Seeing Effort: Assessing Coaches' Prediction of the Number of Repetitions in Reserve Before Task-Failure

Background: A key role of resistance training (RT) coaches is to personalize programs based on their trainees’ abilities and goals. Specifically, coaches often assess how many repetitions in reserve (RIR) their trainees have until task-failure. Coaches can then modify the number of repetitions assigned per set accordingly. However, coaches’ ability to predict the number of RIR is unknown. Methods: Certified RT coaches (n = 259) were randomly assigned to watch a video of one of eight trainees. The trainees performed two sets of barbell squats and preacher biceps-curls, using 70% or 80% of their 1RM, to task-failure. The coaches predicted trainees’ RIR at 33%, 66%, and 90% of the total number of repetitions the trainees completed in each set. We fitted a linear mixed model with various predictors to the prediction errors as the outcomes (i.e., signed and unsigned values of the predicted minus actual repetitions to task-failure). Results: The overall average number of repetitions completed by the trainees was 13.9. The average absolute errors were 4.8, 2.0, and 1.2 repetitions for the 33%, 66%, and 90% time-points, respectively. The absolute prediction error increased for the biceps-curl compared to the squat (1.43, 95% CI [1.13, 1.74]), but decreased for heavier loads (− 1.17, 95% CI [− 2.16, − 0.19]), and in the second set of each exercise (− 1.20, 95% CI [− 1.38, − 1.02]). Surprisingly, coaches’ years of experience had a negligible effect on the absolute error (− 0.020, 95% CI [− 0.039, − 0.0007]). Finally, coaches underpredicted the RIR at early time-points but reverted to slight overprediction at later time-points. Conclusions: Prior coaching experience seems to play a minor role in RIR predictions. However, even short-term exposures to new trainees performing different exercises can substantially improve coaches’ RIR predictions.