Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Science

Program

Kinesiology

Supervisor

Rice, Charles L.

Abstract

The length-tension relationship of the triceps surae (TS) can be altered by changing the knee joint position, ankle joint position or both. However, studies exploring the effect of muscle length on triceps surae (TS) neuromuscular properties have focused only on changes in knee joint position, affecting only two of the three muscle components of the TS. Thus, the purpose of this study is to compare the neuromuscular properties of the three TS muscles during plantar flexion contractions at two ankle joint positions, 20° dorsiflexed (DF) and 20° plantar flexed (PF). Maximal isometric voluntary strength (MVC), voluntary activation, and evoked contractile properties of the ankle plantar flexors were compared between both ankle joint positions. Additionally, motor unit discharge rates (MUDRs) of the soleus, medial (MG) and lateral (LG) gastrocnemii were sampled during plantar flexion contractions at 25, 50, 75 and 100% MVC using indwelling tungsten electrodes. Peak twitch torque and MVC were lower by ~70% and 61%, respectively, whereas maximal rate of torque relaxation was 39% faster in the PF compared with the DF position. Voluntary activation (~95%) was unaffected by changes in ankle joint position. LG MUDRs showed no differences between ankle joint positions, regardless of contraction intensity. Submaximal MG and soleus MUDRs showed no differences between the two ankle joint positions, however at 100% MVC both muscles had 9% and 20% higher rates in the DF position, respectively.

Summary for Lay Audience

The muscles of the calf region of the leg serve a major role in the successful completion of daily tasks, such as standing, walking and running. During walking, these muscles undergo constant changes in length with rotations of the knee and ankle joints. These length changes are associated with different nerve signaling rates, which can increase or decrease muscle activity, affecting how much force the muscles produce and ultimately the successful completion of these daily tasks. This thesis explores how muscle length changes affect muscle contractile properties and nerve signaling rates to the calf muscles in young adults (~24 years of age). The findings in this thesis indicate that the rate of nerve signaling was faster in two of the three muscles of the calf when they were at a longer compared with a shorter length. In the other muscle, there was no difference in the rate of nerve signaling between the two different muscle lengths. This work provides further information regarding the underlying mechanisms of the interaction between nervous system control and muscle function during contractions at different joint configurations, which mimic everyday movements.

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