Skeletal muscle has considerable capacity to increase in mass, both during post-natal growth and even in the adult, to changed physical activity levels. For instance, during the rapid post-natal growth period, the male mouse is adding 25% new muscle per day (Goldspink & Griffin, 1973). In the adult rabbit, the tibialis anterior when subjected to stretch combined with electrical stimulation increases by 35% in mass within 4 days (Goldspink et al., 1992). This latter represents a synthetic rate of 30 000 myosin heavy chain (hc) molecules per nucleus per minute (Goldspink, 1985) which is remarkable but still only about one third of the synthetic capacity during the most rapid post-natal growth period.

As well as changes in muscle mass, changes in muscle phenotype often occur in response to physical signals (Gerlach et al., 1990; Goldspink et al., 1992). Mammalian skeletal muscles consist of populations of slow-contracting, oxidative fibres which are adapted for slow repetitive or postural type contractile activity, and fast-contracting fibres that are recruited for fast phasic movements. The muscle fibre types differ phenotypically in that they express different subsets of myofibrillar isoform genes as well as different types and levels of metabolic enzymes. The inherent ability of skeletal muscle to adapt to mechanical signals is related to its ability to switch on or switch off different isoform genes and to alter the general levels of expression of different subsets of genes.