One of the most widely practiced exercises for improving lower body strength and power is the squat (15), which is also an important exercise in the rehabilitation of lower limb injury (7,18). In an effort to return to preinjury performance levels, athletes can increase the strength of their injured limb; however, overall strength gains may also be accomplished by favoring the uninjured limb (17). This results in a weight-bearing asymmetry (WBA) that could lead to increased risk of reinjury and also attenuate functional efficiency (20). For this reason, previous studies have assessed WBA and interlimb force during the squat in both healthy and clinical populations (2,8,14). Together, these studies demonstrate that significant interlimb asymmetries are common in participants performing the squat exercise.
Although WBA is present in healthy populations, it is dramatically accentuated in patients with lower limb injury. For example, high levels of WBA exist in people who have undergone anterior cruciate ligament reconstruction (ACLR), with the magnitude of WBA related to the time postsurgery (13). At 1.5–4 months post-ACLR, interlimb WBA magnitudes ranging from 33 to 48% have been observed during the squat exercise. This WBA was reduced to ranges of 21–28 and 7–9% in cohorts of participants who have undergone ACLR 6–7 and 12–15 months post-ACLR, respectively. This suggests that, when compared with the typical WBA observed during squatting in healthy populations (≈5–6%), patients who have undergone ACLR possess persistent asymmetry beyond the recommended timeframe for their return to sport. This WBA is accentuated by a person's inability to identify his or her asymmetry, with 87.5 and 50% of untrained and trained participants, respectively unable to perceive their loading asymmetry during a closed kinetic chain lower limb exercise (2).
It has been recommended that further research is needed to determine how important these bilateral imbalances are for future injury risk (14). However, commonly mentioned limitations of these studies are the technical requirements. Typically, accurate interlimb weight-bearing assessment requires 2 force platforms (8,14), with one under each limb to measure force distribution. Consequently, the use of such specialized testing equipment is not feasible in many athletic, clinical, or even research settings because of the considerable expense, cumbersome nature, and trained personnel required to operate the force platforms (10,11).
This study was designed to assess the functional WBA of experienced athletes and untrained adults using Nintendo Wii Balance Boards (NWBBs), a component of the popular computer game Nintendo WiiFit©. Similar to laboratory-grade force platforms, these devices contain 4 load cells that provide force and center of pressure (COP) data. Clark et al. (5) have found the NWBBs to be a reliable and valid tool for accurately assessing COP. The primary aim of this study was to determine if force data outputted from NWBBs located under each foot and displayed in real time to the participant could be used to decrease WBA. A secondary aim was to determine the extent to which interlimb asymmetries exist between highly trained athletes and age-matched, untrained people when squatting and to examine what influence training status had on the response to visual feedback. It is hypothesized that visual feedback will improve WBA and that highly trained athletes will have less WBA than aged-matched untrained adults. It is also hypothesized that, compared with the trained athletes, the untrained sample will produce greater percentage improvements in WBA with visual feedback because of their greater levels of WBA.
Experimental Approach to the Problem
This study consisted of a randomized crossover design protocol, which assessed the performance of squats both with and without visual feedback of WBA. Two different groups of subjects were assessed: (a) young, healthy men with limited experience of the squatting exercise and (b) elite athletes with extensive squatting experience.
A group of 15 trained men (age = 23.1 ± 2.9 years, mass = 91.5 ± 14.1 kg, height = 189.7 ± 6.8 cm) and a group of 32 untrained men (age = 24.3 ± 4.8 years, mass = 77.6 ± 13.1 kg, height = 179.3 ± 7.6 cm) volunteered to participate in this study. The trained population was recruited from an elite Australian Rules Football club, and the participants were currently performing weighted squats as part of their regular in-season conditioning program. The untrained participants were physically active but were not performing the squatting exercise during their weekly physical activity. All the participants provided written informed consent before participation and were free from lower extremity injury and muscle soreness at the time of testing. This study was approved by the Australian Catholic University Human Research Ethics Committee.
The symmetry of weight distribution during squatting tasks was evaluated using NWBBs. The 45-cm by 26.5-cm NWBBs contain 4 microfoil-type strain gauge transducers that are located in each of the 4 corners of the board. These transducers each provide a single measure of force per sample, with the data for all 4 sensors transmitted to a computer via a Bluetooth connection. Data were sampled at 40 Hz, with a 12-Hz lowpass Butterworth filter applied during postprocessing (5).
The reliability of the NWBB system for assessment of weight bearing asymmetry has been previously reported as excellent (ICC range = 0.81–0.91) (4).
The testing protocol involved 2 different squatting conditions performed in a randomized order—squatting with and without visual feedback. In each condition, the participants completed 6 squats at a tempo of 1 repetition per 6 seconds, which was relayed visually to the subject by a light-emitting diode–type indicator and audibly by a low-frequency, low-volume tone, with both feedback protocols built into the software. This feedback was provided at 3-second intervals, representative of the top and bottom of the squat movement. Pilot work demonstrated little difference in natural vs. parallel squat depths, and therefore, the participants were asked to squat to a self-selected (natural) depth in an attempt to replicate a normal squat movement performed in a resistance training session. Specifically, the participants were given the cue: “squat to the depth that feels comfortable for you”.
Before commencing testing, the 2 NWBBs were placed on the ground with a laptop computer positioned on a table at a distance of 1.4 m in front of them and at a height of 0.7 m. The participants were asked to stand behind the NWBB in their relaxed bipedal stance position, and the boards were positioned with the longitudinal and mediolateral axes aligned to the respective axes of the participants' feet. Initially, the participants completed 6 practice squats standing behind the NWBBs to validate the correct interboard width and, where necessary, to adjust this position to ensure a comfortable squatting position. Next, the participants completed an additional 6 squats while standing with a foot on each board and using the visual feedback software (Figure 1). These trials served as both the warm-up and familiarization trials for the participants.
During the feedback trials, custom-made software (Labview 8.5, National Instruments, Austin, TX, USA) was used to display, in real time, the left and right weight distributions of the participant during the squatting exercise. This information was displayed on the laptop computer directly in front of the participant and was linked from the laptop to the NWBB via a Bluetooth connection. In this study, weight-bearing symmetry was defined as the condition when the load on each limb was within ±5% of the participant's body weight. This value was chosen because it represents a minor WBA that would be likely to be attributed to random weight distribution fluctuation observed in quiet stance. Weight bearing was considered asymmetrical if the load on each limb was outside of this region. The weight-bearing vertical bars were green when the squatting movement was symmetrical and turned orange when it was asymmetrical (Figure 2).
The loading condition of body weight only used in this study, which is much lower than a typical load encountered during resistance training, was chosen based on the finding that increased loading had minimal influence on the magnitude of WBA (8). Therefore, this relatively low loading protocol would be reflective of WBA at a variety of squatting loads, while being simple to administer and being low risk for the untrained participants.
The mean mass difference (MMD) as a percentage of body mass provided a measure of the overall WBA and was defined as the average of the absolute difference in weight-bearing load between the limbs as a percentage of body mass at each sample point across the full 36-second trial. This was calculated by obtaining the square root of the squared mean difference between limbs for each testing sample, assessing the average of this difference throughout the trial, and then normalizing this value to body mass. To further determine asymmetries in force patterns during squatting, the symmetry index (SI) was also calculated (9,16). This is simply the mass distributed through the right limb minus the mass distributed through the left limb divided by half of the subject's body mass. This value was multiplied by 100 to express it as a percentage. The difference between the MMD and the SI is that the SI of a periodically fluctuating signal with a mean of zero (e.g., a sine wave) would be zero, whereas for the MMD, any fluctuation away from equal mass distribution is considered to be a positive number regardless of the direction. Finally, the time spent favoring a single limb (TFSL) was also examined, which provided an outcome measure that reflected the time spent outside of the “normal” force thresholds during each trial. A limb was considered “favored” when the WBA exceeded 5% of the body mass through either limb (signified visually when the color of the feedback bar changed from green to orange), and the TFSL was assessed by dividing the time spent outside this normal threshold by the total testing time and multiplying this number by 100.
The Shapiro-Wilk test revealed that the majority of the outcome variables were not normally distributed, and therefore, nonparametric statistics were used. Consequently, medians and interquartile ranges were calculated and reported for the outcome measures. Wilcoxon signed-rank tests were used to identify any differences in squatting performance in response to visual feedback. Secondly, Mann-Whitney U tests were used to evaluate if the trained population compared with the untrained population demonstrated greater levels of symmetry during squatting in both visual conditions. These statistical analyses were performed for each of the 3 outcome measures. Finally, an assessment of the relationship between WBA and the response to feedback was determined using Spearman's rank order correlation analysis. This was performed by correlating the group data for the MMD during the no-feedback condition with the percentage improvement in MMD between the no-feedback and feedback conditions. All statistical analyses were performed at an alpha level of 0.05.
Table 1 presents the results of the entire sample for the measures of WBA in each of the 2 feedback conditions. Visual feedback significantly (p = 0.028) reduced the MMD between the limbs when performing the squatting task, with 14% lower MMD compared with the no-feedback condition. The SI was also significantly reduced during the visual feedback condition (p = 0.007), with a decrease of 41% compared with the no-feedback condition. Although the TFSL displayed a 26% reduction with visual feedback, this result was not significant (p = 0.080).
Group comparisons between visual conditions for the trained and untrained populations for each of the measures of WBA are provided in Figures 3–5. In the untrained sample, significant reductions in MMD (p = 0.045) and SI (p = 0.026) were found in the visual feedback condition compared with that in the no–visual feedback condition (Figures 3 and 4, respectively). Although the reductions in these variables were also observed in the trained sample, these were not identified to be statistically significant (p = 0.363). Similar to findings for the entire cohort, no significant differences in TFSL were observed between visual conditions for either group (Figure 5).
Comparisons between measures of WBA in the trained and untrained samples generally showed a trend toward less WBA in the trained population. Specifically, the trained population showed lower levels of WBA in all outcome measures; however, this was only significant for TFSL both with (p = 0.017) and without (p = 0.015) feedback. This relationship is presented in Figure 5. Despite the significant reductions in the measures of WBA with visual feedback in the untrained population, and nonsignificant difference in the trained population, the percentage improvements in WBA with visual feedback were not significantly different between groups (MMD: p = 0.349, SI: p = 0.910, TSFL: p = 0.570).
To evaluate whether the participants with higher levels of WBA received greater benefit from the visual feedback, a Spearman rank order correlation test was performed between MMD during the no-feedback condition and the percentage improvement in MMD between the no-feedback and feedback conditions. A moderate relationship (p < 0.001, ρ = 0.557) between MMD, and the percentage change in MMD in response to feedback was observed (Figure 6).
To our knowledge, this is the first study to evaluate WBA in trained and untrained populations using an NWBB-based system and to evaluate the influence of real-time visual feedback of weight distribution on measures of asymmetry. The results indicate that interlimb WBA is evident in both trained athletes and untrained adults, and it can be reduced in the untrained adults with real-time visual feedback as prescribed in this study. Reductions in all measures of WBA were also observed in the trained athletes; however, none of these analyses were statistically significant. This is likely because of the relationship between the magnitude of WBA and the response to visual feedback, which indicated that those participants with relatively high levels of WBA during the no-feedback condition recorded the greatest improvement when presented with feedback. Consistent with previous research indicating less weight bearing and other dynamic asymmetries with greater training status (2,4), our results showed reduced WBA in the trained sample for all 3 symmetry measures under both visual conditions. Specifically, all measures of WBA were reduced for the trained compared with the untrained adults; however, only TFSL was significantly lower in the trained athletes for both feedback conditions (no-feedback, p = 0.015; feedback, p = 0.017). This significant difference in TFSL, for which the threshold reflects the values at which visual stimulus of asymmetry was presented to the participant during the feedback trials, indicates that the trained participants received less visual stimulus to modify their technique. Consequently, it is logical that their WBA improvement would have been less than that of the untrained participants, a response that may have been altered if the thresholds for asymmetry feedback were set at a lower percentage of body mass. Despite these apparent differences, between-group comparisons of the percentage improvements in WBA were not significantly different for any outcome measure.
The aforementioned positive relationship between the magnitude of WBA and response to feedback is particularly important in the context of clinical populations, because their increased WBA would imply that they have greater potential to respond to visual feedback. For example, the significantly increased magnitude of dynamic WBA in patients who have undergone ACLR would result in increased visual stimulus alerting the participant to their asymmetry (13). Consequently, the results observed in this study would likely be magnified in a clinical population where high levels of WBA are observed. Given that closed kinetic chain exercises are often used to assess strength levels and readiness to resume sport postinjury (1,6) and that WBA is often observed in response to lower limb injuries and surgical procedures (3,12,13,19), incorporating an NWBB-based system to evaluate functional exercises provides an inexpensive and easy to administer method of assessing WBA.
This study evaluated the acute effects of visual feedback on WBA and found that visual feedback is particularly effective when high magnitudes of WBA are present. However, there is a need for longer-term studies in both healthy and injured athletes to determine whether there is a learning effect while using visual feedback and whether WBA can be improved or corrected over the longer-term using interventions spanning an extended period. Historically, the progress of such research has been limited by the use of the laboratory equipment required; however, the findings of this study suggest that more accessible and portable equipments such as the NWBB may achieve this aim. Further research should also explore any association between WBA and injury risk and whether visual feedback training assists in reducing injury occurrence. Considering the limited number of prospective studies investigating interlimb asymmetries and injury risk in athletes, the clinical relevance of these asymmetries is presently unclear. Although only unloaded squats were examined in this study, given that each NWBB has the capacity to hold 150 kg, it should be possible to use the NWBB under more typical strength training loads. This would allow similar systems to become incorporated into the athlete's normal training routine, providing potential longitudinal tracking of WBA with minimal impingement on the athlete's training routine.
Inexpensive and widely available NWBBs with customized software can be used to improve dynamic WBA. Clinicians and athletic trainers who monitor lower limb asymmetries are able to employ similar systems without the need to attend a research laboratory. This is particularly important in untrained athletes and those rehabilitating from lower limb injuries. Further research should assess the long-term benefits of improved WBA in clinical populations.
Author Clark's position is funded by ASICS Oceania. Ethical approval was provided by Australian Catholic University.
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