Time course of muscle hypertrophy with and without progressive overload

To compare the time course of muscle hypertrophy during resistance training with progressive overload (PROG) or without overload progression (N-PROG), 31 healthy untrained young adults completed a randomized within-subject trial. Each participant trained one leg with PROG and the contralateral leg with N-PROG using unilateral knee extension (3x10-12 repetitions, twice weekly for 12 weeks). Load was progressively adjusted in PROG and kept constant in N-PROG. Muscle cross-sectional area (mCSA) was assessed at Pre and after sessions 6, 12, 18, and 24. A protocol×time interaction was observed for mCSA (p=0.002). Both protocols increased mCSA from Pre to S24 (PROG: 3.36±1.94 cm²; N-PROG: 2.67±1.86 cm²), with no significant between-protocol differences in mCSA at the individual assessment time points. However, the increase in mCSA from S18 to S24 was greater in PROG than in N-PROG (between-protocol difference in change: 1.22 cm², 95% CI 0.59 to 1.84; p<0.001). Repetitions in reserve (RIR) was 0.47±0.61 at S1 and 0.20±0.37 at S24 in PROG, and 1.50±1.11 and 4.08±1.94, respectively, in N-PROG. Overall volume load and perceived exertion were greater in PROG (p<0.001). The PROG protocol maintained high relative effort, and the greater increase in mCSA compared with N-PROG was observed specifically from S18 to S24.