Inter-individual variation in hip flexor length may explain differences in trunk flexion during walking in people with knee osteoarthritis

Abstract

Purpose: Increasing trunk flexion will shift the position of the centre of mass relative to the hip and knee which may lead to changes in muscle activation patterns during walking. Through previous research, we have demonstrated that people with knee osteoarthritis (OA) walk with an increased flexion of the trunk. Our research has also shown that instructing healthy people to increase trunk flexion during walking, by only 5 degrees, produces a distinct increase in duration and magnitude of both the lateral and medial hamstrings. This may indicate that prolonged/increased hamstring activity in people knee OA may, to some degree, result from increased trunk flexion. Importantly, such muscle patterns are known to increase articular load, accelerate the rate of cartilage loss and to be associated with poor long-term clinical outcomes. It is therefore essential that we understand the underlying drivers of altered trunk flexion in people with knee OA. The hip flexor muscle group comprises iliacus, psoas and rectus femoris. As well as functioning to create a hip flexor moment, the relaxed (resting) length of this muscle group could influence the sagittal plane inclination of the pelvis in standing and during movement. Specifically, if the hip flexor muscles are structurally short, this may lead to a relative anterior tilt of the pelvis on the thigh which could, in turn, lead to an increase in trunk flexion. It is therefore possible that the increase in trunk flexion, observed in people with knee OA, is a consequence of short hip flexor muscles. Therefore, this study had two aims. The first was to compare hip flexor length between people with knee OA and healthy control subjects. The second was to understand the association between hip flexor length and trunk inclination during walking, both in healthy people and those with knee OA. Methods: Data was collected from 20 people with radiographically diagnosed OA of the knee and 20 age/BMI matched healthy participants. For each participant, hip flexor length was measured using the Thomas Test. For this test, subjects lay supine with the non-tested leg held against their chest (knee flexed) while the other leg was extended from a flexed position until muscular restriction prevented any further movement. This tested leg hung freely off the edge of the testing plinth while thigh angle, relative to the horizontal, was measured using an inclinometer. Pelvic position was controlled throughout this testing protocol using a pressure biofeedback cuff. An increase in thigh flexion, in the Thomas test, indicates muscular restriction and therefore shortness of the hip flexor muscles, whereas a more extended thigh position, indicates an increased hip flexor length. Measurements were performed on both left and right sides and the average used for subsequent analysis. Following hip flexor testing, kinematic data were collected at a self-selected walking speed. Trunk flexion angles was then calculated, and an average calculated over the period 15-25% of stance as this corresponds to the period of peak knee loading. Independent t-tests were used to compare trunk flexion and hip flexor length between the two groups. A Pearson’s correlational analysis was then used to understand the association between trunk flexion and hip flexor length in each of the two groups separately. Results: Individuals with knee OA demonstrated a more positive Thomas test (p<0.01), mean (SD) = 6.2 (4.2)° compared to the control subjects, mean (SD) = 1.9 (3.7)°, indicating shorter hip flexor muscles in the group with knee OA. The Individuals with knee OA were also observed to walk with 2.8° more trunk flexion (p<0.05). Interestingly, there was a moderate-strong correlation (r=0.67) between hip flexor muscle length and trunk angle during walking in the group with knee OA and a weak-moderate correlation in the healthy group (r=0.45). Conclusions: These data support the idea that people with knee OA exhibit increased muscular restriction of the hip flexor muscle group in static testing and that they walk with more trunk flexion. These data also support the idea that a reduction in the length of the flexor muscles may be a biomechanical driver of increased trunk inclination in people with the disease. Taken together, these findings motivate future research to understand whether interventions that specifically target hip flexor length could lead to reductions in trunk flexion during walking. Such reductions in trunk flexion may lead to changes in hamstring activation patterns and corresponding reductions in the compressive forces at the knee.