Date of Award


Degree Type


Degree Name

Doctor of Philosophy




Dr. Charles Rice

Second Advisor

Dr. Trevor Birmingham

Third Advisor

Dr. Tim Doherty


Studies in humans and animals have derived much understanding of neuromuscular function from isometric (static) contractions. In comparison, fewer studies have evaluated dynamic contractions, which are relevant to everyday movements and activities of daily living. The primary purpose of this thesis was to investigate and compare the contributing factors to fatigue during different voluntary contraction tasks. The interpolated twitch technique is commonly used to assess voluntary activation, but with changes in muscle length, musculotendinous slackness can diminish the amplitude of electrically-evoked twitches used to calculate voluntary activation. This might result in erroneous measurements of voluntary activation. Chapter 2 describes an experiment in which at the short muscle lengths, when voluntary activation is 80% or lower, actual activation will be underestimated. Maximum voluntary isometric contraction (MVC) torque is often used to assess overall neuromuscular function, and any activity-induced decline in MVC torque is indicative of fatigue. However, a reduction in shortening velocity is also an important feature of fatigue. Results from Chapter 3 indicate that shortening velocity was an important and perhaps more sensitive measure of fatigue following both isometric and dynamic contraction tasks than MVC torque per se. These findings are further supported in Chapter 4, in which, following comparable repetitive shortening contraction tasks in two different muscles, shortening velocity was reduced to a greater extent at task failure but was restored more rapidly than MVC torque. Shortening contractions are also characterized by a fatigue-related reduction in joint range of motion (ROM) and it was suggested that the reduction in ROM might be due to length-dependent alterations in torque or contractile slowing with fatigue. Results iii presented in Chapter 5, suggest that length-dependent alterations in torque or contractile slowing cannot explain the fatigue-related reduction in dorsiflexion ROM. Thus, in addition to fatigue-related reductions in torque, decreases in shortening velocity and joint range of motion are important indicators of a fatigue-induced impairment in muscle shortening capacity.



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