Effect of ageing on the myosin heavy chain composition of the human sternocleidomastoid muscle

The slower phenotype with the preferential reduction of the fibres expressing the fastest MyHC-2x provide circumstantial evidence for: (i) more fast-twitch than slow-twitch motor units being lost; and (ii) reinnervation by the surviving motor units. There appears to be no significant influence on muscle fibre size, which is congruent with relatively unchanged SCM activity during life.