Report by George Chen
INTRODUCTION
Swiss balls are large, inflated, rubber and vinyl balls used by physical therapists to enhance the neuro-development of their patients. More recently, the swiss ball has been introduced as a strength-training aid to athletes (4, 12). Using the swiss ball as a multi-purpose bench forces athletes to train in an unstable environment. It is claimed that training in an unstable environment strengthens stabilizer muscles, reduces chance of injury due to repetitive stress, and improves nervous-system function that lead to functional strength gains (4). The shape of the ball also facilitates multi-angle training and allows greater range of motion on some exercises; both potentially important factors in properly training certain muscle groups.
PHYSIOLOGY
The swiss ball can be used as a multi-purpose bench when performing weight training exercises. For example, dumbbell flys and presses can be performed while lying on the ball with additional weight to stabilize the ankles. Having to concentrate and shift their weight to maintain stability on the ball, it's claimed that athletes recruit postural muscles that normally don't come into play in traditional weight training exercises (12). Ultimately, these stabilizers strengthen and enhances one's overall strength.
It's also claimed that swiss ball training improves nervous-system function that results in functional strength gains. The neural component of strength is well established, and maximum strength development requires the development of this component along with that in contractile tissue. Improvements in the neural component of strength can occur through increased IEMG, increased motor unit firing rate, increased recruitment of higher threshold motor units, or improved motor unit coordination (2).
Repetitive stress injuries are believed to occur when training in a restricted range of movement. Machine training and, to some degree, free weight training on stable platforms force the athletes to stress the same ligaments and joint surfaces from repetition to repetition. Because the unstable swiss ball surface, each repetition is different, potentially decreasing the risk of repetitive stress injuries (4).
The shape of the balls allow athletes to perform weight training exercises at a wide variety of angles. For example, starting from a flat bench position on the swiss ball, athletes can drop their hips, roll forward on the ball into an inclined bench position. The posture can be adjusted in any increment because of the continuous surface of the ball and without unloading weight and even in the middle of a rep (which can't be done with traditional benches). It is claimed that this versatility allows the design of more effective workouts.
Also, the shape of the ball enables athletes to train certain muscles through a greater range of motion. For example, while lying with the lower back on the center of the ball, athletes can perform abdominal crunches beginning with the abdominal muscles in the stretched position. This stretched position can't be worked while lying on a flat, horizontal surface. It is claimed that the effect of the pre-stretch and potential strength development in the stretched position can enhance the effectiveness of the exercise and make the task more functionally useful to athletes (4, 12).
Expectation: I expect that swiss ball training has a place in strength training programs for some of the claims mentioned. However, other claims are probably untrue, very difficult to substantiate, or may not provide much benefit over conventional training.
EVALUATION
Muscle activity changes while performing arm elevations on an unstable surface has been studied in humans (6). Compared to the motion on a stable surface, the researchers observed increased co-contraction between soleus and tibialis anterior along with changes in timing of biceps femoris and erector spinae activity in relation to deltoideus anterior (the prime mover) activity. The study concluded that posture control during balancing on an unstable surface consists of adapting the motor program to maintain stability, while the overall postural strategy is maintained. The study supports the swiss ball claim that training in an unstable environment elicits changes in postural muscle activation. However, the postural muscles are distributed throughout the body and their differential recruitment is probably highly task dependent. It is possible that training these muscles directly in a non-postural task may be more effective in developing strength in these muscles. However, recruitment of these muscles in a non-postural task may not be as beneficial to athletic performance since training effects are task specific (2, 11).
It is more likely that swiss ball training improves functional strength through neural mechanisms. Indeed, if the primary benefit of swiss ball training in developing stabilizer strength is due to a better match in task specificity to real world tasks, then the benefit of increased stabilizer strength from swiss ball training is primarily neurological. Swiss ball training may also increase the neural component of strength through improved motor coordination. Gentile (1987) has proposed a taxonomy of tasks based on task function and the environmental context in which the task is carried out. The simplest tasks are closed tasks in which the regulatory stimuli (the pertinent sensory information) are stable and no variability exists between trial to trial. Practicing closed tasks results in fixation, a type of learning that narrows the range of performance. The most complex tasks are open tasks in which the regulatory stimuli are unstable and there is variability between trial to trial. Practicing open tasks results in diversification, the acquisition of a more flexible motor strategy, adaptable to changing environmental conditions. Swiss ball training has been used by physical therapists to challenge their patients toward diversification (16). If swiss ball training with weights results in a similar motor strategy diversification in athletes, the strength gains may be more functional to performance.
The use of swiss ball training to enhance neuro-development in patients is based on a dynamical systems approach (8). Poorly developed nervous systems can constrain movement and perception, limiting the ability of the system to explore functional movement solutions. Introducing instability in the context of motor tasks is one effective way that therapists can provide their patients with the opportunity to discover the dynamics of their own actions. Spontaneous transition from one movement pattern to another can occur when key control parameters of the task reach critical states, allowing the movement to reorganize adaptively (8, 15). Swiss ball training combined with conventional weight training may free the nervous system from the constraints perhaps manifested from conventional training.
The claim that swiss ball training reduces the risk of repetitive stress by increasing movement variability is difficult to substantiate. There appears to be a rationale for unconstraining exercise movement to reduce local ligament and joint wear. However, the mechanism of repetitive-stress or overuse injuries is poorly understood in exercising athletes (14). Injuries can result from training errors, poor performance, poor technique, inappropriate surfaces, or intrinsic factors like malignment and muscle imbalances. Furthermore, rib stress fractures occur in elite rowers (9) and injuries to the shoulder in swimmers (5), both activities which provide variability in repetitive motion. In these cases, the biomechanical factors particular to the motion is believed to dominate the mechanism of injury. Therefore, the swiss ball claim of reducing injury risk from repetitive stress has to be in some doubt.
The swiss ball facilitates multi-angle resistance training and allows some exercises to be performed through a greater range of motion. Although it has not been demonstrated that training at multi-angles or through a greater range of motion increases muscular hypertrophy, these factors can aid overall strength development due to training specificity (2, 11). Dumbbell presses performed in a horizontal position and in an inclined position are two different tasks, and athletes need to train both positions to maximize strength at each angle. A number of studies have demonstrated that training isometric knee extensions at one joint angle primarily increases strength at the joint angle exercised and to a much lesser extent at other joint angles (1, 10, 17). The range of motion that an exercises emphasizes can be important to improvements in functional tasks. For example, the swiss ball facilitates training the abdominal muscles in the stretched position. Abdominal muscle strength in the stretched position is believed to be important in many dynamic athletic movements like the tennis serve and various throwing motions. Therefore, swiss ball training can facilitate functional strength development at multiple angles and ranges of motion from it's versatility in positioning athletes in novel exercise postures. However, some of the benefits can be similarly derived from training with a variety of conventional pieces of equipment.
CONCLUSION
The support for improvements in functional strength from training on an unstable surface like the swiss ball comes primarily from motor control theory and anecdotal experiences of physical therapists. The functional strength gained is most likely due to improved motor coordination in tasks which deviate from traditional weight training exercises. Therefore, it would be difficult to assess the strength gained using standard forms of strength testing. The mechanisms of overuse injuries are poorly understood for many exercises, and the rep to rep variability provided by the swiss ball only addresses one of the many possible causes. Therefore, it is doubtful that swiss ball training can significantly reduce repetitive stress or overuse injuries in general. Training at multiple angles and through a full range of motion is important due to training-effect specificity. Some of the versatility of the swiss ball in aiding athletes to train at multiple angles and through greater ranges of motion can be derived from variety and creativity in traditional weight training exercises. However, a swiss ball does facilitate variety and is a relatively inexpensive alternative to utilizing many specialized benches and platforms.
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Email: gchen@stanford.edu
Homepage: http://www.stanford.edu/~gchen/