Effects of Core Instability on Muscle Activity

The following is an exclusive excerpt from the book Developing the Core, published by Human Kinetics. All text and images provided by Human Kinetics.

Exercises performed on unstable surfaces not only increase core muscle activity but also limb muscle activity and co-contractions (agonists and antagonists together). Triceps and deltoid muscle activity were increased when push-ups and chest presses were performed under unstable versus stable conditions (Marshall and Murphy 2006a, 2006b), while the soleus calf muscle experienced greater activation during unstable squats (Anderson and Behm 2005). In another study by the same group, although isometric chest press strength was decreased, there was no significant difference for limb and chest muscle activity between the unstable and stable conditions (Anderson and Behm 2004). The similar extent of muscle activity but with decreased force with instability suggested that the ability to apply external force by the muscles was transferred into greater stabilizing functions (Anderson and Behm 2004).

The short and long heads of the biceps brachii can both contribute as stabilizers of the shoulder joint, and their role in stabilization increases as joint stability decreases (Itoi et al. 1993). This muscle response to unstable exercises may be especially beneficial in the rehabilitation setting where excessive resistance on an injured joint might increase the chance of injury. Typically an injury forces the person to use less resistance, which results in a lower level of muscle activity. However with an unstable resistance exercise, the muscle activity can be high even with lower resistance so that the repairing connective tissue does not have to contend with high resistance or load.

Thus most studies report a decrease in resistance combined with high limb muscle activity. This suggests a switch from an emphasis on the ability to move loads to protecting the joint (Anderson and Behm 2004).

Co-contractile activity (activity of both the agonist and the opposing antagonist muscles) can increase when playing, working, or training on unstable surfaces. Antagonist activity, in which the opposite muscle resists the intended contraction or movement (e.g., triceps are the antagonists to the agonist biceps during a dumbbell curl), has been reported to be greater when uncertainty exists in the task (De Luca and Mambrito 1987; Marsden, Obeso, and Rothwell 1983). Behm, Anderson, and Curnew (2002) reported that plantar flexion (calf raises) and knee extension muscle actions performed under unstable conditions experienced 30 percent and 40 percent greater antagonist activity than the stable conditions, respectively. The antagonist’s role may have been an attempt to control and protect the limb when producing force. However, the more the antagonist contracts (e.g., triceps during a dumbbell curl), the less resistance can be moved by the agonist (e.g., biceps during a dumbbell curl). Thus while high muscle activity can be achieved with unstable environments, the ability to do work may be impaired as the muscles try to cope with the uncertainty of instability.

Prolonged training can result in lower antagonist activity during lifting (Carolan and Cafarelli 1992; Person 1958). More research is needed to determine if the use of unstable surfaces to improve balance and stability and to decrease movement uncertainty might decrease co-contractions, which may improve movement efficiency. Since people who are confronted with an unstable situation or movement uncertainty adopt a stiffening strategy (Carpenter et al. 2001; Hogan 1984; Karst and Hasan 1987), their coordination, force, power, speed, and other attributes can be adversely affected.

Potentially, an instability training program that first involves static balance and then progresses to dynamic balance activities would improve intrinsic balance. This improvement in balance would increase movement confidence, releasing the neuromuscular system from a stiffening strategy to more unimpeded motion, force, and power development.

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