Supplementary Table S2. Sample audit for the primary and sensitivity analysis datasets.

Analysis	Muscle	Analytical-interval observations	Participants
Primary	Latissimus dorsi	174	13
Primary	Middle trapezius	171	13
Primary	Posterior deltoid	181	14
Primary	Biceps brachii	171	13
Primary	Triceps brachii	178	14
Repetition positions 1 and 2	Latissimus dorsi	174	13
Repetition positions 1 and 2	Middle trapezius	169	13
Repetition positions 1 and 2	Posterior deltoid	179	14
Repetition positions 1 and 2	Biceps brachii	171	13
Repetition positions 1 and 2	Triceps brachii	178	14
Complete cases	Latissimus dorsi	109	7
Complete cases	Middle trapezius	110	7
Complete cases	Posterior deltoid	123	8
Complete cases	Biceps brachii	124	8
Complete cases	Triceps brachii	94	6

Analytical-interval observations refer to the number of lifting or lowering sEMG observations entering each model. The complete-case analysis reflects a stricter subset requiring complete observations for all 4 attachment × 2 phase combinations within each muscle.