The long-term contribution of muscle activation and muscle size to quadriceps weakness following total knee arthroplasty.

Purpose: Many older individuals have persistent quadriceps strength impairments after a total knee arthroplasty (TKA). A combination of muscle atrophy and neuromuscular activation deficits apparently contributes to residual strength impairments. The purpose of this short report is to describe the contribution of quadriceps muscle activation and muscle volume to impaired muscle strength in older individuals an average of 21 months following a TKA.

Methods: Seventeen individuals (males: 3, females: 14; mean age: 68 yrs +/- 8.7; BMI: 33 +/- 4.8 kg/m2; number of TKA: 24; average postoperative months: 21 +/- 11.3) recruited from an orthopaedic surgeon's practice provided their written consent and participated in this study. Quadriceps strength (MVIC) and voluntary quadriceps muscle activation (QA) were measured with use of a burst-superimposition technique in which a supramaximal burst of electrical stimulation is superimposed on an MVIC. Quadriceps volume (QV) was assessed from magnetic resonance images of the quadriceps.

Results: The mean quadriceps strength was 107.3 Nm +/- 36.4 (range: 43.22 - 205.2). The mean QA (as described with a central activation ratio) was 0.97 +/- 0.04 (range: 0.83 - 1.00). The mean QV was 1093 cm3 +/- 311.80 (range: 653.66 - 1706.56). QA and QV explain 85% of the variance in quadriceps strength (R2 = .85, p < 0.001), with QV having the greatest contribution to strength (R2 = .77, p < 0.001).

Conclusions: QV is a much stronger predictor of quadriceps strength than QA in individuals more than 1 year following TKA. Activation levels contributed little to strength one year following TKA, compared to its profound contribution in the first few postoperative months. Physical therapy interventions focused on improving muscle size in this population should be considered more relevant than countermeasures addressing neuromuscular activation.