Humans are living longer than ever – an average of 40 years more than just a couple centuries ago, raising the question of how our quality of life can be preserved into old age. Scientists have found that only about 25 percent of the variation in human longevity is due to genetic factors, which means other elements — like how much we exercise or what we eat — play a large role in driving how we age.

To better understand the link between exercise and aging, biomedical researcher Stephen Harridge of King's College in London and his colleagues looked at whether changes in the characteristics of skeletal muscle result from the aging process itself or the affects of lifestyle choices that are strongly associated with aging — like the tendency to adopt a more sedentary lifestyle. Because of the latter, the current perception of muscle structure and age come from studies in people whose physical activity is low. In these individuals, a number of characteristics "typical" of older muscle have been identified, such as changes to muscle fibers, but few studies have gone as far as to investigate the underlying cause of those changes.

Harridge and his group recruited 125 male and female amateur cyclists aged 55-79, who had been cycling regularly for about 26 years. The scientists predicted that since these athletes would have a similar level of high physical activity, any changes in their muscle as they aged could not be explained by inactivity, but rather than an inherent aging process. 

Biopsies were taken from one of the large muscles at the side of the thigh that we use during cycling — the vastus lateralis. Part of the biopsy was cut into thin slices so that its structure could be assessed under a microscope, and part of it was mashed up into a liquid so that its protein content could be calculated.

To investigate how the structure of muscle differs between cyclists aged 55 - 79 years of age, Harridge needed a set of easily measurable, clear muscle properties: capillaries, muscle fiber, and mitochondria, the highly specialized structures within our cells that use oxygen to make energy.

Capillaries supply the muscle with blood carrying oxygen, which is needed to make energy for muscles to function. Endurance exercise improves the delivery of oxygen to the muscle through increasing the number of capillaries that supply it. When the number of capillaries in the muscle biopsies were evaluated, Harridge and his colleagues discovered that there was no relationship between the age of female cyclists and capillary density, but there was a reduction in capillary density with increasing age in male cyclists. This finding, published recently in the journal Aging Cell, was one of the most important in the study, because it may prove that this change is affected by the aging process, and not because of the interaction between aging and inactivity.

Unlike capillary density, the effect of aging on muscle fiber composition was not as closely linked. Skeletal muscle is composed of two broad fiber types; slow twitch (type I) fibers, which contract slowly and allow us to carry out endurance activities like long-distance running, and fast twitch (type II) fibers, which contract in quick bursts, fatigue rapidly, and are used for power activities like sprinting or weight lifting. Older muscle tends to have smaller type II muscle fibers — partly explaining why elderly people find if difficult to make fast sudden movements. 

The main characteristic of type I muscle is that it uses oxygen, and therefore, it is not surprising that there were more type I fibers in the vastus lateralis of the younger male and female cyclists who participated in this study compared to type II fibers. Harridge and his colleagues discovered that this ratio of more type I to type II fibers stayed the same as cyclists aged. The scientists found that mitochondria, which function better in younger master athletes, also did not change with age among their study participants. These findings suggest that using our muscles to exercise into old age prevents them from deteriorating and maintains their function.

However, one important question still remained for Harridge and his colleagues: How these properties actually relate to the physiological functions that allow us to perform physical exercise — in this case, cycling.

In 2015, Harridge and his lab carried out another study on the same group of cyclists focusing exclusively on physiological functions, those physical mechanisms relevant to endurance and explosive muscle function, and found no strong relationship between them.

Endurance exercise is dependent on aerobic metabolism — the way the body generates energy by burning carbohydrates, proteins, and fats in the presence of oxygen — which can be measured by VO2max, the maximum amount of oxygen a person can use during intense exercise. Back in 2015, the scientists measured VO2max during a continuous progressive exercise test on a cycle ergometer. Harridge and his group discovered that the greater the proportion of type I to type II fibers the participants had, the better their VO2max. They also found that VO2max linearly increased with capillary density in males. In contrast to endurance exercise, explosive physical activities engage fast twitch type II muscle fibers.

To investigate the relationship between type II fibers and the generation of force and high power outputs, the participants were also asked to perform a number of explosive, high-power exercises, such as cycle sprinting and knee extensor exercises. These tests revealed that in male cyclists, type II fiber proportion and size was associated with peak power output during sprints and the rate of force development during maximum voluntary contraction, when extending the knee.

By combining findings from the two studies, Harridge and his colleagues were able to show that there is no age-related decline in selected properties of the vastus lateralis that are relevant to aerobic function and explosive muscle power. Instead, these factors are more influenced by a person's level of activity regardless of age.

While the general benefits of exercise are indisputable, there are a number of important caveats to consider before deciding to become a triathlete in the quest for eternal youth. The age range studied was relatively narrow, and participants already cycled regularly. Therefore, an important question that remains unanswered is whether or not it would be sufficient to take up cycling later in life. 

They also only studied one type of exercise. Could it be the case, for example, that some muscles "age" while others do not depending on the type of frequent physical activity? Indeed, the findings from Harridge's study differ from other studies looking at sprinters, endurance runners, and swimmers or just older people who regularly take part in moderate to vigorous physical activity of different ages. One study showed that in sprinters aged 18-84, for example, an age-related loss of type II fast twitch fibers was observed, suggesting that caution should be taken before concluding that any physical activity can stave of age-related changes in the characteristics of skeletal muscle in old age.