Prolonged Low Frequency Force Depression Assessed Using Isometric and Isotonic Contractions Following a Dynamic Fatiguing Task

Background Neuromuscular fatigue induced by contractile activity can lead to prolonged depression of muscle forces evoked through low frequency electrical stimulations, termed prolonged low‐frequency force depression (PLFFD). PLFFD is assessed as the ratio of forces evoked from low (e.g., 10 Hz) and high (e.g., 50 Hz) frequency stimulations, which has been examined most often using isometric contractions. However, fatigue is task specific, and it is unknown if PLFFD induced via dynamic contractions is accurately characterized using isometric torque, as it is not influenced by the velocity or power components of dynamic movements. Objectives and Hypotheses Our primary objective was to compare isometric and dynamic PLFFD following a concentric fatiguing task in healthy young adults. We hypothesized that both isometric and dynamic PLFFD will be impaired following the task but dynamic PLFFD will be reduced to a greater extent. Methods Young (18‐35 years) males (n=5) and females (n=1) performed a prone, knee straight (180°) concentric plantar flexion fatiguing task (Humac Norm). The task involved maximal velocity isotonic‐like contractions at a resistance of 15% maximal voluntary contraction (MVC) through a 30° ankle range of motion (90° to 120° plantar flexion) until a 75% reduction in peak power. To assess PLFFD, contractions were electrically evoked at 10 Hz and 50 Hz via tibial nerve stimulation using a current that elicited maximal isometric 50 Hz torque. Isometric (joint ankle 90°) and isotonic (5% MVC load) PLFFD were assessed as the ratio of 10 to 50 Hz for peak torque and peak power, respectively. Recovery of isotonic and isometric PLFFD (Figure 1), MVC, and voluntary peak power (5% MVC load) was evaluated at 0‐, 2.5‐, 5‐, 10‐, 20‐, and 30‐minutes post task failure. Results On average (±SD), participants performed 151 ±47 contractions, which resulted in an ~74% reduction in peak power. Isometric (~6%) and isotonic (~8%) 10:50 Hz ratios were similarly reduced following task failure (Figure 2). However, at 2.5‐ and 5‐minutes, isotonic PLFFD was further reduced (~15% and ~21%, respectively), but isometric PLFFD remained relatively unchanged (~0% and ~5%, respectively). By 10 minutes of recovery, isotonic and isometric PLFFD were reduced compared to baseline, which remained until the 30 minute time point (~31% and ~20%, respectively). Voluntary peak power was reduced more than isometric MVC at task failure (~44% vs. ~29%), but both recovered similarly by 30 minutes (~8% vs. ~13%). Conclusions This preliminary analysis observed that PLFFD assessed using isometric and isotonic contractions are similarly reduced following task failure; however, differences in the degree of depression may exist throughout 30 minutes of recovery, particularly within the early (<10 minutes) phase. These results demonstrate that PLFFD can be assessed using isotonic contractions, which may have implications for better understanding fatigue responses during dynamic tasks.

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