Contractile history affects sag and boost properties of unfused tetanic contractions in human quadriceps muscles

The boost of force in unfused tetanic contractions at variable interpulse intervals in fast motor units of the rat medial gastrocnemius

Previously, boost and sag effects seen in unfused tetanic contractions have been studied exclusively at constant stimulation frequency. However, intervals between successive discharges of motoneurons vary during voluntary movements. We therefore aimed to test whether the extra-efficient force production at the onset of contraction (boost) occurs during stimulation with variable intervals, and to what extent it depends on the level of interpulse interval (IPI) variability and history of stimulation. Four sets of three repeated unfused tetani (the first set with constant IPIs at 35 Hz; the other three sets with mean frequency 35 Hz and variable IPIs 28 ± 2 ms, ± 5 ms, and ± 7 ms, respectively) were recorded with 3 min breaks in fast fatigable and fast fatigue-resistant motor units of the rat medial gastrocnemius. We show that boost occurred in the first tetanic contraction of each set for stimulation at both constant and variable IPIs in two fast types of motor units; additionally, it was present in different IPIs variability for a given range. Mathematical decomposition of first (sagging) and second (non-sagging) tetanic contractions into twitch-like responses to successive stimuli in the tested IPIs variability showed that the changes underlying boost are pattern independent. This implies that boost probably occurs in daily activity when motoneurons fire with an unstable firing rate. The effect may therefore contribute to the development and regulation of force in voluntary daily activities, especially in short powerful contractions.

The effect of elevated muscle pain on neuromuscular fatigue during exercise

PURPOSE

Muscle pain can impair exercise performance but the mechanisms for this are unknown. This study examined the effects of muscle pain on neuromuscular fatigue during an endurance task.

METHODS

On separate visits, twelve participants completed an isometric time-to-task failure (TTF) exercise of the right knee extensors at ~ 20% of maximum force following an intramuscular injection of isotonic saline (CTRL) or hypertonic saline (HYP) into the vastus lateralis. Measures of neuromuscular fatigue were taken before, during and after the TTF using transcranial magnetic stimulation (TMS) and peripheral nerve stimulation.

RESULTS

The mean pain intensity was 57 ± 10 in HYP compared to 38 ± 18 in CTRL (P < 0.001). TTF was reduced in HYP (4.36 ± 0.88 min) compared to CTRL (5.20 ± 0.39 min) (P = 0.003). Maximum voluntary force was 12% lower at minute 1 (P = 0.003) and 11% lower at minute 2 in HYP (P = 0.013) compared to CTRL. Voluntary activation was 4% lower at minute 1 in HYP compared to CTRL (P = 0.006) but not at any other time point (all P > 0.05). The TMS silent period was 9% longer at 100 s during the TTF in HYP compared to CTRL (P = 0.026).

CONCLUSION

Muscle pain reduces exercise performance through the excacerbation of neuromuscular fatigue that is central in origin. This appears to be from inhibitory feedback from group III/IV nociceptors which acts to reduce central motor output.