Characterizing healthy knee symmetry using the finite helical axis and muscle power during open and closed chain tasks

Abstract

Understanding healthy joint movement and muscle control, and injurious alterations, is important to determine musculoskeletal contributions to post-injury joint instabilities or altered dynamic joint function. The contralateral limb is often used as a point of reference to determine the effects of knee joint injury. However, it is currently difficult to interpret within subject variability between limbs as this is not well established in the healthy population. There is a continuing need to characterize healthy knee joint mechanics and neuromuscular control to determine the degree of symmetry within healthy individuals. The current study quantified limb symmetry in healthy individuals using the finite helical axis with a unique reference position (rFHA) and electromyography (EMG) approaches, for a closed-chain single leg squat (SLS) and an open-chain seated leg swing. Muscle power and FHA translation, orientation and dispersion were similar between limbs. However, the FHA was located significantly more anterior in the dominant limb relative to the contralateral during both tasks. These between-limb differences in FHA location could be attributed to differences in joint geometry and strength between limbs. This finding provides evidence that healthy knees have asymmetries which have implications for selection of control limbs in studies comparing conditions within and between individuals. Differences identified in dynamic joint function between tasks suggest that the SLS is useful for revealing joint asymmetries due to altered muscular control strategies, while the swing task is expected to highlight asymmetries in joint motion due to altered knee structures following injury.

Introduction

Rupture of the anterior cruciate ligament (ACL) results in a 10 fold increased risk for developing post-traumatic osteoarthritis (PTOA) (Anderson et al., 2011). Altered joint dynamics are speculated to contribute to PTOA development (Andriacchi et al., 2004). Changes in knee joint motion in ACL deficient (ACLD) individuals are likely influenced by injury mechanism, extent of tissue involvement, joint geometry and neuromuscular control strategies. Consequently, quantifying small biomechanical changes in such a heterogeneous group requires appropriate data normalization approaches. Intra-subject normalization is frequently conducted with respect to the individual’s contralateral limb. This approach enables quantification of limb symmetry while accounting for subject specific variation. However, typical variation in inter-limb symmetry in the healthy population is not well established. Consequently, it is currently difficult to determine whether observed within subject variability between limbs is classified as “within a typical healthy range” or “abnormal”. Clearly, there is a continuing need to characterize healthy knee joint mechanics and neuromuscular control to determine the degree of symmetry within healthy individuals.

Typical descriptions of knee kinematics are based on Cardan/Euler or Joint Coordinate System approaches that constrain the description of joint motion to pre-defined planes (Grood and Suntay, 1983). These approaches enable clinically meaningful descriptions of movement within standard planes of motion. However, changes in dynamic knee function and stability typically involve interactions of rotational and translational components across planes of motion, which are challenging to quantify with such constraints. An alternative solution is the finite helical axis (FHA), which enables simultaneous description of joint rotations and translations. The FHA approach utilizes a single axis that moves as the joint articulates, where tibial motion is described as a translation along and a rotation about a moving three-dimensional (3D) axis embedded in the femur (Bull and Amis, 1998). Using a unique reference position approach (rFHA), the FHA may be defined at each timepoint providing continuous representation of joint motion (Bishop et al., 2018). Errors associated with traditional FHA approaches have been substantially reduced using the rFHA approach with subject-specific segment coordinate systems (based on magnetic resonance (MR) imaging derived femur and tibia models) (Bishop et al., 2018). Small differences in 3D dynamic knee joint function (involving combined rolling and sliding) can be uniquely captured with the rFHA. Standard deviations for FHA measures (averaged across trials in a healthy participant) are as follows: location x: 0.44–0.93 mm; location y: 0.92–1.26 mm; orientation: 0.59–2.08°; translation: 0.38–0.92 mm; and dispersion: 0.07–0.51° (Bishop et al., 2018). FHA location describes the position of the FHA within the joint. FHA translation and dispersion describe the linear and angular movements of the FHA throughout a dynamic task, respectively. Consequently, the rFHA approach provides powerful measures to characterize dynamic joint function based on overall position (location and orientation), rotation (dispersion) and translation of the FHA throughout a movement.

The extent of inter-limb joint symmetry is speculated to be dependent on the movement task. Many studies have described the mechanical and muscular constraints of the knee and alterations with injury during gait (e.g. Andriacchi and Dyrby, 2005, Gao and Zheng, 2010, Georgoulis et al., 2003, Yim et al., 2014). While gait is important for daily activities, it is a relatively low demand activity with minimal rotational loading on the knee joint and may not pose sufficient demands on the musculoskeletal system to elucidate potential joint asymmetries in healthy individuals. Specific open and closed chain movement tasks emphasizing the mechanical function of knee structures and contributions of muscle in joint movement control may better reveal joint asymmetries. The closed-chain single leg squat (SLS) is a commonly used task to assess lower limb movement quality in the clinical setting (Ressman et al., 2019) and to assess the risk of sustaining an acute lower extremity injury (Räisänen et al., 2018). Co-contraction across quadriceps and hamstring muscle groups during the SLS are important to provide knee stability and reduce anterior shear force on the joint (Dedinsky et al., 2017, Henning et al., 1985, Lutz et al., 1993, Palmitier et al., 1991, Yack et al., 1993). The SLS is therefore expected to highlight asymmetries in knee movement due to differences in muscular control. Conversely, the open-chain seated leg swing does not require bodyweight support and reportedly results in decreased muscle co-contraction and increased anterior tibial translation relative to closed-chain exercises (Lutz et al., 1993, Wilk et al., 1996, Yack et al., 1993). This task is ideal for examining the unloaded mechanics of the knee and is expected to reveal asymmetries in movement patterns resulting from differences in joint geometry and passive joint structure properties.

Understanding baseline between-limb differences in joint motion (rFHA) and muscular control (EMG) within these tasks in a healthy population is important to enable the interpretation of differences due to various knee joint conditions in future studies. Furthermore, characterising these tasks in healthy individuals will help inform on their potential utility for investigating alterations in joint motion and control associated with the specific conditions. The primary objective of this study was to assess intra-subject limb symmetry in healthy individuals during the SLS and seated leg swing using the rFHA and EMG. A secondary study objective was to identify differences in joint motion and muscle activity between the SLS and seated leg swing in healthy individuals, using rFHA and EMG approaches.