An investigation into the effects of changes in muscle force and muscle length on localised muscle fatigue

Abstract

Introduction: Localised muscle fatigue poses significant challenges to human well-being and performance. However, despite decades of research into muscle fatigue, numerous questions remain unanswered about how such fatigue develops and whether its development and recovery differ between submaximal static and dynamic exertions. The task-dependency principle is known to influence the development of muscular fatigue, with force, range of motion and movement speeds influencing motor unit recruitment strategies. There is, however, a paucity in the literature on how these task-related factors interact with one another. Purpose: This dissertation investigated muscle fatigue under different muscle exertions by applying a novel system for classifying static and dynamic exertions consisting of combinations of muscle length and muscle force criteria. It was hypothesised that the magnitude of localised muscle fatigue is subject to variations (or lack thereof) in the force generated by the muscle, as well as variations in muscle length as a limb moves through a range of motion. Methods: Thirty-six (36) healthy student volunteers participated in this empirical laboratory-based study by performing an elbow flexion/extension exercise protocol designed to fatigue the elbow flexor muscles under three different exertion types on different days: 1) “Varying Length” - participants moved a constant external load (25% of the force generated by their isokinetic maximum voluntary exertions (MVE)) through a range of motion of 80 degrees and at a set takt of 2.6 seconds per cycle, 2) “Pure Static” - this condition entailed no changes in muscle length or force. The elbow flexors had to resist a constant external force corresponding to 25% of the isometric MVE while the elbow flexion angle was set at 90°, 3) “Varying Force” - the elbow joint angle was fixed at 90°, but the force generated by the elbow flexors fluctuated between 15% and 35% of their isometric MVEs and according to the same takt as the “Varying Length” condition. Numerous variables acting as fatigue indicators were recorded before, during, and after the fatigue protocol. Peak torque, time to peak torque, work and power were recorded via isokinetic dynamometry during maximum voluntary exertions before the fatigue protocol, just after its termination, and every minute for the next five minutes during the subsequent recovery period. During the submaximal fatigue protocol itself, EMG of biceps brachii, as well as performance accuracy and variability for cycle time, force, joint angles, and accelerations, were measured during the submaximal exertions at the start of and just before termination of the submaximal fatigue protocol. The exercise protocol was terminated when participants reached a Rating of Perceived Exertion of 9 on the CR-10 scale, and the time to termination was recorded. Analyses using General Linear Models (GLM) were performed to determine the effects of time, as well as exertion type (i.e., “Varying Length”, “Pure Static”, and “Varying Force”) on muscle fatigue, as well as the recovery thereof. The “Pure Static” condition was used as a baseline measurement against which responses of the “Varying Length” and “Pure Static” conditions were compared. Significant differences were identified at p<0.05, and Tukey post hoc analyses determined differences within the main effects (time and condition) and their interactions. Results: The statistical analyses revealed that: 1) The exercise protocol induced localised muscle fatigue of the elbow flexors. This is supported by significant time-related changes in most fatigue parameters calculated during the maximum force exertions before and after the protocol (peak torque, Fatigue Index, work, power), as well as during the submaximal exercise protocol (EMG amplitude, median frequency, and performance accuracy). Conversely, significant improvements were found during the recovery period, although not all variables had returned to pre-fatigue levels after five minutes of rest. Unexpected findings included an unchanged time to peak torque throughout the exercise protocol but significant improvements after rest, as well as greater joint steadiness. 2) Variations in muscle length significantly affected fatigue development. The “Varying Length” condition largely showed greater fatigue for endurance time, number of cycles, peak torque, the Fatigue Index, EMG median frequency, and force accuracy, but not for the time to peak torque, EMG amplitude and cycle-to-cycle variability for force under the “Varying Length” condition, which was lower than under the “Pure Static condition. Similar outcomes were observed during the recovery period. Separation of the concentric from the eccentric movement phases of the “Varying Length” condition revealed a more significant contribution of the concentric muscle actions to fatigue compared to the eccentric exertion phase. All variables that demonstrated significant decrements over the duration of the fatigue protocol showed improvements over the recovery period. The “Varying Length-Concentric” exertion also experienced a significantly larger proportional increase in the time to peak torque over the recovery period. 3) Variations in muscle force had no statistically significant impact on the fatigue responses (endurance time, number of cycles, peak torque, time to peak torque, Fatigue Index, EMG amplitude and median frequency, and cycle-to-cycle variability). Only performance accuracy under the “Varying Force” condition revealed significantly greater deviations from the target joint angle compared to the “Pure Static” condition. Discussion: It was anticipated that dynamic movements, either via changes in muscle length or muscle force, would actively promote blood flow to the working muscles and, therefore, enhance fatigue resistance. The data, however, revealed significantly greater fatigue for the "Varying Length” exertion, compared to the “Pure Static” condition, and no differences in the fatigue responses between the “Pure Static” and “Varying Force” exertions. These unexpected findings can likely be attributed to a variety of physiological and methodological factors. Firstly, the mean loading of 25% of maximum voluntary exertion may have been too low to induce effective occlusion of blood flow, so even though the variations in muscle length and force may have had a peristaltic effect, the impact on muscle fatigue was negligible. Furthermore, it is plausible that even though the external loads manipulated had been relativised to each exertion type’s maximum strength capacity, the true peak torque was not obtained during the maximum exertions prior to the protocol, thereby creating a larger proportional workload for the “Varying Length-Concentric” exertion, but a lower workload for the “Varying Length-Eccentric) exertion. Additionally, as per the length-tension relationship, the internal effort that had to be generated by the elbow flexor muscles under the “Varying Length” condition fluctuated as the forearm moved through its range of motion, thereby placing a greater demand on the metabolic and energetic processes during the concentric movement phases and less under the eccentric and isometric exertions. One important methodological consideration relates to the challenges of comparing different exertion types with one another since they exhibit very different characteristics with regard to their motor strategies, particularly as fatigue develops, and may, therefore, be likened to comparing “apples with pears”. Conclusion: The study concluded that tasks exhibiting variations in muscle length and muscle force do not necessarily enhance fatigue resilience. Given the different mechanical and physiological mechanisms between exertion types and the associated methodological challenges of matching workloads, further research into the effects of task-dependent factors on muscle fatigue is recommended.