Download BioMechanics September 2004

BioMechanics
September 2004
Plyometric concepts reinvent lower
extremity rehabilitation
By: R. Lee Howard, PT, ATC
Plyometrics is a special form of training that has traditionally been used for
developing explosive power in athletes; only recently has its role in rehabilitation and
injury prevention been discussed.1,2 The word plyometrics, derived from plyo
(increase) and metrics (measure), means "measurable increases." This training
method, originally spawned by Yuri Verkhoshansky, was thought to be one of the
primary reasons for Russian track and field success in the 1960s.3
Much of what we read in texts does not represent the true origin of classical
plyometrics.3 Depth jumps are an example of classical exercises where the individual
steps off of a platform that is 36 inches or higher, absorbs the force and quickly
recovers stored energy during a jump upward. These are elite exercises and should
only be performed once proper strength and coordination have been achieved.
Nonetheless, rehabilitation professionals have much to offer using less stressful and
appropriately modified plyometric activities.3 These activities include running, lowintensity hopping and jumping, skipping, lateral changes in direction, slideboarding,
and footwork drills. These type of movements are essential when rehabilitating an
athlete who expects to return to the highest levels of performance.
Plyometric mechanisms
Plyometric training relies on the stretch-shortening cycle (SSC) utilizing the muscle
spindle reflex for energy potentiation.4,5 The stretch occurs during the eccentric
action of the movement, creating tension and storing energy in the series elastic
components (SEC) of the muscle. SECs of the muscle are the spring-like elements
that lie in series to the myofilament (actin, myosin, and tendon), whereas the term
"parallel elastic component" refers to the connective tissues that lie parallel to the
muscle. Focusing training on the SSC to make it faster is thought to enhance the
muscle group to move more quickly and powerfully in response to changes in muscle
length and tension. The stored elastic energy improves power during the concentric
action of the movement, but only when it is performed quickly.6 Energy not used for
function is mechanical energy that dissipates as heat.7
Phases
The intensity of a plyometric exercise is determined by the length of the amortization
phase and coupling time. The amortization phase is the time from the beginning of
the eccentric action to the beginning of the concentric action and is often referred to
as the rate of eccentric action.8 The coupling time (summation phase) is the time
between the end of the eccentric action and the beginning of the concentric action. 8
The summation phase is analogous to the shortening, or recoil, phase of the
movement. The longer the coupling time, the greater the dissipation of kinetic
energy, resulting in decreased concentric power gain. This is critical to understand
because it determines the training effect of the exercise. Classical sports-
performance-oriented plyometrics involves coupling times of less than 0.15 seconds
when performing depth jumps with high force requirements. 3
Strict interpretation of the coupling time creates confusion when applied to
plyometric-like activities such as running and rehabilitation programs. Using this
criterion alone, running would "fit" into this classical example due to its short (<
0.15) coupling in stance. However, running is not considered a high force level
plyometric activity because the action is not of maximum physiological limits but a
cyclic motion that can be maintained. It may be helpful to view running as a means
to improve velocity rather than force when thinking about power (P = F/V: P =
power, F = force, V = velocity) as the force component is too small to lead to an
optimal level of power generation for sports performance. This is a reason that track
and field athletes perform strength training and plyometrics (single-leg bounding,
triple jumps) that are more force oriented to improve running.
Coupling time and rate of force relationships must be taken into consideration
regarding the intensity of plyometrics when used for rehabilitation or sports
performance. Plyometrics geared toward rehabilitation should focus on a progression
from low-impulse activities to more complex and stressful activities, and certainly
would not include depth jumps of great heights. Coupling time will always vary in
rehabilitation because many athletes must begin with form exercises that do not
overload force or velocity components of the power equation. Stabilization hops
require static postures upon landing as opposed to multiple box jumps that will
require a quicker coupling time depending on box height and separation. Thus
coupling time is situation-specific and varies based on the goal of the activity and
healing stage of the athlete.
Criteria
Optimal criteria to begin plyometric activities include absent edema, adequate range
of motion, adequate joint stability, similar limb girth measurements, adequate
isolated strength testing, and adequate balance and functional strength. Examples of
balance and functional testing for the knee include the star balance drill, completing
10 parallel thigh split squats in 15 seconds and drop steps from a 7-inch surface.
Certainly there are other options; the point is to challenge balance strategies and
ensure quick eccentric, isometric, and concentric interactions of motion.
For the lower extremities, consider these guidelines when progressing plyometric
routines:
- engage in double-leg exercises before single-leg ones;
- start with small-amplitude movements;
- emphasize correct form;
- include activities that reinforce natural movement awareness and coordination;
- move from simple to complex activities; and
- initiate stabilization activities prior to dynamic activities.
Following these guidelines will help your athlete avoid unnecessary setbacks during
the recovery process.
The sensorimotor system and
rehabilitation
Proprioception is a variant of the sense of touch that encompasses joint movement
and position.9,10 Proprioceptive feedback is integral to conscious and subconscious
awareness of joint or limb motion. Mechanoreceptors found in the joint, muscle, and
skin provide continuous input regarding tissue deformation into the central nervous
system. Visual receptors and vestibular receptors provide CNS input regarding
balance and body orientation/position. This input is integrated by the CNS and a
motor response is generated. The responses occur through spinal reflexes, cognitive
programming, and brainstem activity.
Reflex muscular stabilization is a reflexive response to joint mechanical loading
(plyometric activities).9,10 Higher CNS levels (cortex, basal ganglia, cerebellum) are
responsible for voluntary movements that are stored as central programs, which
allows for cognitive and noncognitve solutions to movement strategies.4,9,10 It follows
that both anticipatory and reactive responses may be influenced by plyometric
training, which in turn has implications for injury prevention and performance
enhancement. Plyometric activities create a heightened sense of awareness of joint
movement by stimulating the proprioceptive field, in turn improving muscular
cocontraction across active joints via reflex muscle stabilization. Motor learning
occurs through repetition of the program, which may enhance preparatory landing
strategies via higher CNS integration.
Unfortunately, injury distorts mechanical stability while proprioceptive deficits
promote functional instability. The rehabilitation aim is to restore it and plyometrics
may be an effective aid to doing so. 2,9,10
Biomechanics
Several biomechanical factors must be taken into consideration when prescribing
plyometric activities. The body's center of mass must be maintained within its base
of support; if it is not, faulty movement patterns will be produced and efficiency lost.
Upon landing, weight should be distributed on the anterior half or two thirds of the
foot (this depends on whether the landing is one foot or two feet).11 There should be
appropriate flex and recoil according to the activity and purpose of the exercise,
which will result in some exercises having a stiffer appearance and others a softer
look. Ankle hops are a compact movement of low intensity, therefore generating a
large amount of knee flexion on landing becomes less important than the need for
quickness. On the other hand, if the goal is to teach deeper knee bend strategies
when landing, ankle hops with an exaggerated knee flex or squat stabilization jumps
could be used to emphasize deeper flexion control and fluent transition from
eccentric to concentric action.
Regimen design considerations
Designing a plyometric routine for rehabilitation or sports performance is an art. The
following questions should be considered.
- How much volume is necessary?
- At what intensity?
- With what frequency should the routine be performed (sessions per week)?
- What should the duration of the session be?
- Which exercises should be selected?
- What is the injury, if any?
- What stage of rehabilitation is the athlete in?
- What is the athlete's training age (history)?
There is no exact rule to follow when answering these questions but some general
recommendations are provided.
Just as in weight lifting, volume and intensity of plyometrics are inversely related.
Referring to the lower extremities, the higher the number of foot contacts
(equivalent to repetitions in resistance training) the lower the intensity level of the
exercise. The more demanding the exercise, the fewer the number of foot contacts
or seconds for which the exercise should be performed. Intensity should be based on
training age, sport, skill level, and weight of the athlete. If the patient has never
formally trained before, he or she will likely need a basic program. A skilled athlete
needs more challenge and is likely to tolerate more complex drills. An overweight
athlete's program should focus on reactive types of plyometrics such as hexagon
drills or dot drills rather than maximum-level plyometrics such as single-leg
bounding.
Athletes of different genders do not necessarily need drastically different plyometric
approaches. However, our clinical model uses more instruction for the female patient
to control excessive mediolateral knee movement and lack of knee flexion with
jumping and landing in order to guard against possible knee injury. Once proper
technique is achieved, the progression is very similar in female and male athletes.
Clinical application
Three phases of progression may be helpful when instructing a patient in plyometric
activities during rehabilitation. The technical phase, the transitional phase, and the
performance phase (Table 1) are structured to progressively challenge the athlete.
Phase 1 goals are to establish proper technique and enhance the proprioceptive
pathway. The clinician should be teaching key points such as toe-heel landing, triple
flexion (hip and knee flexion, ankle dorsiflexion), triple extension (hip and knee
extension, ankle plantar flexion), and soft landing. Aerobic conditioning should
complement this phase.
Phase 2 goals are to progress the level of difficulty and increase the workload at the
musculotendon/joint complex. Anaerobic and aerobic conditioning should
complement this phase.
Phase 3 goals are to return the athlete to the level at which he or she was
performing prior to injury, if not a higher level. Conditioning should be sport-specific
at this phase.
The range of recommendations across the phases accounts for differences in
diagnoses. For example, a grade 1 ankle sprain would progress more rapidly than a
grade 3 ankle sprain or an ACL reconstruction. A reference that highlights in detail
the different intensities of plyometric activities is Donald Chu's Jumping into
Plyometrics.11 This book should aid the practitioner in selecting appropriate
plyometric exercises in accordance with the magnitude of intensity.
Practical application
Table 2 outlines an example of a practical plyometric routine developed to assist an
injured athlete back into running. A mile run generally consists of 1500 foot contacts,
750 per leg. The exercises are selected to integrate 470 foot contacts per leg, which
would be equivalent to two thirds the foot contacts of a mile. Successfully completing
this routine appears to be a good indicator of an athlete returning to running at half
to three-quarters of a mile distances uneventfully. This is generally a good volume of
running to begin with when trying to integrate the patient/athlete back into
recreational activities or sports participation.
Modification of this volume depends on the sport or activity's physiologic
requirements. Long distance runners may ramp the volume up considerably,
whereas a football participant may only progress to one and a half miles to establish
an aerobic base before beginning interval sprinting better suited for anaerobic
conditioning. This can be used for any sport involving running and is beneficial for
postoperative and nonoperative patients.
Conclusions
Plyometrics are often used without a well-defined goal and little attention is applied
to the scope in which they are used. The interaction of other lifestyle activities,
sports needs, injury history, and rehabilitation phase must be taken into
consideration. Plyometric exercises are safe when supervised by trained
professionals, and in fact research supports the notion that plyometric-like activities
with proper supervision are more likely to prevent rather than cause injuries.3,6
Practitioners should apply these concepts in accordance with sound training
methodologies and critical thinking.
R. Lee Howard, PT, ATC, CSCS, is a physical therapist in the sports physical therapy
department at Wake Forest University Baptist Medical Center. He is also a PhD
student in the exercise and sports science department at the University of North
Carolina-Greensboro.
References
1. Chimera NJ, Swanik KA, Swanik CB, Straub SJ. Effects of plyometric training on
muscle-activation strategies and performance in female athletes. J Athl Train
2004;39(1):24-31.
2. Hewett TE, Lindenfeld TN, Riccobene JV, Noyes FR. The effect of neuromuscular
training on the incidence of knee injury in female athletes: a prospective study. Am J
Sports Med 1999;27(6):699-706.
3. Siff MC. Supertraining, 6th ed. Denver, CO, 2003.
4. Dietz V, Noth J, Schmidtbleicher D. Interaction between pre-activity and stretch
reflex in human triceps brachii during landing from forward falls. J Physiol
1981;311:113-125.
5. Komi PV, Gollhofer A. Stretch reflexes can have an important role in force
enhancement during SSC exercise. J Appl Biomech 1997;13(4):451-460.
6. Cavagna GA, Saibene FP, Margaria R. Effect of negative work on the amount of
positive work performed by an isolated muscle. J Appl Physiol 1965;20:157-158.
7. Cavagna GA. Storage and utilization of elastic energy in skeletal muscle. Exerc
Sports Sci Rev 1977;5:89-129.
8. Radcliffe JC, Farentinos RC. High powered plyometrics. Champaign, IL: Human
Kinetics, 1999.
9. Riemann BL, Lephart SM. The Sensorimotor System, Part 1: The physiologic basis
of functional joint stability. J Athl Train 2002;37(1):71-79.
10. Riemann BL, Lephart SM. The Sensorimotor System, Part 2: The role of
proprioception in motor control and functional joint stability. J Athl Train
2002;37(1):80-84.
11. Chu DA. Jumping into plyometrics, 2nd ed. Champaign, IL: Human Kinetics,
1998.