Muscle atrophy contributes to quadriceps weakness after anterior cruciate ligament reconstruction

Abstract

Objectives: Quadriceps weakness persists after anterior cruciate ligament reconstruction. Muscle atrophy and activation failure may contribute. This study examined the roles of atrophy and activation failure in quadriceps weakness after anterior cruciate ligament reconstruction.

Design: Case series.

Methods: Twenty patients six months post-anterior cruciate ligament reconstruction participated. Atrophy was determined as peak quadriceps cross sectional area from magnetic resonance images. Quadriceps activation was quantified via the central activation ratio, while muscle strength was measured isometrically. All testing was performed bilaterally. Hierarchical linear regression and one-way ANOVAs were performed to examine the relation of muscle strength with activation and atrophy.

Results: Cross sectional area (R(2)=0.307; p=0.011) explained more of the variance in quadriceps strength than central activation ratio (R(2)<0.001; p=0.987). Strength and cross sectional area were lower in the injured (strength: 2.03±0.51Nm/kg; cross sectional area: 68.81±17.80cm(2)) versus uninjured limb (strength: 2.89±0.81Nm/kg; cross sectional area: 81.10±21.58cm(2); p<0.001). There were no side-to-side differences in central activation ratio; however, quadriceps activation failure was present bilaterally (injured: 0.87±0.12; uninjured: 0.85±0.14; p=0.571).

Conclusions: Quadriceps cross sectional area was strongly related to muscle strength six months after anterior cruciate ligament reconstruction and substantial injured versus uninjured limb deficits were demonstrated for strength and cross sectional area. Patients may benefit from exercises aimed at improving quadriceps cross sectional area post-operatively.

Keywords: Anterior cruciate ligament; Central activation ratio; MRI; Muscle; Strength.

Figure 1

Screenshot from strength and QAF testing. The blue line represents the patient's real-time torque output. The solid black line corresponds to the patient's peak value from the MVIC trials and also serves as a threshold for QAF testing. Real-time torque output must cross threshold for the electrical stimulus to be delivered. The dotted line represents the patient's target value, which was set 10% above maximal strength. The black arrow corresponds to delivery of the electrical stimulus. QAF= central activation failure. MVIC= maximal voluntary isometric contraction.

Figure 2

Results of hierarchical linear regression analysis. MVIC= maximal voluntary isometric contraction.

Figure 3

Representative magnetic resonance image demonstrating quadriceps atrophy. Images were obtained from proton density-weighted images with the following parameters: repetition time 200-3000ms, echo time 35ms, slice thickness 6mm, gap between slices 6mm, matrix 364×180, and field of view 480×281mm. VL= vastus lateralis. RF= rectus femoris. VM= vastus medialis. VI= vastus intermedius.