We found that patients who had undergone total knee arthroplasty experienced a profound loss of quadriceps strength, marked failure of voluntary muscle activation, and a decrease in quadriceps cross-sectional area when evaluated one month after surgery. The loss of strength was largely explained by a combination of failure of voluntary muscle activation and atrophy. Failure of voluntary muscle activation explained much more of the strength loss than atrophy did; however, the increased activation failure after total knee arthroplasty was not explained by increased pain.
The loss of >62% of the preoperative quadriceps strength was dramatic and closely matched the 60% loss of strength that we reported previously in a similar study involving a different group of patients11
. Not only was the change in strength after surgery pronounced, but the preoperative quadriceps strength also appears to have been below normal. The preoperative force production reported in the present study (18.1 N of force/body-mass index) was 25% less than the force production reported for healthy older adults who were tested previously in our laboratory (24.2 N of force/body-mass index)16,17
Failure of voluntary muscle activation is likely to have contributed to the low preoperative quadriceps force production. The subjects in the present study had an average central activation ratio of 0.867 at the time of preoperative testing, whereas healthy older adults with no known knee abnormalities have been reported to have an average central activation ratio of 0.95516,17
. Two recent studies involving the use of electrical burst-superimposition strength-testing showed that patients with less advanced knee osteoarthritis (grade-2 or 3 according to the scale of Kellgren and Lawrence18
) did not have such a low level of voluntary muscle activation (as indicated by central activation ratios of 0.92819
). Individuals who undergo total knee arthroplasty represent a population of patients who clearly have substantial deficits in voluntary muscle activation.
Not only was there considerable failure of voluntary muscle activation before surgery, but the degree to which it worsened was remarkable. In contrast, it has been previously reported that patients who had undergone anterior cruciate ligament reconstruction did not exhibit abnormal voluntary activation of the quadriceps muscle eight weeks after surgery20
. Suter et al. reported an unexpected lack of worsening of voluntary muscle activation at six weeks in patients who had undergone arthroscopic surgery for the treatment of anterior knee pain21
. A large reduction in voluntary activation following total knee arthroplasty bodes poorly for the recovery of strength as patients with large activation deficits have been reported to have negligible improvement in strength even after intensive rehabilitation22
Some improvement in voluntary muscle activation is expected during the subsequent recovery period, a point that was not addressed in this investigation. In fact, Berth et al., in a long-term follow-up study of patients managed with total knee arthroplasty, demonstrated that voluntary activation of the quadriceps improves over time5
. Specifically, the level of voluntary activation of the quadriceps improved from 76% preoperatively to 85% at the time of the thirty-three month follow-up. While this improvement was substantial, the intervention of total knee arthroplasty did not result in resolution of activation impairments as the level of voluntary activation of the quadriceps remained much less than that in healthy controls at both testing times.
A relatively small cross-sectional area of the quadriceps at the time of the preoperative assessment also appears to have contributed to the overall reduction in knee extensor strength. The preoperative maximal cross-sectional area in the present study was much lower than the typical cross-sectional areas found in healthy older adults23,24
and was slightly lower than the value found in individuals with less advanced osteoarthritis25
. The change in maximal cross-sectional area was smaller than expected as the average knee extensor strength decreased to less than half of preoperative strength. To our knowledge, the only other investigation that has assessed acute changes in quadriceps cross-sectional area associated with total knee arthroplasty also demonstrated only a small amount of atrophy (a 5% reduction) compared with the preoperative assessment10
Unexpectedly, the change in knee pain did not account for a significant amount of the large reduction in voluntary activation of the quadriceps muscle. A similar moderate relationship between knee pain and muscle activation has been reported in previous investigations of patients managed with total knee arthroplasty11,26
. Most of the activation failure does not appear to be due to knee pain during muscle contraction in this patient population. Assuming that muscle activation will improve as perioperative knee pain subsides, therefore, may not be valid.
In the present study, patients who had been managed with total knee arthroplasty had profound impairment in terms of quadriceps force-producing ability one month after surgery. Both failure of voluntary muscle activation and atrophy contributed to the strength loss; however, the major factor appeared to be failure of voluntary activation. Since activation failure was not strongly related to knee pain after surgery, pain control alone may be insufficient to prevent loss of strength. It appears that efforts that are taken specifically to address deficits in voluntary muscle activation in the early postoperative period may improve the outcome in terms of quadriceps strength. Exploring the use of exercise programs that encourage high-intensity muscle contractions and interventions that facilitate activation (e.g., biofeedback and neuromuscular electrical stimulation) appears to be warranted to counter the large deficit in quadriceps strength following total knee arthroplasty.