A popular resistance-training theory proposes that you should match your rep range to a muscle’s fiber-type composition: train predominantly slow-twitch muscles with lighter loads/higher reps, and more fast-twitch muscles with heavier loads/lower reps. My lab put this idea to the test. In our study (PMID: 32342648), untrained young men trained their calves for 8 weeks using a within-subject design. One leg trained HEAVY (6–10 RM) while the other trained LIGHT (20–30 RM), with sets performed to momentary muscular failure. We specifically compared the soleus, which is predominantly slow-twitch, with the gastrocnemius, which has a more mixed fiber-type composition. What happened? Not much. --Both heavy and light loads produced similar hypertrophy across the calf muscles. --The slow-twitch soleus did not show a preferential hypertrophic response to lighter loads. --The mixed-fiber type gastroc did not show a preferential hypertrophic response to heavier loads. --Isometric strength gains were also similar between conditions. Interestingly, the soleus increased muscle thickness by 7.8%, compared with 8.9% in the medial gastrocnemius and 13.7% in the lateral gastrocnemius, suggesting slow-twitch muscle respond well to training. Overall hypertrophy was similar between heavy and light loading (9.2% vs. 10.7%, respectively). Take home: Our findings challenge the notion that loading ranges should be tailored to a muscle’s fiber-type composition. When sets are performed with a high level of effort, both relatively heavy and light loads appear capable of producing substantial muscle growth, regardless of a muscle’s overall twitch characteristics. However, because we did not directly assess individual fiber hypertrophy, we cannot rule out the possibility that fiber-type–specific differences in growth occurred. physoc.onlinelibrary.wiley.c…