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Cerebral functional anatomy of voluntary contractions of ankle muscles in man

P. Johannsen, L. O. D. Christensen *, T. Sinkjær † and J. B. Nielsen *

1. Cerebral activation elicited by right-sided voluntary ankle muscle contraction was investigated by positron emission tomography measurements of regional cerebral blood flow. Two studies with eight subjects in each were carried out. Tonic isometric plantar and dorsiflexion and co-contraction of the antagonist muscles were investigated in study 1. Tonic contraction was compared with dynamic ramp-and-hold contractions in study 2.
2. All types of contraction elicited activation of the left primary motor cortex (M1). The distance between the M1 peak activation locations for tonic isometric dorsi- and plantar flexion was 17 mm. Co-contraction elicited activation of a larger area of M1 mainly located inbetween but partially overlapping the M1 areas activated during isolated dorsi-/plantar flexion.
3. A voxel-by-voxel correlation analysis corrected for subject covariance showed for dorsiflexion a significant correlation between tibialis anterior EMG level and cerebral blood flow activation in the cerebellum and the M1 of 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 motor area and bilaterally in the premotor and parietal cortices. Isotonic and isometric contractions did not differ except in a small area in the primary somatosensory cortex.
5. One possible explanation of the different cerebral activation during co-contraction compared to that during plantar/dorsiflexion is that slightly different populations of cortical neurones are involved. The more extensive activation during dynamic compared with tonic contractions may reflect a larger cortical drive necessary to initiate and accelerate movements.

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