Effects of strength training on the human corticospinal pathway at a spinal level

Abstract

Strength training consists of high-force muscle contractions. When performed for weeks, strength training improves muscle strength. These early strength gains are not due to changes in muscle, and are believed to arise from adaptations within the nervous system. The aim of this thesis was to determine if strength training of elbow flexor muscles alters the human corticospinal-motoneuronal pathway at a spinal level and if the type of strength training plays a role. In Chapter 2, one session of isometric strength training of the elbow flexors altered the corticospinal-motoneuronal pathway at a spinal level. Increased responses to stimulation of corticospinal axons suggested increased motoneurone excitability and/or increased efficacy of corticospinal-motoneuronal synapses. Ballistic and slow ramp contractions were equally effective at inducing the change. In Chapter 3, static posture of the upper arm and forearm was shown to influence corticospinal excitability to biceps. The effect had a large spinal contribution, with an interplay of multiple reflex inputs to the biceps motoneurones likely contributing. In Chapter 4, forearm posture during one session of isometric strength training of elbow flexors influenced whether training altered the corticospinal-motoneuronal pathway at a spinal level. Training with the forearm pronated facilitated responses to stimulation of corticospinal axons but training with the forearm supinated did not. This suggests a posture-specific effect of strength training on motoneurone excitability and/or efficacy of corticospinal-motoneuronal synapses. In Chapter 5, four weeks of isometric strength training of elbow flexors improved muscle strength, without a change in the muscle. Voluntary activation of the elbow flexors (assessed via cortical stimulation) and biceps electromyographic activity increased after training. However, based on stimulation of corticospinal axons, corticospinal transmission and motoneurone excitability were not affected by training. This suggests that the improvement in muscle activation and strength reflected a cortical mechanism or a spinal-level facilitation that is more active during voluntary contraction compared with rest. Overall, this thesis revealed that strength training of elbow flexor muscles alters the function of the nervous system, with training posture and duration (one session versus four weeks) influencing the nature and magnitude of the changes.