The program of research outlined in this thesis aims to enhance our understanding of the nature of constraints imposed on the acquisition of skill by the organisation of the neuromuscular-skeletal system. The thesis is divided into seven chapters. The first and last chapters provide a general introduction to, and summary of, the research program. The remaining chapters are a series of stand-alone scientific works; each is an integral part of the thesis.
Chapter 2 is a review of the literature pertaining to the influence of the organisation of the neuromuscular-skeletal system upon the acquisition of motor skill. Particular consideration is given to means by which the central nervous system (CNS) can exploit and adapt the functional characteristics of task-specific muscle groups (synergies). This chapter documents the anatomical, mechanical and neural characteristics of the motor apparatus that determine the extent to which muscle synergies are amenable to adaptation during the acquisition of motor skill.
The purpose of the experiment described in Chapter 3 was to determine the nature of practice-related performance adaptations on a task which required appropriately coordinated joint torques in both single and dual degree-of-freedom actions. Two groups of eight individuals practised an isometric, target acquisition task on five consecutive days, each at a different level of torque. For both groups decreases in target acquisition time were primarily associated with increases in the rate of torque development. Skill acquired at the higher torque level was generalised successfully to actions with lower torque demands, however the reverse was not true. Skill was successfully retained at both torque levels after four weeks without practice.
Chapter 4 provides experimental evidence regarding the nature of neuromuscular adaptations that underlie the acquisition, generalisation and retention of skill in single and dual degree-of-freedom isometric actions. Eight individuals completed five consecutive days of practice on an isometric, target acquisition task similar to that used in Chapter 3. Practice-related increases in the rate of torque development were associated with increases in the rate of agonist muscle recruitment. The efficacy of such adaptations was not influenced by differences in the neuro-anatomical properties of the muscle synergies responsible for each action. Performance gains generalised successfully to actions with both greater and smaller torque demands, and were retained four weeks after practice.
In Chapter 5 I sought to identify the nature of the neuromuscular-skeletal constraints upon control strategies implemented by the CNS during the acquisition of skill in two kinematic degrees of freedom. It was anticipated that the divergent mechanical properties of the muscular task groups involved would result in action-specific control strategies and levels of performance. Practice-related performance gains were associated with an increase in the contribution of monofunctional muscles, while the level of activity in bifunctional muscles remained constant Few changes were observed in the timing or rate at which each muscle was first recruited. The results suggest that performance gains were mediated primarily by changes in the spatial organisation of muscle synergies.
Chapter 6 describes an investigation into the role of the primary motor cortex (Ml) in the consolidation of motor skill. Disruption of the Ml contralateral to the moving limb, using repetitive transcranial magnetic stimulation (rTMS), did not produce reductions in performance following practice on a target-directed movement task. The results suggest that the observed neuromuscular adaptations were not amenable to interference using rTMS or that the consolidation of skill in this task occurs outside the Ml.
In conclusion, this program of research demonstrates that the magnitude of practice-related performance gains on tasks that emphasise the rapid and coordinated action of synergistic muscles is not dictated by the specific combination of muscles engaged. The anatomical and mechanical characteristics of each muscle group do however, constrain the manner m which patterns of torque production, movement and muscle activity associated with such actions are modified during the acquisition of skill. Specifically, muscle synergy reorganisation is constrained by the capacity of monofunctional muscles to compensate precisely for torque produced by their multifunctional synergist partners. The cortical origin of such adaptations remains unknown, although it appears likely that motor areas other than the primary motor cortex are critically involved in this process.