Manual material handling activities that involve forward bending and lifting have been identified as risk factors for the development of low back pain, due to the spinal loads and postures experienced during these tasks. Several activities of daily living, such as lifting light-to-moderate objects, gardening, and cleaning, require forward bending and lifting. Many of these tasks can be performed with one hand, therefore allowing for trunk support by placing the free hand on the ipsilateral thigh. This “braced armto-thigh technique” (BATT) could especially benefit individuals with low back pain (LBP). However, the BATT has not been evaluated biomechanically in this specific population, and has not been evaluated when applied to tasks other than lifting. The overall goal of this thesis was to evaluate the effect of a bracing force, applied by the hand on the ipsilateral thigh, on lumbar spine loading and trunk kinematics for symmetrical and asymmetrical bending and lifting tasks, using a newly developed and validated full-body musculoskeletal model with a detailed lumbar spine.

In Study 1 (Chapter 4), an OpenSim full-body model was developed and validated by adapting an existing OpenSim jogging model to be suitable for lifting motions. Muscle activations predicted by the resulting Lifting Full-Body (LFB) model were directly compared to muscle activations measured with electromyography (EMG), during various lifting tasks. Good agreement, both with respect to pattern and timing, was observed for the back musculature. Comparison between model estimates of intradiscal pressures (IDP) and in vivo IDP measurements also showed strong agreement. The spinal loads estimated by the model matched the trends reported for vertebral body replacement (VBR) measurements in older individuals for similar lifting tasks. This study demonstrated that the LFB model is suitable to evaluate changes in lumbar loading during symmetrical and asymmetrical lifting.

In Study 2 (Chapter 5), trunk kinematics and L4/L5 spine loading for the BATT were compared to those of three common unsupported two-handed and one-handed lifting techniques for two loading conditions (2 kg and 10 kg), in 20 healthy participants (30-70 years old) matched in age and gender to 18 participants. The thigh bracing force, measured by a load cell secured to the thigh with a custom apparatus, significantly reduced L4/L5 extension moments, compressive and antero-posterior (AP) shear forces, compared to unsupported lifting techniques. However, the BATT technique also increased asymmetrical L4/L5 moments and trunk angles.

In Study 3 (Chapter 6), the BATT was adapted to three activities of daily living (ADLs) to understand the effect of thigh bracing on lumbar loading and spine kinematics in tasks other than lifting. These three tasks, namely weeding (gardening), reaching for objects in low cupboards, and car egress, were simulated in the laboratory, using custom apparatus, by ten healthy young males. The BATT reduced L4/L5 extension moments, compressive and AP shear forces compared to self-selected techniques.

This thesis presents the first validated full-body OpenSim model suited to estimating lumbar spine loading in symmetrical and asymmetrical lifting tasks, with or without external loads. Using this LFB model, it was demonstrated that the BATT reduces lumbar extension moments, compression and AP shear forces for lifting tasks and other ADLs, compared to unsupported techniques, for healthy and LBP populations.