Mechanisms of Disuse Atrophy in Human Skeletal Muscle (iMOB) (iMOB)

Harnessing Muscle-specific Atrophy Susceptibility to Disentangle the Mechanisms of Disuse Atrophy in Human Skeletal Muscle Atrophy (iMOB)

Loss of muscle can be caused by a variety of stimuli and results in reduced mobility and strength and also impacts whole body health. Whilst it is known that muscles waste the process by which this occurs is not well understood. Furthermore, whilst some muscles waste quickly others seem resistant to the effects of disuse. This study aims to evaluate how quickly changes in muscles start to occur, and investigate the processes which underlie muscle atrophy. By studying muscles which waste quickly and those which are resistant to atrophy this study aims to identify the different processes which lead to muscle loss. This study will also evaluate the differences in muscle changes between young and old people.

Skeletal muscles host ~40% of all protein in the body. Muscles are not only crucial for locomotion but also represent the body's largest metabolically active tissue, glucose disposal site and fuel reservoir for other organs in pathological conditions (i.e., supply of amino acids to the liver for gluconeogenesis). Muscle atrophy is characterized by a reduction in cross sectional area (CSA) and length and occurs in many common illnesses (e.g. cancers (1), renal/heart failure, sepsis, genetic diseases, neurodegenerative disorders etc). It is also prevalent in situations of reduced neural input such as leg casting after fractures (2), bed-rest, spinal cord injury (3), space flight and chronic physical inactivity. Atrophy results in a loss of muscle power and strength (which is related to increased morbidity and mortality (4)) and reduced capacities for whole-body glucose storage and metabolism which causes insulin resistance. Strategies to oppose atrophy are limited but include mechanical loading (5) and the synergistic anabolic effects of nutrients. Although muscle atrophy is of great clinical importance, relatively little mechanistic research has been done in humans. Thus, the aim of this study is to assess the link between the variation in muscle physiological responses to disuse atrophy with variation in protein turnover and molecular-networks. This will not only provide new hypotheses for physiological regulation of human muscle and generate 'intervention targets' derived from clinically relevant human studies, it will also improve understanding of whether the response to disuse is altered with age and determine if mechanistic differences in atrophy resistant and atrophy sensitive muscles might explain inter-muscular variation in susceptibility to atrophy. This study aims to define the molecular and metabolic mechanisms causing disuse atrophy in both young and older individuals and explore how and why some muscles are protected against it. The study will also assess temporal aspects of disuse atrophy (in younger individuals only) to explore the mechanistic basis for the more rapid atrophy observed in the early days of disuse.

The dominant leg of young healthy patients (18-40 years without serious comorbidities) will be immobilised using a fixed knee brace and aircast boot for 15 continuous days

The dominant leg of young healthy patients (18-40 years without serious comorbidities) will be immobilised using a fixed knee brace and aircast boot for 5 continuous days

The dominant leg of aged patients (65-80 years without serious comorbidities) will be immobilised using a fixed knee brace and aircast boot for 5 continuous days

MRI assessment of muscle volume in Tibialis Anterior (TA) and Medial Gastrocnemius (MG) in immobilised vs non-immobilised leg, pre and post immobilisation

Ultrasound scan (USS) assessment of muscle thickness in Tibialis Anterior (TA) and Medial Gastrocnemius (MG) in immobilised vs non-immobilised leg, pre and post immobilisation

Ultrasound assessment of muscle cross surface area, in tibialis anterior (TA) and Medial Gastrocnemius (MG) in immobilised vs non-immobilised pre and post immobilisation

Ultrasound assessment of muscle fibre length in tibialis anterior (TA) and Medial Gastrocnemius (MG) in immobilised vs non-immobilised pre and post immobilisation

Ultrasound assessment of muscle fibre pennation angle in tibialis anterior (TA) and Medial Gastrocnemius (MG) in immobilised vs non-immobilised pre and post immobilisation

IV Pulse tracers (IV tracers to give muscle specific MPB measures of TA+MG muscles in immobilised vs non-immobilised legs)

contrast enhanced ultrasound (CEUS) assessment of muscle blood flow in immobilised vs non-immobilised legs (TA+MG muscle specific)

Doppler assessment of leg blood flow through common femoral artery in fed and fasted states in both immobilised and non-immobilised leg

Measurement of anabolic signalling pathways by western blot (comparison between immobilised vs non immobilised TA + MG muscles)

Measurement of proteasome and lysosomal and related catabolic signalling pathways by western blot (comparison between immobilised vs non immobilised TA + MG muscles)

complete RNA sequencing of immobilised vs non immobilised TA + MG muscles to determine gene set enrichment and pathway analysis

Morphological assessment of muscle fibres by histological techniques (comparing immobilised vs non immobilised TA + MG muscles)

Measurement of mitochondrial respiration to assess different complex activity in immobilised vs non-immobilised TA + MG muscles

Electrically induced maximum force development and fatigability in TA + MG muscles pre and post immobilisation

Assessment of changes in muscle power secondary to immobilisation through 1 rep max (kg) pre and post immobilisation

Cardiopulmonary Exercise Testing (CPET) to assess changes in aerobic fitness (V02 max, anaerobic threshold and Watt Max) following immobilisation

Inclusion Criteria: Group 1 and 2: Male, Age 18-40, BMI 18-35 Group 3: Male, Age 65-80, BMI 18-35 Exclusion Criteria: BMI > 35 / <18 Female Personal or Family History of Venous Thromboembolism Significant medical comorbidities