Studies on the corticospinal control of human walking. I. Responses to focal transcranial magnetic stimulation of the motor cortex

Experiments were done to determine the extent to which the corticospinal tr act is linked with the segmental motor circuits controlling ankle flexors a nd extensors during human walking compared with voluntary motor tasks requi ring attention to the level of motor activity. The motor cortex was activat ed transcranially using a focal magnetic stimulation coil. For each subject , the entire input-output (I-O) curve [i.e., the integral of the motor evok ed-potential (MEP) versus stimulus strength] was measured during a prescrib ed tonic voluntary contraction of either the tibialis anterior (TA) or the soleus. Similarly, I-O curves were measured in the early part of the swing phase, or in the early part of the stance phase of walking. The I-O data po ints were fitted by the Boltzmann sigmoidal function, which accounted for g reater than or equal to 80% of total data variance. There was no statistica lly significant difference between the I-O curves of the TA measured during voluntary ankle dorsiflexion or during the swing phase of walking, at matc hed levels of background electromyographic (EMG) activity. Additionally, th ere was no significant difference in the relation between the coefficient o f variation and the amplitude of the MEPs measured in each task, respective ly. In comparison, during the stance phase of walking the soleus MEPs were reduced on average by 26% compared with their size during voluntary ankle p lantarflexion. Furthermore, during stance the MEPs in the inactive TA were enhanced relative to their size during voluntary ankle plantarflexion and i n four of six subjects the TA MEPs were larger than those of the soleus. Fi nally, stimulation of the motor cortex at various phases of the step cycle did not reset the cycle. The time of the next step occurred at the expected moment, as determined from the phase-resetting curve. One interpretation o f this result is that the motor cortex may not be part of the central neura l system involved in timing the motor bursts during the step cycle. We sugg est that during walking the corticospinal tract is more closely linked with the segmental motor circuits controlling the flexor, TA, than it is with t hose controlling the extensor, soleus. However, during voluntary tasks requ iring attention to the level of motor activity, it is equally linked with t he segmental motor circuits of ankle flexors or extensors.