Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Kinesiology

Supervisor

Dr. Charles L. Rice

Abstract

Muscle contractile properties are history-dependent, and following a conditioning contraction, muscle tissue may be fatigued (slower, weaker) or potentiated (faster, stronger). Postactivation potentiation of evoked contractions, such as the electrically stimulated twitch, has been thoroughly studied. However, the effects of potentiation on voluntary contraction are not well understood, and prior study is largely equivocal.

The following studies propose to determine the effects of potentiation during 1) submaximal contractions at different muscle lengths 2) ballistic contractions following tetanic and voluntary conditioning, and 3) motor evoked potentials following tetanic and voluntary conditioning contractions. Evoked twitch potentiation was assessed with all of the above voluntary measures to compare electrically evoked contractions to those involving the entire neuromuscular system.

Study 1 illustrates that voluntary neuromuscular efficiency of the triceps brachii is greater in a shortened compared to lengthened muscle position. The results of Study 2 indicate that voluntary ballistic peak rate of torque development (RTD) is unchanged following a tetanic conditioning contraction, and is impaired following a voluntary conditioning contraction. This is observed concomitant to a 2-fold increase in twitch torque and peak RTD. Study 3 shows that despite failure of voluntary peak RTD to improve following potentiation, that RTD is enhanced at non-peak time points, implying that performance may adapt differentially throughout the time course of contraction. Study 4 used transcranial magnetic stimulation to provoke a cortical silent period to assess motor cortical inhibition. Following both voluntary and involuntary conditioning contractions, twitch potentiation was observed concurrent to cortical inhibition. This indicates that the conditioning contraction may simultaneously enhance muscular contractile properties and inhibit activity of the motor cortex.

Together, these results indicate a limited opportunity for a conditioning contraction to enhance voluntary contractile properties, despite the substantial enhancement to twitch properties. In addition to the muscular fatigue to which the twitch is subject, the conditioning contraction has centrally inhibitory effects which constrain voluntary contractile performance. This thesis highlights the importance of considering the entire neuromuscular system when assessing contractile adaptations and performance in relation to contractile history.


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