Shoulder muscle activation strategies differ when lifting or lowering a load.

Area of Science:

• Biomechanics
• Human Movement Science
• Musculoskeletal Research

Background:

• Abnormal shoulder and scapular motion can occur when lowering loads.
• Understanding muscle coordination during load handling is crucial for injury prevention.

Purpose of the Study:

• To test the hypothesis that lowering a load involves different shoulder muscle coordination strategies compared to lifting.
• Investigate muscle activation patterns and synergy utilization during lifting versus lowering tasks.

Main Methods:

• Electromyography (EMG) of 13 shoulder muscles was recorded in 30 healthy volunteers.
• Participants lifted and lowered boxes of varying weights (6, 12, 18 kg) between shelves.
• Muscle synergies were extracted using non-negative matrix factorization and analyzed alongside kinematics.

Main Results:

• Four muscles showed greater EMG activity during lowering (anterior deltoid, biceps brachii, serratus anterior, pectoralis major).
• Lifting movements were faster, but hand paths and segment kinematics were similar.
• Muscle synergy structures were largely consistent between lifting and lowering, though synergy #2 (activating antagonists) was more active during lowering in most subjects.

Conclusions:

• Shoulder muscle synergy structures are similar for lifting and lowering tasks.
• Lowering involves greater activation of a 'multiple antagonists' synergy in a majority of individuals.
• Inter-individual differences in muscle coordination strategies, particularly during lowering, warrant further investigation for shoulder musculoskeletal disorder risk.