Author(s): RYAN, O., PIKE, R., RYAN, I., MITCHELL, E., SUTTON, K., SKARABOT, J., PEARCEY, G., Institution: MEMORIAL UNIVERSITY OF NEWFOUNDLAND AND LABRADOR, Country: CANADA, Abstract-ID: 2307

INTRODUCTION:
An anterior cruciate ligament (ACL) rupture affects the neural control of muscles acting about the knee, and there is a higher incidence of injury in females compared to males. Prominent strength deficits and asymmetries are present in the quadriceps and hamstrings after ACL reconstruction, yet despite the significant research on torque steadiness and activation deficits in the quadricep muscles, there exists a gap in the literature on inter-muscle differences during sub-maximal dynamic contractions. Addressing this gap can have clinical implications for predicting and assessing knee function and return-to sport tests for athletes. The purpose of this study was to determine if there are muscle-specific deficits in the coordinated activation of the vastii muscles during isokinetic knee extension, which might contribute to impairments in force control.
METHODS:
Eight female soccer players who underwent ACL reconstruction as well as eight healthy control females participated in this study. Participants performed maximal and sub-maximal contractions to 20% of their maximum under isometric, concentric, and eccentric conditions. Torque output was measured using a Humac Norm isokinetic dynamometer and muscle activity was recorded from the vastus medialis and lateralis using high-density surface electromyography (EMG). Raw EMG signals were bandpass filtered (10-500 Hz), rectified, and then low-pass filtered (6 Hz) to create a linear envelope. Average amplitudes were calculated as mean normalized EMG activity within the middle 5 seconds of the trial. We then computed a ratio of lateralis:medialis EMG to determine whether there were changes in the balance of activation across the vastii muscles.
RESULTS:
Torque steadiness was impaired in the injured leg of people with ACL reconstruction during concentric (p < 0.0001; d = 1.85) and eccentric (p = 0.028; d = 0.8) contractions, but not isometric (p = 0.36), compared to both their uninjured legs. There were no significant differences noted between legs in the controls or between the uninjured leg and controls. In people with ACL reconstruction, irrespective of the task performed, they had an elevated ratio of vastus lateralis to medialis muscle activation (p = 0.049; d = 0.49), which was primarily driven by a tendency for reduced vastus medialis activity.
CONCLUSION:
These results suggest that impaired torque control after ACL reconstruction may be partly mediated by reduced coordinated activity of knee extensor synergists and will help inform rehabilitation strategies.