More Studies Are Needed To Understand Why Physical Performance Declines With Age

As we all know, getting older means a decline in physical performance. While the loss of ability in isometric tasks is well documented by science, explanations are less well-known for dynamic tasks. A recent study attempts to provide some answers to this question.

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Physical Exercise

Physical Exercise

As we age, we generally tire more easily during physical exertion. During the aging process, several global changes occur. In the present case, the changes of interest are those that occur in the neuromuscular system (loss of muscle mass, increase in adipose tissue, loss of motor neurons, longer delays in the transmission of action potentials, reduction of sensory afferents, reduction in the volume of motor areas in the brain, etc.).

All these structural and functional changes generally lead to a deterioration in the contractile capacity of the elderly, which ultimately affects their daily lives. Therefore, it is important to understand these changes in order to delay them. A recent literature review published in the Journal of Applied Physiology explores the issue of capacity loss with age.

How do we measure the loss of capacity?

The most commonly used marker in studies trying to unravel the mysteries of age-related capacity loss is fatigue, defined as “the decrease in an objective measure of performance resulting from a reduction in strength (torque, power) of neuron-dependent activity, regardless of whether the task can be performed well.” Given the differences in performance capacity that may exist between different groups of individuals, fatigue is usually expressed as a percentage reduction from an initial performance.

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It is also known that fatigue is not unique. It depends on several factors (the muscle group involved, the type of contraction, the intensity of the task, the continuity or intermittency of the effort, the measurement criteria used, etc.). Therefore, it is easy to understand that it is very difficult to draw general conclusions about the loss of capacity associated with aging and the scientific explanations for it.

The different types of contractions

For the remainder of this article, it is important to familiarize yourself with the different types of muscle contractions. There are two main types of muscle contractions: dynamic contractions (where the size of the muscle varies) and isometric contractions (where the size of the muscle remains constant).

The dynamic state includes two subtypes of contraction, which give us more detail: concentric contraction (the muscle shortens) and eccentric contraction (the muscle lengthens). Finally, within these two main types, there are two other subtypes of contraction that give us information about the variation in applied force and movement speed during a motor task: isotonic contraction (the applied force remains constant) and isokinetic contraction (the movement speed remains constant).

So far, the literature on fatigue in older people has focused mainly on isometric contractions, leaving out more dynamic efforts that require movement and thus specific mobilization of the joint and tendon system. In fact, dynamic tasks are also more representative of the effort we exert in everyday life. Therefore, it is important to better understand the interaction between age and fatigue during dynamic tasks in order to develop targeted interventions to reduce functional impairment and mobility in older people.

What do we know about fatigue during dynamic tasks?

Current literature agrees that older people are weaker, slower, and therefore less vigorous during concentric contractions than younger people. It is known that these decreases in performance are partly due to intrinsic properties of the neuromuscular system (changes in the myosin chain, higher proportion of slow motor neurons, and atrophy of type 2 muscle fibers) and extrinsic properties of the neuromuscular system (reduced tendon stiffness, altered activation of muscle fibers, variability in contraction of the supraspinatus muscle). How can this be translated into dynamic isotonic and isokinetic tasks?

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Fatigue in isokinetic tasks

Current results on isokinetic tasks are contradictory, i.e., some studies suggest differences in fatigue in these tasks between younger and older people, while others do not. In some studies, the authors point to angular velocity as the main cause of greater fatigue in older people, especially in knee extension exercises, but less so in tasks requiring the use of foot dorsiflexors. Taken together, these results call into question the generally accepted assumption that there is a universal loss of speed with age.

Fatigue in isotonic tasks

On the other hand, the results concerning isotonic tasks are similar: older people are less strong than younger people when a constant maximal force is required when different parameters such as the muscle group involved, the variety of movements performed, or the intensity of the exercise are taken into account. However, some authors find that when performing isotonic and isokinetic tasks on the same groups, they find that the angular velocity of the movement changes the capacity to exert force. In other words, it suggests that when the neuromuscular system becomes “slower,” it may predispose older people to greater fatigue in tasks requiring high speed.

The mechanisms potentially involved

As mentioned above, many changes occur in the neuromuscular system during aging, and changes in the central and peripheral nervous systems may be responsible for decreased performance. However, there is still little consensus on the hypotheses in the literature because the results are inconsistent: decreased willingness to initiate an action, changes in sarcomere excitability, slower muscle recovery, electrical changes in contractile cells, prolonged muscle anemia, accumulation of inorganic phosphate, free radicals, ionic changes, changes in oxidative capacity, etc. Many of these hypotheses are based on the fact that the neuromuscular system is not as efficient as before.

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For many of them, these hypotheses are only supported by statistical correlations, without actually identifying the cellular and molecular mechanisms. Therefore, further work is needed to develop and validate new methods to improve our understanding of fatigue during dynamic tasks.

References

Age-related performance fatigability: a comprehensive review of dynamic tasks

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