Correlation between EMG-based co-activation measures and medial and lateral compartment loads of the knee during gait.

BACKGROUND: Inappropriate tibiofemoral joint contact loading during gait is thought to contribute to the development of osteoarthritis. Increased co-activation of agonist/antagonist pair of muscles during gait has commonly been observed in pathological populations and it is thought that this results in increased articular loading and subsequent risk of disease development. However, these hypotheses assume that there is a close relationship between muscle electromyography and force production, which is not necessarily the case. METHODS: This study investigated the relationship between different electromyography-based co-activation measures and articular loading during gait using an electromyography-driven model to estimate joint contact loads. FINDINGS: The results indicated that significant correlations do exist between selected electromyography-based activity measures and articular loading, but these are inconsistent and relatively low. However despite this, it was found that it may still be possible to use carefully selected measures of muscle activation in conjunction with external adduction moment measures to account for up to 50% of the variance in medial and lateral compartment loads. INTERPRETATION: The inconsistency in correlations between many electromyography-based co-activation measures and articular loading still highlights the danger of inferring joint contact loads during gait using these measures. These results suggest that some form of electromyography-driven modelling is required to estimate joint contact loads in the tibiofemoral joint.

Muscle and external load contribution to knee joint contact loads during normal gait.

Large knee adduction moments during gait have been implicated as a mechanical factor related to the progression and severity of tibiofemoral osteoarthritis and it has been proposed that these moments increase the load on the medial compartment of the knee joint. However, this mechanism cannot be validated without taking into account the internal forces and moments generated by the muscles and ligaments, which cannot be easily measured. Previous musculoskeletal models suggest that the medial compartment of the tibiofemoral joint bears the majority of the tibiofemoral load, with the lateral compartment unloaded at times during stance. Yet these models did not utilise explicitly measured muscle activation patterns and measurements from an instrumented prosthesis which do not portray lateral compartment unloading. This paper utilised an EMG-driven model to estimate muscle forces and knee joint contact forces during healthy gait. Results indicate that while the medial compartment does bear the majority of the load during stance, muscles provide sufficient stability to counter the tendency of the external adduction moment to unload the lateral compartment. This stability was predominantly provided by the quadriceps, hamstrings, and gastrocnemii muscles, although the contribution from the tensor fascia latae was also significant. Lateral compartment unloading was not predicted by the EMG-driven model, suggesting that muscle activity patterns provide useful input to estimate muscle and joint contact forces.

Evaluation of different analytical methods for subject-specific scaling of musculotendon parameters.

Musculoskeletal models are often used to estimate internal muscle forces and the effects of those forces on the development of human movement. The Hill-type muscle model is an important component of many of these models, yet it requires specific knowledge of several muscle and tendon properties. These include the optimal muscle fibre length, the length at which the muscle can generate maximum force, and the tendon slack length, the length at which the tendon starts to generate a resistive force to stretch. Both of these parameters greatly influence the force-generating behaviour of a musculotendon unit and vary with the size of the person. However, these are difficult to measure directly and are often estimated using the results of cadaver studies, which do not account for differences in subject size. This paper examined several different techniques that can be used to scale the optimal muscle fibre length and tendon slack length of a musculotendon unit according to subject size. The techniques were divided into three categories corresponding to linear scaling, scaling by maintaining a constant tendon slack length throughout the range of joint motion, and scaling by maintaining muscle operating range throughout the range of joint motion. We suggest that a good rationale for scaling muscle properties should be to maintain the same force-generating characteristics of a musculotendon unit for all subjects, which is best achieved by scaling that preserves the muscle operating range when the muscle is maximally activated.