By Dr. Geoff Power You may have heard the phrase ‘old man strength’ often used in the context of someone performing a physical task seemingly above the level of performance you would expect for their age. To describe this phenomenon, the urban dictionary has used phrasing such as “The uncanny ability of older men to lift copious amounts of lumber, heavy furniture and beat their sons in arm wrestling”. You may have even overheard or had similar conversations as: “My dad beat me in arm wrestling again”, - ‘of course, he has old man strength”. In this article I will highlight one aspect of the human neuromuscular system which seems to resist the deleterious effects of age, the age-related maintenance of eccentric strength. On one hand, natural adult aging is associated with many ‘negative’ alterations to the neuromuscular system resulting in muscle weakness and impaired functional performance of everyday tasks. Some of these changes include a loss of muscle mass through the death and dysfunctional remodelling of motor units (the basic ‘functional’ contractile unit consisting of the motor neuron housed in the spinal cord and its associated muscle fibres). A reduced ability to activate the remaining muscle mass through a reduction in the fidelity of the neural signals from the from the brain to the muscle. As well, owing to reductions in strength and speed of muscle contraction, power is drastically impaired. Importantly, it is power that is critical to performing daily tasks such as rising from a chair or walking up a flight of stairs. On the other hand, while many of the ‘negative alterations’ to the aging neuromuscular system are evident during static or shortening muscle actions, during lengthening actions (see our previous Knowledge Translation), performance is well-maintained to that of younger adults. A lengthening muscle action (or eccentric contraction) is unique in that the muscle generates an internal tension which is less than the external load, thus the muscle actively lengthens. Eccentric contractions are part of everyday movements such as walking down a flight of stairs, and older adults have elevated performance (relative to other contraction modes) and in some instances equal that of a healthy young adult. Figure: The above graph shows the age-related loss of strength during 3 distinct contraction types relative to a young adult: Static (isometric), shortening (concentric), and lengthening (eccentric). The largest deficits occur over time for the shortening contractions, which is a proxy of power. Static contractions show a similar but less drastic decline, and Eccentric contraction strength is well-maintained far into old age. Some of the suggested mechanisms contributing to a maintenance of eccentric strength in old age include: 1) An increase in non-contractile tissue infiltration such as collagen content/cross-linking. This means less contractile tissue is contributing to force during static and shortening contractions, impairing performance when the muscle is responsible for generating tension. However, more structures now ‘resist’ the external load during the lengthening (eccentric) contraction, increasing performance. The elevated passive stiffness of muscle in old age could also increase the effective ‘storage’ of elastic energy to optimize force production. 2) Eccentric contractions require less voluntary neural activation, meaning there is less of an impairment during lengthening contractions compared to static and shortening contractions. 3) There is an overall slowing of cross-bridge kinetics and increased proportion of weakly bound cross-bridges in old compared to young muscle. As active force generation depends on the number of strongly bound cross-bridges, any reduction or weakening of these cross-bridges will lessen active force generation. However, when external tension is applied, as in the case of a lengthening contraction, this cross-bridge configuration is optimized to resist active lengthening, and generate high forces. Work from our lab investigated the effects of slowing muscle cross-bridge kinetics, and biasing the cross-bridge configuration to more weakly bound states on the age-related maintenance of eccentric strength. We found that we could make young muscle act like old muscle when the muscle temperature was reduced to cause a slowing of muscle contraction. Therefore, if we look to interventions to reverse some of the age-related slowing of muscle contractile properties, we may identify strategies to improve power and performance during activates of daily living. Figure: The relationship between muscle relaxation speed (HRT) and the ratio of eccentric to isometric force. There was a strong relationship between muscle speed and the maintenance of eccentric strength such that individuals with slower muscle properties had a greater eccentric to isometric force ratio. To resist the aging process, there may be some truth in just being stubborn to the natural degeneration of the neuromuscular system. “It's called stubborn-ness. He's more stubborn to push the weights against gravity than gravity is in pulling them down. We should all be so stubborn.” Either way you look at it, aging of the human neuromuscular system is a non-linear process and there are some impressive mechanisms behind the scenes fighting to maintain function and independence into old age.
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September 2023
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