Muscle activity and kinematics underlying complex and simple motor tasks are altered during pain. Although the effects of pain are apparent in everyday life our understanding of the physiological mechanisms that underlie movement adaptations during pain is incomplete. A key issue of current pain literature is the strong focus on effects of pain on voluntary movements. Yet this type of internally initiated, goal directed movement is only one of many examples of how the motor system can act. It remains critical to determine whether motor adaptions to pain are similar in other types of movement responses, such as those related to postural behaviours. Although postural movements use the same sensorimotor system, the movement goals and underlying organization differs from voluntary movements. A key difference is that postural adjustments are primarily aimed at maintaining a position (during static tasks) or a trajectory (during dynamic tasks) against disturbances. Many adjustments that have been proposed for voluntary movements (e.g. inhibition of painful muscles) would be counterproductive for the maintenance of postural stability and orientation. The hypothesis tested in this thesis is that during pain, the goal remains to protect the painful part, but this requires adaptation of muscle activity associated with postural adjustments in a different manner to that during voluntary movement.
Paramount to investigate changes in motor control with a more postural nature was justification and validation of a simple motor control task that is more posturally focused (i.e. maintain joint angle while supporting an inertial load: “position-control” task) and volitionally focused (i.e. maintain target force against a non-compliant resistance: “force-control” task). In this case, if muscle activity is reduced in the “position-control” task, the external force would move the joint (loose control of posture), but in the “force-control” task the consequence would simply be a reduction of the force output, with no consequence to joint position. These tasks are ideal to compare adaptation between movement types as they can be easily manipulated and quantified. Study 1 validated the interpretation of this paradigm by confirming that methodological differences in how the tasks were performed in previous studies (support of limb segments) could not completely explain the differences in task performance.
On the foundation of an underlying difference in control between position- and force-control tasks, subsequent investigations were mounted to provide novel insight into neuromuscular adaptations to pain in postural actions. Study 2 aimed to investigate effects of pain on single motor unit discharge rate (a measure of drive to the smallest unit of the motor system) in knee extensor muscles in a position- and a force-control task. The activity of single motor units that were identified in both position- and force-control tasks (i.e. both tasks performed in a single experimental session) adapted differently to pain between tasks. Although both tasks showed redistribution of activity (with decreased discharge rates of most, and increased discharge of some identified units) the change in activity was more subtle in position- than force-control. This was consistent with the overall hypothesis that adaptation would differ between tasks.
Changes in muscle activation at single motor unit level provided evidence of complex changes in muscle activation that are thought to depend on altered drive to the motoneuron pool from both peripheral and descending inputs. As it is expected that cortical input would be less in postural-type activities, it was expected that the descending cortical input would be less in the position-control task and therefore cortical involvement may be affected differently by pain between the two tasks. Subsequently, Studies 3 and 4 investigated if there was a difference in cortical involvement between the tasks and how pain affected the cortical involvement in both tasks, respectively. Study 3 showed that communication between brain regions (cortico-cortical coherence) was significantly greater in force- than position-control, indicating greater cortical involvement in task performance. Communication between cortex and knee extensor muscles (corticomuscular coherence) did not differ and is best explained by the similarity of visuomotor control aspects of both tasks.
Pain increased cortico-cortical coherence in both tasks in Study 4, but the increase was greater in the force-control task, reflecting greater information processing. In addition, the force-control task showed decreased corticomuscular coherence as well as increased force fluctuations around target, both thought to reflect deterioration of motor performance.
These four studies provide new insights into adaptations in motor function during pain in more postural focused movements at single motor unit level as well as changes in underlying neural drive between postural and volitional movements. More subtle adaptations were observed at the single motor unit and cortical level in the position-control task during pain. Subtlety is fundamental to postural-control as the main aim is to minimize disturbances; the results from these studies are consistent with newer pain adaptation theories and provide a more comprehensive understanding of how and why motor output is affected by pain.