The increased hip extensor moment demonstrated that subjects adopted a strategy to avoid the use of the quadriceps femoris muscle, yet this strategy persisted as quadriceps femoris muscle strength improved. This pattern may be a learned movement pattern that may not resolve without retraining.

The long-term functional abilities of patients after a unilateral total knee arthroplasty (TKA) are influenced by the status of the nonoperated knee at the time of the TKA. We hypothesized that in the 3 years after TKA, the nonoperated limb would become more painful, and the quadriceps muscles would weaken; pain and strength would influence performance on functional testing by 3 years after TKA. Healthy control subjects were tested over the same time interval; we hypothesized the controls would also decline in strength and function over time. Individuals with unilateral knee pain (less than 4/10 on a verbal analog scale) were recruited preoperatively. We tested patients 1, 2, and 3 years after TKA to determine changes in strength, self-report outcome measures, and performance on a stair climbing test and the 6-minute walk test. Control subjects without osteoarthritis were tested twice, 2 years apart. The nonoperated limb of patients with TKA weakened from 1 to 2 years, and further weakened from 2 to 3 years after TKA; by 3 years after TKA, the nonoperated limb was more painful compared to the operated limb. Three years after TKA, nonoperated knee pain contributed 44% of the variability in the 6-minute walk and 33% of the variability in the stair climbing test. Patients with TKA were weaker, slower, and had lower self-report outcome measures compared with control subjects at both time intervals. Control subjects also weakened over time, yet were stable on self-report outcome measures and the 6 minute walk test. Weakening of the quadriceps muscles in all participants represents changes due to ageing; however on average the nonoperated limb weakened over time, possibly representing not only changes resulting from aging, but progression of osteoarthrosis in some patients with unilateral TKA.

Weakness and failure of voluntary activation of the quadriceps femoris muscles often occur after anterior cruciate ligament (ACL) rupture. Side-to-side strength comparisons are used as a measure of progress, and are inaccurate if the quadriceps has activation failure. Burst superimposition testing is commonly used to assess quadriceps strength and activation during a maximal volitional isometric contraction (MVIC), using the central activation ratio (CAR) calculation. A recently developed mathematical model predicts the MVIC from submaximal efforts. The purpose of this study was to compare the CAR calculation to the mathematical model. We hypothesized that the model would be a more accurate predictor of strength than the CAR calculation when voluntary activation failure is present. Data from the involved and uninvolved quadriceps muscles of 100 consecutive subjects with complete, isolated ACL rupture were retrospectively evaluated. Subjects who required multiple trials to produce an MVIC with full activation (true MVIC) were used to compare the CAR calculation, the mathematical model, and this true MVIC. Subjects unable to produce a true MVIC with multiple trials were used to compare the mathematical model to the CAR calculation. Results demonstrate that both methods reliably and accurately estimate the quadriceps weakness associated with ACL rupture. We recommend use of the CAR calculation to provide estimations of true quadriceps strength to facilitate clinical decisions about progress in rehabilitation after ACL rupture.

After unilateral total knee arthroplasty (TKA), rehabilitation specialists often constrain knee angles or foot positions during sit-to-stand, to encourage increased weight bearing through the operated limb. Biomechanical studies often constrain limb position during sit-to-stand in an effort to reduce variability. Differences between self-selecting or constraining position are unknown in persons after TKA. Twenty-six subjects with unilateral TKA participated in motion analysis. Subjects performed the sit-to-stand using a self-selected position (ssSTS); next, trials were collected in a constrained condition (ccSTS), where both knees were positioned with the tibia vertical, perpendicular to the floor. Repeated measures ANOVA (limb × condition) assessed differences between limbs and between conditions. Subjects used greater hip flexion bilaterally during ccSTS (91°) compared to ssSTS (87°; p=0.001) and knee flexion on the non-operated limb was greater during ssSTS (84°) compared to ccSTS (82°; p=0.018). The ccSTS resulted in larger extensor moments on the non-operated limb at the hip (ssSTS -0.473, ccSTS -0.521; p=0.021) and knee (ssSTS -0.431, ccSTS -0.457; p=0.001) compared to the operated limb. The ccSTS exacerbated the asymmetries at the hip and knee compared to ssSTS, and did not improve use of the operated limb. Reliance on the non-operated limb may put them at risk for progression of osteoarthritis in other joints of the lower extremities.