Heavy Resistance Exercise Training in Older Men: A Responder and Inter-individual Variability Analysis

Background Heavy resistance exercise training (HReT) effectively increases muscle mass and strength in groups of older individuals. However, the extent of inter-individual variability in response to HReT, and the possible existence of non-responders, remains unclear. The primary aim was to determine the presence of inter-individual variability in the response to prolonged HReT in healthy older individuals. Secondary aims were to classify individual responsiveness using a combination of gold-standard assessment methods, and to explore training progression and baseline levels as possible response moderators. Methods We conducted a secondary analysis of an 8- and 16-week intervention of thrice weekly HReT (EX) or continuation of a sedentary lifestyle (SED). 58 healthy men (age 72±5) were randomized into EX (n=38) or SED (n=20). Assessments were conducted at baseline, mid-intervention (8wk), and post-intervention (16wk) for five outcomes: Maximal voluntary contraction strength (MVC), rate of force development (RFD), quadriceps cross-sectional area (qCSA), and type I and II myofibre cross-sectional area (fCSA). Training compliance and progression (as measured by 1-repetition maximum (1RM)) were monitored. Inter-individual variability was assessed using the standard deviation of individual responses (SD IR ). Individual change scores relative to a Typical Error (TE) were used to classify individuals as Poor, Trivial, Robust or Excellent responders. Results At the group level, EX increased MVC, RFD, qCSA and type II fCSA by 19±14 %, 58±80 %, 3±4 % and 14±25 %, respectively, with no changes in SED. Inter-individual variability was observed for all outcomes. Individual responses to HReT were outcome- and time-dependent, with 31 participants (82%) being classified as Robust or Excellent responders, and two participants (5%) being Poor responders, following 16wk of HReT. Training compliance and 1RM progression did not account for the observed response variability following HReT. Lower baseline levels were associated with greater improvements; however, this did not account for the observed inter-individual variability. Conclusions This study provides strong evidence of inter-individual variability in response to HReT among healthy older men. Given the rarity of true non-responders, HReT should remain the universally recommended first-line strategy for enhancing muscle mass and strength in this population.