Motor control strategies during double leg squat following anterior cruciate ligament rupture and reconstruction: an observational study

BACKGROUND: Anterior cruciate ligament (ACL) injured individuals often show asymmetries between the injured and non-injured leg. A better understanding of the underlying motor control could help to improve rehabilitation. Double leg squat exercises allow for compensation strategies. This study therefore investigated motor control strategies during a double leg squat with the aim to investigate if individuals with ACL rupture (ACLD), ACL reconstruction (ACLR) and healthy control subjects (CONT) used different strategies. METHODS: 20 ACLD and 21 ACLR were compared to 21 CONT subjects. Participants performed eight continuous double leg squats to their maximum depth, while kinematic and kinetic data were collected. Outcome measures were calculated to quantify the behavior of the injured and non-injured legs and the asymmetry between these legs. RESULTS: Squat depth was significantly reduced in ACLR and ACLD compared to CONT (p < 0.05; 106 ± 17°; 105 ± 21°; 113 ± 21°). Peak knee extensor moments (M( kn(mx) )) were significantly reduced in ACLR and ACLD compared to CONT in the injured leg only (p < 0.05; 0.045 ± 0.015; 0.046 ± 0.016; 0.059 ± 0.022 body weight.height respectively). There was no significant correlation between symmetry of the support moment (SYM( Msup )) and of the % support moment by the knee (SYM%sup( kn )) in CONT (R(2) = -0.07). Data distribution average indicated good symmetry. ACLR showed a significant correlation between SYM( Msup ) and SYM%sup( kn ) (R(2) = 0.561) when two participants who did not recover as well were excluded. ACLR controlled knee moment magnitude using two strategies; 1) transfer of support moment to non-injured leg; 2) transfer of support moment from knee to ankle and/or hip of injured leg. These were combined in different proportions, but with the same effect on the knee moment. ACLD showed no significant correlation between SYM( Msup ) and SYM%sup( kn ) (R(2) = 0.015). Data distribution average indicated reduced symmetry. ACLD therefore used an avoidance strategy: reducing squat depth and subsequently the support moment in the injured leg and the knee contribution. CONCLUSIONS: ACLD and ACLR individuals used different squatting strategies compared to controls, with ACLR using controlled and ACLD using avoidance behavior regarding knee loading. This has major implications for rehabilitation as these kinetic strategies cannot be observed, but result in the injured leg not being exercised as intended.