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Original Research

Acute Stretching Increases Postural Stability in Nonbalance Trained Individuals

Nelson, Arnold G.; Kokkonen, Joke; Arnall, David A.; Li, Li

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Journal of Strength and Conditioning Research: November 2012 - Volume 26 - Issue 11 - p 3095–3100
doi: 10.1519/JSC.0b013e3182430185
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Abstract

Introduction

Three different sensory systems, vision, vestibular, and proprioceptive (muscle spindles and Golgi tendon organs) help a person maintain postural balance, with the proprioceptors exerting their influence through the use of the stretch-reflex response (10). Although it is generally accepted that stretch-reflex response influences balance maintenance, controversy exists concerning the effect that muscle and joint stiffness have on stretch-reflex response and the ability to maintain balance. Sinkjaer et al. (13) proposed that the sensitivity of the stretch reflex can be influenced by muscle stiffness, with stiffer muscles yielding a greater reflex response. This thought was echoed by Petit et al. (11) who suggested that stiffer muscles benefited postural maintenance by allowing for greater or more rapid responses to slight perturbations in muscle length. The postulations of Sinkjaer et al. (13) and Petit et al. (11) are supported by the work of Edwards (5) who through mathematical modeling showed that instability increased with decreased stiffness at either the hip, knee, or ankle joints. In contrast, a practical experiment by Grüneberg et al. (6) did not support the theoretical modeling. These experimenters found that artificially increasing the stiffness at the hip actually decreased stability rather than increasing it.

An alternative increasing artificially stiffness to test the relationship between muscle and joint stiffness is to use stretching activities to decrease muscle stiffness and increase joint range of motion. To date, however, experiments using acute stretching have not been conclusive concerning the relationship between stiffness and the ability to maintain balance. Behm et al. (2) measured the ability to maintain balance while standing on a wobble board before and after either 45-second stretches (stretching the hamstrings, quadriceps, and plantar flexors) or a control (no stretch) session. In comparison with the precontrol sessions, Behm et al. (2) found the control condition to have a significant improvement in balance, whereas the stretch condition had a nonsignificant decrease in balance. On the other hand, Costa et al. (3) measured performance on the Biodex Balance System after 2 different stretching durations (15 or 45 seconds) of the lower limb muscles. They found that the 15 seconds produced a significant improvement in balance scores, with no significant effects with the control condition or the 45-second treatment. Unfortunately, both of these studies have methodological problems. First, neither study provides any verification that the stretching activities actually altered either joint range of motion or muscle stiffness. Second, both studies have oddities with respect to their data. In the Behm et al. (2) study, the prestretch value for the balance score (8.8 ± 1.7 contacts) is better (less in this case) than either the precontrol (10.8 ± 2.0) or the postcontrol value (8.9 ± 1.5 contacts). The same phenomenon is also present in the Costa et al. (3) article. In this article, the pre 15-second stretch stability index (3.73 ± 0.35) is better (higher in this case) than either the precontrol (3.20 ± 0.31) or the postcontrol (3.18 ± 0.24). Having scores that vary in such patterns is not unusual when the participants are not skilled at performing the task. This raises the question of whether or not any of the participants in either study had the experience or skill necessary to ensure repeatable or reliable performances. It is possible that a skilled group would have responded differently to the task and treatments. Finally, Behm et al. (2) tested exclusively males, whereas the Costa et al. (3) only tested women. Thus, one cannot be sure if the varying results obtained are because of differing gender responses.

Given that a decreased ability to maintain balance has been associated with higher injury risk in both the athletic (9,15,16) and the general populations (18), any training modality that improves the ability to maintain balance would appear to be beneficial. However, since the work of Kokkonen et al. (8), static stretching has been shown to have negative effects upon athletic performance (12). Thus, it can be questioned whether one should do acute static stretching before activity to increase one's ability to maintain balance or instead practice activities that result in an increased ability to maintain balance. Therefore, the intent of this study was to determine if an acute bout of stretching can increase the ability to maintain dynamic balance in both a population of college students and a population of athletes with several years of experience practicing the maintenance dynamic balance.

Methods

Experimental Approach to the Problem

To determine whether a stretching program could acutely alter the ability to maintain dynamic balance in both a population of college students and a population of athletes with several years of experience practicing the maintenance dynamic balance, a research paradigm was used that consisted of a familiarization period followed by 2 days of testing. On each testing day, the program was done under a different stretch condition (static stretch vs. no stretch). This experimental design helps reduce the aforementioned confounding problem of learning the task, whereas the 2 different days help show how stretching influence balance, and the 2 different subject groups are used to help show if experienced and novice balancers respond similarly to stretching. For this study, the ability to maintain dynamic balance was determined from the amount of time the person could maintain a stabilometer in the horizontal position during a 30-second period.

The experiment proceeded as follows. Before performing the balance test, the subjects participated in a minimum of 3 accommodation sessions to familiarize themselves with the balance task. Each accommodation session was performed on a separate day. Daily practice of the task consisted of balancing on the stabilometer for 1 minute followed by a 30-second rest, which was repeated a minimum of 10 times. If after 3 practice sessions, a subject's balance times for the last 3–4 practice bouts were not within 2 seconds of each other, the subject was asked to perform additional accommodation sessions until this degree of consistency was achieved. After a person satisfied the familiarization criteria, the person reported to the laboratory for 2 test days. On the test days, the balance tasks were performed after either 20 minutes of quiet sitting (nonstretched [NS]) or 20 minutes of static stretching activities (static stretched [SS]) for 1 of the 2 testing days of each subject, respectively. All tests for a single subject were performed at the same time of the day, and the order of SS and NS was determined for each subject using a balanced design. The entire experiment was completed during the last 2 weeks of February, and each subject was instructed to refrain from exercise 24 hours before each test, and on the second test day to eat and drink the same foods and drinks at the same times as on day 1.

Subjects

Forty-two college students (21 male, 21 female) volunteered for the experiment and represented the group of inexperienced dynamic balancers. All these subjects were classified as recreationally active (i.e., engage in some form of physical activity 2–3 d·wk−1), but none of them were engaged in any regular form of exercise training. The experienced dynamic balancers were recruited from experienced surfers who habituated the major surfing spots on Oahu's North Shore. To qualify as an experienced surfer, the individuals must have surfed a minimum of 5 h·d−1 for 5 d·wk−1 for the last 5 years. Although it was not difficult to find surfers to meet the criteria, the carefree Bohemian-like lifestyle of many of them resulted in several of them being unreliable research participants. Therefore, the final group was made up of 10 men all of whom were current students at the university. Descriptive data for the participants is tabulated in Table 1. The study was approved by institutional review board of Brigham Young University-Hawaii, and each subject submitted both written and oral consent before engaging in the experiment.

Table 1
Table 1:
Subject description.*

Experiment Protocol

The test criterion for dynamic balance was the average time that the subject kept a stabilometer (Model 16020, Lafayette Instrument Inc., Lafayette, IN, USA) level at 180° during two 30-second periods. The task required the subjects to balance with their feet shoulder width apart and parallel to the stabilometer's axis of rotation with the each foot equidistant from the stabilometer axis of rotation. To ensure that foot placement was constant for repetitive measures, a grid was placed on the surface of the stabilometer, and the location of each foot on the grid was recorded. In our laboratory, the reliability for this measure, as assessed by an intraclass correlation, equals 0.89.

To assess alterations in joint range of motion and the effectiveness of the stretching program, each subject performed 2 sit-and-reach tests (3 trials per test) on an Acuflex I sit-and-reach box (Novel Products Inc., Rockton, IL, USA). The first sit-and-reach test was performed before the stretching, and the second sit-and-reach test followed the treatment. The sit and reach was not measured before or after the NS session. This was because of the Costa et al. (3) finding that a few 15-second stretch sessions could influence dynamic balance. Also, in our experience, we have found that doing repeated sit and reach tests can sometimes increase joint range of motion. In our laboratory, range of motion reliability, as assessed by an intraclass correlation equals 0.99.

On entering the laboratory on the test day, the subject sat quietly for 10 minutes. Next, for SS, the subject's sit and reach was measured. This sit-and-reach test was followed immediately by the stretching exercises (see below). After SS treatment, the sit and-reach was measured a second time. For NS, the subject just sat quietly for 20 minutes. Immediately after the second sit-and-reach test (SS) or the 20 minutes sit (NS), the subject stood on the stabilometer platform in the proper position. The stabilometer platform was held at 180° (parallel to the ground) while the subject mounted it, and this support was maintained until the subject was ready to perform the balance task. Once the subject was ready, the support was removed, allowing the platform to readily rock back and forth, and the timers were started. The stabilometer timing system consisted of 4 timers, one which counted down 30 seconds, one which recorded the time the platform stayed on center, one which recorded the time the platform deviated >3° off-center on the left, and one which recorded the time the platform deviated more than 3° off-center on the right. Once the countdown timer was started, the subject tried to keep the platform level for 30 seconds. At the end of 30 seconds, the subject stepped off the platform, and the center, front, and back times were recorded. This test was repeated for a total of 3 trials with a minimum 2 minutes rest between each of the 3 trials. The first trial was designated as a practice turn and was discarded and the final 2 times recorded, and the average of the 2 times was used for analysis. At no time during the study were the subjects allowed to see their times for any of the tasks.

Stretching Protocol

The stretching program consisted of 5 different static stretching activities designed to stretch the major muscle groups of the hip, knee, and ankle. The first stretching exercise was a sit and reach. The subjects sat on the floor with the legs extended and then lowered their heads toward the knees. The second activity was the lotus or butterfly stretch. Here, the subjects sat on the floor in the lotus position and then proceeded to lower their heads to the floor. For the third activity, the heel cord or calf stretch was performed. The subjects first stood with one foot flat on the floor and the other foot placed on a block so that the ball of the foot was 10 cm above the heel. The subject then leaned forward. The fourth exercise was a standing half lotus. While standing with one foot flat on the floor, the subject placed the opposite leg in a lotus position on a table. The subjects then alternated lowering their head toward either the foot or the knee of the leg resting on the table. The fifth and final exercise was a quadriceps stretch. The subjects stood with their back to a pommel hors, and then placed the dorsal side of one foot on the pommel horse by flexing at the knee joint. From this position, the subject leaned backwards.

During the stretch treatment, each person performed all 5 stretches 3 times unassisted and, except for the heel cord stretch, 3 times assisted. The 3 repetitions for a specific exercise were completed before another exercise was performed. For each of the unassisted exercises, the subject assumed the appropriate position and then leaned or lowered as far as possible, placing the musculature on stretch. The stretch was then held and the subject tried to remain motionless for 15 seconds. The activity was then repeated 2 more times with a 15-second recovery period between the repetitions. After the 5 exercises were performed unassisted, the exercises were performed again in the same order, but with assistance from an investigator, except for the heel cord stretch, which was repeated again 3 times unassisted. The assisted activities were performed in the same manner and for the same length of time as the unassisted. However, for the assisted condition, one of the experimenters pushed the person until he or she verbally acknowledged that a noticeable stretch was felt in the muscle. The experimenter then held the subject's body at that position for the 15 seconds. The stretching exercises were usually completed in 20 minutes. After the stretching bout, each person relaxed for 10 minutes before repeating the sit-and-reach test.

Statistical Analyses

The PRE-SS vs. POST-SS sit-and-reach scores for students and surfers were compared independently via a paired t-test. Before comparing balance time between SS, NS, students, and surfers, a 2-way (male students vs. female students, SS vs. NS) analysis of variance (ANOVA) with repeated measures (SS vs. NS) was used for analysis to determine if there were differing responses because of gender. There was no significance found between genders, so the male and female scores were combined, and this group's performance was compared with that of the surfers. The effect of SS and NS on both student and surfers and the comparison of students' balance performance to that of the surfers was determined using a 2-way (surfers vs. students, SS vs. NS) ANOVA with repeated measures (SS vs. NS). When appropriate, Bonferroni t-tests were used for post hoc analysis. The significance level was set at alpha = 0.05. Finally, a generalized eta squared after the equations of Bakeman (1) was used for effect size determination.

Results

Flexibility

For the students, the stretching exercises (S) yielded a significant (p < 0.05) increase of 6.5 ± 2.9 cm (mean ± SD) in the sit and reach (Table 2). For the surfers, the stretching exercises (S) also yielded a significant (p < 0.05) increase of 6.5 ± 1.6 cm in the sit and reach (Table 2).

Table 2
Table 2:
The influence of the stretching program upon sit and reach.*

Balance Time

Balance times for all the conditions and trials are presented in Table 3. The main effect for activity level (surfers vs. students) was significantly different (F[1, 82] = 44.5, p < 0.0001,

) with the surfers showing a longer balance time. The main effect for stretch condition was also significant (F[1, 82] = 9.3, p < 0.0031,

) with the S being greater than NS. A significant interaction (F[1, 82] = 28.7, p < 0.0001,

) between activity level and stretch condition was also found. Post hoc analysis showed that stretching helped the students to improve their balance times, but stretching had no effect upon surfers' balance times.

Table 3
Table 3:
The influence of the stretching program on balance time.*

Discussion

As mentioned previously, the intent of this study was to determine if an acute bout of stretching can increase the ability to maintain dynamic balance. The observed data from this study agree with the findings of Costa et al. (3) in that increased flexibility, especially for people unaccustomed to performing dynamic balance tasks, after acute stretching yields a greater increase in balance time. This finding suggests that increased balance instability may be because of enhanced joint stiffness. It is postulated that the enhanced ability to maintain dynamic balance after an increased flexibility is because of a desensitized stretch reflex. The effect that a desensitized stretch reflex would have on balance can be exemplified by a child's teeter-totter game. Suppose 2 children start play on a teeter totter with one person in contact with the ground and the other high in the air. If the child on the ground pushes off with enough force, the positions of the children can be reversed. Now, if the child in contact with the ground always responds with a maximal force output, the teeter totter will alternate between extremes and equilibrium cannot be established. An overly responsive stretch reflex could cause a similar loss of equilibrium. A less responsive stretch reflex, however, could make it easier to establish equilibrium. Support for this postulation can be seen in the work of Solomonow et al. (14). These researchers reported that repeated stretching of the viscoelastic tissue of the human spine desensitized the mechanoreceptors within the tissue. Although their study directly concerned spine tissue, they did observe a sharp decline in a mechanoreceptor's sensitivity within 20 seconds of cyclic loading the tissue. Maintaining balance on the stabilometer for 30 seconds is also a type of cyclic loading of the lower extremity muscles, especially because the balancing motion is essentially a teeter-totter game played by one's feet. Thus, it is possible that during the balance testing, the mechanoreceptors within the lower extremity experience a continual desensitization process, further suppressing the stretch reflex with each succeeding trial until finally reaching a minimum level of the mechanoreceptor sensitivity. Additional evidence of enhanced motor control via acute stretching has been reported by other investigators. Hutton et al. (7) found that after postcontraction potentiation, people were not able to reproduce their accuracy on a learned force output test without stretching the potentiated muscle. Similarly, van Deursen et al. (17) reported that desensitizing the stretch reflex via vibration allowed the individuals to perform a tracking task more accurately. Not surprisingly, when the subjects were interviewed after completing both sessions to determine which condition they felt allowed for the best performance. The majority of the students chose the S condition. In choosing the S condition, the subjects explained that it was easier to maintain balance because they felt that they had better control of their off-balance reaction. In other words, the subjects felt they were able to better control the magnitude of the force output, and thus made smaller deviations to either side.

On the other hand, the finding that acute stretching does not influence a surfer's ability to maintain postural stability is also not too surprising. In their review, DiStefano et al. (4) provide evidence to show that a healthy person can train to improve their ability to maintain both static and dynamic stability on both stable and unstable surfaces. Moreover, they cite studies that indicate that improve ability to maintain stability can be transferred to multiactivities. Thus, a surfer who learns to stand upright on a multidirectional moving surf board should also be able to keep a stabilometer balanced in the horizontal position. Furthermore, DiStefano et al. (4) reported that the studies investigating the mechanisms behind enhanced balancing ability suggest that increased balancing ability arises from an improved ability to decrease center of pressure excursions. In other words, both acute stretching and balance training result in better balance through enabling a people to exert less force on the surface upon which they stand. Interestingly, during post hoc interviews of the surfers, some of them indicated that part of the success in not “wiping out” came from learning to be “light on their feet”. Apparently, these individuals had already learned to decrease force output and did not need the augmented ability to decrease force derived from the stretching.

In summary, increased flexibility of the leg musculature appeared to dampen the stretch-reflex response to postural deviations. This damping of the stretch reflex improved the ability to maintain balance on a teetering platform. Therefore, it is possible that unsteadiness seen during unsupported standing could be reduced with acute stretching exercises. It should be noted, however, that this study used an acute set of stretching exercises that did not specifically target a particular joint, and it remains to be seen whether this benefit would also arise from a chronic stretching program that targets a particularly stiff joint (e.g., ankle). In addition, although stretching may benefit a person in an acute setting, it would appear that it does not replace or enhance the benefits derived from actual balance training.

Practical Applications

The results of this study, coupled with those of Costa et al. (3), suggest that acute stretching before performing any activity would enhance that individual's ability to maintain postural balance. Based on this finding, it would appear that acute stretching should be performed before any athletic event that has a likelihood of upsetting normal postural balance. However, numerous studies show that acute stretching is likely to significantly decrease other performance variables to include strength, power, mechanical efficiency, and aerobic efficiency (for a review, see [12]). Moreover, stretching did not impart any benefit to people who were experienced doing balancing tasks. Thus, it would appear that adding some form of balance training to a workout program would be a more beneficial than relying on acute stretching to increase an athlete's ability to maintain balance. On the other hand, if a person is involved with the training or rehabilitation of individuals who are not engaged in athletics and have stability problems when doing activities of daily living (e.g., elderly), then acute stretching could be an important part of their daily activities. Just before standing, if these individuals did simple lower limb stretches while sitting it could impart an increased capacity to maintain balance and perhaps reduce their risk of falling.

Acknowledgments

This study received neither funding nor technical or equipment support from any source, and the authors do not have any professional relationships with any companies or manufacturers who might benefit from the results of this study. Also, the results of this study do not constitute endorsement of any product by the authors or the National Strength and Conditioning Association.

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Keywords:

flexibility; muscle stiffness; stretch reflex; postural control; surfers

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