Effects of posture and trunk muscle coordination on multi-joint isometric lifting strength: Implications for individualised movement assessment and intervention

Abstract

The manner in which movement is executed can influence biomechanical demand and consequently the development of musculoskeletal disorders. Although modifying movement execution can help regulate biomechanical demand by influencing tissue loading and tolerance, it can also influence physical performance by influencing the ability to exert force. Movement-based interventions to regulate biomechanical demand that hinder physical performance can limit acceptability, sustainability, and effectiveness of the intervention. Conversely, interventions to enhance physical performance that modify movement execution in a way that imposes higher than necessary biomechanical demand, can hinder physical development and long-term performance. However, the development of comprehensive movement assessments and interventions that consider both the impact on biomechanical demand as well as physical performance is challenged by a scarcity of knowledge of how movement execution can influence physical performance in multi-joint tasks. The global aim of this thesis was to investigate how modifying posture and trunk muscle coordination can influence the ability to exert force during isometric lifting. Four studies were conducted to address the global thesis aim. Study 1 investigated the effects of modifying trunk inclination and low back curvature on isometric lifting strength as previous research has yet to clearly dissociate how these distinct postural characteristics could influence the ability to exert force in multi-joint tasks such as lifting from the ground. Results showed that modifying trunk inclination and low back curvature can influence strength in multi-joint tasks substantially and to a similar extent. However, the effect of modifying these postural characteristics interact and vary greatly across individuals in magnitude (up to 620N of isometric lifting force) and direction (increase/decrease). Thus, a single postural profile cannot be generalized as the strongest for every individual and the ability to exert force in multi-joint tasks cannot be inferred solely from posture. Additionally, the individually varying effects of posture on strength suggests that individuals can adapt through movement training to be stronger in postures that are favourable for tissue loading and tolerance. Assessing the L4/L5 joint contact forces of the observed postures suggested that the effect of posture on biomechanical loading is more consistent, and the biomechanical demand imposed by flexing the low back outweighs any potential acute gain in isometric lifting strength. Together this supports the recommendation to avoid highly flexed low back postures during demanding physical activities. Study 2 compared the immediate effects of a simple verbal directive and detailed trunk muscle bracing coaching on isometric lifting strength, low back postural displacement and trunk muscle co-contraction. Prior research has suggested that cueing co-contraction of all the trunk muscles can enhance the ability to exert force in multi-joint tasks but has not yet isolated the effect of modifying trunk muscle coordination on the ability to exert force which may potentially depend on the approach used to cue trunk muscle coordination. Detailed coaching which included a combination of verbal and physical cues was more effective than the simple verbal directive at increasing trunk muscle co-contraction (group mean co-contraction for the baseline, directive and coached condition was 10.1%, 11.0% and 13.5% respectively) and decreasing low back postural displacement (group mean change in low back flexion angle normalized to each individual’s maximum flexion range-of-motion for the baseline, directive and coached condition was 21.4%, 19.5% and 17.2% respectively). However, both cueing approaches immediately reduced isometric lifting strength to a similar extent (group mean peak isometric lifting force for the baseline, directive and coached condition was 1194 N, 1109 N and 1096 N respectively). Results indicate that detailed coaching is more effective than simple verbal directives at modifying trunk muscle coordination to restrict low back postural displacement but cautions cueing trunk muscle coordination for the first time in situations where peak force production is desired. Results also suggest that future research should confirm acquisition and transfer of the targeted trunk muscle coordination patterns as the full potential impact of modifying trunk muscle coordination may not be completely apparent from observing the immediate responses to cues. Motivated by the individually varying effects of posture on the ability to exert force observed in Study 1, Study 3 evaluated the extent to which proxies for leverage derived from kinematic quantities can statistically explain the individual variation in the effects of posture on the ability to exert isometric lifting force. Prior research has used kinematic-based proxies to describe and make inferences about how posture can influence leverage in multi-joint tasks. However, these approaches do not capture all the mechanics involved in multi-joint kinetics and the extent to which they may explain the individually varying effects of posture on the ability to exert force in a multi-joint task has not yet been tested. As expected, based on fundamental biomechanical principles, the kinematic-based proxies for leverage that were investigated (joint-to-external force moment arms and predicted joint-angle-dependent torque-generating capacities) explained a very low proportion of variation (<17%) in the effects of posture on isometric lifting force. In contrast, variables derived from both kinematic and kinetic measurements such as the net joint reaction moments calculated using a rigid linked segment model and inverse dynamics explained a higher proportion of variation in the effects of posture on the ability to exert isometric lifting force (approximately 80%). These results indicate that simplified kinematic-based approaches cannot be used to assess the effects of posture on leverage in multi-joint tasks on an individual basis. Instead, variables derived from both kinematic and kinetic measurements such as the net joint reaction moments show promise for being used in development of quantitative assessments of multi-joint leverage. Study 4 investigated whether individuals maintain their potential for physical performance when given instruction to avoid rounding the low back during light mass lifting. Compared to prior movement-based interventions such as the squat lift technique that vaguely describes whole body posture, targeting a key postural feature such as low back flexion is theoretically expected to afford greater flexibility to self-organize the rest of the body linkage to regulate biomechanical demand, without hindering physical performance. However, using simple verbal directives to cue specific movement features during low demand tasks may not acutely prompt individuals to prioritize physical performance as they self-organize, rendering the intervention ineffective. Although aggregate group level results indicated that low back postural instruction targeting biomechanical demand decreased low back flexion during crate lifting and increased isometric lifting strength in postures replicating those exhibited during crate lifting, there was high heterogeneity in responses. Among the 37 of 40 participants classified as individuals who could potentially benefit from decreasing their low back flexion, 15 and 22 participants were respectively classified as successful and unsuccessful in decreasing low back flexion in response to the instruction, to be within a range that minimizes passive tissue strain. Though replicating the crate lifting posture to assess isometric strength emerged to be challenging, there were individuals who increased (n=8), decreased (n=9) and did not change (n=23) isometric strength, independent of their low back postural response. Hence, although most participants appeared to maintain physical performance potential when given simple verbal directives that target low back flexion to regulate biomechanical demand, some participants responded in a manner that decreased performance potential and many did not successfully decrease their low back flexion within a range that minimizes passive tissue strain. This suggests a need for more detailed movement coaching and training in research and practice to effectively modify movement behaviour without hindering physical performance potential. Overall, the results from this thesis indicate that the effects of modifying posture and trunk muscle coordination on the ability to exert force in multi-joint tasks is complex and can greatly vary across individuals. Additionally, modifying movement execution may need to go beyond simple verbal directives to provide detailed movement coaching. The findings support that considering the effects of movement execution on physical performance has the potential to advance movement assessment and intervention strategies. Yet, there is a need to develop approaches to capture the individually varying effects of movement features such as posture on the ability to exert force in multi-joint tasks to develop strategies that can effectively regulate biomechanical demand without hindering physical performance as well as enhance physical performance without imposing unnecessary biomechanical demands.