1. Forearm muscle substrate utilization was studied during exercise induced by median nerve stimulation in five patients with paretic forearm muscles due to upper motor lesions and in five healthy control subjects.
2. The patients showed lower oxygen uptake during exercise, larger lactate production and higher blood lactate/pyruvate ratio in comparison with the control group. Free fatty acid uptake and release did not differ between the groups but the patients showed a markedly lower capacity to oxidize the free fatty acids taken up by the muscle.
3. The results are interpreted to indicate a more pronounced shift to the reduced state of the muscle-cell cytoplasmic redox systems in the patient group, possibly caused by a disturbance in muscle-cell oxygen utilization.