Development of an Alternative Protocol to Study Muscle Fatigue

When measuring real-time in vivo muscle fatigue with electromyography (EMG), data collection can be compromised by premature sensor removal or environmental noise; therefore, the objective of this study was to develop a postmortem in vivo methodology to induce muscle fatigue and measure it using EMG. Barrows (N = 20) were stratified by weight and randomly allocated into one of two treatments. The treatments consisted of barrows being subjected to a hog electric stunner super-contraction cycle (ES) or not (CON) postmortem. The right hind limb bicep femoris (BF) and semitendinosus (ST) were selected for ambulatory movement simulation using electronic muscle stimulation (EMS). Muscle workload during EMS was measured with EMG using median power frequency (MdPF) and root mean square (RMS) as indicators of action potential velocity and muscle fiber recruitment. Ambulatory movement was induced and recorded for 20 min with a 4:4 duty cycle at 70 Hz. Muscle biopsies were collected pre- and post-EMS for metabolite analyses to corroborate muscle fatigue onset. There was a TRT × Muscle interaction for normalized RMS percentage (p < 0.01), where BF from CON barrows had greater values (p < 0.01). There were no interactions or TRT main effects for the MdPF normalized value (p ≥ 0.25), but there were Period and muscle effects on MdPF (p < 0.01). Bicep femoris had smaller (p < 0.01) MdPF than ST. The percentage of MdPF decreased (p < 0.01) by Period 5 compared to the other Periods, which did not differ from each other (p ≥ 0.38). There were TRT × Muscle and Muscle × Period interactions for ATP muscle concentration (p ≤ 0.03). The concentration of CON BF ATP was greater (p < 0.01) than that of ES BF and CON and ES ST, which did not differ from each other (p ≥ 0.11), but the APT concentration tended to differ between ES BF and ES ST (p = 0.06). Semitendinosus ATP concentration decreased (p < 0.01) post-EMS compared to ST pre- and BF pre- and post-EMS (p ≥ 0.29), but BF and ST concentration tended to differ pre-EMS (p = 0.07). The data indicated that EMS is a valuable tool for replicating ambulatory movement or physical activity, but super-contraction is not a means to accelerate postmortem muscle fatigue onset. Therefore, further refinement, such as longer EMS stimulation time, should be considered.