Human movement is inherently variable. In the performance of complex, multi-joint tasks, it is possible to consistently achieve an accurate outcome (i.e. low variability of the goal: VARgoal), with many different combinations of joint movements and patterns of muscle activation (i.e. variability in the elements of the movement: VARelements). It has been proposed that when the nervous system is challenged by acute experimental pain, VARelements might increase to search for a new, less painful movement strategy and then decrease if a less painful solution is found. The changes to VARelements found in situations of chronic musculoskeletal pain are more diverse. In chronic pain VARelements might be reduced, increased, not changed, or a complex interaction of these possible adaptations. All previous studies that investigated VARelements during pain evaluated multi-joint tasks (e.g. walking, pointing) that involve multiple elements. It was unclear whether VARelements would be altered in a similar manner for simple tasks with fewer elements and thus limited potential for VARelements to change.
For the series of studies included in this thesis, a simple movement task was developed that involved radial-ulnar deviation of the wrist between two target angle regions. Kinematic data were collected with 3-dimensional recording systems and VARelements were considered in wrist flexion-extension and forearm pronation-supination. The effect of pain on VARelements during performance of the radial-ulnar deviation task was evaluated in Studies 1-3, under various pain conditions.
Study 1 investigated the influence of acute experimental pain, induced with injection of hypertonic saline, on VARelements during performance of the repetitive radial-ulnar deviation task. This study showed that, unlike that observed in more complex multi-joint systems, VARelements was reduced during acute pain in the simple task with limited elements that could change. The most likely explanation was that the motor system constrained movement in an attempt to reduce pain or exert greater control over joint motion.
On the foundation of differences in the changes to VARelements for complex and simple tasks during acute pain, Study 2 investigated whether VARelements would initially increase during acute pain to gain exposure to different movement options in a search for a less painful solution. An experimental paradigm was developed where the simple task provoked moderate pain for most movements, but a less painful or non-painful solution was available that was likely to be experienced as a result of VARelements with repetition of the task. We found participants searched for, and found, a less painful movement strategy, but VARelements was not used as part of this search. Participants did not select the strategy provided as the least painful solution by the experimental paradigm, but found a less painful strategy with gradual changes to wrist/forearm position over multiple repetitions to explore alternative movement options. The changes to VARelements when participants performed the simple task in Studies 1 and 2 were not consistent with the strategies observed in previous studies of multi-joint tasks.
In Study 3 participants with chronic lateral epicondylalgia (LE) and pain-free controls performed the radial-ulnar deviation task whilst gripping a load cell to a standardised force, which provoked pain for LE participants. We found no difference of VARelements between the LE group and controls at the start or end of the trial, but in the LE group, VARelements in the flexion-extension direction decreased over time. Participants with chronic LE moved the wrist into a more flexed wrist position and reduced VARelements to allow performance of the radial-ulnar deviation task in a less painful manner.
Based on the results of Studies 1-3 and previous investigations, it was clear that VARelements could be altered during pain, but two fundamental questions remained unclear. First, what is the time-course of changes to VARelements when acute pain is sustained, and second, are the changes to VARelements in acute pain and chronic pain related? To answer these questions a model of pain that induces acute pain that is sustained for several days was needed.
Study 4 investigated whether an intramuscular injection of nerve growth factor (NGF) into an elbow/forearm muscle induced sustained pain that was provoked by movement and muscle contraction/stretch. Pain that was provoked by movement of the upper limb and by contraction/stretch of the injected muscle was sustained for six days. These features indicate that intramuscular injection of NGF induces pain that responds in a manner that is typical of clinical pain, and is a suitable model to study the effect of sustained lateral elbow pain on VARelements.
These four studies provide insight into the relationship between VARelements and pain during a simple task with few elements, and offers an avenue for future work using NGF as a sustained pain model for LE. When challenged by pain, the motor system does not use VARelements to search for a less painful solution for simple tasks with less capacity to change and considers multiple factors in addition to minimisation of pain and injury when selecting a movement strategy.