Abstract
BackgroundGenerating a force at the hand requires moments about multiple joints by a theoretically infinite number of arm and shoulder muscle force combinations. This allows for learning and adaptation and can possibly be captured using the complexity (entropy) of an isometrically generated force curve. Patients with Subacromial Pain Syndrome have difficulty to explore alternative, pain-avoiding, motor strategies and we questioned whether loss of motor complexity may contribute to this. We assessed whether patients with Subacromial Pain Syndrome have reduced entropy of an isometrically generated abduction and adduction force curve. MethodsForty patients and thirty controls generated submaximal isometric ab- and adduction force at the wrist. The force curve was characterized by the magnitude of force variability [standard deviation and coefficient of variation], and the entropy (complexity) of force variability [approximate entropy]. FindingsPatients showed reduced entropy both during the abduction (−0.16, confidence interval: [−0.33; −0.00], p: 0.048) and adduction task (−0.20, confidence interval: [−0.37; −0.03], p: 0.024) and reduced force variability during abduction (standard deviation: −0.006, confidence interval: [−0.011; −0.001], p: 0.013 and coefficient of variation: −0.51, confidence interval: [−0.93; −0. 10], p: 0.016). InterpretationsIsometric force curves of patients with Subacromial Pain Syndrome show reduced complexity compared to asymptomatic controls, which may indicate more narrow and stereotype use of motor options. In future studies, it should be investigated whether the finding of reduced force (motor) entropy indicates functional decline, contributing to decreased ability to acquire and optimize motor strategies in Subacromial Pain Syndrome. Level of evidenceLevel II prognostic study.
Highlights
Healthy physiological systems have an infinite number of solutions for a given task, resulting in a measurable complexity of the system's output (Lipsitz, 2002; Lipsitz and Goldberger, 1992)
We propose that loss of motor output complexity may contribute to the perpetuation of pain in patients with Subacromial Pain Syndrome (SAPS), as patients may fail to explore alternative motor strategies and avoid subacromial pain (Moseley and Hodges, 2006a)
We extend the analyses of variability by quantifying the complexity of isometric force curves using Approximate Entropy (ApEn) in patients with SAPS and controls (Pincus, 1991)
Summary
Healthy physiological systems have an infinite number of solutions for a given task, resulting in a measurable complexity of the system's output (Lipsitz, 2002; Lipsitz and Goldberger, 1992). Few studies have looked into this aspect of motor control in SAPS by analysing the dispersion of force output using measures like the standard deviation (SD) or coefficient of variation (CV) (Bandholm et al, 2006; Camargo et al, 2009; Maenhout et al, 2012; Zanca et al, 2010). We will determine force entropy during isometric adduction, to provide insight into whether a potential loss of force entropy is specific to the abduction movement, or more systemic for the arm
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