Ageing alters the myosin heavy chain composition of single fibres from human skeletal muscle

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Abstract
The myosin heavy chain composition of single fibres (n = 1088) was analysed with an electrophoretic technique in biopsy material from m. vastus lateralis (n = 5) and m. biceps brachii (n = 4) of young (23–31 years old) and elderly men (68–70 years old). In m. vastus lateralis, elderly subjects had a higher proportion of fibres showing a coexistence of myosin heavy chain types I and IIa (20 ± 3% vs 8 ± 1%, P < 0.05) and of myosin heavy chain types IIa and IIb (33 ± 2% vs> 12 ± 4%, P < 0.05). In contrast, the young subjects had a higher proportion of fibres containing only myosin heavy chain type I (50±5% us 33 ± %, P < 0.05) and type IIa (26 4 3% vs> 12 ± 2%, P < 0.05). A similar pattern of myosin heavy chain expression was found in single fibres from m. biceps brachii, with the exception that the elderly subjects had a lower proportion of fibres with coexistence of types IIa and IIb (23 ± 1% vs 34 ± 20, P < 0.05) and a higher proportion of fibres containing only myosin heavy chain type IIa (25 ± 5 % us> 12 ± 20, P < 0.05). Three fibres from m. biceps brachii contained all three isoforms. These results indicate that coexistence of myosin heavy chain isoforms in single fibres is present in skeletal muscles of young adults, and that there is an increased occurrence of this phenomenon with ageing. Whether this reflects an ongoing transition process or a ‘dynamic equilibrium’ between the fibre populations is presently unsettled.