Resistance training improves single muscle fiber contractile function in older women

The purpose of this study was to 1) examine single cell contractile mechani cs of skeletal muscle before and after 12 wk of progressive resistance trai ning (PRT) in older women (n = 7; 74 +/- 2 yr) and 2) to compare these resu lts to our previously completed single cell PRT work with older men (n = 7; 74 +/- 2 yr) (Trappe S, Williamson D, Godard M, Porter D, Rowden G, and Co still D. J Applied Physiol 89: 143-152, 2000). Knee extensor PRT was perfor med 3 days/wk at 80% of one-repetition maximum. Muscle biopsies were obtain ed from the vastus lateralis before and after the PRT. Chemically skinned s ingle muscle fibers (n = 313) were studied at 15 degreesC for peak tension (P-o), unloaded shortening velocity (V-o), and power. Due to the low number of hybrid fibers identified post-PRT, direct comparisons were limited to M HC I and IIa fibers. Muscle fiber diameter increased 24% (90 +/- 2 to 112 /- 6 mum; P < 0.05) in MHC I fibers with no change in MHC IIa fibers. P-o i ncreased (P, 0.05) 33% in MHC I (0.76 <plus/minus> 0.04 to 1.01 +/- 0.09 mN ) and 14% in MHC IIa (0.73 +/- 0.04 to 0.83 +/- 0.05 mN) fibers. Muscle fib er V-o was unaltered in both fiber types with PRT. MHC I and IIa fiber powe r increased (P < 0.05) 50% [11 <plus/minus> 2 to 17 +/- 2 muN.fiber length (FL).s(-1)] and 25% (40 +/- 8 to 51 +/- 6 muN.FL.s(-1)), respectively. Howe ver, when peak power was normalized to cell size, no pre- to postimprovemen ts were observed. These data indicate that PRT in elderly women increases m uscle cell size, strength, and peak power in both slow and fast muscle fibe rs, which was similar to the older men. However, in contrast to the older m en, no change in fiber V-o or normalized power was observed in the older wo men. These data suggest that older men and women respond differently at the muscle cell level to the same resistance-training stimulus.