Investigating The Mechanistic Role Of Muscle Growth On Strength In Response To Isometric Handgrip Training

Muscle growth is a purported mechanism for exercise-induced changes in strength. Prior research in the elbow flexors was unable to identify muscle growth as a mechanism for strength gains. It is possible that ‘learning” the elbow flexion movement may have masked the ability to detect a role of growth on strength. If muscle growth is indeed playing a role in strength adaptations, then its importance should be able to be demonstrated in a simpler movement. PURPOSE: Determine if muscle growth mediates strength changes associated with isometric handgrip (HG) training. METHODS: A total of 179 participants completed a 6-week study, with 135 individuals performing isometric HG training over 18 sessions (3x/week). Participants were randomly assigned to one of four groups: 1) low-intensity (4x2 minutes of 30% MVC; LI, n = 47), 2) low-intensity with blood flow restriction (LI + 50% arterial occlusion pressure; LI-BFR, n = 41), 3) maximal effort (4x5 seconds of ≥100% MVC; MAX, n = 47), and 4) non-exercise control (CON, n = 44). Forearm muscle thickness (MTH) and HG strength were measured before and after the intervention. A mediation model (adjusted for pre-MTH, and pre-strength) was constructed to determine if changes in MTH might explain changes in muscle strength. Effects of each training group were evaluated relative to a control. Data are presented as coefficient (95% CI). RESULTS: Only LI-BFR had evidence of an increase in MTH from training (a1-3 paths) [LI: -.0113 (-.060, .037) cm, LI-BFR: .0630 (.012, .113) cm, MAX: .0166 (-.032, .065) cm]. No evidence suggested a relationship between the change in MTH and maximal HG strength (b path) [ΔMTH: .5083 (-3.679, 4.696) cm]. Importantly, there were no significant mediations via indirect effects (a1-3 x b) on the change in MTH mediating a change in strength for any of the training groups [LI: -0.0057 (-0.141, 0.141) kg, LI-BFR: .0320 (-.289, .390) kg, MAX: .0084 (-.157, .165) kg]. CONCLUSION: Despite what others consider to be a mechanism, our data suggests that the augmentation of muscle size through resistance training does not influence the change in strength. This remains applicable even if the exercise is simplified such as HG training and suggests that the changes in maximal strength observed herein were driven by mechanisms other than growth.