Purpose: The present study examined the effect of extreme old age on the plasticity of the rat plantaris muscle in response to an increase in mechanical load.

Methods: Male Fischer 344 rats, aged 7 months (adult) and 25 months (old) underwent bilateral surgical ablation of the gastrocnemius muscle to functionally overload (OV) the fast-twitch plantaris muscle for 8 wk

Results: At 27 months of age, plantaris wet weight and cross-sectional area (CSA) were unaffected by age, but aging decreased peak isometric tension (Po) 27% (P < 0.05). Plantaris muscle myosin heavy chain composition indicated a loss of faster myosin heavy chains (MHC) isoforms with concomitant increases in slower MHC in old rats (P < 0.05). In adult rats, OV increased muscle CSA and Po 72% and 83%, respectively (P < 0.05). Similarly, OV increased CSA and Po 69% and 73%, respectively, in old rats (P < 0.05). Average fiber CSA increased 57% and 68% in adult-OV and old-OV rats, respectively (P < 0.05).

Conclusion: Collectively, our data indicate that plantaris muscle mass and plasticity in response to increased mechanical load are well conserved in very aged male Fischer 344 rats.