Download acute effects of static stretching on maximal eccentric torque

Journal of Strength and Conditioning Research, 2006, 20(2), 354–358
q 2006 National Strength & Conditioning Association
ACUTE EFFECTS OF STATIC STRETCHING ON
MAXIMAL ECCENTRIC TORQUE PRODUCTION
IN WOMEN
JOEL T. CRAMER,1 TERRY J. HOUSH,2 JARED W. COBURN,3 TRAVIS W. BECK,2
GLEN O. JOHNSON2
AND
Department of Health and Exercise Science, University of Oklahoma, Norman, Oklahoma 73019; 2Department of
Nutrition and Health Sciences, University of Nebraska-Lincoln, Lincoln, Nebraska 68588; 3Department of
Kinesiology, California State University, Fullerton, Fullerton, California 92834.
1
ABSTRACT. Cramer, J.T., T.J. Housh, J.W. Coburn, T.W. Beck,
and G.O. Johnson. Acute effects of static stretching on maximal
eccentric torque production in women. J. Strength Cond. Res.
20(2):354–358. 2006.—The purpose of this study was to examine
the acute effects of static stretching on peak torque (PT) and the
joint angle at PT during maximal, voluntary, eccentric isokinetic
muscle actions of the leg extensors at 60 and 1808·s21 for the
stretched and unstretched limbs in women. Thirteen women
(mean age 6 SD 5 20.8 6 0.8 yr; weight 6 SD 5 63.3 6 9.5 kg;
height 6 SD 5 165.9 6 7.9 cm) volunteered to perform separate
maximal, voluntary, eccentric isokinetic muscle actions of the
leg extensors with the dominant and nondominant limbs on a
Cybex 6000 dynamometer at 60 and 1808·s21. PT (Nm) and the
joint angle at PT (8) were recorded by the dynamometer software. Following the initial isokinetic assessments, the dominant
leg extensors were stretched (mean stretching time 6 SD 5 21.2
6 2.0 minutes) using 1 unassisted and 3 assisted static stretching exercises. After the stretching (4.3 6 1.4 minutes), the isokinetic assessments were repeated. The statistical analyses indicated no changes (p . 0.05) from pre- to poststretching for PT
or the joint angle at PT. These results indicated that static
stretching did not affect PT or the joint angle at PT of the leg
extensors during maximal, voluntary, eccentric isokinetic muscle actions at 60 and 1808·s21 in the stretched or unstretched
limbs in women. In conjunction with previous studies, these
findings suggested that static stretching may affect torque production during concentric, but not eccentric, muscle actions.
KEY WORDS. voluntary, eccentric isokinetic, peak torque, joint
angle at peak torque, dominant, nondominant
INTRODUCTION
s part of a warm-up routine, static stretching
often is performed prior to exercise (1) and/or
athletic performance (3, 19, 35, 39). It is believed that 2 primary benefits can be achieved
from pre-exercise stretching: (a) improving
performance (35, 38, 39) and (b) decreasing the risk of
injury (13, 14, 18, 32, 33, 38). In theory, decreases in muscle stiffness as a result of stretching would reduce the
amount of force required to move the limb through a
greater range of motion, which would allow for a stretching-induced increase in performance (36). Recent evidence, however, has challenged this hypothesis and has
suggested that an acute bout of pre-exercise stretching
may compromise muscle performance capabilities during
explosive jumping (7, 9, 27, 45, 46) and sprinting (37);
may reduce maximal strength and/or power during dynamic constant external resistance (DCER) (17, 22, 30),
concentric isokinetic (10, 11, 15, 24, 29), and isometric (2,
A
354
5, 16, 28, 31, 32, 43) muscle actions; and may decrease
performance during balance and reaction-time tasks (4).
Collectively, these studies have proposed 2 primary mechanisms to explain the so-called stretch-induced force deficit: (a) mechanical factors, such as stretching-induced alterations in the length-tension relationship (9–11, 15, 16,
22, 29, 43), and (b) neural factors, such as decreases in
reflex sensitivity (32, 42) and/or muscle activation (4, 5,
16, 31) that may be related to central nervous system
inhibitory factors (2, 10, 11).
Recent studies, however, have observed no effects of
stretching on maximal voluntary strength of the plantar
flexors (23), 100-yd dash times (12), vertical jump kinetics
(20), vertical jump performance (41), range of motion and
foot speed while kicking a football (44), or tennis-serve
performance (21). Thus, conflicting evidence exists regarding the acute effects of stretching on performance capabilities. In addition, most previous studies have examined the acute effects of stretching on performance in
men and women combined (15, 16, 20–22, 24, 28–30, 46),
men only (2, 4, 5, 9, 11, 12, 23, 31, 32, 43–45), or boys
(37), yet only a few studies have focused on women only
(7, 10, 41) or girls (27). Although no gender differences
have been reported thus far (15, 16, 20–22, 24, 28–30, 46),
more evidence is needed regarding the acute effects of
stretching in women. Furthermore, the stretching-induced force deficit has been observed only during concentric-only muscle actions (10, 11, 15, 24, 29), countermovement jumping (9), or vertical jumping (7, 26, 45, 46). No
previous studies have examined the effects of static
stretching on eccentric torque production, which is an important component of many athletic events. Therefore,
the purpose of this study was to examine the acute effects
of static stretching on peak torque (PT) and the joint angle at PT during maximal, voluntary, eccentric isokinetic
muscle actions of the leg extensors at 60 and 1808·s21 for
the stretched and unstretched limbs in women.
METHODS
Experimental Approach to the Problem
This study was designed (a) to extend the findings of our
previous studies (10, 11) to include the acute effects of
static stretching on eccentric torque production, (b) to test
the hypothesis of Nelson et al. (29) that the acute effects
of static stretching are velocity-specific by examining eccentric torque production at both 60 and 1808·s21, (c) to
test the previous hypotheses (2, 10, 11) that the stretch-
EFFECTS
ing-induced force deficit (if present) may be related to a
central nervous system inhibitory mechanism by examining eccentric torque production of the stretched and unstretched leg extensor muscles, and (d) to test the hypothesis (9–11, 16, 28, 29, 43) that the decreases in the
force-producing capabilities of a muscle due to stretching
(if any) are related to alterations in the length-tension
relationship.
Subjects
Thirteen women (mean age 6 SD 5 20.8 6 0.8 year;
weight 6 SD 5 63.3 6 9.5 kg; height 6 SD 5 165.9 6
7.9 cm) who were recreationally active (defined as 1–5
hours of exercise or recreational sport activity per week)
but not involved in intercollegiate athletics, volunteered
to participate in this experiment. The study was approved
by the University Institutional Review Board for Human
Subjects, and all subjects completed a health history
questionnaire and signed a written informed consent prior to testing.
Procedures
Approximately 1 week prior to the experimental trial,
each subject participated in a familiarization trial to practice the maximal, voluntary, eccentric isokinetic muscle
actions at 60 and 1808·s21. During the second laboratory
visit, each subject started the experimental trial by completing a 5-minute warm-up at 50 W on a stationary cycle
ergometer prior to the initial isokinetic testing. Before
(pre) and after (post) the static stretching exercises, the
PT generated during maximal, eccentric isokinetic muscle
actions of the leg extensors was measured separately for
the dominant (based on kicking preference) and nondominant limbs using a calibrated Cybex 6000 dynamometer
(Lumex, Inc., Ronkonkoma, NY) at randomly ordered velocities of 60 and 1808·s21. The subjects were in a seated
position with a restraining strap over the pelvis and
trunk in accordance with the Cybex 6000 User’s Guide
(Cybex 6000 Testing and Rehabilitation User’s Guide, Lumex). The input axis of the dynamometer was aligned
with the axis of the knee, and the contralateral leg was
braced against the limb stabilization bar. Three submaximal warm-up trials preceded 3 maximal muscle actions
at each velocity, with the highest PT selected as the representative score. A 2-minute rest was allowed between
testing at each velocity, and a minimum of 5 minutes was
allowed between testing for each limb. The joint angle at
PT (8) was provided by the Cybex 6000 software.
Immediately following the prestretching isokinetic
tests, each subject underwent 4 static stretching exercises
designed to stretch the leg extensor muscles of the dominant limb only. The static stretching procedures used in
this study have been described in detail elsewhere (10,
11, 29). Four repetitions of each stretching exercise were
held for 30 seconds at a point of mild discomfort, but not
pain, as acknowledged by the subject. Between each
stretching repetition, the leg was returned to a neutral
position for a 20-second rest period. The total stretching
time (mean 6 SD) was 21.2 6 2.0 minutes.
Each subject performed an unassisted stretching exercise followed by 3 assisted stretching exercises. For the
unassisted stretching exercise, the subject stood upright
with 1 hand against a wall for balance. The subject then
flexed the dominant leg to a knee joint angle of 908. The
ankle of the flexed leg was grasped by the ipsilateral
OF
STRETCHING
ON
ECCENTRIC TORQUE 355
hand, and the foot was raised so that the heel of the dominant foot approached the buttocks. Following the unassisted stretching exercise, the remaining stretching exercises were completed with the assistance of the primary
investigator.
The first assisted stretching exercise was performed
with the subject lying prone on a padded table with the
legs fully extended. The dominant leg was flexed at the
knee joint and slowly pressed down so that the subject’s
heel approached the buttocks. If the heel was able to contact the buttocks, the knee was gently lifted off the supporting surface, causing a slight hyperextension at the
hip joint, to complete the stretch. To perform the second
assisted stretching exercise, the subject stood with her
back to a table and rested the dorsal surface of her dominant foot on the table by flexing the leg at the knee joint.
From this position, the dominant leg extensors were
stretched by gently pushing back on both the knee of the
flexed leg and the corresponding shoulder. The final assisted stretching exercise began with the subject lying supine along the edge of the padded table with the dominant leg hanging off of the table. The dominant leg was
flexed at the knee and the thigh was hyperextended
slightly at the hip by gently pressing down on the knee.
Immediately after the stretching exercises, each subject
sat quietly for 4.3 6 1.4 minutes (mean 6 SD) before performing the poststretching isokinetic tests for the dominant (stretched) limb and 13.2 6 2.5 minutes before testing the nondominant (unstretched) limb. Because the primary purpose of this study was to examine the effects of
the stretching on the stretched limb, the isokinetic testing
always was performed first on the dominant limb.
Statistical Analyses
Previous test-retest reliability from our laboratory for PT
during maximal, eccentric isokinetic leg extension muscle
actions indicated that for 8 male subjects measured 48
hours apart, the intraclass correlation coefficients (r)
ranged from 0.88–0.97 with no significant (p . 0.05) differences between mean values for test vs. retest at either
velocity (60 or 1808·s21).
Two separate 3-way repeated measures analysis of
variance (ANOVAs; time [pre- vs. poststretching] 3 limb
[stretched vs. unstretched] 3 velocity [608·s21 vs.
1808·s21]) were used to analyze the PT and the joint angle
at PT data. An alpha of p # 0.05 was considered statistically significant for all comparisons.
RESULTS
Table 1 contains the mean 6 SEM values for PT and the
joint angle at PT for the dominant and nondominant
limbs during the pre- and poststretching isokinetic assessments. For PT, the analyses indicated no significant
3-way (time 3 limb 3 velocity) or 2-way (time 3 limb;
time 3 velocity; limb 3 velocity) interactions and no significant main effects for time or limb, but did indicate a
significant main effect for velocity. There was a decrease
in PT (collapsed across time and limb) from 60 to 1808·s21
(Figure 1a), but there were no significant differences in
PT from pre- to poststretching.
For the joint angle at PT, the analyses indicated no
significant 3-way (time 3 limb 3 velocity) or 2-way (time
3 limb; time 3 velocity; limb 3 velocity) interactions, no
significant main effect for time, but significant main effects for limb and velocity. The joint angle at PT (col-
356
CRAMER, HOUSH, COBURN
ET AL.
TABLE 1. Peak torque (Nm) and the joint angle at peak torque (8) values.
Prestretching
Dominant leg
608·s
Peak torque (Nm)
Mean
SEM*
21
204.1
11.3
1808·s
Poststretching
Nondominant leg
21
608·s
21
1808·s
Dominant (stretched leg)
21
608·s
21
1808·s
21
Nondominant (unstretched) leg
608·s21
1808·s21
191.2
11.8
189.6
10.6
187.1
13.0
201.8
15.6
178.8
16.9
197.6
12.7
185.1
12.6
The joint angle at peak torque (8)
Mean
68.2
62.8
SEM*
2.5
2.7
58.9
3.1
58.7
2.3
69.3
2.0
64.5
2.8
65.1
2.3
58.2
2.5
* SEM 5 standard error of the mean.
DISCUSSION
FIGURE 1. (a) The marginal means for peak torque (collapsed
across limb, Nm). (b) The joint angle at peak torque (8). Values
are mean 1 SEM. There were no stretching-related changes in
peak torque or the joint angle at peak torque (p . 0.05). * The
marginal means for peak torque (collapsed across time and
limb) were greater at 608·s21 than at 1808•s21 (p # 0.05). **
The marginal means for the joint angle at peak torque
(collapsed across time and velocity) were greater for the
dominant than the nondominant limb (p # 0.05). † The
marginal means for the joint angle at peak torque (collapsed
across time and limb) were greater at 608·s21 than at 1808•s21
(p # 0.05).
lapsed across time and velocity) was greater for the dominant than for the nondominant limb. In addition, the
joint angle at PT (collapsed across time and limb) was
greater at 608·s21 than at 1808·s21 (Figure 1b). There were,
however, no significant differences in the joint angle at
PT from pre- to poststretching.
It is traditionally believed that there are 2 primary benefits associated with stretching: (a) improving performance (35, 38, 39) and (b) decreasing the risk of injury
(13, 14, 18, 32, 33, 38). Most previous studies, however,
have reported acute decreases in the performance capabilities of a muscle as a result of stretching (5, 7, 9–11,
15–17, 22, 24, 27–32, 43, 45, 46), whereas others have
observed equivocal performances immediately following a
bout of stretching (20, 21, 41, 44). Of the investigations
that have reported stretching-induced decreases in
strength or power for isolated muscle groups, the
strength-testing protocols have included isometric (5, 16,
28, 31), concentric isokinetic (10, 11, 15, 24, 29), or DCER
(17, 22, 30) muscle actions. The unique contribution of the
present study, therefore, was that the static stretching
did not elicit changes in maximal, eccentric isokinetic
torque (Figure 1a). Thus, the present study extended the
findings of previous studies (5, 10, 11, 15–17, 22, 28–31)
and suggested that the acute effects of static stretching
may be mode-specific, affecting isometric and concentric
strength, but not eccentric torque production.
Two primary hypotheses have been proposed to explain the stretching-induced decreases in force (or torque)
production during isometric (5, 16, 28, 31), concentric isokinetic (10, 11, 15, 24, 29), and/or DCER muscle actions
(17, 22, 30): (a) mechanical factors, involving alterations
in the muscle’s length/tension relationship (9–11, 15, 16,
22, 29, 43) and (b) neural factors, such as decreased muscle activation and/or altered reflex sensitivity (2, 4, 5, 10,
11, 16, 31, 32, 42). Previous studies have reported stretching-induced increases in the joint angle at which maximal
isometric (16, 28) and concentric isokinetic (10) force is
produced. In a previous study, however, we reported no
changes in the joint angle at PT during concentric isokinetic muscle actions (11). It has been suggested that the
shape and parameters of the isokinetic joint angle-torque
relationship can be used as global indicators of the
length-tension relationships of the active sarcomeres (6,
25). Therefore, any stretching-induced changes in the
joint angle at PT may reflect stretching-induced changes
in the length-tension relationship. In the present investigation, however, we observed no changes in the joint
angle at PT during the maximal eccentric muscle actions
following the static stretching (Figure 1b). These findings
were consistent with those from our previous study (11)
that reported no change in the joint angle at PT following
static stretching.
EFFECTS
It also has been hypothesized that neural factors contribute to the stretching-induced decrease in force, which
may be due to a central nervous system inhibitory mechanism (2, 10, 11). Previous studies have reported decreases in strength (2, 10, 11) and muscle activation (2, 11) for
both the stretched and the unstretched limbs. Because
there were stretching-induced decreases in the unstretched limb, it was hypothesized that some residual neural
inhibition could have occurred at the level of the spinal
cord in response to the stretching (2, 10, 11). In the present study, however, there were no changes in PT from
pre- to poststretching for either the stretched or the unstretched limb during the eccentric muscle actions. These
findings suggested that no neural inhibition occurred as
a result of the static stretching. It is known, however, that
maximal eccentric muscle actions require lower levels of
muscle activation, but yield higher levels of torque production when compared to concentric muscle actions (8,
34, 40). Therefore, the lack of change in eccentric torque
production in the present study may be related to the
unique motor control strategies that are used to modulate
torque production during eccentric muscle actions.
A recent study by Nelson et al. (29) reported decreases
in maximal, concentric isokinetic PT at 60 and 908·s21 (7.2
and 4.5% decreases, respectively), but no changes at 150,
210, or 2708·s21 after an acute bout of static stretching. It
was concluded that the stretching-induced decreases in
PT may be velocity-specific, occurring primarily under the
high torque production conditions associated with the
slower velocities (60 and 908·s21), but not the lower torque
production conditions at the faster velocities (150, 210,
and 2708·s21) (29). Several subsequent studies, however,
have reported stretching-induced decreases in concentric
torque production for the leg extensor muscles at 60, 240,
and 3008·s21 (10, 11, 24) and for the forearm flexor muscles at 30 and 2708·s21 (15). These studies have suggested
that stretching-induced decreases in PT may not be as
velocity-specific as suggested by Nelson et al. (29). In addition, the results of the present study indicated no
stretching-induced decreases in eccentric PT at either 60
or 1808·s21.
PRACTICAL APPLICATIONS
The results of this study indicated that the static stretching did not alter maximal, voluntary eccentric isokinetic
PT or the joint angle at PT at 60 or 1808·s21 for the
stretched or the unstretched leg extensor muscles in
women. In conjunction with previous studies (2, 5, 10, 11,
15, 16, 22, 24, 28–31, 43), these findings suggested that
the effects of static stretching may be mode-specific, affecting concentric, but not eccentric, muscle actions.
These findings may have implications for athletes who
stretch prior to athletic events. Furthermore, in contrast
to similar studies that have reported stretching-induced
force deficits during isometric (5, 16, 28, 31), concentric
isokinetic (10, 11, 15, 24, 29), and DCER (17, 22, 30) muscle actions, the lack of change in eccentric torque production in the present study may be related to the unique
motor control strategies that are used to modulate torque
production during eccentric muscle actions. Future studies should investigate the motor control strategies involved during eccentric torque production in response to
an acute bout of stretching.
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ECCENTRIC TORQUE 357
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Acknowledgments
This study was funded, in part, by the National Strength and
Conditioning Association Student Grant Program.
Address correspondence
[email protected].
to
Dr. Joel T. Cramer,