A gender comparison of reactive knee stiffness regulation strategies under cognitive loads
Date
2010
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Context: Unintentional injuries can occur due to errors in coordination and failure to anticipate or react to sudden joint forces. Neuromuscular control strategies facilitate joint stabilization by regulating knee stiffness, referred to as dynamic restraint; however, a concurrent cognitive load may disrupt or alter reflexes and the execution of routine motor programs. Furthermore, it is suggested that gender differences exist in cognitive faculties such as visual spatial and language skills, as well as lower extremity muscle activation strategies. Purpose: The purpose of this study was to assess whether preparatory and reactive knee stiffening strategies are affected differently in males and females exposed to gender biased cognitive loads. Participants: Twenty males (20.450±1.572 yrs; 88.153±17.866 kg; 179.395±6.235 cm) and 20 females (20.050±1.234 yrs; 58.768±6.970 kg; 163.887±6.392 cm) healthy volunteers with no current injury or previous surgery to their dominant lower extremity participated in the study. Interventions: The independent variables were the type of cognitive tasks administered, which included the Benton Judgment of Line Orientation (JOLO), Symbol Digit Modalities Test (SDMT), Serial 7’s, and a control condition. A custom Stiffness and Proprioception Assessment Device (SPAD) was used to measure reactive knee stiffness. Participants were seated in the SPAD with the testing leg in 30̊ of knee flexion. They were then instructed to perform one of the three cognitive tasks or the control condition. During this period (approximately 10 seconds) they were instructed to react to a randomly timed knee flexion perturbation (excursion = 40̊, velocity = 100̊/sec, acceleration = 1000̊/sec2). Reactive stiffness was measured from the starting position of 30̊ knee flexion to the end of the 40̊ perturbation. Surface electromyography (EMG) was used to measure muscle activation at the medial and lateral quadriceps and hamstrings muscles. Data was processed using customized LabVIEW software (National Instruments, Austin, Tx). An analysis of variance with repeated measures was used to analyze the differences in reactive knee stiffness and muscle activation strategies between the cognitive tasks and a control condition. Main Outcome Measures: Stiffness, normalized to body weight, was calculated as Δ torque (Nm) / Δ position (degrees) and neuromuscular control was assessed through the amplitude and timing of quadriceps and hamstring EMG data. Results: Reactive knee stiffness values were significantly less during the cognitive tasks compared to the control condition (JOLO = 0.034 ± 0.014 Nm/deg/kg, SDMT = 0.037 ± 0.013 Nm/deg/kg, Serial 7’s = 0.037 ± 0.012 Nm/deg/kg, control = 0.048 ± 0.011 Nm/deg/kg). Females had greater overall stiffness than males. The quadriceps muscles had faster and greater activation than the hamstring muscles; however, no gender differences were observed. No overall differences were found among the cognitive loading tasks. Conclusion: Optimum muscle activation and stiffness strategies are necessary to properly stabilize joints during movements. Cognitive tasks may decrease the ability of healthy individuals to reactively stiffen their knee joint in response to sudden external perturbations. These data support the significant role neurocognitive processes may have on unintentional musculoskeletal injury because cognitive loading appears to interfere with the normal force attenuating properties of eccentric muscle contractions. This also suggests caution should be used when implementing cognitive loading as a modality to increase the level of difficulty during prevention and rehabilitation programs as it may expose individuals to greater risk of unintentional injury.