Experimental muscle pain increases normalized variability of multidirectional forces during isometric contractions

Abstract

Pain elicits complex adaptations of motor strategy, leading to impairments in the generation and control of steady forces, which depend on muscle architecture. The present study used a cross-over design to assess the effects of muscle pain on the stability of multidirectional (task-related and tangential) forces during sustained dorsiflexions, elbow flexions, knee extensions, and plantarflexions. Fifteen healthy subjects performed series of isometric contractions (13 s duration, 2.5%, 20%, 50%, 70% of maximal voluntary force) before, during, and after experimental muscle pain. Three-dimensional force magnitude, angle and variability were measured while the task-related force was provided as feedback to the subjects. Surface electromyography was recorded from agonist and antagonist muscles. Pain was induced in agonist muscles by intramuscular injections of hypertonic (6%) saline with isotonic (0.9%) saline injections as control. The pain intensity was assessed on an electronic visual analogue scale (VAS). Experimental muscle pain elicited larger ranges of force angle during knee extensions and plantarflexions (P<0.03) and higher normalized fluctuations of task-related (P<0.02) and tangential forces (P<0.03) compared with control assessments across force levels, while the mean force magnitudes, mean force angle and the level of muscle activity were non-significantly affected by pain. Increased multidirectional force fluctuations probably resulted from multiple mechanisms that, acting together, balanced the mean surface EMG. Although pain adaptations are believed to aim at the protection of the painful site, the current results show that they result in impairments in steadiness of force.

Modulation of motor variability related to experimental muscle pain during elbow-flexion contractions

“…The selected period avoided errors in the initial part of the contraction. Force standard deviation (FSD) was calculated to quantify force variability in the task-related direction, and tangential force variability was estimated using the total excursion of the center of pressure (CoP) (Mista et al, 2013;Salomoni & Graven-Nielsen, 2012a, 2012b. In addition to the CoP, a two-dimensional histogram of tangential force components was derived using 5 bins to represent the range of the force.…”

“…Even though the chosen motor strategy allowed subjects to achieve the motor task, a shift from the ''painless'' motor pattern has been inferred as a deviation from the optimal performance (Hodges & Tucker, 2011;Mulder, Zijlstra, & Geurts, 2002). The compensatory strategies require more effort and have mental (Mulder et al, 2002), metabolic (Schulte et al, 2004), and mechanical (Salomoni & Graven-Nielsen, 2012a) costs on the system. Consequently, the system can solve an immediate impairment using a different motor strategy, but this adaptations could lead to chronic musculoskeletal disorders in the long term under maintenance/repetition of the noxious stimulus (Hodges & Tucker, 2011).…”

“…Previous studies inducing experimental muscle pain based their hypotheses on the pain adaptation theory, predicting reduced agonist muscle activity and increased antagonist muscle activity (Ervilha et al, 2005;Salomoni & Graven-Nielsen, 2012a;Schulte et al, 2004). However, several reports have found unchanged muscle activation during m. biceps brachii pain (Salomoni & Graven-Nielsen, 2012a;Schulte et al, 2004) and neuromuscular facilitation of both elbow flexor and extensor muscles, contrasting with the principles of the pain adaptation theory (Hodges & Tucker, 2011).…”

“…However, several reports have found unchanged muscle activation during m. biceps brachii pain (Salomoni & Graven-Nielsen, 2012a;Schulte et al, 2004) and neuromuscular facilitation of both elbow flexor and extensor muscles, contrasting with the principles of the pain adaptation theory (Hodges & Tucker, 2011). Most studies have quantified the level of muscle activity based on linear analysis of dynamic changes of the muscle activity during a motor task.…”

“…Thus, the strategy used to execute a motor task depends on the visual information available (Hong, Brown, & Newell, 2008;Prodoehl & Vaillancourt, 2010), resulting in changes to the distribution of the load across muscles and joints. Most experimental muscle pain studies have been limited to provide visual feedback of the task-related force (Bandholm et al, 2008;Ervilha et al, 2005;Salomoni & Graven-Nielsen, 2012a) without taking into account the information of tangential force components related to the task. Inclusion of tangential components in the visual feedback increases the required force precision that may trigger a different motor response to pain, since muscle adaptations are restricted by voluntary adjustment of the force in all directions.…”