After nine weeks of lifting weights—whether heavy or light—men who work out casually don’t get bigger pectoral muscles, even though other muscles in their body do grow. This might mean some muscles just don’t respond the same way, or maybe they’re not doing the exercises right.

A randomized controlled trial with recreationally trained males assigned to either high-load resistance training, low-load resistance training, or a control group (no training), with pectoralis major and other muscle thicknesses measured via ultrasound before and after nine weeks, controlling for volume, frequency, and diet.

A longitudinal cohort study tracking the same recreationally trained males over nine weeks of structured high- or low-load resistance training, measuring pectoralis major and other muscle thicknesses at multiple time points to assess growth trajectories and regional differences.

A case-control study comparing individuals who showed no pectoralis major growth after nine weeks of training (cases) with those who did (controls), matched for training load, volume, and experience, to identify differences in technique, muscle activation, or biomechanics.

An analysis of existing datasets from prior resistance training studies to identify whether pectoralis major growth consistently lags behind other muscles under similar training conditions.