['Patients after total knee arthroplasty (TKA) are known to exhibit deficits in quadriceps muscle activation. The purpose of this study was to determine if quadriceps activation levels in patients after TKA at the beginning of rehabilitation would influence quadriceps strength after rehabilitation.', 'A secondary analysis of data from a prospective, randomized, longitudinal clinical trial.', 'Institutional clinic and research laboratory.', 'Patients who underwent unilateral TKA (Men= 102; Female= 84).', 'Voluntary activation of the quadriceps during maximal voluntary isometric contractions (MVIC) was measured using the central activation ratio (CAR). Hierarchical multivariate regression analysis was used to determine if CAR prior to treatment could predict MVIC after the strength training intervention.', 'After controlling for age, sex, and initial strength levels (R(2)= 0.548; p<0.001), the predictability of quadriceps strength after the 6-week intervention did not change when pain during MVIC (R(2)= 0.551; p= 0.317) and pre-rehabilitation activation levels (R(2)= 0.551; p= 0.818) were introduced into the regression.'] Do activation deficits limit quadriceps strength training gains in patients after total knee arthroplasty?