Purpose/background: Deficits in functional abilities persist after total knee arthroplasty (TKA), while static measures of knee extensor strength (e.g. isometric contractions) are related to functional performance, little is known about the associations between functional ability and dynamic knee extensor strength (e.g. power). With the growing rate of these procedures, in a progressively younger and more active cohort, a better understanding of the functional importance of dynamic strength (muscle power) is needed. The purpose of this study was to examine the associations between functional performance and peak knee extensor power (isokinetic and isotonic measures) from patients after unilateral TKA.
Design: Cross-sectional, controlled laboratory study, with correlation and regression analyses.
Setting: Institutional clinic and research laboratory.
Participants: Patients 6 months after TKA (N=24, 12 men and women), most of whom were mildly to very active. A normal control group without knee pain (CON; N=22, 10 men and 12 women) was also assessed for comparison.
Main outcome measures: Static and dynamic strength measures were assessed during normalized voluntary isometric contractions (NMVIC), isokinetic contractions at three velocities (60, 90, and 120 deg/s), and isotonic contractions against three body weight normalized resistances (20, 30 and 40% BW). Functional performance was assessed using the timed up-and-go (TUG), stair climbing test (SCT), and 6- minute walk (6MW). Analyses of the relationships between functional performance measures and peak knee extensor NMVIC and power were performed. Regression analyses predicting functional performance from power were also performed after controlling for NMVIC.
Results: Peak power across isokinetic velocities, isotonic resistances, and NMVICs were correlated with the functional performance measures for the TKA group. Unlike the TKA group, functional performance was not significantly associated with peak power across all isokinetic velocities and isotonic resistances (e.g no significant associations between peak isotonic power and 6MW distance). In the TKA group, inclusion of the isotonic power against 30% BW, after controlling for NMVIC, improved the predictability of all three functional performance tests; TUG (p= 0.022), SCT (p=0.006), and 6MW (p=0.001).
Conclusions: Measurements of knee extensor power may be a useful tool for clinicians when assessing and setting milestones during rehabilitation.
Level of evidence: Prospective cohort study, level II.
Keywords: Dynamic strength; knee replacement; outcome assessment.