THE INFLUENCE OF QUADRICEPS FEMORIS ACTIVATION ON QUADRICEPS STRENGTH OUTCOMES.

Abstract

Purpose/Hypothesis: Patients with knee osteoarthritis (OA) and total knee arthroplasty (TKA) have significant impairment in quadriceps strength and quadriceps-demanding functional tasks. Activation deficits as well as muscle atrophy appear to be vital players in the significant weakness exhibited by these patients. Muscle activation deficits can be detrimental to quadriceps strengthening after TKA, impeding the success of traditional strengthening paradigms. The purpose of the current investigation is to determine if muscle activation can be improved after TKA and whether patients with low level activation have greater difficulty improving quadriceps strength. Number of Subjects: 73 patients with primary, unilateral knee OA (mean age=64.5plus or minus 8.9 years; mean BMI=30.4 ± 4.3). Materials/Methods: Testing occurred an average of 11plus or minus 7 days before scheduled TKA, 27plus or minus 3 days after TKA prior to the commencement of outpatient PT, at the midterm of rehabilitation, and 3 months, 6 months, and 1 year after TKA. Quadriceps strength and voluntary muscle activation (quantified by central activation ratio (CAR)) were measured using a burst superimposition technique. CAR >0.94 was classified as high muscle activation, CAR >0.84 and <0.94 was considered moderate activation, and CAR <0.84 was considered low activation. All patients completed 6 weeks of outpatient PT with a primary emphasis on intensive quadriceps strengthening. Group 1 had high CAR, group 2 had moderate CAR, group 3 had low CAR pre and low CAR at 1 year, and group 4 had low CAR pre and moderate or high CAR at 1 year. Paired t-tests were used to determine changes in CAR. Strength outcomes were analyzed using a repeated measures ANOVA (group x time) with Bonferroni post-hoc comparisons. Alpha was set at p<0.05 to determine significance. Results: Prior to TKA 44% of patients had high CAR, 25% had a moderate CAR values, and 31% had low CAR values. One year after TKA, 50% of patients had high CAR, 28% had moderate CAR values, and 22% had low CAR values. 57% of patients who had low activation preoperatively demonstrated significant improvement in CAR 1yr after TKA (t=−6.93, p<0.001) and 43% of those patients had high CAR levels after TKA (t=−8.76, p<0.001). There was no significant difference in preoperative strength between group 3 (mean=13.86plus or minus 7.44 N/BMI) and group 4 (mean=13.84plus or minus 5.47 N/BMI) (F=0.007, p=0.934). Group 3 (mean=13.38plus or minus 6.05 N/BMI) was weakest 1yr postoperatively. Group 4 (mean=19.76plus or minus 8.58 N/BMI) exhibited similar quadriceps strength to group 1 (mean=23.32plus or minus 8.54 N/BMI) at 1 year (F=6.13, p<0.01). Conclusions: Improvement in CAR is possible after TKA and contributes to improved quadriceps strength outcomes after TKA. Contrary to this, failure to improve CAR after TKA leads to poor strength recovery and persistent quadriceps weakness at 1 year post-operatively. Clinical Relevance: Rehabilitation interventions to improve quadriceps strength should aim to improve muscle activation through the use of modalities such as biofeedback and electrical stimulation, especially in individuals with low CAR.