We tested the hypothesis that the nervous system, and the cortex in particular, is a critical determinant of muscle strength/weakness and that a high level of corticospinal inhibition is an important neurophysiologic factor regulating force generation.
A group of healthy individuals underwent 4-weeks of wrist-hand immobilization to induce weakness. Another group also underwent 4-weeks of immobilization, but they also performed mental imagery of strong muscle contractions five days/wk. Mental imagery has been shown to activate several cortical areas that are involved with actual motor behaviors- including premotor and M1 regions. A control group, who underwent no interventions, also participated in this study. Before, immediately after, and one-week following immobilization, we measured wrist flexor strength, VA, and the cortical silent period (SP; a measure that reflect corticospinal inhibition quantified via transcranial magnetic stimulation).
Immobilization decreased strength 45.1±5.0%, impaired VA 23.2±5.8%, and prolonged the SP 13.5±2.6%. Mental imagery training, however, attenuated the loss of strength and VA by ~ 50% (23.8±5.6% and 12.9±3.2% reductions, respectively), and eliminated prolongation of the SP (4.8±2.8% reduction). Significant associations were observed between the changes in muscle strength and VA (r=0.56) and SP (r=-0.39).
These findings suggest neurological mechanisms, most likely at the cortical level, contribute significantly to disuse-induced weakness, and that regular activation of the cortical regions via imagery attenuates weakness and VA by maintaining normal levels of inhibition.