Does It Matter How Much and How Often a Muscle Is Trained? An Open, Testable Model of Muscle Growth from Weekly Sets and Sessions

Coaches decide how many sets each muscle gets per week and over how many sessions to spread them. Trials find that weekly sets matter and that, at equal weekly sets, frequency makes little difference. The Weekly Net Stimulus model holds that frequency is a major lever. Each workout’s stimulus shows diminishing returns per set, and muscle is lost on the days outside 48-hour post-workout windows. The loss rate is calibrated so that three sets once a week only maintains muscle. The model’s predictions are in arbitrary units. I put the model in testable form and tested it two ways. First, in a dose-response model fitted to 224 effects from 36 studies. Doubling weekly sets from 6 to 12 added 1.7 percentage points of growth over about 10 weeks (95% interval 1.1 to 2.4). Moving 12 weekly sets from one session to three changed growth by +0.3 points (−1.6 to +2.1). At 9 weekly sets, once-weekly training grew 4.6 points (0.6 to 8.7) more than the model’s published version allows, and 3.2 points (0.1 to 6.5) more than its second-curve version allows. Within the model’s recovery limits the margins were 2.0 (−0.5 to 4.7) and 1.2 (−1.0 to 3.4). Second, in the four volume-equated trials that compared one weekly session with three and measured muscle size directly. The model predicted standardised advantages for three sessions of 1.08 (published version) and 0.66 (second-curve version). The trials found 0.25 (95% CI −0.14 to 0.65). Without the between-session loss, the same curves predicted 0.52 and 0.25. With its published curve and loss rate, and marginally with its second curve, the model predicts more benefit from frequency than the trials show. Nothing in the trials requires a between-session loss. I state one prediction for a 12-month trial and provide an open calculator.