Accumulation and Recovery of Prolonged Low-Frequency Force Depression at Different Intensities of Repetitive Isometric Contractions

Abstract

Prolonged Low Frequency Force Depression (PLFFD) may impact performance in the workplace by influencing musculoskeletal disorder (MSD) risk or reducing force stability. PLFFD is a reduction in low-frequency stimulated force with little change in high-frequency stimulated force for long period after contraction. The objective of this research was to measure changes in PLFFD over exposure and recovery from a half-shift of an isometric contraction task at two intensities on separate visits, and to determine whether there is a relationship between PLFFD and force stability. Participants repetitively supported a weight with their elbow flexors for 4 consecutive 1-hour work-segments. Participants performed a low-force high-duty cycle and a high-force low-duty cycle workload-matched protocol on two different days. PLFFD and force stability were measured in the biceps brachii muscle throughout task exposure and recovery. PLFFD was measured as the ratio of elbow flexion force produced at low (10 Hz) and high (100 Hz) frequency transcutaneous stimulations on both the left (non-intervention) and right (intervention) arms. A repeated measures ANOVA detected a progression of PLFFD in the intervention arm through intervention and recovery, but no protocol-effects were detected. Force stability metrics of variability (normalized standard deviation) and unsteadiness (average rate of change) of force during an isometric elbow flexion force matching task were poorly predicted by PLFFD and better predicted by changes in Maximum Voluntary Force (MVF). This research expanded on incidental findings of PLFFD from past research, and reinforced relationships between muscle fatigue and force stability. PLFFD did not recover at different rates depending on exertion intensity, nor impact force stability metrics, therefore likely not affecting worker performance. Muscle fatigue was shown to impact force unsteadiness to a greater extent than force variability, leading to the suggestion that force stability changes caused by muscle fatigue should be considered when designing a workplace.