Muscle growth from strength training varies among individuals, but recent research suggests the differences may be less pronounced than previously believed. This topic was a focal point at a conference last fall organized by Juha Ahtiainen, a muscle researcher at the University of Jyväskylä in Finland, where experts examined "response heterogeneity"—the varied muscle-building responses to identical training programs.

According to a meta-analysis cited by the researchers, most people can expect to gain between 1.3 and 1.8 kilograms of muscle after eight to twelve weeks of systematic strength training. However, results can range widely: some participants in controlled studies build twice that amount, while others show little to no muscle gain. This variability has historically raised concerns about "non-responders," those thought to derive no benefit from resistance training.

Newer research challenges the notion of true non-responders. A 2024 study from Brazil found that increasing the volume of workouts—from one to four sets per exercise—converted 80 percent of apparent non-responders into individuals who made measurable gains. Another study highlighted that when multiple measures such as muscle size, strength, physical function, and muscle fiber characteristics are considered, all participants exhibit improvement in at least one area.

The scientists also noted that natural fluctuations unrelated to training, such as measurement error or lifestyle changes, can influence muscle size, complicating the assessment of individual responses. Overall, the experts contend that while genetics and biology affect muscle-building capacity, the differences are more subtle than once assumed.

Certain group-level patterns emerged from the discussions. Older adults tend to experience slower muscle gains for similar workout stimuli, whereas women, relative to their generally smaller body sizes, gain muscle proportionally at rates comparable to men. This holds true even after menopause despite hormonal shifts. Surprisingly, individuals with slow-twitch muscle fibers—typically associated with endurance athletes—build muscle at similar rates to those with predominantly fast-twitch fibers, who are often stronger and more explosive. Training adaptations differ accordingly, with slow-twitch dominant individuals performing higher repetitions at lighter loads to achieve comparable fatigue.

While exercise details such as load, volume, and specific movements can be optimized for better results, the American College of Sports Medicine’s recently updated guidelines emphasize that consistent participation in resistance training remains the most critical factor. The consensus from the conference is clear: anyone who commits to regular strength training will see benefits, even if others progress more quickly.