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
Level of Evidence: Prognostic Level I. See Instructions
to Authors for a complete description of levels of evidence.