Skeletal muscle tissue is sensitive to the acute and chronic stresses associated 
with resistance training. These responses are influenced by the structure of 
resistance activity (i.e. frequency, load and recovery) as well as the training 
history of the individuals involved. There are histochemical and biochemical 
data which suggest that resistance training alters the expression of myosin 
heavy chains (MHCs). Specifically, chronic exposure to bodybuilding and power 
lifting type activity produces shifts towards the MHC I and IIb isoforms, 
respectively. However, it is not yet clear which training parameters trigger 
these differential expressions of MHC isoforms. Interestingly, many programmes 
undertaken by athletes appear to cause a shift towards the MHC I isoform. 
Increments in the cross-sectional area of muscle after resistance training can 
be primarily attributed to fibre hypertrophy. However, there may be an upper 
limit to this hypertrophy. Furthermore, significant fibre hypertrophy appears to 
follow the sequence of fast twitch fibre hypertrophy preceding slow twitch fibre 
hypertrophy. Whilst some indirect measures of fibre number in living humans 
suggest that there is no interindividual variation, postmortem evidence suggests 
that there is. There are also animal data arising from investigations using 
resistance training protocols which suggest that chronic exercise can increase 
fibre number. Furthermore, satellite cell activity has been linked to myotube 
formation in the human. However, other animal models (i.e. compensatory 
hypertrophy) do not support the notion of fibre hyperplasia. Even if hyperplasia 
does occur, its effect on the cross-sectional area of muscle appears to be 
small. Phosphagen and glycogen metabolism, whilst important during resistance 
activity appear not to normally limit the performance of resistance activity. 
Phosphagen and related enzyme adaptations are affected by the type, structure 
and duration of resistance training. Whilst endogenous glycogen reserves may be 
increased with prolonged training, typical isotonic training for less than 6 
months does not seem to increase glycolytic enzyme activity. Lipid metabolism 
may be of some significance in bodybuilding type activity. Thus, not 
surprisingly, oxidative enzyme adaptations appear to be affected by the 
structure and perhaps the modality of resistance training. The dilution of 
mitochondrial volume and endogenous lipid densities appears mainly because of 
fibre hypertrophy.