Inter-day ranking of biceps brachii sEMG amplitude during four arm curl exercises.

Surface electromyography (sEMG) is often used to estimate muscle activation for exercise selection, but session variability may limit absolute amplitude comparisons, while a rank-based approach may offer a more reliable alternative. This study examined the inter-day reliability of biceps brachii sEMG amplitude rank order across four arm curl exercises (concentration, dumbbell, hammer, and incline) in 15 adults (8 trained and 7 untrained, 37 ± 15 years) performing seven repetitions per exercise across four sessions at ~33% of peak isometric force. Amplitudes were normalized to single-peak isometric and dynamic (dumbbell curl) amplitudes. Exercise effects were significant for both methods, but session and interaction effects were not. Concentration produced significantly greater amplitudes than hammer, while isometric normalization was more reliable. Kendall's W showed a stable rank order across sessions (W = 0.90, permutation p = 0.0013), with hammer lowest, dumbbell/incline tied, and concentration first. Rank order was stable for exercise pairs with meaningful amplitude differences, but not for closely ranked exercises. However, one-third of participants showed only fair-to-moderate individual concordance across sessions. Although sEMG-based rank order can reliably distinguish exercises at the group level, individual-level reliability warrants verification before applying group findings to exercise prescription for an individual's goals at light-to-moderate loads.