Quantifying voluntary knee strength deficits and muscular contribution to torque in an anterior cruciate ligament-injured adolescent population using a musculoskeletal model

Background

Surface electromyography is commonly used to elucidate the effect of anterior cruciate ligament injury on neuromuscular function. For comparisons, electromyography is normalized to a known value, such as peak activation during maximum voluntary isometric contractions. However, a knee injury may compromise one's ability to achieve a true maximal effort. A simple musculoskeletal model may provide insight into injury related strength deficits.

Methods

Thirty-nine anterior cruciate ligament injured adolescents (14-16 years; 25 females) and 39 matched controls (25 females) completed maximum voluntary isometric knee extension and flexion contractions on an isokinetic dynamometer. A participant-specific musculoskeletal model used normalized electromyography of knee joint muscles to determine a theoretically ideal torque for each contraction type, assuming agonist muscles were fully activated. Strength deficit ratios expressed peak experimental torque relative to theoretically ideal torque. Individual muscle contribution to experimental torque were also computed.

Findings

Injured participants demonstrated significantly lower experimental torque than controls, with percent group mean difference of 17.8 % for knee extension (Injured:2.33 ± 0.89 vs Controls:2.88 ± 0.56 Nm/kg) and 16.7 % for flexion (Injured:1.22 ± 0.44 vs Controls:1.49 ± 0.27 Nm/kg). Group mean differences in strength ratios reduced to 6.3 % for extension (Injured:0.69 ± 0.11 vs Controls:0.74 ± 0.08) and 10.0 % for flexion (Injured:0.56 ± 0.15 vs Controls:0.63 ± 0.12). No between-group differences in muscular contribution to peak experimental extension torque were observed. Injured participants had lower medial gastrocnemius percent contribution to peak experimental flexion torque.

Interpretation

Isometric strength tests may not adequately identify strength deficits in adolescent anterior cruciate injured populations. Simplified modelling frameworks may be more appropriate for evaluating the relationship between neuromuscular control and functional outcomes.