. If eccentric exercise can cause an increase in passive tension at longer lengths, the length-tension curve should be shifted to the right. This adaptation to the titin filament could be the mechanism for the second shift in optimum length. It should be noted that this increase in passive tension after a training period with multiple eccentric training sessions, should not be confused with the well documented increase in passive tension that occurs immediately and up to 10 days after a single bout of eccentric exercise. The increase in passive tension immediately after eccentric exercise is due to disruption of the excitation-coupling process. The increase in passive tension that occurs after an eccentric-based training programme of 6–8 weeks is thought to be caused by an increase in the contribution of the passive elements. 5. Implications for Athletic Performance and Injury Prevention 5.1 Muscle Injury Prevention There is a growing interest in the effects of eccentric exercise on muscle strain injuries, specifically for the hamstrings.[9,50] The design of the biarticulate hamstring muscles places them at a high risk of injury. During the late swing phase in running, the simultaneous actions of hip extension and knee flexion actively stretch the hamstrings to greater lengths. Because of the prevalence of hamstring injuries in sport, finding methods of decreasing injury rates is a high priority. Hamstring strain injuries alone account for 6–16% of all injuries in Australian Rules football,[75] soccer,[76] basketball,[77] cricket[78] and rugby union.[79,80] In addition, the risk of re-injuring the hamstrings is even higher (12–31%).[76,81] Previous injury is considered one of the greatest risk factors for re-injury. It has been © 2007 Adis Data Information BV. All rights reserved. 819 proposed that hamstring injuries most often occur when they are being actively lengthened,[82,83] or when they need to switch from an eccentric contraction to a concentric contraction after being actively lengthened.[84] The long head of the biceps femoris is the most often injured hamstring muscle.[83] Unlike the semitendinosus and semimembranous, the biceps femoris is actively lengthened throughout the latter half of the swing phase during sprinting.[83] This makes the biceps femoris more susceptible to muscle strain injuries. Many believe that athletes who produce peak torque at shorter muscle lengths are more likely to get injured.[9,10,47,50] A shorter optimum length would mean that more of the muscles operating range would be on the descending limb of the length-tension curve. Brockett et al.[50] explored this idea by measuring optimum lengths in athletes who have had previous hamstring injuries in one leg (experimental) versus their uninjured leg (control). The mean optimum angle was 12.7° shorter for the injured leg, although strength values (hamstring/ quadriceps ratio) were similar. Based on these findings, the authors suggested that optimum length may be a greater risk factor for muscle strain injuries than strength ratios. The conclusions were that a shorter optimum length would place an athlete at greater risk for muscle strain injuries. Another interesting adaptation that occurs after a single bout of eccentric exercise is called the ‘repeated bout effect’. If a second bout of eccentric exercise is performed that is similar to the first, muscle damage is significantly reduced.[6,18,85] This adaptation can last for several weeks, and possibly up to 6 months.[85] The repeated bout effect can occur even if there is slight muscle damage.[18] The rightward shift in the length-tension curve is considered one of the possible mechanisms for the repeated bout effect.[7] Several authors believe that during eccentric exercise, damage occurs on the descending limb of the length-tension curve.[42,50,53] The deSports Med 2007; 37 (9) 820 Brughelli & Cronin scending limb is the region on the curve that is beyond the optimum length of myofibril overlap. Some have suggested that muscle strain injuries occur on the descending limb of the length-tension curve.[9,50] By shifting the optimum length to greater lengths, the descending limb may not be reached during subsequent eccentric activities, i.e. sprinting, kicking and jumping. Although there is a growing interest in eccentric exercise for preventing muscle strain injuries, very few studies have actually assessed injury rates after a period of eccentric exercise. There currently are only three published studies[49,86,87] and one unpublished study[9] that have reported a reduction in hamstring injuries with eccentric exercise (table II). It should be noted, however, that >700 athletes were monitored during these four studies, and they all reported significant drops in hamstring injury rates. 5.2 Eccentric Exercise and Athletic Performance Despite the recent interest in eccentric exercise and the length-tension relationship, there is a gap in the literature on the effects of eccentric exercise on athletic performance. Intuitively, one would think that if the mechanical properties of muscle are changing, then the functional properties would also change. Also, since human movement typically takes place when the muscles are on or near their optimum lengths,[29,57] performance may be altered if the optimum length is shifted. However, this is not the case for all muscles. Some muscles operate solely on the descending or ascending limbs of the length-tension curve.[29] It has been known for some time that eccentric contractions can produce the greatest amounts of force, recruit fast twitch fibres with little effort, and place greater strain on the muscle to induce favourable adaptations.[14,15,29,31] Many have suggested that more research should be performed with eccentricbased training programmes.[13,29,32] However, re© 2007 Adis Data Information BV. All rights reserved. search of this nature has proved problematic because multiple spotters are needed for heavy eccentric lifts; specialised and expensive equipment is often required; there is a risk of DOMS; and there is a risk of injury. These limitations, unfortunately, led practitioners and scientists to suggest that eccentric exercise should only be performed by advanced athletes. However, given the resurgent interest in eccentrics, exercises are being developed for subjects over a range of abilities. The challenge for clinicians and strength and conditioning practitioners is to be more inventive in their prescription of eccentric exercise. Much of the literature addressing injury and athletic performance uses the Nordic hamstring curl or similar derivations. Such exercises have limitations. First, it is an open-chain/bilateral exercise. It is very likely that the stronger leg (longer optimum length) will be strained more than the weaker leg (shorter optimum length). Both Brockett et al.[47] and Clark et al.[48] reported differences in optimum length between legs. After 4 weeks of training with the Nordic hamstring exercise, Clark et al.[48] reported that the imbalance became larger between the legs. It was thought that the leg with a longer optimum length would receive more strain and continue to adapt to a longer optimum length, while the other leg would not adapt as much. Secondly, this exercise is a single joint exercise. The hamstrings are a biarticulate muscle group, with both hip extension and knee flexion functions. Injuries most frequently occur when the hamstrings are being actively stretched by simultaneous hip flexion and knee extension (during running or kicking). Multi-joint exercises would probably be more effective at improving performance and preventing muscle strain injuries in the lower body. Thirdly, the subjects are unable to support their own bodyweight at around 30° of knee extension. As the subjects lower their bodyweight, gravity becomes a factor, and is a major factor at 30°. If the effects of gravity could be manipulated, it would be possible to go beyond 30°, since this is Sports Med 2007; 37 (9) © 2007 Adis Data Information BV. All rights reserved. Pilot study. b Eccentric exercise groups (11 clubs participated) union players 2002–4 English Premier rugby (n = 220) Rules football players Amateur Australian SSN group (n = 400) SSS group (n = 288) S group (n = 296) Control group (n = 106) NH group (n = 114) straight leg deadlifts, knee curls, NH football players (n = NR) Additional eccentric exercises: Australian Rules Control group (n = 15) Yo group (n = 15), Yo curl Results SSN group: 0.39 hamstring injuries per 1000h 6–7 reps (weekly basis) per 1000h Additional NH exercise 2–3 sets of SSS group: 0.59 hamstring injuries (weekly basis) 1000h S group: 1.1 hamstring injuries per sustained a hamstring injury Control group: 13.2% of the players hamstring injury NH group: 4% of players sustained a hamstring injuries 2003 experimental season: 2 hamstring injuries 2002 experimental season: 5 injuries 2001 control season: 16 hamstring hamstring injury Control group: 10 of 15 received a hamstring injury Yo group: 3 of 15 received a Additional hamstring stretching (weekly basis) Concentric and eccentric exercise 12wk, regular training exercise (12 sets of 6 reps) 12wk, 5 sessions of additional NH Pre-season training (not detailed) 10wk, regular training Yo training 10wk, 4 sets of 8 reps of additional Exercise protocol hamstring. NH = Nordic hamstring; NR = not reported; reps = repetitions; S = strengthening; SSN = stretching, strengthening and NH; SSS = static stretching and strengthening; Yo = Yoyo Preliminary results. a Brooks et al.[87] Gabbe et al.[86]b Proske et al.[9]a Competitive soccer Askling et al.[49] players (n = 30) Subjects Study Table II. Preventing hamstring strain injuries in sport Length-Tension Relationship with Eccentric Exercise 821 Sports Med 2007; 37 (9) © 2007 Adis Data Information BV. All rights reserved. Physically active men (n = 9) and women (n = 22). Each served as own control Amateur Australian Rules football players (n = 9). No control group Physically active men (n = 34) High-school basketball players (n = NR) Competitive soccer players (n = 23) Benn et al.[72] Clark et al.[48] Colliander and Tesch[88] LaStayo et al.[13]a Mjolsnes et al.[89] 10wk, 2–3 times/wk 10wk 2–3 sets of 6–12 reps (NH) 10wk 2–3 sets of 5–10 reps (LC) NH exercise LC group LC exercise Regular training Control group NH group 8wk, 3 times/wk for 30 min 4–5 sets of 12 max reps Ecc/Con group, Con/Ecc isokinetic dynamometer HE group 12wk, 3 times/wk Con group, Con only isokinetic dynamometer 2–3 sets of 6–8 reps, 1–3 times/wk SLS experimental leg NH group (n = 9) 10wk, 2–3 times/wk 10wk, regular training Control group (n = 15) SL control leg 10wk, 4 sets of 8 reps (Yo) protocol Exercise Yo group (n = 15) Eccentric exercise groups ↔ LC ↔ NH: 11% ↑ in H/Q ratio Hop Hz H/Q ratio ↔ HE: 12% ↑ (no SD reported) Hop Hz VJ HE: 8% ↑ (no SD reported) Ecc/Con group: 7.9% ↑ (ES: 0.92) Con group ↔ NH: 6.6% ↑ (no SD reported) SLS: 8.6% ↑ (ES = 0.25) SL: 6.6% ↑ (ES = 0.2) Control ↔ Yo: 2.4% ↓ in sprint times (ES = 0.8) Results VJ VJ VJ VJ VJ VJ 30m sprint 30m sprint type Con = concentric; Ecc = eccentric; ES = effect size; HE = high-force eccentric ergometer; H/Q = hamstring-quadriceps; Hz = frequency; LC = leg curl; max = maximum; NH = Nordic hamstring; NR = not reported; reps = repetitions; SD = standard deviation; SL = single-leg squat; SLS = single-leg squat with extra stretch load; VJ = vertical jump; Yo = Yoyo hamstring; ↑ indicates increase; ↓ indicates decrease; ↔ indicates no change. Preliminary results. Competitive soccer players (n = 30) Askling et al.[49] a Subjects Study Table III. Enhancing athletic performance with eccentric exercise 822 Brughelli & Cronin Sports Med 2007; 37 (9) Length-Tension Relationship with Eccentric Exercise around the same joint angle that most people produce peak torque during isokinetic testing. Fourthly, after a few weeks of this exercise, subjects are able to lower their bodyweight beyond 30° (personal observations). If an imbalance in optimum length is occurring between legs it would not be appropriate to overload this exercise, which is the normal procedure for training athletes, until this imbalance has been corrected. Given the development of exercises and equipment that safely overload the eccentric contractile ability of muscle, greater interest in the shift in optimum length after eccentric exercise and its effects on athletic performance may become of increasing interest. How the shift in optimum length affects functional and athletic performance, i.e. stiffness/compliance, storage and release of elastic energy, fast stretch-shortening cycle performance, slow stretch-shortening cycle performance, kinetics (force, work, power, rate of force development, and kinematics (acceleration, peak velocity) need to be investigated. It would appear that the shift causes active stiffness to decrease at shorter muscle lengths (due to sarcomereogenesis)[2,10,53] and passive stiffness to increase at longer muscle lengths (due to titin elasticity).[11,12,56] A more compliant muscle-tension unit is capable of storing more elastic energy, while a stiffer muscle-tension unit is capable of producing a faster rate of power output. If the muscle is more compliant at the beginning of the stretch, it would be possible to store more elastic energy. Also, if stiffness increases at the end of the stretch, more energy could be released at a higher rate. Thus, performance of the stretch-shortening cycle would be greatly enhanced. It would be possible that each adaptation would have a role in enhancing athletic performance. The training studies that have assessed athletic performance after eccentric exercise have found that performance does improve. Eccentric exercise has been shown to improve sprint times,[49] jumping © 2007 Adis Data Information BV. All rights reserved. 823 ability,[12,48,72] optimum hopping frequency[12] and overall strength (table III). However, of these studies, two were unpublished,[11,12] one was a pilot study[48] and one used a combination of eccentric/ concentric exercise versus concentric only.[88] Given these limitations, there would seem a need for a great deal more research in this area. 6. Conclusions There has been a re-emergence of interest in eccentric exercise over the last decade. The ability to shift the optimum length with eccentric exercise could have several implications for performance and injury prevention. Altering the mechanical properties of muscle could have profound effects on athletic potential. Also, by allowing the muscle to operate and maintain stability at longer lengths could decrease injury rates. Several studies have reported either an increase in athletic performance or a decrease in injury rates with eccentric exercise. Despite the recent interest in eccentric exercise, there are a few limitations in the literature. Currently, there are no randomised, controlled training studies that have reported a shift in optimum length after eccentric exercise. Only one training study (pilot) reported a shift after 4 weeks. There is also a lack of studies on athletic performance after eccentric exercise. Most studies in the past only performed heavy eccentric exercise or plyometrics and examined the effects on performance. There is only one type of exercise that is currently being used to prevent injuries and enhance performance, i.e. the Nordic hamstring exercise or Yoyo hamstring curl. This type of exercise has a few inherent flaws. Being a bilateral and open-chain exercise, the hamstrings may not be worked equally between legs. Thus, imbalances may occur over time. New exercises need to be developed that do not have these limitations. From the literature and a functional standpoint, lower-body eccentric exercises that are designed to increase the optimum length, prevent muscle injuSports Med 2007; 37 (9) 824 Brughelli & Cronin ries and enhance athletic performance should include as many of the following recommendations as possible: multi-joint movements; high/moderate eccentric force activities; muscles operating at long lengths; closed-chain exercises; high/moderate velocity; easy to implement exercises; cost-effective exercises; high volume (if an acute shift is desirable); and unilateral exercises. Exercise design should take a multi-joint and multi-movement approach. This will allow greater prevention against active strain injuries to a larger number of muscles, and possibly improve performance. Acknowledgements The authors received no funding for the preparation of this article and have no conflicts of interest directly relevant to its contents. References 1. Allen D. 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