Compensatory hypertrophy and training effects on the functioning of aging rat m. plantaris

The influence of age, compensatory hypertrophy and training on isometric contraction characteristics of rat m. plantaris were investigated in 5-, 13- and 25-month-old rats. Each age group was subdivided into Control Not Trained, Control Trained, Operated Not Trained and Operated Trained groups. Run training was applied. The Operation i.c. denervation of synergists induced a 30% compensatory hypertrophy (P < 0.001). Age and training did not significantly affect muscle weight. The tetanic force was highest in 13-month-old and lowest in 25-month-old muscles as was the tetanic force/muscle weight (P < 0.001). Trained and hypertrophied muscles had increased tetanic force (P < 0.01), but tetanic force/muscle weight was not significantly affected. Twitch contraction time was longer in hypertrophied muscles than in controls (P < 0.001). Half relaxation time decreased with training (P < 0.05). Resistance to fatigue was increased in hypertrophied muscles as compared to controls (P < 0.04) and was lower in 5-month-old than in 13-month-old muscles, but their fatigue resistance did not differ from the 25-month-old muscles. Each age group showed comparable effects of training and hypertrophy on the contraction characteristics. These results indicate that the force generating capacity of the M. plantaris was optimal in the 13-month-old animals compared to the other age groups. Furthermore, our data show that aged muscles react in a similar way to increased functional demand as muscles of younger age.