An Investigation of Experimental Muscle Pain on Neuromuscular Fatigue and Endurance Performance

The determinants of exercise-induced fatigue are a contentious topic within exercise physiology and the limitations to endurance performance are still not fully understood. Exercise-induced pain is often present during high intensity exercise and typically occurs in parallel with exercise-induced fatigue. Therefore, it is possible that exercise-induced pain may be in part responsible for the development of neuromuscular fatigue and be a limitation to endurance performance. Therefore, the purpose of this thesis was to experimentally increase muscle pain and investigate the development of neuromuscular fatigue during endurance exercise. Experimental muscle pain was caused with an intramuscular injection of hypertonic saline into the vastus lateralis whereas an isotonic injection served as the non-painful control.Four experimental studies were conducted for this thesis. The first study aimed to investigate the test-retest reliability of an isometric time to task failure (TTF) of the knee extensors and measures of neuromuscular fatigue. This was to determine if these measures were sufficiently reliable to investigate the effects of pain on fatigue and endurance performance. The second study was to determine the effect of localised muscle pain on the performance of an isometric TTF of the knee extensors and measures of neuromuscular fatigue. Study three was to explore if non-local pain could also affect isometric endurance performance and the development of neuromuscular fatigue. The final study sought to investigate if whole-body, self-paced cycling exercise was also impaired by elevated muscle pain.

The results of study one showed that an isometric knee extensor TTF displayed good reliability displayed by a coefficient of variation = 5.1% [95% CI: 2.9 - 7.3] and standard error of measurement = 21 s. Similarly, measures of neuromuscular fatigue displayed good reliability in the presence of exercise-induced fatigue (all coefficient of variation < 10%). Study two revealed that elevated muscle pain reduced isometric endurance time by 16% and this was due to a reduction in maximal strength and a reduction in voluntary activation (exacerbated central fatigue). Similarly, study three showed that non-local pain can reduce endurance time by 10% due to a decreased voluntary activation. Short duration self-paced cycling exercise remained largely unaffected in the presence of elevated muscle pain and the development neuromuscular fatigue was also unaltered.

In conclusion, muscle pain appears to exacerbate the development of neuromuscular fatigue and impose a significant limitation to single-limb time to exhaustion exercise. However, self-paced exercise appears to be less affected by elevated pain and requires further investigation.