Resistance exercise induces strength gains prior to muscle hypertrophy. The mechanism for this early strength gain is not well understood. Multiple studies regarding animal models have reported increased muscle fiber number and altered muscle fiber size following voluntary resistance exercise. These investigations lacked direct performance measurements and analyzed muscles only after the onset of muscle hypertrophy. Consequently, whether these morphological changes contribute to early strength gains is unknown. Our group developed a voluntary weight-lifting model for rats which measures performance and induces increased muscle mass after two months of training. PURPOSE: To characterize muscle morphology and performance after one month of volitional training - a time period with the potential to precede muscle hypertrophy. METHODS: Rats (8 per group) were exposed to squat-type lifts of a weighted (70g or 700g) ring 5 days per week. Sections of soleus (SOL), medial gastrocnemius (MG), lateral gastrocnemius (LG), plantaris (PL), and tibialis anterior (TA) muscles were analyzed by stereology. Two-way ANOVA was used for statistical analysis; significance was set at p < 0.05. RESULTS: Despite the absence of muscle hypertrophy, 700g load training induced significant effects compared with cage control conditions. Fiber number per unit area increased by 16% for SOL muscles (212 +/- 7 vs. 183 +/- 9 fibers/mm2, p = 0.03) and 23% for TA muscles (349 +/- 11 vs. 283 +/- 19 fibers/mm2, p = 0.01). Fiber number per section, evaluated for SOL muscles, increased by 18% (2749 +/- 42 vs. 2324 +/- 121 fibers/section, p < 0.05). These results were accompanied by decreases in muscle fiber area of 15% for SOL muscles (4449 +/- 127 vs. 5234 +/- 277 um2, p = 0.03) and 21% for TA muscles (2787 +/- 91 vs. 3510 +/- 219 um2, p = 0.01) - a finding consistent with muscle fiber splitting. No chronic degeneration/regeneration was observed excluding such phenomenon as a requirement for the morphologic changes. Peak lifting forces increased by 21% (12.4 +/- 0.2 vs. 10.3 +/- 0.5 N, p = 0.01) - possibly an outcome of adaptive remodeling at the neural, capillary, or connective tissue level during fiber splitting. CONCLUSION: These findings indicate that modulation of muscle fiber number should be considered as a potential mechanism for early exercise-induced strength gains.