Healthy ageing is associated with the loss of muscle mass and physical function. As a result, older people are limited in their independence. The aging of muscles typically begins around the age of 30. From this age onward, muscle strength, muscle mass, and the maximum oxygen uptake of muscles decrease. The reasons for this are not entirely clear, but it seems to be partly related to how oxygen moves from our blood vessels to the muscles and how muscles burn energy. The precise role of age and physical fitness, as well as whether exercise can counteract the effects of ageing, is still unknown. Therefore, in this study, we aim to investigate the muscle function of both physically active and inactive young and middle-aged individuals. We hypothesise that endurance training can mitigate some of the effects of ageing.

Healthy ageing is associated with a loss of muscle mass and physical function. This loss of physical function is underpinned by reductions in characteristics such as muscle strength, power, and maximal oxygen uptake (V̇O2max; reflecting exercise capacity). However, the causal contributors to these age-associated impairments, and the role of exercise training status in mitigating them, remain poorly defined. Skeletal muscle mitochondrial function has been proposed to be a key contributor to age-associated effects on physical function, however many conflicting results are present in the extant human literature. Moreover, diffusion of oxygen from capillaries to mitochondria is a key determinant of V̇O2max, however, whether the skeletal muscle diffusive capacity for oxygen (DmO2) declines with age is unknown. A new technique utilizing near-infrared spectroscopy (NIRS) will enable the non-invasive assessment of skeletal muscle diffusive capacity in young and elderly subjects for the first time to resolve this issue. The primary aims of this study are therefore to 1) compare DmO2 derived via NIRS between young sedentary, young endurance-trained, older sedentary, and older endurance-trained subjects; 2) to compare non-invasive (i.e. with NIRS and 31phosphorous magnetic resonance spectroscopy [31P-MRS]) and invasive (i.e. measures of mitochondrial morphology and respiration obtained by skeletal muscle biopsy) markers of mitochondrial function between the same groups, and 3) to assess the relationships between DmO2, mitochondrial measures and assessments of capillarization with functional measurements of muscle strength, power, and V̇O2max.

Inclusion Criteria:

In order to be eligible to participate in this study, young sedentary participants must meet all of the following criteria:

• Aged between 18-30 years
• Male or female
• Not currently engaging in any formal exercise training or competitive sports
• No chronic health conditions likely to affect exercise tolerance or the physiological responses to exercise

In order to be eligible to participate in this study, young trained participants must meet all of the following criteria:

• Aged between 18-30 years
• Male or female
• Currently engaging in formal training (at least 3 times per week) in competitive endurance sports
• No chronic health conditions likely to affect exercise tolerance or the physiological responses to exercise

In order to be eligible to participate in this study, older sedentary participants must meet all of the following criteria:

• Aged between 50-65 years
• Male or female
• Not currently engaging in any formal exercise training or competitive sports
• No chronic health conditions likely to affect exercise tolerance or the physiological responses to exercise

In order to be eligible to participate in this study, older trained participants must meet all of the following criteria:

• Aged between 50-65 years
• Male or female
• Currently engaging in formal training (at least 3 times per week) in competitive endurance sports
• No chronic health conditions likely to affect exercise tolerance or the physiological responses to exercise

Exclusion Criteria:

• Age that falls outside of 18-30 years (young groups) or 50-65 years (middle-aged groups)
• Inability to provide informed consent
• History of claustrophobia
• Ineligibility to perform the exercise test described in this study protocol or follow instructions
• Taking any medications known to interfere with the physiological responses to exercise, e.g. e.g. systemic corticosteroids, statins, SGLT2 inhibitors, GLP1 receptor agonists
• Contraindication for MRI (e.g. pacemaker, claustrophobia)
• Being under investigation for non-diagnosed disease at the time of investigation
• Body Mass Index (BMI) >30 due to adiposity, since this is known to cause difficulties in obtaining muscle biopsies and NIRS measurements
• Pregnancy
• Are current smokers or have been a regular smoker within the last 12 months

15 sedentary and 15 endurance-trained young and middle-aged individuals will be recruited per age group (i.e. a total of 60 individuals spanning 4 groups). An equal balance of the sexes will be aimed for in each group. All participants will be recruited from specific social media groups, through personal networks of friends, family and colleagues, flyers and posters at local sports clubs and on the VU Campus. All testing will be conducted at the VU or in the Department of Radiology and Nuclear Medicine at Amsterdam UMC location AMC. Therefore, participants should be currently residing in or willing to travel to Amsterdam.