Sex differences in passive and active stiffness of the knee flexor muscles during dynamic perturbation test: principal component analysis

Purpose: The sensorimotor system is a subcomponent of the comprehensive motor control system of the body. However, the complex nature of the sensorimotor system makes it difficult to interpret findings for clinical application. The purpose of this study was to utilize principal component analysis (PCA) to identify sex differences and relationships between sensorimotor variables during a dynamic perturbation.

Materials and methods: Thirty physically active individuals (15 males and 15 females) were blindfolded and positioned on an isokinetic dynamometer with their knee flexed to 70°. At random, the dynamometer moved rapidly towards knee extension. Subjects were asked to resist the dynamometer as it would randomly and rapidly move towards knee extension. Torque and position values were used to calculate stiffness values.

Results: PCA revealed sex differences in two principal components (PCs): PC2 in female was comprised from higher position, torque, and time values (p = .038), PC4 in females was comprised from higher active stiffness and lower short-range stiffness values (p = .032) compared to males. Torque at the resting position was correlated to the short-range passive stiffness (ρ = 0.539, p = .002), time to peak torque (ρ = -0.375, p = .003), and reactive stiffness (ρ = 0.526, p = .041).

Conclusions: Females had later reaction time and lower short-range passive stiffness and they resisted the dynamometer by their voluntary activation compared to the males thus requiring muscle activation for meaningful response. In addition, the higher resting muscle activities may correlate to short-range passive stiffness and quicker active stiffness. Abbreviations: ACL: anterior cruciate ligament; EEG: electroencephalogram; EMG: electromyography; ICC: intraclass correlation coefficient; MDC₉₅: minimally detectable differences at 95% confidence intervals; PC: principal component; PCA: principal component analysis; POS₅₀: position value at 50 ms; POS₁₀₀: position value at 100 ms; POS_prop: position value at TIME_prop; POS_pk: position value at TIME_pk; POS_prop-pk: position difference between POS_prop and POS_pk; SEM: standard error of measurements; STIFF₅₀: short-range-stiffness at 50 ms; STIFF₁₀₀: short-range-stiffness at 100 ms; STIFF_reac: reactive knee stiffness (stiffness between TIME_prop to TIME_pk); TIME_prop: threshold-to-detect passive movement as the time point; TIME_pk: time at which peak hamstrings torque occurred; TIME_prop-pk: time between TIME_prop to TIME_pk; TORQ₀: torque value at time zero; TORQ₅₀: torque value at 50 ms; TORQ₁₀₀: torque value at 100 ms; TORQ_prop: torque value at TIME_prop; TORQ_pk: torque value at TIME_pk; TORQ_50diff: torque difference between TORQ₀ and TORQ5₀; TORQ_100diff: torque difference between TORQ₀ and TORQ₁₀₀.