Supplementary Table S4. Final model structures used for primary inference. Muscle Primary model structure Model-checking note Latissimus dorsi sEMG amplitude ~ attachment × phase + (1 | participant) Participant-level random effects were retained to account for repeated observations. More complex random-effects structures were considered and simplified when necessary after singular-fit diagnostics. Inference should be based on Type III tests and estimated marginal mean comparisons. Middle trapezius sEMG amplitude ~ attachment × phase + (1 | participant) Participant-level random effects were retained to account for repeated observations. More complex random-effects structures were considered and simplified when necessary after singular-fit diagnostics. Inference should be based on Type III tests and estimated marginal mean comparisons. Posterior deltoid sEMG amplitude ~ attachment × phase + (1 | participant) Participant-level random effects were retained to account for repeated observations. More complex random-effects structures were considered and simplified when necessary after singular-fit diagnostics. Inference should be based on Type III tests and estimated marginal mean comparisons. Biceps brachii sEMG amplitude ~ attachment × phase + (1 | participant) Participant-level random effects were retained to account for repeated observations. More complex random-effects structures were considered and simplified when necessary after singular-fit diagnostics. Inference should be based on Type III tests and estimated marginal mean comparisons. Triceps brachii sEMG amplitude ~ attachment × phase + (1 | participant) Participant-level random effects were retained to account for repeated observations. More complex random-effects structures were considered and simplified when necessary after singular-fit diagnostics. Inference should be based on Type III tests and estimated marginal mean comparisons. This table clarifies that repetition-level observations were modelled with participant-level clustering rather than treated as independent participants.