Effects of local muscle temperature manipulations on neuromuscular function

Abstract

Human muscle can operate through a wide range of temperature; however optimal function may occur throughout a much narrower range. Muscle cooling results in an impairment in muscle contractile properties and maximal force, whereas heating the muscle fosters faster and more powerful contractions. However, what neural compensatory mechanisms exist such that the muscle can still function adequately throughout a wide range of temperatures are unknown and forms the purpose of this dissertation. To this end, muscle contractile and motor unit properties of the flexor carpi radialis were examined during three separate projects involving forearm temperature manipulations. Chapter 4 investigates the effects of local forearm cooling on motor unit properties during an isometric wrist flexion contraction to 50% of baseline maximal force. Chapter 5 builds upon Chapter 4 to include local heating and contraction intensities above and below the motor unit recruitment range of the flexor carpi radialis. Finally, Chapter 6 investigates how different muscle temperatures affect manual performance – assessed through a staircase isometric force tracking task. Local cooling did not affect the ability to perform voluntary contractions to 50% of baseline force, but motor control was achieved through changes in the relationship between motor unit firing rate and recruitment threshold, indicating either faster motor unit firing rates and/or earlier motor unit recruitment to accomplish a task at the same absolute force (Chapter 4). However, these differences were not present when force requirements were made relative to muscle capacity of the respective temperature conditions. We found that motor units were recruited earlier in the cold when contraction intensity was above the motor unit recruitment range (Chapter 5). The altered relationship between motor unit firing rate and recruitment threshold observed in Chapter 4 with muscle cooling at an absolute force level did not affect isometric force tracking ability (Chapter 6). Collectively, this thesis found that the motor unit recruitment threshold may be depressed in the cold due to cutaneous stimulation, and that manual function during an isometric force tracking task involving relatively light loads is not impaired with muscle temperature changes.