John H Hollman1, Takashi Nagai2, Nathaniel A Bates3, April L McPherson4, Nathan D Schilaty5. 1. Program in Physical Therapy, Mayo Clinic School of Health Sciences, Mayo Clinic College of Medicine and Science, Rochester, MN, USA; Department of Physical Medicine and Rehabilitation, Mayo Clinic, Rochester, MN, USA. Electronic address: hollman.john@mayo.edu. 2. Military Performance Division, US Army Research Institute of Environmental Medicine, Natick, MA, USA. 3. Department of Orthopaedics, The Ohio State University, Columbus, OH, USA. 4. United States Olympic & Paralympic Committee, Colorado Springs, CO, USA. 5. Department of Orthopaedic Surgery, Mayo Clinic, Rochester, MN, USA; Department of Physiology and Biomedical Engineering, Mayo Clinic, Rochester, MN, USA.
Abstract
BACKGROUND: Anterior cruciate ligament (ACL) injuries impair knee extensor and flexor force generation and may alter force variability. Fractal scaling exponents quantify signal complexity and reflect neuromuscular system adaptability. The purpose of this study was to evaluate force variability magnitudes and fractal scaling exponents in persons with ACL injuries. METHODS: Twenty-four individuals with ACL injury (time from injury: 55 ± 66 days) and 25 uninjured controls completed 10-s isometric knee extension and flexion contractions on a dynamometer at 10%, 25%, 35%, and 50% of peak force. The middle 8-s of data were used to calculate coefficients of variation and fractal exponents. Injured and non-injured limbs as well as dominant and non-dominant limbs in the control group were compared with ANOVA (P < 0.05). FINDINGS: Peak knee extensor and flexor forces were 19% and 10% lower in the injured limb of ACL-deficient participants (P = 0.014 and P = 0.036, respectively). Fractal scaling exponents of knee extensor force signals at 25% and 35% peak force in injured limbs were higher than in non-injured limbs (P = 0.008 and P = 0.027, respectively). The fractal scaling exponent of knee extensor force signals was greater in injured limbs of ACL-deficient participants than in dominant limbs of the control group at 35% peak force (P = 0.046). The magnitude of variability did not differ between limbs in ACL-deficient participants or between the injured and control groups. INTERPRETATION: Altered fractal exponents in knee extensor force signals represent sensorimotor and neuromuscular system deficits in individuals with ACL injury. Overall, fractal analysis identified both between-limb and between-group differences.
BACKGROUND: Anterior cruciate ligament (ACL) injuries impair knee extensor and flexor force generation and may alter force variability. Fractal scaling exponents quantify signal complexity and reflect neuromuscular system adaptability. The purpose of this study was to evaluate force variability magnitudes and fractal scaling exponents in persons with ACL injuries. METHODS: Twenty-four individuals with ACL injury (time from injury: 55 ± 66 days) and 25 uninjured controls completed 10-s isometric knee extension and flexion contractions on a dynamometer at 10%, 25%, 35%, and 50% of peak force. The middle 8-s of data were used to calculate coefficients of variation and fractal exponents. Injured and non-injured limbs as well as dominant and non-dominant limbs in the control group were compared with ANOVA (P < 0.05). FINDINGS: Peak knee extensor and flexor forces were 19% and 10% lower in the injured limb of ACL-deficient participants (P = 0.014 and P = 0.036, respectively). Fractal scaling exponents of knee extensor force signals at 25% and 35% peak force in injured limbs were higher than in non-injured limbs (P = 0.008 and P = 0.027, respectively). The fractal scaling exponent of knee extensor force signals was greater in injured limbs of ACL-deficient participants than in dominant limbs of the control group at 35% peak force (P = 0.046). The magnitude of variability did not differ between limbs in ACL-deficient participants or between the injured and control groups. INTERPRETATION: Altered fractal exponents in knee extensor force signals represent sensorimotor and neuromuscular system deficits in individuals with ACL injury. Overall, fractal analysis identified both between-limb and between-group differences.