Apoptosis or programmed cell death is a genetically controlled response of cells 
to commit suicide and is associated with DNA fragmentation or laddering. The 
common inducers of apoptosis include Ca2+i and oxygen free radicals/oxidative 
stress, which are also implicated in the pathogenesis of exercise-induced 
myopathies. To examine training-induced apoptosis, Thoroughbred horses were 
subjected to 3 months training programme on a treadmill. At the end of the 
training programme venous blood samples were taken for a creatine kinase (CK) 
assay. In addition, muscle biopsy samples were obtained for a membrane lipid 
peroxidation measurement by malondialdehyde (MDA) assay and for apoptosis 
detection. Apoptosis was studied by visualising the apoptotic myocytes on the 
paraffin sections by the modified TUNEL method. DNA laddering was evaluated by 
subjecting the DNA obtained from the biopsies to 1.5% agarose gel 
electrophoresis. There was a significant increase (P<0.05) of protein-bound MDA, 
and a nonsignificant trend (P = 0.14) for the control group to have higher 
levels of CK compared to the trained group. Under light microscopy, percentage 
of the TUNEL positive cells was higher (P<0.001) in the training group. This 
result was corroborated with the findings of DNA fragmentation by gel 
electrophoresis, which showed higher ladders of DNA band at the same group. In 
conclusion, these results clearly demonstrate that there is training-induced 
apoptosis in skeletal muscle. It is probable that apoptosis allows the 
work/recovery/rebound/supercompensation cycle, when unaccustomed muscle cells 
activate programmed cell death and are replaced by new and stronger cells, which 
is the mechanism for training-induced increases in fitness.