Altered neuromuscular control in the vastus medialis following anterior cruciate ligament injury: A recurrence quantification analysis of electromyogram recruitment

Abstract

Background: Neuromuscular deficits exist following anterior cruciate ligament (ACL) injury. To observe these deficits, we examined nonlinear characteristics of vastus medialis electromyography (EMG) signals during submaximal isometric knee extensor contractions. Our purpose was to examine if determinism and entropy in EMG signals reflected neuromuscular control deficits in individuals with ACL-deficient limbs.

Methods: 24 participants (12 male, 12 female, mean age = 18.8 ± 3.1 years) with unilaterally injured ACLs and 25 age-similar healthy controls (11 male, 14 female, mean age = 18.8 ± 3.1 years) volunteered. Isometric knee extensions were tested at 10%, 25%, 35%, and 50% maximum voluntary contractions. Surface electrodes adhered over the vastus medialis captured EMG signals. EMG data were processed with recurrence quantification analyses. Specifically, determinism (an index of system predictability) and entropy (an index of system disorder) were calculated from recurrence plots.

Findings: Determinism and entropy in EMG signals were lower in the injured than uninjured limb, and lower than that from healthy controls (P < .05).

Interpretation: Vastus medialis EMG signals from the injured limb were less predictable and less complex than those from healthy limbs. The findings reflect impaired neuromuscular control in the injured limb's quadriceps and are consistent with a 'loss of complexity' hypothesis in physiologic signals emanating from pathologic states. Determinism and entropy in EMG signals may represent biomarkers of one's neuromuscular control system.

Keywords: Anterior cruciate ligament; Electromyography; Entropy; Isometric contraction; Quadriceps muscle.

Fig. 1.

Representative plots of (A) knee extensor force output during a participant’s submaximal contraction at 50% effort, and (B) the same participant’s corresponding EMG signal from the vastus medialis muscle during the trial. EMG data from the middle 8 seconds of the 10 second isometric contraction were extracted for processing and analysis.

Fig. 2.

A raw EMG signal (A) is plotted in a phase space reconstruction with a time delay of 10 data points (B), from which a recurrence plot is composed (C), defined with a radius of 10% of a standard deviation.

Fig. 3.

Box plots of determinism in the EMG signal from the vastus medialis at four volitional force levels during submaximal, isometric knee extensor contractions.

Fig. 4.

Box plots of entropy in the EMG signal from the vastus medialis at four volitional force levels during submaximal, isometric knee extensor contractions.