Recently, the importance of balance training has become a prevalent point of interest for fitness professionals and consumers alike. Although the average person may take it for granted, the ability to balance is crucial for normal functional activity and advanced movements common in athletics (19,23,24). Although often used synonymously, balance and sensorimotor control are not the same. Balance is the process by which the individuals control their center of mass with respect to their base of support (9,26). Sensorimotor control is the precursor to good balance and provides postural control by generating a musculoskeletal response to external perturbations (12). Originally defined by Sherrington in the early 1900s, proprioception, a component of sensorimotor control, is used to describe sensory information originating from the joints, muscles, tendons, and associated deep tissues. Mechanical deformation of the aforementioned structures will then result in a muscular response known as neuromuscular control (8,20). The speed at which the body responds to these kinesthetic and joint position cues can be improved with balance training or hindered as a result of injury or inactivity (14).
Because of the high incidence of lateral ankle sprains in athletics, such injuries have become the most prevalent musculoskeletal injury in athletics (8,14). When an individual sustains an ankle injury, it is not uncommon for the surrounding musculature and associated neural structures to be affected (8). If the functions of these neural structures are not restored, the resultant deficits can lead to a chronic instability of the ankle, predisposing the injured person to possible future injuries (8,18). By incorporating a rehabilitation protocol into the recovery period, and even following, any deficits in neuromuscular function should be regained and the individual can return to the functional level necessary for his/her activity demands (8,18).
Although it may seem commonplace, the importance of addressing proprioception and postural control in either a fitness or rehabilitation protocol is still a relatively new concept and current research is ongoing (8,25). Furthermore, the increased interest in these areas has led to advances in proprioception and balance training equipment, and more of this equipment is being used (5,21,25). Various balance platform systems have been created to increase the ankle proprioception in injured patients (5,21,25). Subsequently, more advanced computerized mechanical platforms have been designed to challenge both the muscular and visual systems in response to calculated perturbations (6,7,16,25). Although there are many different balance training products available, this study will focus on the Both Sides Utilized (BOSU) balance trainer (Bosu Fitness, LLC, SanDiego, CA, USA), which has become increasingly popular over the past several years.
The BOSU balance trainer is a device that combines a solid plastic base with an inflatable bladder, resembling a halved Swiss ball. Designed for the athletic and recreationally active population, the BOSU balance trainer is unique in that it can be used with either the flat or bladder side on the ground, making it a versatile piece of balance training equipment and hypothetically resulting in varied difficulty depending on which side of the device is being used. Consequently, it has become quite popular in fitness gyms, strength and conditioning rooms, athletic training rooms, and physical therapy clinics. However, although there has been a recent surge in research on balance training for performance enhancement, injury prevention, and rehabilitation, limited exploration of specific balance training devices and the effects they have on the human body have been reported (1,3,13-15,17).
The popularity and the relatively widespread general acceptance of the BOSU balance trainer's ability to generate results has not been quantitatively reported in recent literature. Furthermore, few studies even list this piece of equipment as being used in reported postural control, proprioception, or ankle rehabilitation protocols (25). Given the scarce mention of this device in the professional literature, the BOSU balance trainer needs to be further examined. Therefore, the purpose of this study was to quantitatively determine if one side of the BOSU balance trainer produces greater ankle muscle activation than the subsequent side. Such information may be beneficial for athletes and coaches attempting to improve balance and for clinicians interested in decreasing the risk of injury and during the rehabilitation of various lower extremity injuries.
Experimental Approach to the Problem
This study used a repeated measures control group design. Each subject randomly completed a balance task on one side of the BOSU balance trainer and then repeated the balance task on the opposite side to determine if there are differences in ankle muscle activation between the 2 sides of the device. The independent variables were the 2 conditions being observed (condition 1: bladder side up; condition 2: bladder side down). The dependent variable was average electromyographic (EMG) activity for the tibialis anterior, peroneus longus, and medial gastrocnemius during each condition.
Twenty, recreationally, physically active, college-aged men (age: 21.4 ± 1.4 years; height: 179.4 ± 8.6 cm; mass: 80.0 ± 12.1 kg) volunteered to participate in this study. Recreational physical activity was defined as any type of physical activity participated in at least 3 times per week for a minimum of 30 minutes. The activities performed by the subjects all included some form of aerobic exercise, such as running, bicycling, or swimming at noncompetitive level. Individuals with any history of lower extremity injury within the past 2 years of this study, history of balance training, or who had any disorder affecting balance or sensorimotor control were excluded. Each subject was informed of the experimental risks and signed an informed consent document before the investigation as approved by the university Institutional Review Board.
Before data collection, all subjects were encouraged to ask any questions regarding the study and their participation. After all questions were answered to the subjects' satisfaction, they were prepared for electrode placement. Disposable pre-gelled Ag-AgCl electrodes with a circular contact area of 1 cm diameter on a 41-mm-diameter moisture resistant backing (BIOPAC Systems, Inc., Goleta, CA, USA) were applied to the skin after the areas were shaved, abraded, and cleansed with 70% isopropyl alcohol. Two electrodes were placed on tibialis anterior, peroneus longus, and medial gastrocnemius, 2 cm apart (center to center) and parallel to the muscle fibers. To measure peroneus longus activity, the electrodes were placed 3 cm distal to the fibular head and the lateral malleolus. To measure tibialis anterior and medial gastrocnemius activity, the muscles were palpated and the electrodes were placed over the area of greatest bulk. One electrode was placed over the tibial tuberosity of the same leg and used as a ground. The leg used in this study was determined by asking each subject which leg they preferred to kick a ball.
Once prepared, each subject was connected to the MP-100 System (BIOPAC Systems, Inc.) to record his ankle muscle activity at a frequency of 1,000 Hz per channel and signals were amplified (DA100B; BIOPAC Systems, Inc., Santa Barbara, CA, USA) from the disposable surface electrodes. Before collecting surface EMG data from each condition, all subjects were required to perform a 5-second maximal voluntary isometric contraction (MVIC) for each of the 3 muscles. These initial measures were used as the norm for each subject.
After the collection of MVIC EMG, each subject was ready to begin their trials for each condition. Condition 1 was defined as having the BOSU balance trainer with the base (hard flat side) down and performing the single-leg stance on the bladder side. This is the manufacturer's recommended use for the BOSU balance trainer with regard to the lower extremity (4). The second condition consisted of using the BOSU balance trainer with the base up and the inflatable bladder on the ground. Each subject was required to perform 3 trials for each condition. All 6 trials were randomized, and each lasted 10 seconds.
The same BOSU balance trainer was used for the entirety of the study. Before each subject's participation, the investigators placed the device on the bladder side and measured for the manufacture's recommended height of 25.4 cm to ensure proper inflation (4). Before beginning the trials, subjects were allowed to practice each condition to the point of being able to complete the task for at least 10 seconds. Once they displayed the ability to complete each condition, they were not allowed any further practice. Subjects were instructed to perform a barefoot single-leg stance with their eyes open, hands on their hips, and the opposite leg flexed under the body, but not touching the balancing leg, which was allowed to be slightly flexed. They were advised to focus on a specific point in front of them. If at any point the subject broke form, the data were discarded and the trial was repeated. A break in form included removing the hands from the waist, touch down of the non-balancing leg, touching the balancing leg, leaving the surface of the BOSU balance trainer, or extending the non-balancing leg or trunk and upper extremity outside the borders of the BOSU balance trainer. Subjects were allowed to rest for at least 30 seconds between each trial and were allowed additional time if necessary. However, no subjects requested any additional time.
A statistical package for the social sciences 14.0 for Windows (SPSS, Inc., Chicago, IL, USA) was used for statistical analysis. Electromyographic data from each subject's 3 muscles were analyzed using a repeated measures analysis of variance (ANOVA; p ≤ 0.05) to determine significant differences between conditions 1 and 2 for each muscle.
Mean and SDs for the average EMG for each ankle muscle can be viewed in the Table 1. The repeated measures ANOVA yielded no significant differences between the 2 sides of the BOSU balance trainer for the values of the tibialis anterior (p = 0.54), peroneus longus (p = 0.74), and medial gastrocnemius (p = 0.65).
The results of this study imply that there are no significant differences in ankle muscle activity when using alternate sides of the BOSU balance trainer. This study is unique in that it attempts to provide quantitative data to confirm the anecdotal evidence, suggesting that the BOSU balance trainer is a versatile and effective piece of equipment. The present literature focuses on the outcomes of implementing balance training, but there is sparse research documenting the actual physiological effects that specific balance training devices generate.
Only one readily available article documents the use of the BOSU balance trainer, which is somewhat surprising given that this device has been around since 1999, when it was invented by David Weck, and has seen so much recent popularity. Yaggie and Campbell (25) incorporated the BOSU balance trainer into their balance training regimen, using exercises that were both commercially available with the product and consistent with those described in the literature (2,10,11,22,25). Their subjects underwent a 4-week training program to determine if there were any benefits on selected functional skills. After the training period, single-leg stance duration on the BOSU balance trainer was used as one of these functional skills to determine if any benefits were observed. They found significance with their “time on ball” measures and were able to determine that there were improvements in postural control and sport-related activities (25). Based on this previous investigation and the results of our study, it appears that the BOSU balance trainer does provide some balance training benefits; however, the side of the device used does not appear to matter.
McGuine and Keene (14) and Verhagen et al. (22) took different approaches and described the benefits of incorporating an external balance device into a training program for the prevention of ankle sprains. Both studies used a protocol that began with simple single-leg tasks and, over time, progressed to more advanced exercises using a wobble board. Similar results were found, in that both investigations document a significant decrease in ankle sprains in the intervention groups when compared with the control groups (14,22). Paillard et al. (17) used a balance device to determine postural performance and strategy differences between national and regional soccer players. A seesaw was placed on a force platform and used to measure anterior/posterior and medial/lateral control between groups. Using the information gathered, the authors were able to determine that national players were the more stable athletes. Based on these past studies, it appears important for athletes to incorporate balance training, such as the use of a BOSU balance trainer or other balance exercises, into their exercise regimen as a means of improving athletic performance and decreasing the risk of injury.
One study that investigated the efficacy of a specific balance training device was done by Blackburn et al. (3). These authors reported on the effect of using exercise sandals during a rehabilitation program as a means of increasing ankle muscle activity. They ultimately concluded that using the exercise sandals did increase the ankle muscle activity while performing functional activities (3). However, Michell et al. (15) conducted a study that incorporated the exercise sandal into a rehabilitation protocol and tested against a control group. Their findings indicate that the balance device did contribute to significant gains but not any more significant than functional balance training alone without the exercise sandals (15). Our study reported similar results showing good activity while using the BOSU balance trainer but no difference in activity based on which side of the device was used.
There are several limitations to this study. The first is related to subject participation. The EMG data suggest that some subjects' trials were able to produce more ankle muscle activity than the MVIC. Although not uncommon, this can occur because of the lack of familiarity with the motion needed to produce the MVIC or can be the unfortunate result of poor motivation to produce a maximal contraction. Each subject was coached and encouraged throughout each MVIC measure to try to avoid this problem. There are also some inherent problems with surface EMG that can produce these findings. The lower extremity comprises many overlapping muscles, and although the 3 muscles used are superficial, there is still some cross talk from other muscles that can cause altered EMG activity. Furthermore, it is necessary to realize that this study only made observations of 3 ankle muscles in the lower extremity. That does not rule out changes occurring in other muscles, and hip and core musculature should also be recorded in the future. Finally, there was a limited ability to control the amount of pressure and displacement in the BOSU balance trainer. Measuring the BOSU balance trainer height before each subject participated was used in an attempt to keep this constant. However, with the varying weights of each subject, the displacement of the bladder would be higher for heavier subjects, consequently lowering the BOSU balance trainer to the ground in each condition. In condition 1, this would increase the instability of the device. Conversely, this would generate more surface area in the second condition, giving the subject a larger base of support.
Using the BOSU balance trainer for exercise and rehabilitation is one way to create a unique and diverse training experience for a client or patient. With regard to ankle muscle activity, our study demonstrates that there is no benefit to the amount of ankle muscle activity that results from flipping the BOSU balance trainer onto the bladder side. As such, athletes using this device as a means for improving balance, for injury prevention, or for rehabilitating a lower extremity injury may use either side of the BOSU balance trainer with similar results.
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