The purpose of this investigation was to determine the effects of a 12-week progressive resistance training program (PRT) on single myofiber calcium sensitivity in older women. Five healthy older women between the ages of 67-82 with a mean age of 72.8±2.7 yr. participated in this study. The training regimen consisted of bilateral isotonic knee extensions. The subjects performed 3 sets, the first two sets consisted of 10 repetitions and the last set was performed to volitional exhaustion at 80% of their 1RM 3 days/week. Muscle biopsies of the vastus lateralis were obtained and single muscle fibers were isolated. The fibers were mounted and fiber length and diameter were determined. The experimental sequence for each fiber was the determination of maximal isometric tension (PO) at pCa 4.5 (pCa = -log [Ca 2+]), and then subsequent activations of the fiber submaximally with free Ca 2+ concentrations of pCa 6.8, 6.5, 6.2, 6.0, 5.8, 5.5, 5.2, 5.0, and 4.7. Due to the small sample size of the myosin heavy chain (MHC) type II single fibers that were studied, only the MHC type I fibers were included for analysis. The MHC type I single fiber diameter increased significantly (p<0.05) from 90.47±3.90 µm to 102.47±2.27 gm pre-to-post PRT, respectively. The Po increased approximately 22% (p<0.05) in the MHC type I fibers pre- to-post PRT. The mean MHC type I fiber Ca 2+ activation threshold increased (p<0.05) from pCa 6.83±0.02 to pCa 6.91±0.01 preto-post PRT. In addition, the mean half-maximal activation of the type I fibers increased (p<0.05) with PRT (5.50±0.02 and 5.70±0.03, pre and post, respectively. The slope of the tension-pCa relationship below (n2) and above (ni) half-maximal activation were also examined to predict molecular cooperativity during cross-bridge interaction. The slope of the Hill plot for n1 did not change significantly with the PRT. However, the slope of the Hill plot for n2 demonstrated a significant increase (p<0.05) from 1.70±0.11 to 2.43±0.09 pre-to-post PRT. In conclusion, the results of this investigation indicate that myofibril Ca 2+ sensitivity and activation properties appear to exhibit a significant role in the mechanisms involved with skeletal muscle adaptability in older women following PRT. / School of Physical Education