| Summary: | Systematic resistance training (RT) is a well-established method to induce neuromuscular adaptations. However, studies consistently report heterogeneous responses among individuals, including potential “non-responders” or negative responders to standardized training protocols. This variability has driven interest in assessing individual-level responses to personalize RT interventions. Notedly, evidence for individual response is commonly inferred from variability in intervention effects, but true differences in trainability (i.e., caused by the training intervention) must isolate subject-by-training interaction variance from random error and within-subject variability. Quantification of variability also aids with categorizing individual training responses. Dichotomic methods classify individuals as either “responders” or “non-responders” based on a response threshold of relevance. Alternative methods consider the continuous range of likelihood of response by adding confidence intervals (CI) to each response estimate. The study of individual responses demands specific methodological and analytical considerations, but rigorous application of these in RT studies is currently limited. The purpose of this study was to integrate several methods for the assessment of variability and individual responses in skeletal muscle strength and size to a RT intervention.
A total of 134 individuals were randomized to either a group undergoing a control period followed by a 12-week RT period (CPG, n= 60) or a group only performing the training period (INT, n = 74). All participants completed two supervised, whole-body RT sessions per week. Muscle strength (i.e., leg press one-repetition maximum, 1RM) and size (i.e., vastus lateralis cross-sectional area, VL CSA) were measured at three timepoints. Statistical analyses evaluated random error and variability, group-level training efficacy and individual responses to the RT intervention (standard deviation of individual responses, SDIR). Response rates were compared between dichotomic (i.e., zero-, variance- and practical relevance based- thresholds) and continuous (i.e., group- and individual-based CI’s) response categorization methods.
Both group-level efficacy analyses showed significant increases in 1RM and VL CSA compared to control conditions (i.e., period/group) (p = < .01), supported by a large proportion of response in muscle strength and size (~ 90-100%). SDIR estimates indicated the presence of true individual responses. However, these were similar to practical relevance/variation thresholds and the effect size magnitude was small-to-moderate for both outcomes. Dichotomic methods tended to inflate response rates, while continuous methods reduced the risk of false positive responders by considering uncertainty around the response estimate. No negative responders were found, but “uncertain” response varied across methods (1RM = 0 to 21; VL CSA = 0 to 27).
Our findings indicated that the 12-week RT intervention is an effective strategy to increase muscle strength and size untrained young- and middle-aged adults. While individual responses were present, their magnitude may be of limited practical significance. These findings align with emerging evidence highlighting the role of random error and within-subject variation as preferential contributors to the observed variability following a RT intervention. Alternatively, RT studies classifying training response may consider the concurrent use of continuous methods accounting for uncertainty at the sample- and individual-level, as well as response thresholds based on sample- and/or population-specific values. This study highlights the relevance of adopting an integrative, objective methodological approach for the assessment of RT efficacy and variation, both at the group and individual level.
Keywords: Individual response to training, muscle strength, response categorization, proportion of response, random variation.
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