Disuse of anterior tibial muscle during locomotion and increased proportion of type II fibres in hemiplegia

Abstract

The tibialis anterior (TA) is a muscle activated mainly during walking. Its use during the step cycle was studied in 10 patients (55.8 ± 8.8 years) with chronic hemiplegia (duration 3–18 years) and related to the muscle fibre composition, size and expression of isoforms of myosin heavy chains (MHCs). In the average step cycle the integrated surface EMG of the paralysed TA did in the majority of the hemiplegic patients not exceed 10% of that recorded during maximal contraction of the normal leg. The type I fibre percentage in the paralysed TA subject was 57.4% as compared with 79.4% in normal muscles ( P < 0.05). The range of axonal conduction velocities in the peroneal nerve did not differ in paralysed and non-paralysed leg, suggesting that there was no selective loss of one class of motoneurons. The type II fibres consisted of IIA (66%) and IIB (31%), in contrast to the normal TA muscle where less than 1% of the muscle fibres are of type IIB. The incidence of fibres in the biopsies with both slow and fast MHCs had a mean value of 3.5% (range 0.7–9%). The type I and type II muscle fibres had normal sizes with cross-sectional area 4511 ± 962 μm 2 and 6181 ± 1062 μm 2. No selective type II atrophy was seen. Occasional hypertrophic type I and II fibres were seen in 4 patients. The findings indicate that mainly the low threshold motor units are active in the hemiplegic patients and suggest that the selective disuse of high threshold motor units predisposes for a transformation from type I to type II fibres with the occurrence of type IIB fibres.