Stretching is commonly performed before exercise in an attempt to improve muscle flexibility, reduce the risk of skeletal muscle injury, and enhance performance. Although these are the reasons that many individuals participate in stretching, current research provides inconsistent evidence of the effectiveness of stretching in injury prevention or in improving muscle performance (13-17,22,23). The effects of either acute or chronic stretching on the development of muscle force and power was recently reviewed by Shrier (26). The review concluded that chronic stretching may actually enhance muscle force production through stretch induced hypertrophy, but acute bouts of stretching may be detrimental to exercise performance. A number of studies have shown that stretching prior to exercise reduces muscle force production as well as nervous activation of muscle (2,3,10,11,21,27,29). These results clearly have implications for those looking to maximize muscle force and power and warrant studying the effects of longer term stretching programs on muscle force development.
One of the reasons that stretching prior to exercise may be disadvantageous is that it may reduce the sensitivity of the muscle proprioceptors to stretch and hence stretch reflex activity. Stretching is often completed for the purpose of reducing passive muscle stiffness, and it has been proposed that this may occur through the modulation of peripheral and/or central nervous function (3,10,21). This has been shown to occur through decreased alpha motor neuron activity and decreased output from, or sensitivity to, muscle and joint proprioceptors (11,29). It is thought that attenuated motor neuron activity reduces resistance to stretch in the relaxed state and that attenuated proprioception can reduce stretch-induced reflexive muscle contraction. These changes would therefore reduce passive muscle stiffness, allow a greater muscle length to be achieved, and would thus increase joint range of motion. These sound like advantageous changes, but the decrease in voluntary as well as reflex-induced muscle contraction has the potential to negatively affect physical performance. This decreased reflex sensitivity has been shown to last for a number of days after muscle stretch and may therefore question the use of chronic stretching by strength and power athletes (4).
Muscle's ability to produce force can be affected by stretching because the capacity of the muscle to store energy elastically and the force developed by contraction sum to produce the net muscle force. Reduced muscle stiffness due to plastic deformation of the in-series or in-parallel elastic components leads to a decrease in energy storage. Specifically, stretching may be able to disrupt myosin cross-bridge formation by breaking regions of contracture; distend the connective tissue of the endomysium, perimysium, and epimysium; and lengthen in-series proteins like titin (30). These notions have been supported by the work of Magnusson et al. (18-20) who demonstrated that acute bouts of static stretching can reduce muscle stiffness and work absorption for up to 1 hour post-stretching. The reduced stiffness lowers the work absorption capacity of the muscle and indicates that the muscle is less capable of storing energy to be used elastically. Consequently, this would negatively affect force production during ballistic movements that are commonly used during vigorous physical activity. If chronic stretching can elicit a lasting reduction in stiffness, it could negatively affect muscle performance in the long term.
A common assumption among many athletes, coaches, therapists, and physicians is that stretching is a key component of the warm-up routine prior to exercise. However, it seems as if there are both neurological and musculotendinous changes that occur with acute bouts of stretching that may negatively affect active force production (3,11,25,29). The literature clearly indicates that stretching immediately prior to exercise may be contraindicated when high amounts of force or power are needed. What is not clear is whether these effects are lasting and whether stretching modality influences muscle force. For example, the speed of ballistic stretching and its transient nature may preserve stretch induced muscle contraction making it more appropriate before exercise than static stretching. Therefore, the purpose of this investigation was to determine whether muscle force, work, power, and the length-tension relationship are affected by routine stretching. This is important for those partaking in or directing training designed to optimize muscle force and power as it is desirable to know the effects of regular stretching on these measures. The investigators hypothesized that although acute bouts of stretching have been shown to negatively affect strength and power, chronic stretching will not decrease strength, power, work or alter the length tension relationship of muscle. Additionally, it was thought that due to the dynamic nature of ballistic stretching, that differences in the parameters of muscle force production may exist between those completing static and ballistic stretching.
Experimental Approach to the Problem
This study was a randomized clinical trial with a repeated-measures design. All participants completed a habituation protocol that included assessment of voluntary peak torque (PT) in order to familiarize them with the test and minimize the strength gains due to the testing protocol. One week after habituation and following random assignment to groups, participants from the experimental and control groups returned for assessment of initial PT, PT angle (PTA), rate of torque development (RTD), and work (W) in the hip extensors. These measures were chosen as PT, RTD, and W have all been related to physical performance in tasks demanding high muscular forces, and PTA was monitored to assess changes in optimal muscle length. Following baseline testing, all participants were advised not to participate in any organized lower body weight training or stretching and to avoid strenuous activity throughout the duration of the study. For the next 4 weeks the 2 stretching groups met 3 times per week and followed a protocol that included 10 minutes of low-intensity cycle ergometry, followed by investigator-led static or ballistic stretching. The frequency and volume of stretching were designed to mimic that which might be conducted by those participating in a stretching program designed to improve flexibility. Individuals assigned to the nonstretching group were instructed to maintain their normal activities. Following the 4 weeks of stretching, PT, PTA, RTD, and W were reassessed 48 hours after the last training session using the same test protocol completed before training. If chronic stretching had any effect on measures of voluntary force production, differences should have existed between the 2 stretching groups and control groups following training.
Twenty-nine male subjects who were healthy and recreationally active (age, 18-60 years) and had not participated in an organized strength training or flexibility program in the previous 6 months were randomly assigned to control (n = 10), static (n = 9), or ballistic stretching (n = 10) groups. All participants completed a questionnaire designed to assess previous participation in stretching and strength training and the frequency, intensity, and duration of physical activities routinely performed. Participants who had done routine lower body strength or flexibility training in the previous 6 months, were competitive athletes, or were high volume exercisers were excluded from the study. The sample size was chosen based on an estimation indicating that to be able to detect a 10% change in peak force and a 5% change in PTA, 9 participants would be needed per group. Subjects on average were 31.6 ± 15.2 years old and had a body mass of 81.1 ± 15.0 kg and a height of 1.76 ± 0.06 m; there were no statistical differences between groups in age, body mass, or height at the beginning of the study (Table 1). The research protocol was approved by the University of Vermont Institutional Review Board, and the project, including its risks and benefits, was explained to the participants who gave their written informed consent.
A custom hip extensor torque apparatus was built to measure muscle torque and hip angle (Figure 1). It was designed with a platform for the subject to lie on in the supine position; an adjustable resistance arm, straps to secure the ankle, contralateral leg, and pelvis; and a Cybex II isokinetic dynamometer. The resistance arm was instrumented with a load cell (Transducer Techniques, Temecula, CA) to measure force and the axis of the Cybex dynamometer was fit with an electrogoniometer (Radio Shack, Fort Worth, TX) to measure hip angle. Analog data from the load cell and electrogoniometer were imported to a personal computer via a BIOPAC MP30 data acquisition system (BIOPAC, Goleta, CA), digitized, processed, and stored on the computer's hard drive using data acquisition software (BIOPAC PRO software). The load cell was calibrated at 0 N (0 kg) and at 98 N (10 kg) using a 10-kg mass applied inline with gravity and the sensing axis of the load cell. The electrogoniometer was calibrated at 0.08 rad (5°) from horizontal and at 1.57 rad (90°) from horizontal. Both force and hip angle data were sampled at 50 Hz and were smoothed every 10 samples using a moving average technique. All torques were gravity corrected to account for the contribution of the leg's mass to total torque at different hip angles.
Measures of Voluntary Torque Development
To measure the force-generating capacity of participants, the maximal voluntary torque production of the hip extensors was assessed. The active torque tests were performed prior to the 4 weeks of training and were repeated 48 hours after the last stretching session. These 2 days without training were used to minimize the acute effects of the last bout of stretching on the measures of muscle force. Starting from a flexed position, participants performed a maximal isokinetic hip extension at an angular velocity of 1.05 rad·s−1 (60°·s−1). The procedure was completed using the resistive torque apparatus described previously and was completed on the right leg of all participants. Participants were placed in the supine position with the axis of rotation of the resistance arm aligned with the greater trochanter of the femur (Figure 1). The resistance arm was secured to the lower leg at a distance of 5 cm proximal to the medial malleolus, and the moment arm length was recorded. The participant's knee was braced at approximately 3.0 rad (170°) using an adjustable rigid frame knee brace (Donjoy, Vista, CA). This was done to limit change in hamstring length as a result of knee flexion and to limit the knee's posterior joint capsule's effect on range of motion. The participant's torso was secured to the table by a nylon strap placed around the pelvis, and the contralateral leg was secured to the table in the same manner proximal to the knee. The hip was flexed to a position equivalent to 90% of the individual's maximal range of motion as measured during the habituation protocol. At this point, the participant was instructed to maximally extend the hip. Five repetitions of the test were completed separated by 1 minute, and the scores of the last 4 trials were averaged and used for analysis.
Peak torque was determined by finding the highest torque of the torque vs. position (hip angle) curve and PTA was measured as the hip angle at which PT occurred (Figure 2). Muscular W was calculated by integrating the torque vs. position curve to obtain the area under the curve. The RTD was measured by determining the slope of the linear region of the torque vs. time curve from the onset of force production to the beginning of the plateau in torque.
To assess the reliability of the torque measures over time, a follow-up test was completed after the initial testing session and prior to training. This analysis indicated that for PT the intraclass correlation coefficient was rxx = 0.95, PTA elicited rxx = 0.89, and W had rxx = 0.95. Based on these results, it can be expected that if moderate differences between groups did exist over time, the measures would have been sensitive enough to detect them. The observed power to detect differences over time was 0.37 for PT, 0.31 for RTD, 0.67 for W, and 0.05 for PTA while the power to detect differences between groups over time was 0.06 for PT, 0.05 for RF, 0.07 for W, and 0.1 for PTA. This indicates that either the sample size used in this study was insufficient to detect significant differences between groups or that the intervention has no effect on the dependent variables as the authors have hypothesized.
Stretching of the hip extensors was supervised by the investigators and was completed while participants stood with their feet slightly narrower than shoulder width and involved flexing at the waist by extending the hands toward the toes. Participants were instructed to keep their backs straight and to maintain a slight bend in the knees. Static stretching involved slowly stretching the muscles until a point of mild discomfort was felt, at which time the participant held the stretch in this position for 30 seconds. Static stretching participants were watched carefully to prevent any ballistic movements. Ballistic stretching used the momentum created by repetitive bouncing movements to produce muscle stretch. Participants were instructed to move into and out of the stretch each second pushing to a feeling of mild discomfort with each bounce. Each group performed 10 sets of stretching separated by 30 seconds. Stretching was completed 3 times per week over 4 weeks for a total stretch duration of 3600 seconds and was consistent for both stretching groups. A 4-week duration was chosen because a number of studies have demonstrated that this duration is sufficient to produce lasting changes in joint range of motion, peak passive torque, and W absorption (6,19). It is therefore plausible that these lasting effects to passively stretched muscle could have potential negative effects on actively contracting muscle.
To assess the reliability of the measures over time, intraclass correlation coefficients were calculated from the 2 tests completed prior to training. For all data, the normality of the data was confirmed with visual plots of the data and with estimates of skewness and kurtosis. Levene's statistic was calculated to verify the homogeneity of variance assumption. A 1-way analysis of variance test was used to determine whether participant characteristics differed by group at the beginning of the study. To compare the response to the individual stretching protocols, changes over time in PT, PTA, W, and RTD were compared between the control, static stretching, and ballistic stretching groups using repeated-measures analysis of variance (RMANOVA) using a 3 × 2 (group × time) analysis. Following this analysis, it was apparent that there were no differences in the response to training between the 2 stretching groups, and the data from these 2 groups were combined to simplify presentation of the data. Then another RMANOVA was used to determine whether the main effect for time or the group × time (2 × 2) interactions were significant between those that stretched and those that did not. The rejection criterion for all tests was set at P ≤ 0.05.
Mean scores for all groups both before and after training for PT, RTD, PTA, and W can be seen in Table 2. In agreement with previous work from our laboratory and the literature related to relaxed muscle length, PTA varied little over time with a −1.6 ± 6.6% decrease in the static group and increases of 0.86 ± 4.1% in the ballistic and 0.18 ± 8.7% in the control groups with no statistical differences between groups (P = 0.71). After training, PT increased by 5.3 ± 19.0% in the static group, by 7.8 ± 12.7% in the ballistic group, and by 6.1 ± 17.9% in the control group with no statistical differences between groups (P = 0.93). W increased by 3.9 ± 7.0% in the static group, by 14.7 ± 27.4% in the ballistic group, and by 5.5 ± 9.5% in the control group and was not different between groups (P = 0.88). RTD increased by 4.8 ± 22.7% in the static group, by 3.6 ± 28.0% in the ballistic group, and by 9.7 ± 24.0% in the control group with none of the differences being statistically significant (P = 0.98).
Initial reviews of the data indicated that there were no statistical differences between the ballistic and static stretching groups for any of the measures and therefore data from the 2 groups were combined. Repeated-measures analysis of variance indicated that the main effect for time was not different for PTA (P = 0.95), PT (P = 0.11), or RTD (P = 0.12) but was significant for W (P = 0.014) (Figures 3-6). Similarly, there were no statistical differences over time between the control group and those who stretched for PTA (P = 0.70), PT (P = 0.75), RTD (P = 0.87), or W (P = 0.70).
The results of this study indicate that 4 weeks of hamstring flexibility training has little effect on peak hamstring force, work capacity, power, or optimal muscle length. This is important as it appears that routine stretching has no apparent negative effects on muscle force development that could possibly occur because of reduced reflex activity or decreased work absorption. The effect of the 4-week stretching protocol employed in this study on measures of flexibility and exercise tolerance has been previously reported (16). This study demonstrated that 4 weeks of either static or ballistic stretching has the capacity to promote significant increases in joint range of motion. These changes occurred through increases in stretch tolerance and were not due to changes in muscle stiffness or work absorption as is often assumed.
The current investigation focused on whether chronic stretching would alter parameters of voluntary force development including PT, W, RTD, and PTA. It would be expected that if chronic stretching were capable of inducing changes in muscle length, the length-tension relationship of muscle would change, resulting in a change in optimal muscle length. Results indicate that the change in PTA between the stretching groups and controls was not different (Figure 3). This is in contrast to Weir et al. (29) who demonstrated that PT occurred at a longer muscle length following an acute bout of stretching but agrees with the results of Cramer et al. (9) who showed no change in the PTA following stretching. Both of these studies monitored the effects of an acute bout of stretching on force development, but muscular adaptation to a month-long stretching program might be expected to be different. Following chronic stretching, if an increase in the actual length of the muscle occurred, it would be anticipated that the optimal hip angle for force production would have increased as well. The lack of change in PTA and the lack of change in muscle stiffness seen using this protocol suggest that there were no lasting changes in muscle length as a result of the 4 weeks of stretching (16). It is possible that either chronic stretching has little effect on muscle length or that the stretching program was not of sufficient duration or intensity to produce measurable changes.
The current study demonstrated increases in PT and W that were not statistically different between groups (Figures 4 and 5). Theoretically, if the stretching program used in the current study had a negative effect on muscle force production, measures of PT and W should have been significantly less in the stretching groups than in controls. Interestingly, the 2 stretching groups and the control group all showed an increase in PT and W, most likely a training effect due to repetition of the strength tests. These results are similar to those from Handel et al. (12) who examined the effect of 8 weeks of proprioceptive neuromuscular facilitation stretching on flexibility and voluntary force generation. If chronic stretching negatively affects force development, it would be unlikely to see the increases in strength seen in these studies.
Many competitive sports and daily activities require the rapid application of muscle force (RTD). It has been demonstrated that acute bouts of stretching can negatively affect maximal muscle force, proprioception, and motor unit activation (3,10,21). These impairments would most likely lead to a decrease in the rate of muscle force production, which is closely tied to performance in activities requiring rapid acceleration (5). A number of studies have looked at the practical application of stretching prior to physical performance measures that rely on the rapid generation of muscle force, but few have studied the effects of longer duration stretching programs. One study using top-level college sprinters showed that passive stretching increased the time to run 20 m (23), while other studies showed a significant decrease in vertical jump (7,8,28,31). The current study indicates that in addition to having no effect on PT or the amount of W done, chronic stretching has little effect on the RTD. This is supported by Guissard and Duchateu (11) who showed no change in RTD following 6 weeks of passive stretching. These results may therefore alleviate some of the concerns of individuals who are looking for maximal muscle power during exercise yet desire to maintain overall joint range of motion.
In the current study, both static and ballistic stretching were used. A decrease in alpha motor neuron and 1a afferent activity from muscle spindles following static stretching indicates that this stretching modality may be contraindicated prior to exercise (3). The author's observation of the warm-up of elite athletes prior to both power and endurance events indicates a preference by some athletes for the use of dynamic and ballistic muscle movements with little dedicated static stretching. It was hypothesized that the dynamic nature of ballistic stretching may mitigate the decrease in muscle activation seen following static stretching and preserve muscle force production (3,10,29). The current study showed no differences in measures of force production between static and ballistic stretching following 4 weeks of stretching. Since the time between the last stretching session and completion of the test protocol was 48 hours, the acute effects of static or ballistic stretching on measures of muscle force production could not be evaluated. While a number of studies have looked at the effects of warm-up modality on performance, few have conducted rigorous analyses of neuromuscular function. Although not demonstrated in this study, it is possible that the dynamic nature of ballistic stretching could increase the sensitivity of muscle spindles to stretch and/or potentiate muscle force production through an increase in the activity of alpha motor neurons. This may make this modality of stretching preferable prior to exercise requiring high levels of muscle force and power.
The lack of effect seen in the current study supports the authors' hypothesis that moderate duration stretching programs have little influence on muscle force, power, W, and the length tension relationship of muscle. Additionally, there were no differences between static or ballistic stretching that would suggest that one modality is superior to the other with regard to its effect on muscle strength or power.
It is possible that long-lasting changes in the ability of muscle to store elastic energy when stretched, as well as muscle excitability, could be negatively influenced by stretching. However, the results from this study should alleviate some of the concerns related to the use of routine stretching as those who performed stretching had similar performance to those who did not. Additionally, the type of stretching used in a routine program may not be of great importance with respect to muscle force and power production as there were no differences between those who did static or ballistic stretching. The authors suggest that routine stretching be completed following exercise and passive stretching of the muscles should be avoided prior to activities requiring high muscle force and power, including both training and competition. Before exercise that requires high muscular forces, individuals may perform dynamic sport-specific exercises to increase blood flow, metabolic activity, temperature, and compliance of the muscle. The negative effects of acute bouts of passive muscle stretching on muscle force development should not be used as a reason to avoid the maintenance of muscle flexibility as a number of studies have related poor flexibility to an increase in injury risk (13,14,22). Although a lack of flexibility has been shown to increase injury risk, the link has not yet been made to support the notion that stretching reduces injury as is commonly believed (24).
This publication was made possible by the Vermont Genetics Network through NIH grant P20 RR16462 from the INBRE Program of the National Center for Research Resources.
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