MULTI-DIRECTIONAL HIP STRENGTH, PHYSICAL FUNCTION AND BALANCE IN PEOPLE WITH UNILATERAL KNEE OSTEOARTHRITIS COMPARED TO HEALTHY AGE-MATCHED CONTROLS: A CROSS-SECTIONAL STUDY

Purpose: Hip abduction strength is 7-23% weaker in people with Knee Osteoarthritis (KOA) compared to healthy controls. Evidence on hip adduction is conflicting and there is insufficient evidence in the sagittal and transverse planes. Hip strengthening in addition to quadriceps strengthening elicits superior benefits in pain, function and quality of life outcomes than quadriceps exercises alone. The best type of hip exercise is unknown. Understanding associations between multi-planar hip strength, function and dynamic balance in people with knee osteoarthritis (KOA) may direct more targeted exercise prescription, improving patient outcomes. The aims of this study are to 1) Determine if hip strength and balance differs between limbs in people with unilateral KOA and matched controls and, 2) explore whether hip strength is related to function and balance in people with unilateral KOA. Methods: 42 participants were recruited including 21 (13 men; 62.85 ± 8.63 years) with unilateral KOA and 21 age- and sex-matched healthy controls (13 men, 62.09 ± 10.29 years). Hip flexion, extension, abduction, adduction, external and internal rotation strength was measured using hand-held dynamometry. Torque was normalised to body weight. Function was measured using 40-metre fast-paced walk test (40mFPWT), 30s chair-stand test, and stair-climb test (SCT). Balance function was assessed using the Star Excursion Balance Test (SEBT) in the anterior, postero-medial and postero-lateral directions. Distance reached was normalised to leg length (mm/m). Statistical analyses included mixed model analysis of variance (ANOVA), post-hoc analyses, and Pearson’s correlations (α = 0.05). Results: Multi-planar hip strength and balance function is reduced in people with unilateral KOA. KOA participants demonstrated lower hip adduction strength in the symptomatic compared to non-symptomatic limb (mean difference 0.09, 95% CI 0.01-0.17 Nm/kg). Hip abduction (0.29, 0.04-0.54 Nm/kg), adduction (0.31, 0.07-0.57 Nm/kg) and extension (0.24, 0.01-0.48 Nm/kg) strength was lower in the symptomatic limb of people with unilateral KOA compared to healthy controls. Balance is reduced on the symptomatic limb compared to the non-symptomatic limb of unilateral KOA participants in all three directions (anterior 0.04, 0.02-0.06mm/m; postero-medial 0.04, 0.02-0.07mm/m; postero-lateral 0.22, 0.05-0.39mm/m, p<0.05). Balance on the symptomatic limb of people with unilateral KOA was reduced in the anterior (0.13, 0.05-0.21 mm/m) and postero-medial (0.22,0.1-0.34 mm/m) directions compared to healthy controls (p<0.05). There was a moderate to strong correlation in all planes of hip strength to 40mFPWT, SCT and balance in people with unilateral KOA (p<0.05). Hip extension strength had the strongest correlation to 40mFPWT (r=0.82) and SCT (r = 0.72) in the KOA group, however only moderate correlation to 40mFPWT (r=0.52, p>0.05) and no correlation (r=0.24, p>0.05) to SCT in the healthy control population. Hip external rotation had the strongest correlation with 30sCST (r=0.52) in the KOA group, and no correlation with 30sCST in the healthy controls (r=0.01, p<0.05). All planes of hip strength were correlated with all 3 directions of balance in the unilateral KOA group (p<0.05). In people with unilateral KOA, hip external rotation strength had the strongest correlation to anterior (r=0.82) and postero-medial (r = 0.72) balance directions, whereas hip internal rotation had the strongest correlation with postero-lateral (r = 0.52) balance. There was no correlation between multi-planar hip strength and balance in the healthy controls (p<0.05). Conclusions: When designing exercise programs for people with KOA clinicians should consider multi-planar hip strength deficits regardless of the person’s functional limitations. Functional limitations of walking and stairs are most strongly correlated with sagittal plane hip extension strength. In contrast, functional limitations of sit to stand and balance are more correlated with rotational hip plane strength. Targeting direction of hip strengthening exercises to functional limitations may lead to greater benefits in pain and function for the individual with KOA.