The contribution of fast corticospinal input to the voluntary activation of proximal muscles in normal subjects and in stroke patients

Although it is well known that the corticospinal system exerts more influence over distal (hand and fingers) than proximal (elbow and shoulder) upper limb muscles, differences in the importance of this system for voluntary activation of these muscle groups have not been demonstrated directly. Two investigations were carried out to provide a quantitative comparison of the contribution of fast corticospinal inputs to voluntary activity in proximal and distal muscles of normal subjects. The first study confirmed that the rate of increase in the amplitude of EMG responses to transcranial magnetic stimulation (TMS) with voluntary activation of the muscles was significantly greater in a hand muscle (first dorsal interosseous, 1DI) than in biceps, which was in turn greater than that for deltoid. The second study demonstrated that this result reflected a genuine difference in corticospinal influence over these muscles and was not due to differences in the pattern and type of motor unit recruitment in proximal vs distal muscles. The voluntary activation of a pair of low-threshold single motor units (SMUs) in 1DI and deltoid was compared with their response to TMS. In both muscles only a small amount of additional effort was required to recruit the second SMU; increments were typically within 1% of maximum voluntary contraction, as assessed from EMG measurements. Subjects were asked to voluntarily discharge the lower threshold SMU at a steady rate, and then the threshold of responses of this SMU and that of the second unit to TMS were determined. In 1DI, only small increments in TMS intensity above the threshold for the first SMU were required to activate the second unit [mean 1.4% maximum stimulator output (MSO), SD±1.0%, n=7 subjects]. In contrast, in deltoid a significantly greater intensity increase was needed (mean 6%, SD±1.2%, MSO n=7, P<0.001). Similar results were obtained when TMS thresholds of motor unit pairs were assessed in relaxed subjects. These experiments support the hypothesis that the fast corticospinal input that can be activated by TMS is of greater importance for the voluntary activation of hand than of shoulder muscles. This hypothesis served as a basis for testing deltoid responses in three stroke patients. In two patients smaller responses to TMS were obtained on the affected side than on the unaffected side during the production of equivalent voluntary contractions, suggesting that the patients achieved these contractions using inputs other than the fast corticospinal elements excited by TMS.