what IA says
Evidence and Debate in Humans
The evidence for muscle hyperplasia in humans is scarce, indirect, and highly controversial [1] [2] [3] [4] [12].
Cross-Sectional Studies on Bodybuilders: Some studies comparing elite bodybuilders to sedentary individuals have found that bodybuilders possess a greater number of muscle fibers [2] [13] [14]. This has led to the hypothesis that long-term, high-intensity resistance training might induce hyperplasia. However, a significant limitation of these studies is their cross-sectional design; they cannot determine if the bodybuilders developed more fibers through training or if they were genetically predisposed with a higher baseline number of muscle fibers [2] [7]. Many other studies have found no significant difference in fiber number between bodybuilders and untrained individuals, suggesting that hypertrophy is the primary mechanism for their increased muscle mass [7].
Longitudinal Training Studies: Most longitudinal resistance training studies in humans, which track changes in muscle fiber number before and after a training intervention, have failed to show a direct increase in muscle fiber number [2] [15].
A meta-analysis published in 2025, which included 11 studies assessing muscle fiber number pre and post resistance training, concluded that resistance exercise did not significantly alter the number of muscle fibers in humans, suggesting hyperplasia is unlikely to occur to a meaningful extent [4].
Myofiber Splitting: The observation of "split" or "branched" muscle fibers in human muscle biopsies, particularly in highly trained individuals or those with certain myopathies, has been proposed as evidence for hyperplasia [16] [17]. However, recent detailed investigations using serial cryosections and advanced microscopy techniques suggest that these "branches" are more likely a result of incomplete fusion of myotubes during regeneration after muscle injury, rather than true fiber splitting leading to an increased fiber count [16]. This implies that what appears to be splitting is actually a physiological feature of healthy muscle regeneration, where immature myotubes fuse with existing myofibers [16].
Anabolic Steroids: The use of anabolic steroids has been linked to a higher number of muscle cells in some studies, suggesting a potential role for pharmacological agents in inducing hyperplasia [2] [7] [18]. This might also contribute to the "muscle memory" phenomenon, where individuals who have previously used steroids retain muscle gains more easily [7].
Eccentric Exercise: While eccentric exercises (muscle lengthening under tension) are highly effective for promoting muscle hypertrophy, the evidence for their ability to induce hyperplasia in humans is limited and inconclusive [3]. Some experts believe it's unlikely, while others suggest it's a possibility, especially with extremely heavy loads [3].
