Assessment of neuromuscular function and fatigue in different elderly populations

The age-related loss of muscle strength, namely dynapenia, is a condition resulting from a loss of muscle mass, motor control impairments and/or innervation problems (activation inhibition), which can be exacerbated by ongoing pathological conditions such as osteoarthritis or musculoskeletal disorders. As the population ages, the need for effective treatments and (surgical and non-surgical) interventions become more apparent. However, before any such measure can be implemented, a deeper understanding of the mechanisms behind dynapenia is necessary. This thesis was carried-out within three research projects focused on the investigation of the mechanisms underpinning the loss of both muscle force and neuromuscular function in the elderly (with a specific focus on arthritic patients), and conducted at the IRCCS Rizzoli Orthopaedic Institute (Bologna, Italy) and at the LISiN laboratory (Torino, Italy). While the three studies shared the methodology, as dynamometry and electromyography data were collected for all subjects, the aims and specific instrumentation and techniques varied. In the two studies conducted in Bologna, the subjects (20 healthy young adults, 5 elderlies and 12 individuals elected for total knee arthroplasty) performed maximum voluntary isometric contractions on a dynamometer, followed by superimposed neuromuscular electrical stimulation trial, and underwent – on a separate day – a magnetic resonance of the lower limbs. The third study, conducted in Turin, focused on the assessment of fatigue mechanisms during dynamic contractions. High-density EMG data were collected on a total of 121 subjects (40 healthy elderly – 76.80 ± 4.45 y.o. –, 55 middle-aged – 59.38 ± 9.01 y.o. – and 26 trained old subjects – 70.19 ± 3.42 y.o. –). A semi-automatic method was implemented to segment each contraction from the dynamometer data of each trial. In general, elderly individuals showed reduced muscle volumes and muscle strength compared to healthy young(er) adults (i.e., the quadriceps muscles were smaller in size: 1482.2 ± 127.0 cm2 vs 1673.7 ± 475.1 cm2). Nonetheless, all participants showed low levels of muscle co-contractions and minimal voluntary activation deficits (quantified comparing voluntary and involuntary contractions), suggesting a substantially unimpaired muscle control, independently on their age. The fatigue assessment, conducted on the 121 subjects who performed two sets of dynamic contractions (40 maximal knee extensions each set), showed that ageing negatively impacted on muscle power (66.0 ± 10.9 W), but that training is beneficial as trained old individuals performed better (114.5 ± 12.6 W) than middle-aged healthy participants (89.55 ± 9.5 W). Mechanical and myoelectric fatigue manifested similarly, with a reduction of power output accompanied by a decrease of RMS and an increase of MNF values within all contractions. However, trained old individuals recovered a higher power percentage between the two sets of contractions (35.2 ± 5.0%) than elderly (15.3 ± 4.6%) and even than middle-aged (22.8 ± 5.0%), confirming the effectiveness of the training. Similar results, i.e. better recovery in trained elderly, were obtained for the myoelectric manifestations of fatigue. The methodologies implemented in the cross-sectional study provide a framework for the longitudinal assessment of changes in neuromuscular activation and fatigue in elderly during static and dynamic contractions.