During everyday life, humans experience fluctuations in temperature which can influence muscle performance. For example, cooling the muscle has been shown to slow muscle contractile speed and decrease strength.
Mathew Debenham, PhD Candidate
School of Health and Exercise Sciences, The University of British Columbia Okanagan
- During everyday life, humans experience fluctuations in temperature which can influence muscle performance. For example, cooling the muscle has been shown to slow muscle contractile speed and decrease strength.
- A key determinant of muscle function is the optimal control of the motor unit which is the motor nerve (in the spinal cord) and all the muscle fibers it innervates. Muscle force is modulated by the recruitment (number of active motor units) and firing rate (frequency of activation) of those active motor units.
- Currently, it is unclear how changing muscle temperature affects motor unit behaviour during submaximal contractions.
- 20 healthy young adults (10 males and 10 females, mean age: ~24 years) completed 3 testing sessions that involved evaluating the function of a forearm muscle that flexes the wrist.
- The forearm was cooled (∼13°C), heated (∼44°C), or remained at a neutral temperature (∼33°C) by pumping water through a soft plastic tubing wrapped around the participant’s forearm for 25 minutes.
- Isometric maximal voluntary contractions (MVCs; strength), motor unit recordings during submaximal efforts, and muscle contractile properties from electrically-stimulated contractions were evaluated during the three conditions.
- Participants performed 4 ramp contractions (2 targeting 30% MVC; 2 targeting 60% MVC) by flexing their wrist from rest at a rate of 10% MVC/s until they reached their target, remained at this force for 15-30 s, and then decreased back to rest. Motor unit firing rates and recruitment thresholds (i.e., force at which a motor unit is activated) were examined in combination with muscle activity (an indirect measure of neural drive).
What the researchers found
- Cooling decreased and heating increased the electrically stimulated contractile speed of the muscle compared to a neutral temperature. Strength (i.e., MVC force) was reduced for muscle cooling compared to heating and neutral. Muscle activity increased during the submaximal ramp contractions with muscle cooling compared to neutral and heating, despite a lower absolute contraction force.
- Cooling the muscle decreased recruitment threshold of motor units during the 60% MVC ramp contractions compared to neutral, but not for the 30% MVC ramp contraction. Motor unit firing rates did not change with alterations in temperature.
- Temperature did not affect the relationship between motor unit recruitment threshold and firing rates for both submaximal ramp contraction intensities. However, the relationship between motor unit potential amplitude and firing rate was altered when the muscle was cold compared to warm, which could indicate differential temperature effects on motor units of varying sizes.
- Muscle cooling decreased strength and contractile speed, but increased muscle activity. Additionally, mean motor unit recruitment thresholds were reduced for the higher intensity submaximal ramp contraction during cooling compared to the warmer temperatures.
- The influence of acute, local cooling on motor unit behaviour during moderate-intensity voluntary contractions is likely owing to sensory feedback arising from the slower muscles or altered skin temperature.
- When muscles are cold, exercise performance may be impaired compared to when muscles are warm. Specifically, strength decreases, muscles contract slower, and submaximal-effort muscle activity increases, which may indicate a greater neural drive owing to an alteration in motor unit recruitment for a similar exercise intensity.
- Maintaining a warmer or neutral local muscle temperature may be beneficial when exercising in a colder environment.
Mallette MM, Cheung SS, Kumar RI, Hodges GJ, Holmes MWR, Gabriel DA. [The effects of local forearm heating and cooling on motor unit properties during submaximal contractions.] Exp Physiol. 2021; 106(1): 200-211