Larger muscles are stronger than smaller muscles; but not all variations in muscle strength are due to variations in muscle size. Ageing causes both muscle size and muscle strength to fall; but the fall in strength is greater than the fall in size (Newman et al. 2003). Decline in muscle quality is reflected in the ratio of force to CSA, termed ‘specific muscle force’, although shortening speed and power output may also decline. The evidence for age related decline in specific muscle force comes both from relatively small studies with individual muscles or muscle groups (e.g. Phillips et al. 1993) and also from larger studies in which muscle mass is measured for the whole body or lower extremity (e.g. Newman et al. 2003). The decline in specific force is not due to loss of actomyosin from the muscles, because the ability to resist stretch is not lost (Phillips et al. 1991). This also suggests that activation failure is unlikely to be the cause of the reduced specific force.Permeabilised (‘skinned’) muscle fibres can in principle be used to test whether changes in the intracellular milieu is responsible for force loss. Most published results do show a decline with age in the specific force of skinned muscle fibres (e.g. Yu et al. 2007), but there are some exceptions (e.g. Trappe et al. 2003), which leaves open the possibility that the responsible factors may be removed by skinning in some conditions. In humans the loss of specific muscle force has a different time course in men and women (Phillips et al. 1993). In women there is a relatively abrupt force loss at the menopause, which can be reversed by hormone replacement therapy (Skelton et al. 1999). In men the force loss occurs more gradually. Other factors which can change specific force are inorganic phosphate and temperature. Sudden changes in these variables alter force on a timescale short compared to the lifetime of crossbridges. (Crossbridges are the structures within muscle which develop force between the actin and myosin filaments.) At least with temperature change, force can be altered without changing the stiffness of the muscle. This shows that attached crossbridges in muscle exist in a mixture of states exerting different forces. Recently Ochala et al. (2007) showed that the specific force loss in skinned muscle fibres is not accompanied by any change in stiffness, suggesting that the ageing effect also is altering the proportions of these different force producing crossbridge states. If this is so there might be an interaction between temperature change and the process that reduces specific force with age. It is hoped to communicate some new experimental results on this point.