Constrained handgrip force decreases upper extremity muscle activation and arm strength

Area of Science:

• Occupational biomechanics
• Human factors engineering
• Ergonomics

Background:

• Industrial tasks often involve repetitive shoulder movements combined with physical and mental demands.
• The shoulder's high mobility makes it susceptible to injury, particularly under demanding task conditions.

Purpose of the Study:

• To investigate the impact of simultaneous gripping at a set force on muscle activity and maximal arm strength in various directions.
• To compare the effects of specified grip force versus preferred (self-selected) grip force on upper extremity performance.

Main Methods:

• Ten female participants performed maximal arm exertions at different heights and directions.
• Grip forces were applied at either a specified 30% maximum voluntary grip or a preferred self-selected level.
• Electromyography (EMG) was used to record activity from eight muscles in the right upper extremity.

Main Results:

• Preferred grip forces varied based on arm height and direction, being significantly greater, lower, or similar to specified grip forces.
• Specifying grip force, regardless of its magnitude, led to an 18-25% decrease in maximal arm strength and a 15-30% reduction in muscle activity.
• These findings indicate an interfering effect when grip force is visually matched to a target, impacting overall arm performance.

Conclusions:

• Task constraints, such as specific gripping requirements, can reduce peak attainable force and alter muscle activation patterns.
• The degree of interference varies with task demands, necessitating careful consideration when establishing occupational safety thresholds.
• Understanding these biomechanical interactions is crucial for designing safer and more efficient industrial work environments.