Relative contributions of neuromuscular factors to muscle strength decline with age
Creators: name: Siddiqi, Ariba Moti
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400305
subject: Biomedical instrumentation
classificationCode: 460207
subject: Modelling and simulation

description: Aging is associated with numerous changes in the human body. One of the most prominent alterations is in the decline of neuromuscular function leading to loss of muscle strength. Muscular weakness in the lower limb muscles is a risk factor for falls in the elderly, with the Tibialis Anterior (TA) playing a pivotal role. This loss in strength has been identified as being due to several age-associated neuromuscular alterations. These alterations can be unique for each muscle, depending on their functional role and composition. Nonetheless, the individual contributions of the altered neuromuscular properties to muscle strength decline remain unclear. A need to delineate these influences has been identified in order to provide a more targeted strength preservation strategy for the elderly.<br><br>This thesis has investigated the relative influence of neuromuscular properties on age-associated muscle strength decline in the Tibialis Anterior using a new improved computational surface electromyogram (sEMG) and muscle force model. It has further used this model to estimate the neuromuscular properties and expected strength loss in the older cohorts from the age-associated sEMG changes. It has also investigated the inhomogeneous aging of the Triceps Surae (TS) in comparison to the Tibialis Anterior using sEMG. <br>A series of experiments were performed with healthy young cohorts (age range: 20-30 years) and older cohorts (age range: 60-85 years). They performed isometric dorsi- and plantarflexion of the ankle at six fractions of their maximal voluntary contraction (MVC), and their exerted force and sEMG from the TA and TS muscles was recorded. The power spectrum (PSD) and bispectrum of the sEMG was computed for each participant.<br><br>This study has found significant age-associated changes in the sEMG’s PSD, and Gaussianity where older cohorts had higher maximal power and increased non-Gaussianity of their sEMG. These changes were attributable to the age-associated physiological motor unit remodelling reported in literature.<br><br>This study has also found the TA to have significantly higher age-associated increase in the maximal power of the PSD in comparison to TS. Within the Triceps Surae complex, the Soleus and Lateral Gastrocnemius were not significantly affected in comparison to Medial Gastrocnemius. These differential changes were considered to be due to different fibre composition, and rates of age-associated neuromuscular changes.<br><br>The simulation studies performed with the sEMG model found the firing rate as the most influential property, followed by muscular factors. The model was also able to estimate the aging condition that described the older cohorts of this study. The older cohorts’ TA were characterised to have moderate motor unit remodelling which corresponded to a 40% loss of motor units with half the number of fast fibres.<br><br>This study highlights that firing rate is the most influential property regulating muscle strength. It also establishes that moderate motor unit remodelling in the TA may not be accompanied with strength loss. This was also found in the TS muscles, which was differently altered to the TA. This information can be used to provide more targeted strength preservation strategies for the prevention of falls in the elderly.