The effects of age and low-intensity endurance exercise on the contractile properties of single skinned fast- and slow-twitch skeletal muscle fibres.

Aged (25 month old) rats were subjected to a low-intensity exercise programme, consisting of 10-weeks endurance swimming. At the conclusion of training (at the age of 27 months), fresh single skinned muscle fibres were prepared from the extensor digitorum longus (EDL) and soleus (SOL) muscles of the hindlimb and activated in Ca(2+)- and Sr(2+)- buffered solutions to measure selected isometric contractile characteristics. The major fibre population of the SOL from trained aged animals demonstrated a higher threshold [Ca2+] and [Sr2+] for contraction and a reduced sensitivity to Sr2+. Few changes in contractile characteristics were observed in the EDL muscle fibres from exercised rats, except for fibre type specific changes in the steepness of the force-pCa and force-pSr relationship. The effect of ageing on the contractile characteristics of skinned muscle fibres was also investigated by comparing the data obtained from aged sedentary rats with that from adult (6-9 month old) sedentary rats. Ageing was shown not to have affected the single fibre contractile properties of the SOL, but did affect one of the two fibre populations of the EDL. This was illustrated by the higher sensitivity to Ca2+ in the aged muscle fibres with a concomitant decrease in the cooperative interactions within the thin filament during tension activation.