The ankle is an important area of research in the rehabilitation field. A recent review of the overall incidence of ankle injury in sport found that it was the most frequently injured region of the body, accounting for 34.3% of all injuries (11). What is concerning is that approximately 40%-75% of individuals who experience an ankle sprain will have ongoing symptoms (such as pain) or instability (1,13). Moreover, there is evidence to suggest that dysfunction at the ankle joint may be associated more proximal adaptations (2), which in some cases may predispose individuals to developing low back pain (25). The application of "core stability" training (12,35) as a panacea for all prevention and rehabilitation programs currently lacks justification; however, the linkages between ankle and low back function make this a worthy area for research.
Chronic instability of the ankle is caused by either mechanical instability or functional instability or a combination of both (24). Mechanical instability is said to occur because of anatomical changes that occur postinjury such as pathological laxity, synovial changes, and development of degenerative joint disease (17). Recent evidence found that individuals with mechanically unstable ankles exhibited greater frontal and less sagittal plane ankle displacement compared with individuals with functional ankle instability (FAI) (4). More commonly investigated, FAI is related to the neuromuscular control of the ankle and is characterized by impaired joint kinesthesia and altered muscle recruitment patterns (17), often reported as a feeling of the ankle "giving way" (39). It has been suggested that in subjects with FAI, altered afferent input from the ankle is responsible for whole-body nervous system adaptive strategies, resulting in proximal joint adaptations to compensate for impaired lower limb balance (2).
Altered postural reflexes in joints remote to the injured area is not a new suggestion (38). There is some experimental evidence for different proximal adaptations in muscle recruitment patterns of individuals with FAI. Beckman and Buchanan (2) speculated that in patients with FAI, increased gamma drive from the central nervous system to the hip musculature, manifested as faster latency responses of the hip muscles to ankle inversion, is an adaptive strategy to reduce the load on the ankle. Other alterations about the hip in patients with FAI have been identified, including delayed gluteal recruitment during hip extension, altered recruitment of rectus femoris during walking, and use of a hip-focused movement strategy to maintain balance (6,9,26). In one particular study showing proximal complications associated with lower limb injury, Nadler et al. (25) found that athletes with a lower extremity impairment (defined as either an overuse type injury or a knee/ankle joint instability) were more likely to experience low back pain. A potential causative link has been suggested in recent research that found athletes with greater hip abduction and external rotation strength were less likely to sustain a future lower limb injury (22). None of the aforementioned studies have provided an effective measurement of both ankle and trunk stability. The mechanistic link between hip, trunk, and ankle stability has recently been explored in an investigation which found that the major adaptation to experimentally increased hip and trunk stiffness was an increased likelihood of a loss of balance (15). What has not been shown is whether or not patients with FAI manifest impairments in specific objective measures of both ankle and trunk postural stability. The two specific measures used in this study were the time to stabilization (TTS) of the lower limb and the latency response of the trunk muscles during a sudden unloading task.
Recently, TTS has been reported to be an appropriate measure of lower limb function that can discriminate between individuals with and without FAI (33,36). Individuals are required to maintain balance during the transition from a dynamic to static state (14), thereby replicating conditions commonly observed in sport. Measurements are made from the force traces recorded during a single-leg landing task (33,36). Improvements in TTS of individuals with FAI after coordination training have been documented (32).
Trunk stability has been described as a context-dependent variable that is fraught with error in its measurement and scope of interpretation (29). One particular test that evaluated the latency response of the trunk muscles to a sudden unloading task found that athletes with delayed reflexes were more likely to experience a future episode of low back pain (7). The same method was used in this study. To the best of our knowledge, this measure has not been used in a study group with FAI.
Therefore, the purpose of this study was to examine the differences between participants with and without FAI for measures of lower limb and trunk stability. Previous research has shown that TTS times can discriminate between patients with and without FAI; however, it is unclear whether these patients also exhibit altered trunk postural reflexes that may be associated with a whole-body nervous system adaptation to the injury. The hypothesis of this study was that patients with FAI would have delayed TTS and trunk muscle latency responses time compared with healthy controls. Information about altered low back function in the FAI participants will provide useful data for future prospective studies evaluating links between core stability and the lower limb.
Twenty-four subjects participated in this investigation; 12 healthy individuals (23.7 ± 3.5 yr, 171 ± 9.3 cm, 67.9 ± 11.7 kg; 7 females) and 12 individuals with FAI (29.1 ± 9.1 yr, 70.9 ± 12.1 kg, 173 ± 7.2 cm; 7 females). There were no between-group differences for the demographic variables. All subjects were free from any acute injury within the last 3 months. Inclusion criteria for the FAI group were that they had experienced at least one ankle sprain in the last year that had required them to have medical treatment and miss at least 1 d of work or training, had ongoing feelings of giving way or instability of the ankle (19), had no current pain (visual analog scale [VAS] score less than 2 out of 10), had not had any formal rehabilitation in the last 3 months, and had no significant structural changes postinjury that would lead to a classification of mechanical instability (confirmed by consultation with their treating physical therapist and review of medical records for previous scans and diagnosis information). Exclusion criteria for both groups included mechanical ankle instability (as previously described), bilateral ankle instability, benign hypermobility syndrome that may influence pathological joint laxity, any neurological or vestibular disorder, any previous lower limb surgery, use of orthotics, any open foot wound, any obvious bony or musculoskeletal deformity or asymmetry between the limbs, current level of back disability of greater than 15 (using the Oswestry Disability Index [ODI] ), or had experienced an episode of back pain in the last year that required a day off work or missed training session. Benign hypermobility was assessed using the Beighton hypermobility scale (3). Informed written consent was provided by each subject before their participation in the study. The local ethics committee approved all procedures used in this study.
Subjects reported to the university research laboratory for testing. Self-report questionnaires were administered before the physical testing, which included the ankle and trunk measures of stability. Measures of ankle and trunk stability were administered in a random order among subjects.
The Foot and Ankle Disability Index (FADI) is a 26-item questionnaire, and the FADI sport is an eight-item questionnaire with each item being scored from 0 (unable to do) to 4 (no difficulty at all). The four pain-related items are scored from 0 (unbearable) to 4 (no pain). The FADI and the FADI sport are scored separately and are converted to a percentage score, 100% corresponds with no dysfunction (16). The FADI is able to detect functional limitation in participants with FAI and is able to discriminate from healthy participants, with intraclass correlation coefficients (ICC) greater than 0.80 (16). The Cumberland Ankle Instability Tool (CAIT) is a valid and reliable tool to measure FAI (18) and consists of nine items with a maximum score of 30. Lower scores reflect a higher degree of FAI. Sensitivity was shown to be 82.9% and specificity 74.7%. Test-retest reliability was excellent with an ICC of 0.96. (18). The visual analog pain score (VAS) was calculated from a 10-cm scale with the points "least possible pain" and "most possible pain" at either end. The participant rated their pain on the line that corresponds to a millimeter reading and hence a level of pain. VAS scores are believed to produce greater accuracy than point scales (8). The Oswestry Disability Index (ODI) was used to assess self-rated low back disability (10). This is a 10-item questionnaire that rates an individual's self-perceived low back disability as a percentage score, with higher scores indicating greater disability.
Time to Stabilization
For the TTS method, maximal vertical jump testing (Vertmax) was performed, then measures of static stance were recorded from a single-leg stance on a force plate for a 5-s period. After appropriate familiarization with the jump protocol, subjects completed three successful jump protocol trials on either leg. For the FAI group, the order of leg tested was counterbalanced between affected and unaffected sides to avoid any learning or fatigue effects, whereas for the normal group, this was based on dominance. If a subject lost balance and touched the floor with the contralateral limb, if a short additional hop occurred upon landing, or if the subject missed landing on the force platform, the trial was discarded and repeated. The average TTS calculated from the force trace obtained in the x- and y-axes for three successful trials for each subject was used for data analysis.
Maximum vertical jump.
Subjects' Vertmax was measured using a Vertec vertical jump device (Sports Imports, Columbus, OH). Vertmax was determined as the difference between the subjects' maximum standing reach height and the subjects' maximum leap height. Measurements were made at 1.27-cm increments. The largest difference between the standing reach and the leap height of three trials was used during subject testing.
The TTS method was based on the jump protocol used by Ross and Guskiewicz (30). A Bertec triaxial force plate (Bertec Corporation, Columbus, OH) was used to measure duration of instability at a frequency of 200 Hz. The force plate data were A/D converted, and a personal computer using the Vicon Workstation v5.1 (Vicon, Singapore) analog was used for data acquisition, processing, and analysis. Subjects started in a standing position 70 cm from the center of the force plate (Fig. 1). Each subject was required to jump off both legs and touch an overhead marker placed at a position equivalent to 50% of the subject's Vertmax before landing on the force plate. Each subject was to land on the test leg, stabilize as quickly as possible, and balance for 20 s with their hands on their hips and looking straight ahead. Upon completion, the subject returned to the start position. All subjects were instructed to jump with their head up and hands in a position to touch the designated marker on the Vertec.
Data from the TTS protocol were analyzed in both the anterior-posterior (y-axis) and the medial-lateral (x-axis) movement planes using an unbounded third-order polynomial (UTOP method ). The UTOP method calculates the time it takes for the initial ground reaction force (GRF) upon landing to become similar to the GRF measured during quiet single-leg stance. Collected GRF traces in each movement direction are initially rectified, and from the initial impact, a UTOP is fitted to the GRF data. TTS is calculated from the equation for the polynomial as the point in time where the fitted line intersects the horizontal line representing the quiet single-leg stance (Fig. 2).
Trunk Muscle Reflexes
After appropriate familiarization, trunk muscle reflex testing to a sudden unloading paradigm was performed. The order of presentation was randomized and counterbalanced among subjects between the flexion and the extension movement directions. The average latency time across the three trials for the muscle response to perturbation in each direction was used for analysis.
After appropriate skin preparation to reduce impedance below 5 kΩ, pairs of Ag/AgCl electrodes (3M Red Dot, St. Paul, MN; 2-cm contact diameter) were applied bilaterally to the erector spinae (ES) and the rectus abdominis (RA) muscles before trunk muscle reflex testing. ES electrodes were placed in a longitudinal arrangement 3 cm lateral to the L4 spinous process, and RA electrodes were placed 3 cm lateral to the umbilicus (7). All EMG signals were collected using a g.tec (Guger Technologies OEG Herbersteinstrasse 60, 8020 Graz, Austria) BSamp biosignal amplifier system (16 bit A/D conversion, common mode rejection 110 dB, input impedance >110 MΩ). EMG signals were band-pass filtered between 20 and 500 Hz and recorded at 2 kHz.
Trunk sudden unloading protocol.
The protocol used in this study was adapted from method used by Radebold et al. (27). The original test protocol and adapted setup using a dynamometer are shown in Figure 3. Subjects were placed in a seated position with the hips and legs stabilized to restrict movement and the trunk fixed to the moveable lever of the device to allow isometric trunk flexion and extension movements to be performed. The test position was set at the subjects' neutral spine posture. Resistance to active force generated by the subjects was provided and measured by the dynamometer. Release of the dynamometer produced sudden unloading, resulting in displacement of the trunk and initiating reactive responses from the trunk muscles. Each subject performed three trials at the predetermined force level in each movement direction (flexion or extension). This was 65 N for males and 40 N for females representing approximately 20% of the maximal isometric exertion averaged for flexion, extension, and lateral bending (27). Output from the dynamometer was connected to a Powerlab (Analog Digital Instruments, Sydney, Australia) data acquisition system. The output from the dynamometer consisted of angular position, force output (calibrated for weight of device and subject), and movement velocity. The order of movement direction presentation was randomized between each subject to minimize learning and/or fatigue effects. Force output was displayed on a computer screen in view of the subject, and they were instructed to produce the required kinetic output in each direction as set to the target level on the screen. Once achieved, the system was released within a 2- to 5-s period. Ear plugs were used to restrict the subjects hearing when the button release was used to trigger the perturbation so that they could not anticipate the movement.
Onset times of ES (in response to flexion unloading) and RA (in response to extension unloading) were calculated as the point after which the root mean square of a 25-ms period of activity of the rectified EMG signal was increased by 1.5 SD above the prerelease level of activity (Fig. 4).
The Statistical Package for the Social Sciences (version 15; SPSS Inc., Chicago, IL) was used for the data analysis. Self-report measures were compared between groups using independent t-tests. Preliminary analysis indicated no difference between dominant and nondominant limbs for the control group; therefore, TTS times in the normal subjects were averaged for both sides of the body as per previous work (36). Dependent t-tests were used to compare TTS times in the x- and y-axes between the injured and the uninjured limb for individuals with FAI. The Kolmogorov-Smirnov test was used to identify that the TTS and the latency times were normally distributed in each group, and therefore when all group results were combined, parametric analysis methods were used. ANOVA procedures were used to compare TTS times in the x- and y-axes between groups (injured limb for FAI group, average of both limbs for control group). A group by direction ANOVA was used to analyze latency times and angular displacement for the trunk testing. Linear regression analysis was used to evaluate the relationship between trunk latency times and TTS times. Significance level of this study was P < 0.05.
Subjects with FAI had worse perceptions of their injured ankle compared with the uninjured ankle (P < 0.001). Subjects with FAI had worse perceptions of their ankle disability compared with the control subjects for the FADI, FADI sport, and CAIT self-report assessment tools (Table 1). Self-rated low back disability (ODI) and current pain levels (VAS) were not different between groups and were at very low absolute levels.
Time to stabilization.
Vertmax was not different between groups. Average jump height was 40.2 ± 8.5 cm for the control group and 43.4 ± 10.4 cm for the FAI group (P = 0.55). Medial to lateral (x-axis) TTS results discriminated between the injured and the uninjured limb in participants with FAI with an average TTS of 6.00 ± 2.8 s for the injured limb compared with 2.5 ± 1.7 s for the uninjured limb (P = 0.001). There was no difference between the injured (2.2 ± 1.5 s) and the uninjured limb (2.1 ± 1.6 s) in the anterior to posterior (y-axis) results (P = 0.95). The x-axis TTS results discriminated between the injured limb of the FAI group and the healthy control group (F(1,22) = 12.7, P = 0.002). Average TTS time in the x-axis for the control group (pooled results for dominant and nondominant legs) was 2.91 ± 1.04 s (Fig. 5). Anterior to posterior (y-axis) TTS results did not discriminate between groups (F(1,22) = 0.59, P = 0.45).
Trunk muscle testing.
Significantly delayed reflex times were measured for subjects in the FAI group (Fig. 6) for both the flexion (F(1,22) = 7.6, P = 0.01) and the extension (F(1,22) = 4.5, P = 0.04) movement conditions. The calculated effects sizes for between-group comparisons were 0.99 and 1.03 for flexion and extension, respectively. No between-group differences were observed for resultant trunk motion (F(1,22) = 0.086, P = 0.77), although the extension movement resulted in greater total displacement (pooled group results-flexion angle = 8.5° ± 9.2°; extension angle = 25.1° ± 13.6°, F(1,22) = 52.3, P < 0.001).
Linear regression analysis identified a significant relationship between the TTS times and the latency response during trunk extension (F(2,21) = 3.7, P = 0.043; Fig. 7). The adjusted r2 was 0.19, with the TTS time in the x-axis being identified from the regression analysis as the significant predictor within the model (P = 0.019). The regression relationship between the TTS times and the latency during trunk flexion was not significant (adjusted r2 = 0.08, F(2,21) = 1.9, P = 0.17).
The results of this study provided support for use of TTS measures as an effective method to discriminate between individuals with and without FAI (36). Furthermore, support for the experimental hypothesis of this study was found in that individuals with FAI also had delayed trunk muscle reflex responses compared with healthy control subjects. Regression analyses demonstrated that the TTS times explained 19% of the variance in trunk extension times, with greater medial to lateral TTS, predicting greater trunk extension latency times. The results of this study cannot be used to conclude a cause or effect relationship between impaired ankle and trunk stability, although these findings may provide reasoning for why individuals with FAI tend to develop chronic low back pain (25). Prospective studies are still required to examine whether or not impaired trunk function is a causative or a compensatory finding for individuals with FAI.
Time to stabilization.
Medial to lateral TTS times revealed a difference between the groups with FAI subjects having a more delayed time in comparison to the control subjects (6.0 ± 2.8 s vs 2.9 ± 1.0 s). Anterior-posterior TTS times did not discriminate between groups, which is in contrast to a previous report that found between-group differences were only in this movement plane (medial-lateral differences were apparent but were not statistically significant 36). The average medial to lateral TTS time we have recorded for individuals with FAI of 6.0 ± 2.8 s is longer than that previously reported in the literature, which range between 2.48 and 2.70 s (5,31,36). The average medial to lateral TTS time of both legs for the control group (2.9 ± 1.0 s) is longer than that reported for individuals with stable ankles in some research (34) but is less than others (36). The average anterior to posterior TTS time we have recorded for approximately 2.5 s for both groups is commensurate with previously reported values (5,31,36). It may be likely that our FAI subjects are more impaired than previous studies and therefore have a more delayed medial to lateral TTS. The use of different self-report questionnaires makes an accurate comparison of disability level difficult. However, it is also likely that methodological differences in the analysis of the TTS results lead to the different absolute times reported.
There are several different suggestions as to how TTS results should be calculated, which explains some of the variation in times presented from different studies. Our study calculated static stability before the dynamic jump protocol using the range of variation method (31,37), whereas other research has looked to refine this method to take into account the greater inherent instability an individual with FAI would have that may confound TTS calculations (34). It was suggested that the former method may not be an appropriate calculation method (34) to discern between-group differences. Furthermore, Ross et al. (33) have suggested that calculation of the TTS time should be based on the resultant vector of the landing GRF, whereas Wikstrom et al. (37) suggested calculation of a dynamic postural stability index, a composite score from the landing forces in all movement directions. The calculation of TTS based on a composite calculation rather than separation into the different force vectors may allow greater comparison of results between different research groups by controlling for experimental variation that may influence the specific vector results (instruction, type of surface). Despite methodological variations, the majority of times we have recorded in the current study are within the range reported in the literature. The greater TTS time in the medial to lateral direction for our FAI subjects may be reflective of a different disability status; however, we are unable to compare our subjects owing to the differences in descriptive data collected.
Trunk muscle reflexes.
This is the first study that we know of to have identified delayed trunk muscle reflexes in a group of individuals with FAI. Furthermore, this is the first study we know of to show an association between measures of both ankle and trunk stability in patients with FAI. This study has shown that TTS times in the x-axis are significantly associated with trunk latency times during the extension task. Significant relationships between TTS times and latency times in the flexion movement were not identified; however, we cannot discount a type II error in this study owing to the relatively small sample size. Previous research in controls found reflex times during extension and flexion of approximately 70 and 60 ms, respectively, (27,28). This is slightly different to the current study where controls had average responses of 106 and 79 ms for flexion and extension. In patients with low back pain, average onset times were 80 to 90 ms (27,28) compared with the current study where patients with FAI had average onset times of 198 and 133 ms, respectively, for flexion and extension. The initial force levels before "release" were the same as previous research, although the different experimental setup has influenced the absolute difference to previous results. Previous research had the subject in a semiseated suspended position compared with the current study where a dynamometer was attached to the trunk via placement on the back. In addition to measurement of trunk muscle reflex time, the protocol adopted for the current study also allowed continuous measurement of trunk position and force level from the output of the dynamometer. Although it may have been possible that resultant motion would also have been greater in patients with FAI, in combination with the delayed trunk reflexes, the current study found no difference in the amplitude of the resultant trunk movement between subjects with and without FAI. A potential limitation is that the measurement of angular displacement used the dynamometer position rather than the joint kinematics of the subject. Therefore, this is an imprecise measure of trunk movement. The greater displacement measured during trunk extension compared with flexion cannot be interpreted as direction-specific instability. The difference in displacement between directions is probably owing to the posterior position and inertia of the dynamometer trunk attachment, thereby increasing the electromechanical delay to the extension perturbation.
The proximal adaptation we have measured in this study, especially considering the different experimental measurement context for the between-group difference, suggests a centrally mediated adaptation in postural reflexes. Beckman and Buchanan (2) suggested that the proximal hip adaptation they measured was compensatory in nature. We cannot be sure as to whether or not the delayed trunk reflex is allowing compensatory balance strategies to take place. The mechanistic link between trunk and ankle stability (15) would suggest that a delay in initiating the increase in trunk stiffness would allow more time for adaptive movement strategies, such as that exhibited about the hip, to occur. If this change in proximal function is an adaptive response to maintain balance, then the question is raised as to whether impaired trunk reflexes should ever be a specific focus of a rehabilitation program for FAI. Indeed recent research has shown that impaired whole-body postural stability as well as local ankle joint proprioception can improve with exercise that has no specific trunk focus (21). However, the association of delayed trunk reflexes with future low back pain means that this finding has important consequences to be considered.
FAI and low back pain.
It is important to reiterate that this study has not established whether the delayed trunk muscle finding is a cause or effect of the ankle instability. From a rehabilitation perspective, the delayed trunk latency responses in the FAI subjects are interesting and novel findings as it may help to explain why individuals with FAI tend to develop low back pain (25). It has been found that individuals with delayed trunk muscle reflexes in response to a sudden unloading task were more likely to develop a future episode of back pain (7). Therefore, the immediate consequence of the finding of this study is that the FAI subjects are more likely to develop low back pain than the control group. This does not mean that every individual with FAI will develop low back pain and conversely that individuals without FAI will not.
Owing to the identified relationship between the two measures of ankle and trunk stability, popular recommendations for individuals with FAI would be to commence a "core stability" training program to offset the future likelihood of low back pain. However, there is uncertainty about what effect this type of intervention has (35). Recently, it was found that a core stability training program using the Swiss ball for low back pain did not immediately improve trunk muscle latency responses during a rapid upper limb movement task and that the chronic improvement observed in latency times was irrespective of the type of intervention received (23). Research looking at specific localized muscle contractions of the abdomen has shown that certain stabilization techniques increase compressive loading and that some muscles purported to have important roles in trunk stability (such as the transverses abdominis) actually have very minimal contribution (20). Therefore, a recommendation for the FAI subjects in this study to embark upon rigorous "core stability" training would be unjustified until we better understand the causative links between ankle and trunk stability. Studies that investigate neuromuscular adaptations in FAI after rehabilitation do not include proximal measures (21). Therefore, we do not know whether an indirect training program has led to proximal adaptation and then if the improvement after rehabilitation is perhaps owing to addressing these problems more so than correcting the specific joint impairments.
Of particular interest is that the average CAIT score for the control group (25.3 ± 3.4) is slightly below the optimal discrimination score (27.5) established for this questionnaire (18). Scores on the FADI were comparable to previous research for healthy individuals (16). Participants with a CAIT score of 28 or over are suggested to be unlikely to have FAI, whereas participants with a score of 27 or lower are more likely to have FAI (18). This result may suggest that some of the individuals in the control group may in fact have a minimal level of FAI despite the group mean being higher than those specifically included for their lower limb impairment. The CAIT has only recently been developed as a measurement tool for FAI owing to issues those authors raised with the use of the Functional Ankle Instability Questionnaire and the Ankle Joint Functional Assessment Test (AJFAT) (18). Although recent evidence has suggested that the AJFAT can more accurately discriminate between individuals with and without FAI compared with TTS results (33), there has been no direct comparison of the AJFAT to the CAIT in regards of what is the most appropriate self-report assessment tool for FAI. Determination of what questionnaire is more important for self-reporting of FAI was not a purpose of this study, although the results have shown that the CAIT can indeed discriminate between individuals deemed to have FAI and those who do not.
An important question for this study is did we accurately represent individuals with FAI. CAIT scores (17.3 ± 5.1) placed the FAI subjects we recruited within the lower range as compared with previous research (18); however, scores on both scales of the FADI were above that reported in individuals with chronic ankle instability, indicating that our subjects were not as affected (16). However, it was not clear from the previous research whether or not individuals with mechanical instability were included in their group, thereby confounding comparisons between studies (16). Our inclusion/exclusion criteria were relatively similar to previous research for FAI (16,34,36). In terms of functional physical performance, the two groups were unable to be discriminated with the vertical jump test. When considering that we were able to discriminate between these two groups in terms of self-report, ankle, and trunk measures whereas minimal levels of actual pain were reported with the low VAS scores, we do believe that we have been able to capture as accurately as possible the characteristics of individuals with FAI.
SUMMARY AND CONCLUSIONS
This study has provided further support to show that the jump protocol with analysis of TTS can discriminate between subjects with and without FAI. Furthermore, it was found that individuals with FAI had delayed trunk muscle reflexes to a sudden perturbation, which supports theoretical and experimental descriptions of proximal adaptations associated with ankle injury. Moreover, we found that TTS times in the x-axis were significantly correlated with the trunk latency responses during extension. Although a cause or effect relationship between impaired trunk function and lower limb stability cannot be established here, previous research suggests that the subjects with FAI are more likely to experience a future episode of back pain owing to the delayed trunk reflexes.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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