Strength is often associated with muscle mass. When we picture strength, we picture a bodybuilder or football player – someone with big muscles. What we less often associate with strength, is its connection to the brain. There are “neural factors” that play a role in muscle performance. For example, in order to get a muscle to fire, motor units must be recruited systematically to produce the action you want. So what happens if our muscles can’t produce any action? Following an injury that requires immobilization, the muscle goes through a period of disuse and becomes weak. Part of this loss in strength occurs as a result of the decline in physical work or motion at the level of the muscle, but recent studies suggest there may also be a neurological component that we can target in order to minimize weakness following immobilization.
This concept was explored in a recent article published in J Neurophysiol by Clark and colleagues. The aim of their study was to determine whether or not the motor cortex plays a role in regulating muscle strength/weakness following immobilization-induced loss of strength via mental imagery (MI). In this study, 29 healthy participants were immobilized at the wrist and hand for 4 weeks, and 14 of those participants performed MI exercises 5 days/week. Measurements of isometric muscle strength, voluntary activation, and silent period during 15% max voluntary isometric contraction were recorded at baseline, immediately after immobilization, and 1 week after immobilization. Mental Imagery exercises consisted of participants performing 52 imagined contractions of the wrist flexors, initiated by commands such as “imagine that you are pushing in against a handgrip as hard as you can.” To get an idea of the degree to which MI effects post-immobilization weakness, results were compared to a control group that did not complete any visualization exercises during the 4-week immobilization.
Interestingly enough, MI was proven to be highly effective in reducing loss of strength and voluntary activation by 50% following immobilization when compared with the control group. Although details of the mechanism may not be clear, is is clear that MI in some way reduces cortical inhibition (by activating areas in the motor cortex), thereby limiting loss of strength. Based on these findings, it may be time we start looking at strength with a broader lens, with consideration of this neurological component. From a clinical standpoint, this study highlights mental imagery as a potential treatment option during periods of immobilization to reduce the detrimental effects that disuse can bring after the fact.