Low frequency fatigue in human quadriceps is fatigue dependent and not task dependent

Area of Science:

• Exercise Physiology
• Muscle Physiology
• Biomechanics

Background:

• Low-frequency fatigue (LFF) is typically associated with low-intensity, long-duration contractions.
• Previous studies did not control for fatigue levels, potentially confounding results on task dependency.

Purpose of the Study:

• To investigate if low-intensity/long-duration contractions induce greater LFF than high-intensity/short-duration contractions when quadriceps muscle fatigue is matched.
• To determine the influence of task intensity and duration on LFF under controlled fatigue levels.

Main Methods:

• Eighteen healthy males performed isometric quadriceps contractions at 35% and 65% of maximal voluntary contraction (MVC) until fatigue reached 45% MVC.
• Muscle fatigue was assessed using the double pulse to single pulse torque ratio (D/S ratio), electromyography (EMG) amplitude, and median power frequency (MPF).

Main Results:

• Both 35% and 65% MVC tasks significantly increased the D/S ratio, indicating LFF, with no significant difference between the two conditions immediately or 5 minutes post-fatigue.
• Endurance time was significantly longer for the 35% MVC task (124s) compared to the 65% MVC task (63s).
• EMG and MPF analyses did not reveal significant differences in muscle activation or central fatigue between the two fatiguing protocols.

Conclusions:

• Low-frequency fatigue is dependent on the level of muscle fatigue achieved, not solely on the intensity or duration of the contraction task.
• These findings clarify the task dependency of muscle fatigue, suggesting that the degree of fatigue is a critical factor in LFF development.