Complexity of Knee Neuromuscular Control in Uninjured Physically Active Adults: Right/Left and Dominant/Nondominant Asymmetry Analyses

Context: Motor pathways include upper motor-neurons from the cerebrum and brainstem and lower motor-neurons from the spinal cord. Together, these physiological components are effectors of knee neuromuscular control. Because multiple components are involved, each with an output that is asynchronous to the others, ‘whole-system’ output is characterized by irregular temporal behavior and signal fluctuations. The irregular temporal behavior of physiological signals is analyzed using ‘complexity’. Complexity-based measures reflect the ability to adapt motor output rapidly and accurately in response to external perturbations and provide physiological information missed by magnitude-based (variability) measures.Objective: To characterize side-to-side symmetry of knee neuromuscular control (sub-maximal isometric knee extension constant-force task) using variability (coefficient of variation [CV%]) and complexity (approximate entropy [ApEn], detrended fluctuation analysis [DFA α]) measures.Design: Cross-sectional.Setting: Laboratory.Patients or Other Participants: Sixteen (male/female n=11/5; age 24.0±5.3yr; height 1.74±0.08m; body-mass 68.3±11.1kg).Main Outcome Measure(s): Right/left and dominant/nondominant group-level (t-test) and individual-level (absolute-asymmetry [%]) comparisons. A limb-symmetry-index was calculated for each variable and clinically-significant absolute-asymmetry defined (>15%). Clinically-significant absolute-asymmetry prevalence (%) was computed for each variable.Results: The only significant side-to-side difference was for right/left DFA α (P=.000). Maximum absolute-asymmetries were (right/left, dominant/nondominant): CV 18.2%, 18.0%; ApEn 34.5%, 32.3%; DFA α 4.9%, 5.0%. Clinically-significant absolute-asymmetry prevalence was (right/left, dominant/nondominant): CV 43.8%, 43.8%; ApEn 62.5%, 50.0%; DFA α 0.0%, 0.0%.Conclusions: Different side-to-side comparison methods yield different findings. Large proportions of participants demonstrated wide ranges of side-to-side absolute-asymmetries. The finding of a significant difference for the right/left DFA α comparison but not for the right/left ApEn comparison suggests that different complexity variables assess different aspects of complexity. Consideration for how side-to-side comparisons are performed (right/left, dominant/nondominant) is required. Approximate entropy and DFA α assess different aspects of complexity and both should be used alongside other traditional magnitude-based measures when studying knee neuromuscular control.