Background On Neuromuscular Fatigue
Neuromuscular fatigue is one of several factors contributing to fatigue in swimming. Peripheral fatigue refers to a process along the motor pathway. This typically occurs at or distal to the neuromuscular junction (within the contracting muscle) whereas central fatigue refers to processes occurring proximal to the neuromuscular junction (within the central nervous system).
Electrical stimulation of the muscle is believed to measure both peripheral and central fatigue. For example, if a reduction in torque occurs in response to stimulation at rest following a maximal contraction, then peripheral fatigue is suggested. Central fatigue occurs if there is a reduction in voluntary activation. Unfortunately, the specific location of central and peripheral fatigue is not specific, one limitation of this technique.
Burnely et al. looked to see if during an intermittent isometric quadriceps contractions performed above the critical threshold affected peripheral and central fatigue. If task failure of maximal voluntary contraction (MVC) would be required to achieve the target torque if task failure altered the amplitude of the electromyography (EMG).
What was done
Nine men were subjected to eight sessions of training with at least 24 hours in between. These sessions included familiarization and trials below and above the critical threshold. Critical threshold (CT) is a percent of the maximal voluntary contraction where fatigue is not believed to occur. Torque, EMG, and time to failure were measured.
All trials above critical threshold resulted in significant decrease in the maximal voluntary contraction torque and in doublet torque indicating peripheral fatigue. Voluntary activation also occurred in trials above the critical threshold, but it was not statistically significant, indicating central fatigue also occurred.
In trials below the critical torque, specifically performing 60 minutes of contractions at 10% critical threshold, the mean MVC torque was significantly greater than the submaximal torque, the exact opposite of above critical threshold. The doublet torque was higher in both 10 and 20% critical threshold and slightly higher than the above critical threshold trial.
This study suggests during contractions below the critical threshold, maximal voluntary contraction torque declined modestly. This implies that the rate of fatigue development does not increase proportionately as the torque requirements as a fraction of MVC are increased, but increases suddenly when CT is exceeded. Below the critical threshold, peripheral fatigue developed slowly and the neuromuscular adjustments required to maintain the target torque were modest. Above the critical threshold, the MVC and potentiated doublet torque declined progressively and at a disproportionately faster rate than below CT until task failure occurred.
This is one of the first studies looking to isolate peripheral and central fatigue. This looked at an isometric contraction of one muscle group, very different from sports, but the preliminary findings suggest at below CT peripheral fatigue is dominant and above CT central fatigue may be the driving force. Therefore, these systems must be trained accordingly to prevent swimming fatigue.
- Burnley M, Vanhatalo A, Jones AM. Distinct profiles of neuromuscular fatigue during muscle contractions below and above the critical torque in humans.J Appl Physiol. 2012 May 3.
Originally Posted June 2012