Alterations in single muscle fiber calcium sensitivity with resistance training in older women

Abstract.

The purpose of this investigation was to determine the effects of a 12-week progressive resistance-training program (PRT) on single muscle fiber calcium sensitivity in six older women (73±2 years). Muscle biopsy samples of the vastus lateralis were obtained pre- and post-PRT. Chemically skinned single muscle fibers (n=274) were dissected and studied. The experimental sequence for each fiber was the determination of peak maximal isometric tension (P _o) at pCa 4.5 (pCa=–log[Ca²⁺]), and then subsequent submaximal activations of the fiber at nine Ca²⁺ concentrations (pCa 6.8 to 4.7). Myosin heavy chain (MHC) I fiber (slow-twitch) diameter increased 16% (P<0.05) with no change in MHC IIa fibers (fast-twitch) pre- to post-PRT, respectively. P _o in MHC I fibers increased 34% (P<0.05) as a result of the training with no change in MHC IIa fibers. The mean MHC I Ca²⁺ activation threshold (minimal amount of Ca²⁺ necessary to induce tension) increased from 6.83±0.02 to 6.91±0.01 (P<0.05), as did the mean half-maximal activation (pCa₅₀), 5.51±0.02 to 5.71±0.03 (P<0.05) with PRT. The slope of the Hill plot above (n ₁) the pCa₅₀ for MHC I did not change significantly with the PRT. However, the slope of the Hill plot below (n ₂) the pCa₅₀ for MHC I demonstrated an increase (P<0.05) with training. There were no differences with MHC IIa fibers with PRT for any of the variables measured. In conclusion, the results of this investigation indicate that myofibril Ca²⁺ sensitivity and activation properties are altered in MHC I, but not MHC IIa fibers with PRT in older women. The alterations in the MHC I Ca²⁺ properties appear to have an effect on the mechanisms involved with skeletal muscle adaptability in older women following PRT.