One of the most remarkable findings to emerge from medical science in recent years is the strong association that exists between age-related declines in hand grip strength and host of other indices of health and well-being. In recognition of its power to predict health outcomes that only become evident several decades later, Richard Bohannon has described maximum grip strength as a “vital sign” for middle-aged and older adults. It is perhaps natural to assume that a diminution in grip force simply reflects changes in physical capacity arising from the weakening of muscles with age. Indeed, low grip strength is an element of most clinical classifications of frailty, which also include markers such as slow walking speed and low levels of physical activity.
An indication that such assumptions are simplistic is provided by the additional observation that there is a very clear relationship between maintenance of grip strength and preservation of cognitive function. As Ian Deary has put it, “People who grip better, think better.” In striking contrast, there is no evident relationship between cognitive function and factors such as muscle mass/quality that account for some of the variations in strength that distinguish individuals. At first glance, the absence of an association between muscle status and cognitive function when one exists between grip strength and cognitive function may appear curious. It does, however, serve to highlight the weakness of our usual suppositions about the nature of grip strength.
If we each consider our own circle of acquaintances, we could doubtless rank those of our own age in terms of their probable physical strength. In doing so, we would likely use clues such as their height and weight, the apparent girth of their muscles, and so on. There are other factors not visible to the eye, such as muscle fiber type (which, in Olympic sprinters, differs from that of world-class marathon runners), which also determine differences in strength between individuals. It is, however, fallacious to assume that the factors that dictate variations across individuals of a given age are the same as those which account for changes in the strength of each person over the course of their lifetime. Indeed, by and large, the declines in grip strength that are experienced as we age are not due primarily to changes in the state of our muscles.
It is not unusual — even in medical and scientific texts — to see tests of maximum grip strength described as “very simple tasks that only involve one muscle group.” This is far from the truth. Charles Darwin recognized that the capacity of the human hand for prehensile movement is among the most highly-evolved functions in the animal kingdom. While the structure of the hand is fixed, by engaging the muscles that control its articulations in myriad ways, an astonishing variety of actions may be performed. Even young children can be threading the eye of a needle one moment and swinging from monkey bars the next. The extraordinary power, subtlety, and flexibility of the human hand is realized through the brain’s exquisite control of muscle activation.
If we are aiming to generate maximum force — for example, during a test of grip strength — it might be assumed that the muscles that flex our fingers would be activated by the central nervous system (CNS) to the greatest degree possible. This is not, however, what occurs. There is no natural action in which a single muscle is engaged in isolation. It is always the case that the CNS recruits groups of muscles acting in concert to achieve the intended outcome. These muscle collectives are referred to as synergies. In the context of a synergy, each muscle plays its part in a precisely orchestrated ensemble piece. In practice, this means that in seeking to generate maximum grip force, some muscles act to stabilize the position of the hand, while others orient the fingers with respect to one another.
Even muscles with actions that extend, rather than flex the fingers, are engaged. The actual force that is generated depends on the ability of the brain to generate a synergy that best balances these various requirements. The implications of this constraint are revealed in various ways. For example, the force that can be applied by each individual finger declines as the number of other fingers that contribute to the grip is increased. When all four fingers are engaged together, the maximum force that can be applied by each finger tends to be about half of that which can be generated when it is used in isolation. This is in spite of the fact that the intrinsic capacity of the muscles that move the fingers remains constant. It has been established that the limiting factor is neural, rather than perhaps being due to the mechanics of the hand. In other words, the CNS simply does not activate the muscles to their maximum degree. The fact that the magnitude of this “multi-finger force deficit” is larger in older persons than in the young suggests that we should look to the brain, rather than to brawn, in seeking to account for age-related declines in grip strength.
Among the many strands of evidence illustrating that individual changes in grip strength over time can be determined to a large degree by the sufficiency of brain rather than muscle function, two are particularly compelling. The first concerns studies in which the purpose has been to increase the muscle mass of older persons, with the intended objective of increasing their capacity to engage in everyday tasks. A typical approach is to provide testosterone replacement therapy for a period of several months. Although this enhances lean body mass and reduces fat mass, it fails to bring about an accompanying increase in grip strength. The second source of evidence is derived from experiments showing that substantial increases in grip strength can be achieved in the absence of changes in muscle.
This is not new information. It has been known since demonstrations by Edward Scripture in the Yale Psychological Laboratory toward the end of the nineteenth century that grip strength can be enhanced solely through training performed by the opposite limb. In such circumstances, there is no alteration in the state of the muscles of the untrained limb. Since the observed increase in grip strength is therefore entirely unrelated to muscle mass/quality, it must be concluded that the CNS, in particular, acting via enhanced control of muscle activation by the brain, is the primary agent of change.
Once it is recognized that the factors that explain differences in grip strength between people are not necessarily those that determine the changes that occur during the lifetime of an individual, an entirely new interpretation of the associations between grip strength and various indices of health and well-being is indicated. In particular, given its determining role in coordinating muscle activation, it is to the integrity of the brain that we must look for an explanation. In short, individual changes in grip strength are likely to be a marker of brain health. Indeed, it is only on this basis that it becomes possible to explain the relationship that exists between maintenance of grip strength and preservation of cognitive function.
Since the advent of techniques such as magnetic resonance imaging (MRI), it has been feasible to establish the areas of the brain and their connecting tracts that play a role in regulating various aspects of human behavior. As a consequence, it is now apparent that the coordination of muscle activation and many facets of cognition depend on the same brain networks. This is an important finding, as neural degeneration does not affect the brain in a random way. Rather, it proceeds in an orderly and sequential manner, altering brain networks that regulate closely related functions. It is therefore natural that the neurodegenerative processes that are a feature of aging should have a corresponding effect on our grip strength and our mental faculties. Indeed, to the extent that the integrity of the brain depends ultimately on cardiovascular sufficiency and the status of other organ systems, the power of grip strength to serve as a more general marker of good health – a vital sign for middle-aged and older adults, can also be better understood. For further reading, see: (https://doi.org/10.1016/j.neurobiolaging.2018.07.023).