Abstract

Objective: The aims of this study were to quantify the effects of unilateral posterior-anterior mobilization on force production, failure point and muscle activity of the hamstrings during the Nordic hamstring exercise (NHE) and explore individual differences in responses.
Methods: In a replicated randomized crossover trial, twenty-four participants (age [mean ± SD]: 27 ± 6 y, body mass: 82 ± 17 kg, stature: 181 ± 8 cm) completed two standardized intervention (L4/5 zygapophyseal mobilizations) and two control conditions. The failure point of the Nordic hamstring exercise was determined with 3D motion capture. Peak force, knee flexor torque and electromyography (EMG) of the Biceps Femoris were measured. Data analyses were undertaken to quantify mean intervention response and explore any individual response heterogeneity.
Results: Mean (95% confidence interval) left limb force was higher in intervention vs control by 18.7 (4.6–32) N. Similarly, right limb force was higher by 22.0 (3.4–40.6) N, left peak torque by 0.14 (0.06–0.22) Nm and right peak torque by 0.14 (0.05–0.23) Nm/Kg. Downward Force (DWF) angle was decreased in intervention vs control by 4.1° (0.5–7.6) on the side of application. Both peak EMG activity (p=.002), and EMG at the DWF (Right) (p= .020) increased in the intervention condition by 16.8 (7.1–26.4) and 8.8 (1.5–16.1) (mV), respectively. Mean downward acceleration angle changed by only 0.3° (-8.9–9.4) in intervention vs control. A clear response heterogeneity was indicated only for force right (participant x intervention interaction: P=.044; Response heterogeneity SD = 34.5 (5.7–48.4) N). Individual response heterogeneity was small for all other outcomes.
Conclusions: Following UPA mobilization, immediate changes in bilateral hamstring force production and peak torque occurred during the NHE. The effect on the NHE failure point was unclear. EMG activity increased on the ipsilateral side. Response heterogeneity was generally similar to the random trial-to-trial variability inherent in the measurement of the outcomes.