Exploring Localized Muscle Fatigue Responses at Current Upper-Extremity Ergonomics Threshold Limit Values

Area of Science:

• Ergonomics and Occupational Health
• Muscle Physiology
• Biomechanical Engineering

Background:

• A threshold limit value (TLV) equation was developed to prevent localized muscle fatigue (LMF) during repetitive upper limb tasks.
• The equation predicts acceptable maximal voluntary contraction (% MVC) levels for specific duty cycles (DCs).
• Experimental validation of this TLV curve is needed to guide practical application.

Purpose of the Study:

• To evaluate localized muscle fatigue (LMF) responses under three different upper-extremity ergonomics threshold limit value (TLV) duty cycles (DCs).

Main Methods:

• Eighteen participants performed intermittent isometric elbow flexion at low (20% DC, 29.6% MVC), medium (40% DC, 19.7% MVC), and high (60% DC, 13.9% MVC) workloads defined by the ACGIH TLV equation.
• Localized muscle fatigue (LMF) in the biceps brachii was monitored using strength (MVC), electromyography (EMG) amplitude, mean power frequency (MnPF), and psychophysical ratings.

Main Results:

• Significant differences in biceps mean power frequency (MnPF) and maximal voluntary contraction (MVC) were observed across all tested TLV workloads.
• The high duty cycle (DC) condition resulted in the most substantial declines in both MnPF and MVC.

Conclusions:

• Working at different duty cycles (DCs) along the ACGIH TLV curve does not yield equivalent prevention of excessive localized muscle fatigue (LMF).
• Higher DC workloads induce a more pronounced LMF response.
• Current TLVs for repetitive upper-extremity work may overestimate safe contraction thresholds, particularly at higher DCs.