Cerebral functional anatomy of voluntary contractions of ankle muscles in man

1. Cerebral activation elicited by right-sided voluntary ankle muscle contr action was investigated by positron emission tomography measurements of reg ional cerebral blood flow. Two studies with eight subjects in each were car ried out. Tonic isometric plantar and dorsiflexion and co-contraction of th e antagonist muscles were investigated in study 1. Tonic contraction was co mpared with dynamic ramp-and-hold contractions in study 2. 2. All types of contraction elicited activation of the left primary motor c ortex (M1). The distance between the M1 peak activation locations for tonic isometric dorsi- and plantar flexion was 17 mm. Co-contraction elicited ac tivation of a larger area of M1 mainly located inbetween but partially over lapping the M1 areas activated during isolated dorsi-/plantar flexion. 3. A voxel-by-voxel correlation analysis corrected for subject covariance s howed for dorsiflexion a significant correlation between tibialis anterior EMG level and cerebral blood flow activation in the cerebellum and the M1 o f the medial frontal cortex. For plantar flexion a significant correlation was found between soleus EMG and cerebral activation in the left medial S1 and M1, left thalamus and right cerebellum. 4. The activation during dynamic isotonic and isometric dorsi- and plantar flexion was significantly more extensive than during tonic contractions. In addition to M1, activation was seen in the contralateral supplementary mot or area and bilaterally in the premotor and parietal cortices. Isotonic and isometric contractions did not differ except in a small area in the primar y somatosensory cortex. 5. One possible explanation of the different cerebral activation during co- contraction compared to that during plantar/dorsiflexion is that slightly d ifferent populations of cortical neurones are involved. The more extensive activation during dynamic compared with tonic contractions may reflect a la rger cortical drive necessary to initiate and accelerate movements.