Early quadriceps strength loss after total knee arthroplasty. The contributions of muscle atrophy and failure of voluntary muscle activation

Abstract

Background: While total knee arthroplasty reduces pain and provides a functional range of motion of the knee, quadriceps weakness and reduced functional capacity typically are still present one year after surgery. The purpose of the present investigation was to determine the role of failure of voluntary muscle activation and muscle atrophy in the early loss of quadriceps strength after surgery.

Methods: Twenty patients with unilateral knee osteoarthritis were tested an average of ten days before and twenty-seven days after primary total knee arthroplasty. Quadriceps strength and voluntary muscle activation were measured with use of a burst-superimposition technique in which a supramaximal burst of electrical stimulation is superimposed on a maximum voluntary isometric contraction. Maximal quadriceps cross-sectional area was assessed with use of magnetic resonance imaging.

Results: Postoperatively, quadriceps strength was decreased by 62%, voluntary activation was decreased by 17%, and maximal cross-sectional area was decreased by 10% in comparison with the preoperative values; these differences were significant (p < 0.01). Collectively, failure of voluntary muscle activation and atrophy explained 85% of the loss of quadriceps strength (p < 0.001). Multiple linear regression analysis revealed that failure of voluntary activation contributed nearly twice as much as atrophy did to the loss of quadriceps strength. The severity of knee pain with muscle contraction did not change significantly compared with the preoperative level (p = 0.31). Changes in knee pain during strength-testing did not account for a significant amount of the change in voluntary activation (p = 0.14).

Conclusions: Patients who are managed with total knee arthroplasty have profound impairment of quadriceps strength one month after surgery. This impairment is predominantly due to failure of voluntary muscle activation, and it is also influenced, to a lesser degree, by muscle atrophy. Knee pain with muscle contraction played a surprisingly small role in the reduction of muscle activation.

Figures

Fig. 1

A sample of quadriceps force production during a burst-superimposition test of a subject who was tested four weeks after total knee arthroplasty. CAR = central activation ratio.

Fig. 2

Illustration showing the mean percent changes (and standard errors) in quadriceps strength, voluntary muscle activation, and maximal cross-sectional area, normalized to the initial condition. NMVIC = normalized force of maximal voluntary isometric contraction (calculated as the units of force, in Newtons, divided by body-mass index [weight in kg divided by the height in meters squared]). CAR = central activation ratio (with a value of 1.0 representing complete activation). CSA = maximal cross-sectional area (in centimeters squared).

Fig. 3

Illustration depicting the results of multiple regression analysis of the relative contributions of loss of cross-sectional area and voluntary muscle activation to the change in strength. The relative change from the preoperative value to the postoperative value was quantified as a percentage of the preoperative value ([preoperative value – postoperative value]/preoperative value). Y = Predicted percent loss of quadriceps strength, A = percent loss in central activation ratio, and B = percent loss in cross-sectional area.

Fig. 4

Illustration depicting the results of linear regression analysis of the contribution of the change in knee pain during strength-testing to the change in voluntary activation of the quadriceps muscle. CAR = central activation ratio. MVIC = maximal voluntary isometric contraction. Change was determined by subtracting the preoperative value from the postoperative value. Negative values for the central activation ratio represent a reduction in voluntary activation of the muscle compared with the preoperative value. Negative values for knee pain represent a reduction in the knee pain associated with maximal voluntary isometric contraction compared with the preoperative value.