Age-related muscle wasting and weakness (sarcopenia) ultimately lead to a dependent lifestyle. The general thought is that age-related reduction in force is primarily the consequence of a loss of muscle mass, due to both loss of fibres and fibre atrophy, and to a limited extend to a reduction in specific force of the remaining muscle tissue. To investigate the quantitative contribution of each of these factors to the age-related muscle weakness 9-month- (adult, n=9) and 25-month- (old, n=10) old male C57BL/6j mice were studied. Maximal isometric tetanic force of the m. plantaris was measured in situ(1). All mice received a subcutaneous injection of 0.06 ml 1% temgesic as an analgesic and 4% isoflurane, 0.1L*min-1 O2 and 0.2L*min-1 normal air for anaesthesia (1.5-2.5% during operation). Wet muscle mass was measured directly after excision. The fibre type composition was determined by immunohistochemical staining of 10-µm sections of the muscle using antibodies, against type I, IIA, IIX and IIB, respectively. An ANOVA was used to determine age-related differences. Data are expressed as mean±SD. The maximal isometric force of the plantaris muscle of old mice was 13% lower than that of adult mice (968±122 vs. 840±109mN; p<0.05). The m. plantaris, gastrocnemius medialis and soleus muscle masses were 5-8% lower in old than adult mice (24.2±2.3 vs. 22.4±2.4mg, 65.2±5.9 vs. 60.08±5.3, 13.3±1.0 vs. 12.2±1.3; p<0.05, respectively). Fibre number was similar in the m. plantaris of adult and old mice (Adult vs. Old: 1033±195 vs. 1109±174). A shift towards a slower phenotype was observed, reflected by a lower proportion of type IIB fibres (46.5±4.6 vs. 38.4±4.5%; p<0.05) in the older animals. Fewer than 1% type I fibres and similar proportions of type IIA and IIX fibres in adult and old mice were observed (Adult vs. Old: 26.8±4.4 vs. 30.2±5.1% and 25.6±2.9 vs. 29.1±3.4%, respectively). Although the specific force (40.94±3.72 vs. 37.58±4.31N/gr; p=0.108) did not differ significantly between m. plantaris muscles from adult and old mice, linear regression suggested that 60% of the variance of force was explained by specific force (R2=0.598) and the other 40% by muscle mass (R2=0.392). The most important outcome of this study is the reduction in force generating capacity with age, accompanied by a shift towards a slower phenotype. Moreover, age-related reduction in muscle force was explained by reductions in muscle mass (40%) and s