Adjustments in the force-frequency relationship during passive and exercise-induced hyperthermia

Area of Science:

• Exercise physiology
• Skeletal muscle function
• Thermoregulation

Background:

• Understanding how exercise in heat impacts muscle contractile function is crucial for athlete performance and safety.
• Fatigue and hyperthermia are known to affect muscle force production.

Purpose of the Study:

• To investigate the influence of fatiguing cycling exercise in the heat on the force-frequency relationship of knee extensor muscles.
• To compare the effects of exercise-induced hyperthermia (ExH) with passive heat hyperthermia (PaH).

Main Methods:

• Participants underwent percutaneous electrical stimulation of knee extensors to assess contractile function.
• Measurements included potentiated twitch force and force production at various stimulation frequencies (10, 20, 50, 100 Hz).
• Assessments were performed before and after both exercise in heat and passive heat exposure.

Main Results:

• Both ExH and PaH decreased the 20:50 Hz force ratio, indicating low-frequency fatigue.
• This decrease was significantly greater following ExH compared to PaH.
• A rightward shift in the force-frequency relationship was observed after both conditions, and this shift was exacerbated by ExH.
• Peak twitch force declined after ExH.

Conclusions:

• Fatiguing exercise in heat leads to skeletal muscle fatigue, reducing force summation.
• The observed rightward shift in the force-frequency relationship is more pronounced with exercise-induced hyperthermia than passive heat exposure.
• ExH impairs contractile function more severely than PaH.