Why Does Grip Strength Correlate with Working Memory Function in Old Age?
Many aspects of aging are correlated with one another. A simple model of aging as a collection of end results that are produced by the accumulation of forms of biochemical damage to cells and tissues would suggest that all age-related conditions will be at much the same stage in a given individual: it is all a matter of how much damage that individual accumulates over time. Aging isn't that simple, however. While it still arises from comparatively simple root causes, the aforementioned biochemical damage to cells and tissues, each specific consequence of that damage sits within a complicated, interacting network of cause and effect. A consequence can interact with its cause, and with other consequences, and with downstream effects that it causes itself, making them worse, or accelerating their progression. There is plenty of opportunity for feedback loops to form and for specific narrow aspects of aging to race ahead of others in any given individual, or for some parts of this network and consequent age-related conditions to be tightly coupled to one another versus loosely coupled to one another.
So when we ask why grip strength in older people appears to be correlated with working memory, one can start with the idea that perhaps the nervous system is subject to damage that degrades all of its capacities, whether in the memory systems of the brain, or in the innervation and control of muscles. Or perhaps chronic inflammation affects muscle tissue maintenance and the neurogenesis needed for memory through similar effects on the function of stem cell populations in muscle and brain. Or there could be many other reasons why these two aspects of aging are more tightly coupled to one another. In today's open access paper, researchers dig in to specific workings of the brain and muscle in their consideration of the correlation between aging of these two portions of physical function.
Does muscle strength predict working memory? A cross-sectional fNIRS study in older adults
This study investigated the correlation among muscle strength, working memory (WM), and cortical hemodynamics during the N-back task of memory performance, and further explored whether cortical hemodynamics during N-back task mediated the relationship between muscle strength and WM performance. We observed that muscle strength (particularly grip strength) predicted WM of older adults in this cross-sectional study, which validated our hypothesis and expanded on previous research findings. Studies demonstrated that grip strength predicted executive function decline in patients with mild cognitive impairment. Other cross-sectional studies showed that grip strength and lower limb strength also predicted cognitive impairment. Previous research revealed that grip strength was positively linked to cognitive functions such as WM, language fluency, and word recall.
The reason why grip strength predicted working memory might be the control of muscles by the nervous system. Grip strength was influenced not only by muscle volume but also by the central nervous system, conversely, neurologic deterioration not only contributed to cognitive decline but might also be a factor in strength loss. This was consistent with the findings of the present study, where we found that greater muscle strength was associated with higher levels of activation in specific regions of the prefrontal cortex (PFC)/a> and better WM performance. The greater the muscle strength, the stronger the activity of the left dorsolateral prefrontal cortex (L-DLPFC) at a low WM load (i.e., 0-back). At moderate, high WM load (i.e., 1-, 2-back), the greater the muscle strength, the more active areas - additionally right dorsolateral prefrontal cortex (R-DLPFC), right frontopolar area (R-FPA), and left frontopolar area (L-FPA). Some studies suggested that the PFC played a crucial role in high grip strength performance, indicating that it may be the connection between grip strength and executive function. A systematic review found that resistance exercise improved brain function, particularly changes in the PFC, accompanied by improvements in executive function. Our findings further validated that a certain level of muscle strength was beneficial for brain health.
Furthermore, our finding that higher WM load was associated with fewer activation areas supported our hypothesis and was consistent with the compensation-related utilization of the neural circuit hypothesis, which suggested that older adults showed over-activations at a lower WM load, and under-activations at a higher WM load. Previous research found that higher levels of oxyhemoglobin concentration in the PFC of older adults during cognitive tasks were associated with better cognitive performance, particularly in the DLPFC, which was closely linked to WM. Additionally, studies showed that the level of PFC activation increased with increasing WM load in older adults, and then tended to stabilize or decrease. Older adults exhibited greater DLPFC activation than younger adults during WM tasks, and meta-analysis showed that when young people and older adults had the same cognitive performance, young people exhibited greater activity in left ventrolateral prefrontal cortex (L-VLPFC), while older people exhibited greater activity in L-DLPFC. These findings suggested that older adults could compensate for cognitive performance by activating more task-related brain regions, supporting the assumption of a positive neurobiobehavioral relationship between cortical hemodynamics and cognitive performance.
However, our study found cortical hemodynamics during N-back tasks did not mediate the relationship between muscle strength and WM performance. It can be inferred that an increase in muscle strength was associated with prefrontal cortex activation, thereby promoting positive effects on brain health.
Muscular power output in the elderly can be impaired by dysfunctional motor units due to several reasons:
1. Motor Unit Denervation: Motor units consist of a motor neuron and the muscle fibers it innervates. With age, there is a natural process of motor unit denervation where motor neurons die, and the muscle fibers they control are left without neural stimulation. This denervation leads to muscle weakness and impaired power output.
2. Motor Unit Remodeling: In response to denervation, the body tries to adapt by reinnervating muscle fibers with new motor neurons. However, this process is not always efficient, leading to motor unit remodeling. The new motor units might not match the size and type of the original ones, resulting in less effective muscle contractions and reduced power output.
3. Decreased Motor Neuron Firing Rates: With age, there is a decrease in the firing rates of motor neurons. Motor neurons need to fire at higher rates to generate stronger muscle contractions. When the firing rates decline, the muscle fibers they control produce weaker contractions, leading to reduced power output.
4. Sarcopenia: Sarcopenia is the age-related loss of muscle mass and strength. As muscle mass decreases, the number of functioning motor units also decreases, leading to a decrease in overall muscle power. Sarcopenia often results from a combination of motor unit denervation and a decline in the regenerative capacity of muscle tissue.
5. Neuromuscular Junction Dysfunction: The neuromuscular junction is the point where the motor neuron meets the muscle fiber. With age, there can be dysfunction at this junction, leading to inefficient transmission of neural signals to the muscle. This inefficiency results in weaker muscle contractions and diminished power output.
6. Altered Muscle Fiber Composition: Aging can lead to changes in the composition of muscle fibers. There is a shift from fast-twitch (Type II) muscle fibers, which are responsible for powerful contractions, to slow-twitch (Type I) fibers, which are more fatigue-resistant but produce weaker contractions. This shift further reduces the overall power output of muscles in the elderly.
The loss of power output in the elderly can be connected to neurotransmitter depletion. Neurotransmitters are chemical messengers that play a crucial role in transmitting signals from motor neurons to muscle fibers, allowing muscles to contract. Several factors related to neurotransmitter depletion can contribute to the decline in power output among the elderly:
1. Reduction in Acetylcholine Levels: Acetylcholine is a neurotransmitter that transmits signals across the neuromuscular junction, enabling muscle contractions. As individuals age, there can be a reduction in acetylcholine synthesis and release. This decrease in acetylcholine levels impairs the communication between motor neurons and muscle fibers, leading to weaker and less powerful muscle contractions.
2. Dopamine Depletion and Motor Control: Dopamine is a neurotransmitter involved in motor control and coordination. Its depletion, which commonly occurs in aging and neurodegenerative disorders, can lead to impaired muscle coordination and reduced power output. The loss of dopamine can affect the initiation and execution of movements, making it challenging for elderly individuals to generate strong and coordinated muscle contractions.
3. Neuromuscular Fatigue and Serotonin: Serotonin is a neurotransmitter that influences mood and also plays a role in regulating fatigue during exercise. In the context of muscle function, excessive serotonin release can occur during prolonged or intense physical activity, leading to central fatigue. In the elderly, the regulation of serotonin levels might be altered, contributing to quicker onset of neuromuscular fatigue and reduced power output during physical activities.
4. Gamma-Aminobutyric Acid (GABA) and Muscle Tone: GABA is the primary inhibitory neurotransmitter in the central nervous system. It helps regulate muscle tone and prevents excessive muscle contractions. With age, there can be changes in GABAergic signaling, leading to altered muscle tone and reduced ability to generate powerful muscle contractions. This decreased muscle tone can impair the force production necessary for activities requiring significant power output.
5. Neurotransmitter Imbalance and Motor Unit Recruitment: Imbalances in neurotransmitter levels can affect the recruitment of motor units. Motor units are groups of muscle fibers controlled by a single motor neuron. Proper recruitment of motor units is essential for generating power. Disruptions in neurotransmitter balance can lead to improper motor unit recruitment, resulting in inefficient and weaker muscle contractions.
The correlation between grip strength and working memory function in old age can be attributed to several interconnected factors:
1. Common Neural Network: Both grip strength and working memory function are controlled by specific brain regions, including the prefrontal cortex. These areas are involved in motor control and cognitive processes. As people age, there is a natural decline in neural connectivity and functioning. Changes in these brain regions can affect both grip strength and working memory, leading to a correlation between the two.
2. Neuroplasticity: Neuroplasticity refers to the brain's ability to reorganize and form new neural connections throughout life. Engaging in activities that require grip strength, such as regular exercise or certain hobbies, can stimulate neuroplasticity. Similarly, cognitive activities that challenge working memory, like puzzles or learning new skills, also promote neuroplasticity. When individuals engage in activities that enhance both grip strength and working memory, the brain's plasticity can positively influence both functions, leading to a correlation between them.
3. Vascular Health: Adequate blood flow is crucial for optimal brain function. Grip strength is often considered a marker of overall health, including cardiovascular health. Good vascular health ensures that the brain receives sufficient oxygen and nutrients, promoting the functioning of brain regions responsible for working memory. Common vascular risk factors, such as high blood pressure or diabetes, can negatively impact both grip strength and cognitive function, strengthening the correlation between the two.
4. Inflammatory Markers: Chronic inflammation is associated with both muscle weakness and cognitive decline. Inflammation can affect the structure and function of neurons as well as muscle tissues. Individuals with higher levels of inflammation may experience reductions in both grip strength and working memory function, leading to a correlation between the two.
5. Physical Fitness and Lifestyle Factors: Regular physical activity has been shown to have positive effects on both muscle strength and cognitive function. Exercise promotes the release of neurotrophic factors, which support the growth and maintenance of neurons. Additionally, a healthy lifestyle, including a balanced diet and sufficient sleep, can impact both grip strength and working memory function, creating a link between the two variables.
6. Motor-Cognitive Interaction: The relationship between motor skills (such as grip strength) and cognitive functions (such as working memory) is bidirectional. Engaging in activities that require fine motor skills can stimulate cognitive processes, and vice versa. Activities that challenge grip strength often involve coordination and attention, which are cognitive functions associated with working memory. This interaction between motor and cognitive skills strengthens the correlation between grip strength and working memory function in old age.