Formulation and exploration of novel, intramuscular pressure based, muscle activation strategies in a spine model

Optimization models are often devised to assess spinal stability via estimating individual muscle forces. However, neglecting muscles' fluidic behavior remains an approximation due to the role of muscle pressure in force transmission. The purpose of this study was to leverage a validated Finite Element (FE) model of the spine, inclusive of Intra-Muscular Pressure (IMP), to explore muscle activation strategies towards maintaining equilibrium spinal stability. Three conventional strategies governing minimizing muscle effort, minimizing IVD compressive forces, and maintaining stability at all costs were first investigated to explore model's validity. Thereafter, two novel IMP-based strategies were devised and explored, specifically minimizing and maximizing IMP. The model was previously shown valid in light of in vivo and in silico observations with an average discrepancy of 6%. This being the case, the conventional strategies dictated efficacy in muscular activations whilst maintaining an equilibrium stable position, as quantified in the present paper, with a difference of 9.8% from documented data. In addition, the explored novel IMP-based strategies suggested the presence of a threshold individual muscles IMP, approximately 272 mmHg for the longissimus muscle for example, beyond which muscles potentially start to share radial loads with surrounding tissues, whilst limiting the contraction of the underlying muscles. In conclusion, this study theoretically supports the possibility of activation strategies based on muscular pressure, which the developed, verified, and validated FE spine model was leveraged to investigate. The explored novel IMP-based strategies may have significance in informing clinical applications such as motion analysis and functional electrical stimulation of muscles.