The objective of this study was to determine differences in electromyographic (EMG) activity of the soleus (SOL), vastus lateralis (VL), biceps femoris (BF), abdominal stabilizers (AS), upper lumbar erector spinae (ULES), and lumbo-sacral erector spinae (LSES) muscles while performing squats of varied stability and resistance. Stability was altered by doing the squat movement on a Smith machine, a free squat, and while standing on two balance discs. Fourteen male subjects performed the movements. Activities of the SOL, AS, ULES, and LSES were highest during the unstable squat and lowest with the Smith machine protocol (p < 0.05). Increased EMG activity of these muscles may be attributed to their postural and stabilization role. Furthermore, EMG activity was higher during concentric contractions compared to eccentric contractions. Performing squats on unstable surfaces may permit a training adaptation of the trunk muscles responsible for supporting the spinal column (i.e., erector spinae) as well as the muscles most responsible for maintaining posture (i.e., SOL).

The most predominant literature regarding balance has emphasised the physiological mechanisms controlling stability. Topics range from extrinsic factors (environment) to intrinsic factors (i.e. muscle coordination, vestibular response). Balance is achieved through an interaction of central anticipatory and reflexive actions as well as the active and passive restraints imposed by the muscular system. However, less research has attempted to document the effects of balance on performance measures (i.e. force, power). Furthermore, short- and long-term adaptations to unstable environments need more substantial research. While force and other performance measures can be adversely affected by a lack of balance, the transferability of instability training to activities of daily living and sport is not precisely known. The applicability of instability and resistance training using unstable platforms or implements may have strong relevance in a rehabilitative or athletic setting. Therefore, a comprehensive review of the literature in this area may possibly be of benefit to practitioners who deal with the general population, athletes or persons debilitated by balance and/or stability disabilities.

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Anderson, K.G., and D.G. Behm. Maintenance of EMG activity and loss of force output with instability. J. Strength Cond. Res. 18(3):637-640. 2004.-Swiss Balls used as a platform for training provide an unstable environment for force production. The objective of this study was to measure differences in force output and electromyographic (EMG) activity of the pectoralis major, anterior deltoid, triceps, latissimus dorsi, and rectus abdominus for isometric and dynamic contractions under stable and unstable conditions. Ten healthy male subjects performed a chest press while supported on a bench or a ball. Unstable isometric maximum force output was 59.6% less than under stable conditions. However, there were no significant differences in overall EMG activity between the stable and unstable protocols. Greater EMG activity was detected with concentric vs. eccentric or isometric contractions. The decreased balance associated with resistance training on an unstable surface may force limb musculature to play a greater role in joint stability. The diminished force output suggests that the overload stresses required for strength training necessitate the inclusion of resistance training on stable surfaces.

There are many instances in daily life and sport in which force must be exerted when an individual performing the task is in an unstable condition. Instability can decrease the externally-measured force output of a muscle while maintaining high muscle activation. The high muscle activation of limbs and trunk when unstable can be attributed to the increased stabilization functions. The increased stress associated with instability has been postulated to promote greater neuromuscular adaptations, such as decreased cocontractions, improved coordination, and confidence in performing a skill. In addition, high muscle activation with less stress on joints and muscles could also be beneficial for general musculoskeletal health and rehabilitation. However, the lower force output may be detrimental to absolute strength gains when resistance training. Furthermore, other studies have reported increased cocontractions with unstable training. The positive effects of instability resistance training on sports performance have yet to be quantified. The examination of the literature suggests that when implementing a resistance training program for musculoskeletal health or rehabilitation, both stable and unstable exercises should be included to ensure an emphasis on both higher force (stable) and balance (unstable) stressors to the neuromuscular system.