EMG Onset after a 10-Week Stretching Intervention: A Comparison between Older and Younger Adults.
Flexibility improves following a stretch training program in both younger and older individuals (7,31). Range of motion (RoM) depends to a large extent on the ability of the muscle to stretch (2,17). Several different aspects of muscle flexibility are responsible for achieving the maximum RoM. These aspects appear to be more mechanical factors associated with a subjective stretch tolerance that limits the maximum RoM (25,34). The primary mechanical factor includes parameter of "stretching tension", such as stiffness. When a muscle is stretched, there is an exponential increase in the stress-strain curve, where stiffness reflects the physiological response of the muscle during a stretch (6). In addition to the mechanical factors, previous studies have also investigated neurophysiological aspects of muscle flexibility.
When a muscle is stretched, there may be a physiological response reflex that can be measured using an electromyographic signal (EMG activity). The EMG activity is elicited during a muscle stretch by the muscle spindles that represent a portion of the neuromuscular system. The muscle spindles are the primary stretch receptors of the skeletal muscles and are arranged parallel to the muscle fibers (2,24). The EMG activity during a stretch is triggered by impulses from the intrafusal fibers of the muscle spindles. Stretching of the intrafusal fibers activates the primary nerve endings and results in action potentials within the muscle. A muscle stretch can be considered passive (no EMG activity) when the EMG signal does not move significantly from its baseline value. EMG onset is defined as EMG activity that deviates significantly from its baseline (5,15). Blazevich et al. (3) described a stretch reflex feedback mechanism underlying the EMG activity during a muscle stretch in order to prevent excess stretch and injury to the muscle.
Although previous studies have reported that EMG activity does not represent a limiting factor for flexibility (20,21,25), more recent research (3) has shown that there is earlier EMG activity in young adults with lower flexibility than in individuals with greater flexibility. Similarly, in a comparison of younger and older adults, Ryan et al. (27) demonstrated that there was earlier EMG onset in older individuals with less flexibility compared to younger adults.
Based on the knowledge that there are changes to the neuromuscular system in older adults as well as reduced flexibility (10,16,26), the purpose of this study was to investigate the change in maximum range of motion and EMG onset after a 10-wk stretch training program.
This study complied with the Declaration of Helsinki. It was reviewed and approved by the Ethics Committee of Faculty 5, Empirical Human Sciences at Saarland University (Application 15-5). The subjects signed a declaration of informed consent after having been informed of the details of the study.
The required sample size was determined a priori using the program G*Power (8). Determination of the sample size was based on other similar experimental studies and the effect sizes anticipated in this case for the change in maximum RoM (11,30). The calculation of sample size was based on an alpha level of P<0.05 and a power (1-[beta]) of 0.8. A total of 10 subjects were required per experimental group (one group of older adults, and one group of younger adults). Twelve older (age: 65.1 [+ or -] 7.9 yrs) and 13 younger (age: 24.0 [+ or -] 4.0 yrs) subjects were included in the statistical analyses. Both experimental groups were free of diagnosed degenerative disorders of the musculoskeletal system and had no past injuries to the ischiocrural muscles. Also, none of the subjects had undertaken a regular stretching program in the past. The subjects were all recreational athletes who were involved in physical activity on no more than 3 d*[wk.sup.-1] and for no longer than 60 min*[d.sup.-1].
The study took place over a total of 12 wks. Subjects were habituated to the test situation on two separate familiarization occasions during the first 2 wks, which was followed by the training phase. Subjects undertook a 10-wk static self-stretch training (3 times*[wk.sup.-1], 3 reps of 60 sec per leg) of the ischiocrural muscles. The stress parameters were based on the training recommendation for older individuals (9,12). Subjects performed the stretches by standing in front of a chair and placing the leg to be stretched on the chair. They were then to bend the upper body forward until intense stretch pain could be felt in the back of the thigh that would be categorized as between 7 and 8 on a pain scale of 0 to 10 (0 = no stretch pain, 10 = intolerable stretch pain). Over the entire intervention period, the subjects kept a log of their stretch training. This log was used as a check for compliance. Both experimental groups performed stretches once or twice per week under supervision and at least once at home. Data were recorded using a passive self-stretch of the ischiocrural muscles by way of a specially-designed instrumented Straight Leg Raise Test (ISLRT), which was equipped with an electromotor for continuous variation of speed (Figure 1).
Prior to collection of the data, the EMG electrodes were attached to the subjects following the methods of De Luca (19) and Hermens et al. (14). Before the electrodes were affixed to the skin, the subject's hair was removed from the area using a disposable razor. The shaved area of skin was then scrubbed with an abrasive paste (Everi, Spec Medica) and, then cleaned with an alcohol-based skin antiseptic (Kodan Tinktur Forte, Schulke, and Mayr). The EMG conductors included two disposable electrodes (Ambu, Blue Sensor N) placed in the direction of the muscle fibers on the belly of the long head of biceps femoris. Another disposable electrode was attached to the tibial ridge and served as a reference electrode. The distance between electrodes on the biceps femoris muscle was 20 mm. Once the EMG electrodes were attached to the muscle, the EMG signal was checked using frequency analysis for interfering signals and visually verifying the raw EMG. There was then a 3-min warm-up on a bicycle ergometer (Ergo-Fit, Ergo 1500 Cycle) at a cadence with 70 rev*[min.sup.-1] (RPM) and one Watt per kilogram body weight (1 W*[kg.sup.-1] BW). The intensity was based on the performance level of the older group.
The older subjects underwent an annual medical exam in which a stress ECG was carried out. In some subjects, an intensity greater than 1 W*[kg.sup.-1] body weight was enough to pose a critical stress for the cardiovascular system. The warm-up was to proceed consistently for each subject, thus the intensity was selected for the older and the younger experimental groups. Following the warm-up, the subjects performed a pre-stretching (6 x 10 sec per leg static self-stretch). The pre-stretching consisted of the same exercise as the stretch training during the treatment phase. According to Gajdosik et al. (11), the goal of this pre-stretching was to reduce the "tissue force-relaxation". Immediately after the pre-stretching, the subject was positioned on the ISLRT. Each subject was place in a standardized position on the back. The right leg was placed on a rack device with the knee flexed, and the left leg was set on a U-shaped foot construction on the lever with the knee in the extended position (13). The upper body and pelvis were then fixed in place using tension straps.
Finally, the positioned leg was fixed at the groin to prevent retroversion of the pelvis during the stretch procedure. In order to preclude visual distractions, the subjects' eyes were covered with a sleeping mask during the stretch procedure (Figure 1). The left leg was tested in a passive self-stretch of the ischiocrural muscles using a speed of 1.5 deg*[sec.sup.-1]. The aim of this low speed was to avoid having muscle reflexes triggered due to a speed that was too high (24). Each subject controlled the electromotor using a 1-way joystick (Saia-Burgess Electronics, Switzerland). The subjects were asked to stretch the leg until it was no longer possible due to the pain threshold. Subsequently, the lever arm was lowered by the subject at a speed of 3.75 deg*[sec.sup.-1] until it reached the original position. This procedure was performed twice, and the mean of the two values was calculated.
Joint angle was measured using a potentiometer built into the rotary axis of the lever arm (Biovision, Wehrheim). The EMG signal and joint angle were recorded simultaneously using an A/D converter (Biovision, Wehrheim) at 1000 Hz. The data were logged and stored in ASCII format using DasyLab (National Instruments Ireland Resources Limited, Version 10) and later processed using MatLab (MathWorks Inc., Version R2014a).
The EMG signals were filtered with a 10 Hz high pass filter and a 500 Hz low pass filter, as well as a Butterworth 4th order filter. Following the methods of Blazevich et al. (3) and Hodges et al. (15), the EMG onset was defined as the EMG activity that deviated from baseline by at least three standard deviations for 100 ms.
The dependent variables are maximum range of motion ([RoM.sub.max.]), absolute EMG onset ([EMG.sub.onset]) and relative EMG onset ([EMG.sub.onset-%]), which was calculated as a percentage of the [RoM.sub.max.] (Figure 2). Reliability of the dependent variables was calculated using Pearson correlation coefficients during the familiarization trials at two appointments spaced 1 wk apart for both age groups: [RoM.sub.max.-young] (r(13) = 0.95, P<0.001); [RoM.sub.max.-old] (r(12) = 0.83, P<0.001); [EMG.sub.onset-young] (r(10) = 0.88, P<0.001); [EMG.sub.onset-old] (r(9) = 0.77, P<0.05); [EMG.sub.onset-%-young] (r(10) = 0.79, P<0.001); [EMG.sub.onset-%-old] (r(9) = 0.73, P<0.05). The varying degrees of freedom of the correlation coefficients were explained in that three of the younger and three of the older subjects showed no EMG onset during the muscle stretch.
Statistical calculations were carried out using SPSS (IBM Corp., Version 23). Differences in the dependent variables between the age groups were initially tested using independent t-tests. Variations in the variables for both groups were analyzed with repeated measures variance analysis. The level of significance was set at an alpha level of P<0.05. Results are presented as mean [+ or -] standard deviation.
The dependent variables did not differ significantly between age groups in the pre-test data: [RoM.sub.max.] (t(23) = 1.04, P = 0.31); [EMG.sub.onset] (t(16) = -0.09, P = 0.93; [EMG.sub.onset-%] (t(16) = -0.43, P = 0.67). The results of the ANOVA separated by age group are presented in Table 1.
The maximum range of motion ([RoM.sub.max]) significantly changed over the 10-wk stretch training period, which was independent of age group. For the statistical analysis of the EMG parameters ([EMG.sub.onset] and [EMG.sub.onset-%]), the subjects were included only if there was an EMG onset noted during the pre- and post-test. There were three younger and four older subjects for whom no EMG onset was recorded during muscle stretch during at least one measurement point. Only relative EMG onset ([EMG.sub.onset-%]) showed a significant change independent of age group. Overall, no significant interactions between age group and time were identified.
This study investigated the change in maximum range of motion and EMG onset following the 10-wk stretch training program in younger and older adults. The primary result is that the maximum range of motion, absolute EMG onset and relative EMG onset did not differ significantly between the two age groups.
During the 10-wk stretch training program, the maximum range of motion of the ischiocrural (hamstring) muscles improved as expected in both age groups (7,31). The absolute EMG onset did not differ between the age groups and did not change significantly between the pre-and post-test measurements. These results are consistent with studies of the calf muscles (4) and the hamstrings (21) after 3 wks of stretch training. Moreover, the relative EMG onset decreased in both age groups between the pre- and post-test measurements. An unexpected result was that the older age group showed a later relative EMG onset compared to the younger group. In contrast to these results, Ryan et al. (27) showed that after a one-time stretch of the calf muscles in both a younger and older group of individuals, the relative EMG onset was earlier in the older group. These authors speculated that greater muscle stiffness in the older subjects may have contributed to an earlier reflex response by the muscles. Unlike Ryan et al. (27), in the present study, a warm-up and pre-stretching were allowed immediately prior to the collection of the data for EMG onset. It is possible that the warm-up and pre-stretching led to an acute reduction of stiffness in both age groups and, therefore, affected the EMG onset. Knudson (18) demonstrated that the warm-up effect can change the stiffness of a muscle.
In addition, pre-stretching immediately prior to collection of the data could lead to a "tissue force-relaxation", in the sense of Gajdosik et al. (11). This acute relaxing effect is also known as the "creeping effect" (28,29,32). In contrast to the assumptions of Ryan et al. (27), in the present study the later relative EMG onset in the older subjects could be explained in that the younger subjects were able to achieve a greater maximum range of motion in the pre- and post-test measurements. The relative EMG onset depends on the maximum range of motion. The younger age group achieved greater maximum range of motion compared to the older age group at both measurement points. Due to the lower range of motion in the older adults but with similar absolute EMG onset in both age groups, the later relative EMG onset in the older age group is thus not unexpected.
Similar to other studies, not all subjects showed an EMG onset during the muscle stretch (3,25,27,30). This was the case for the younger and the older age group. At 1.5 deg*[sec.sup.-1], the stretch speed during the passive stretch on the straight leg raise test device may have been too slow and, consequently, resulted in no muscular reflex response in some cases due to varying speed sensitivities of the primary endings of the muscle spindles. During a stretch, the primary endings of the muscle spindles are sensitive to the speed of the change in length (22,23). For example, Toft et al. (33, p. 490) noted: "Stretch reflexes were not elicited since the stretch velocity was low (<2 deg*[sec.sup.-1])". Another possibility is that the subjects with less muscle stiffness also showed lower or even no EMG activity at all. Abellaneda et al. (1) documented a higher EMG activity in individuals with greater stiffness of the calf muscle compared to individuals with lesser stiffness. Interestingly, however, Magnusson et al. (20) found that neither subjects with greater or lesser stiffness of the ischiocrural muscles exhibited EMG onset during a passive knee extension test. On the basis of the present study, it is not possible to unambiguously determine why some subjects do not show muscle activity during a stretch. According to Magnusson et al. (20,21) and McHugh et al. (25) for younger adults, this study does identify that EMG onset does not influence maximum range of motion in younger and older adults.
Overall, younger and older adults showed no significant differences in either the maximum range of motion or EMG onset after the 10-wk stretch training program of the ischiocrural muscles. Maximum range of motion increased in both the younger and the older subjects. The absolute EMG onset was similar in both groups and remained virtually unchanged over the intervention period. There was a non-significant reduction in relative EMG onset in both age groups and EMG onset appeared at a later time in the pre- and post-test measurements in the older age group. Since the relative EMG onset depends on the maximum range of motion and the younger age group achieved a greater RoM at all measurements, this resulted in an earlier relative EMG onset for the younger age group.
Future study should address why some subjects fail to show an EMG onset during a muscle stretch. This study involved recreational athletes with no apparent restrictions in flexibility. Further work should also focus on individuals with initial conditions that are weaker.
We would like to thank Dr. Markus Schwarz and Christian Kaczmarek for their support with the older experimental group.
Address for correspondence: Thomas Haab, Sport Science Institute, Saarland University, Geb. B 8.1, 66041 Saarbruecken, Germany, Email: email@example.com.
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Thomas Haab (1,2), Matthias Massing (1), Georg Wydra (1)
(1) Sport Science Institute, Saarland University, Saarbruecken, Germany, (2) LUNEX University, Differdange, Luxembourg
Table. 1 Results of the ANOVA and Descriptive Data for Pre- and Post-Test. Young Old [RoM.sub.max.] ([degrees]) (n = 13) (n = 12) Pre 93.0 [+ or -] 15.4 86.7 [+ or -] 14.9 Post 101.4 [+ or -] 13.0 94.7 [+ or -] 15.7 [EMG.sub.onset] ([degrees])* (n = 10) (n = 8) Pre 68.7 [+ or -] 17.9 69.2 [+ or -] 5.2 Post 68.8 [+ or -] 18.1 70.7 [+ or -] 4.8 [EMG.sub.onset-%] (%)* (n = 10) (n = 8) Pre 74.3 [+ or -] 16.6 77.3 [+ or -] 11.6 Post 68.2 [+ or -] 17.0 71.8 [+ or -] 6.1 ANOVA F P [RoM.sub.max.] ([degrees]) Age Group 1.3 0.3 Pre Time 56.3 < 0.001 Post Age Group x Time < 1.0 0.9 [EMG.sub.onset] ([degrees])* Age Group < 1.0 0.9 Pre Time < 1.0 0.7 Post Age Group x Time < 1.0 0.7 [EMG.sub.onset-%] (%)* Age Group < 1.0 0.6 Pre Time 8.8 < 0.01 Post Age Group x Time < 1.0 0.9 [RoM.sub.max] = Maximum Range of Motion; [EMG.sub.onset] = Joint Angle at EMG Onset; [EMG.sub.onset-%] = EMG Onset Expressed as a Percentage of [RoM.sub.max.] (*) Statistical results only for subjects who showed EMG activity during the muscle stretch at pre- and post-test
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|Author:||Haab, Thomas; Massing, Matthias; Wydra, Georg|
|Publication:||Journal of Exercise Physiology Online|
|Date:||Aug 1, 2017|
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