Based on the prior physiological underpinnings of muscle mechanics and measurement techniques, we can examine studies that measure hypertrophy and changes in muscle strength. It is important to consider the population, training status, and measurement techniques when investigating the relationship between hypertrophy and strength gain. Because the contributions to strength outcomes are multifactorial and may vary with training age, a critical analysis of the literature should be performed.

Strength can be developed through a variety of RT methods and may be mediated by neural factors, skill acquisition, mechanical changes, and morphological alterations. It has been proposed that early changes in strength are primarily driven by neural factors and skill acquisition, and later changes are primarily mediated by muscular hypertrophy [25, 26]. If that is true, then one would anticipate the association between hypertrophy and strength gains to increase as training age increases. Because the training process is a “long-term investment,” it is necessary to understand when particular physiological mechanisms may play a larger role compared to others in the improvement of strength.

When examining short-term studies where subjects are naïve to RT, relative increases in strength outpace relative increases in muscle size. This is readily observable in a multiple studies where strength increases in the first 4–6 weeks with little-to-no appreciable hypertrophy [27, 28], and is supported by several studies that examined strength changes resulting from short-term training interventions (12–24 weeks). In these studies, the correlations between muscle hypertrophy and strength changes are low, with hypertrophy accounting for as little as 2–28% of the variance in strength improvement [13, 29,30,31,32]. Contrasting these findings are studies with homogeneous groups of trained individuals (> 1 year RT), which suggest that, as training status increases, hypertrophy accounts for a greater percentage of the variance in strength gain [29, 33].

As opposed to short-term studies in untrained individuals, longer-term studies on trained individuals suggest that hypertrophy—as assessed via changes in body mass and fiber cross-sectional area—account for ~ 65% of the variance in strength gain [34, 35]. Such findings in trained individuals are further supported by the work of Appleby and colleagues, who evaluated professional rugby players over 2 years of RT [36]. Investigators found that squat strength increases were strongly correlated with relative changes in lean mass index (R2 = 44–77%) [36].

The findings of the aforementioned studies in trained individuals are corroborated by cross-sectional studies with elite athletes, which suggest that muscle size accounts for an even larger percentage of variance in strength (R2 ≥ 70%) [37,38,39,40,41]. While these data do not necessarily suggest that changes in muscle size are related to changes in strength, they do still indicate that athletes with more muscle mass in relation to their height have a competitive advantage in the sports of powerlifting and Olympic weightlifting. The tightness of these associations in light of the mechanistic theory, we believe, suggests that hypertrophy provides lifters with a competitive advantage.

Finally, while the studies presented hitherto address the question of whether those who gain more muscle also get stronger (between-subject), it has been suggested that the question of greater interest is if an individual will get stronger as they gain more muscle (within-subject) [42]. Indeed, such analyses explain a much greater percentage of variance in strength gain than do between-subject analyses [42, 43]. While this work remains in its infancy and has room for improvement, both methodologically and statistically, we believe the early results to be both intriguing and thought-provoking in helping to elucidate the strength-hypertrophy relationship.