Progressive Overload Affects the Magnitude of Muscle Hypertrophy

PURPOSE: We investigated the influence of progressive overload (PO) on resistance exercise-induced muscle growth. METHODS: Fifty-five untrained young women participated in the study. Each participant's arm was randomly assigned to one of the following conditions: 1) PO ( n = 37), 2) nonprogressive overload (N-PO, n = 32), and 3) time-matched nonexercise control (Control, n = 41). The training conditions performed unilateral elbow extension exercise in three sets of 8-12 repetitions, 3 d·wk -1 , for 8 wk. The PO condition increased the load whenever it reached the upper limit of the repetition range, whereas N-PO remained training with the same load and number of repetitions throughout the 8 wk. Triceps brachii (TB) muscle thickness was assessed using ultrasonography at 60% (TB60) and 70% (TB70) of the upper arm length, and the sum of both sites was calculated (∑TB). RESULTS: TB60 muscle thickness increased in both training conditions compared with Control, with greater increases in PO relative to N-PO (PO = 0.40 cm, +21.4% > N-PO = 0.22 cm, +11.3% > Control = 0.02 cm, +1.3%; P < 0.001). TB70 muscle thickness increased in both training conditions compared with Control, with greater increases in PO relative to N-PO (PO = 0.38 cm, +25.0% > N-PO = 0.19 cm, +12.0% > Control = 0.03 cm, +2.5%; P < 0.001). ∑TB increased in both training conditions compared with Control, with greater increases in PO relative to N-PO (PO = 0.79 cm, +22.9% > N-PO = 0.41 cm, +11.6% > Control = 0.05 cm, +1.8%; P < 0.001). CONCLUSIONS: Muscle growth appears to be more pronounced when resistance exercise is progressively overloaded. Notably, training without overload progression is sufficient to induce muscle hypertrophy in young, untrained women. Whether resistance training without increasing overload remains effective in promoting muscle growth when performed for longer periods remains an open question and warrants further investigation.