Muscle Growth Responds to Effort, Not Load
Heavy weight is better, but it's not the only way.
New research from McMaster has found that after 10 weeks of training, young males built muscle at virtually identical rates whether they lifted heavy weights for low reps or lighter weights for high reps, provided they trained to failure on both. The study, published in The Journal of Physiology, suggests that your genetics and biological makeup matter far more than the load on the bar. This adds to the growing body of research demonstrating that effort matters much more than the weight on the bar for “growing bodies”.
The research examined 20 healthy, recreationally active young men over 10 weeks of structured resistance training. Each participant trained both arms and legs three times weekly, but with a change: One limb performed high-load training (8-12 reps at 70-80% of max strength), while the opposite limb performed low-load training (20-25 reps at 30-40% of max strength). Critically, both conditions required lifters to reach muscular failure.
Results across multiple measurement methods were strikingly consistent. Arm lean mass increased 4.8% on average, and leg lean mass increased 3.1%, with no meaningful difference between high-load and low-load training. Ultrasound measurements of muscle thickness showed the same pattern. Muscle fiber analysis from biopsies revealed gains of 12% for type I fibers and 17.9% for type II fibers, again independent of load.
The data revealed something equally important: between-person variability dwarfed within-person variability. This means the difference in how much one person responds versus another vastly exceeds any difference created by switching between heavy and light loads. One participant might gain 13.8% of arm cross-sectional area while another gained only 1.1%, but that same high responder gained similar percentages in the legs. Your response pattern stays consistent across your body.
Aim
To determine whether external load (heavy vs light) or limb location (upper vs lower body) drives the differences in muscle growth between individuals during resistance training. The researchers hypothesised that identical twins would show more similar growth patterns within each person than between different people, suggesting biological factors dominate over training variables.
Methods
Twenty healthy young men (average age 22 years) participated in three supervised training sessions weekly for 10 weeks. The study used a unilateral design: each participant trained one arm and one leg with high load (HL: 8-12 reps at 70-80% 1RM), while the opposite arm and leg were trained with low load (LL: 20-25 reps at 30-40% 1RM). The assignment was randomised regardless of limb dominance.
Each session consisted of three sets of knee extensions and dumbbell preacher curls, separated by 90 seconds of rest. Every repetition followed a 2-second eccentric phase, no pause, and a 2-second concentric phase with a full range of motion. Participants pushed to failure on every set, defined as the inability to complete another rep with proper form.
Researchers measured muscle growth using multiple methods. Dual-energy X-ray absorptiometry (DXA) quantified lean mass in the arms and legs. B-mode ultrasound captured the cross-sectional area and muscle thickness at the biceps and quadriceps. Muscle biopsies from the quadriceps analysed individual fiber sizes. Strength increased through 1-repetition max tests and isometric force measurements.
To assess protein synthesis (the mechanism driving growth), researchers used deuterated water labelling at week 1 and week 10. Participants consumed a loading dose, then daily maintenance doses for nine weeks. Saliva samples tracked isotope enrichment to measure how quickly new muscle protein was being built.
All participants consumed 50 grams of whey protein daily (25 grams post-exercise, 25 grams pre-sleep) to ensure adequate protein intake above 1.6 grams per kilogram of body weight.
Results
After 10 weeks, high-load and low-load training produced virtually identical muscle growth across all measurement methods. Lean mass increases were identical (0.17 kg arms, 0.29-0.32 kg legs). Ultrasound measurements showed 1.3 cm² gain in biceps for both groups and no difference in quadriceps. Muscle fiber analysis revealed no statistical difference between high-load (811 micrometres squared type II fiber growth) and low-load (712 micrometres squared) conditions.
Strength increased in both groups without meaningful differences between high and low loads. Biceps curl strength improved 3.8 kg (high load) versus 3.2 kg (low load). Knee extension strength improved 24 kg (high load) versus 22 kg (low load).
