Author:

Abstract:

Life expectancy is rising globally, which is a human success story. However, aging is accompanied by a loss of functional performance capacity and a higher risk of falling, leading to a loss of autonomy. Due to these changes, the rising medical costs and the higher need for primary health care, a major clinical and economic burden for the society exists. Therefore, detection of early declines in muscular function during aging and prevention of the loss in functional performance capacity are of paramount importance. However, the current methodologies used in clinical practice are rather limited to capture the complex functioning of muscles during human movements and hence the age-related decline in muscular function. Factors such as response time, contraction type, movement complexity and acceleration rate are shown to have a clear influence on muscular function. However, how these factors are affected by aging is not well understood. Therefore, we aimed to identify age-related changes in muscular function and get more insight into the functional outcomes of the aging process by investigating the effect of aging on muscular function with the focus on response time, contraction type, movement complexity and acceleration rate. In addition, we aimed to gain a better insight into the underlying mechanisms of the age-related changes in muscular function, which could help to develop effective strategies to counteract these age-related deteriorations. Generally, this doctoral thesis, which consists of three chapters, aims to create awareness in clinical practice about less known age-related problems that can have a high impact and to bring new insights into the field of aging research to help finding solutions for the loss of functionality. Chapter 1 focusses on factors such as response time and contraction type. More specifically, the reliability and the age-related decline of time-dependent measures of knee extensor function obtained through isotonic testing is investigated. In article 1, it is shown that maximal and time-dependent measures of muscular function such as pP and RPD are good to highly reliable in older adults. Therefore, pP and RPD can be considered as consistent measures of muscular function. In article 2, it is found that RPD differentiates more between young and older adults compared with pP, due to an additional age-related increase in the time to pP. Furthermore, all power-related parameters show a strong relationship with functional performance tests, such as 7.5-meter fast walk, timed-up-and-go and stair climbing, in well-functioning community-dwelling older adults. These results underline the importance of time-dependent measures to detect age-related changes in muscular function, in particular they emphasize the inability to generate power rapidly at older age. Chapter 2 focusses on movement complexity. In article 3, the effect of age on power production of the leg extensors in single- versus multi-joint movements is evaluated across the adult life span. In both single- and multi-joint tests, it is shown that the age-related decline in RPD exceeds the decline in pP. In addition, it is found that RPD declines more in the multi-joint compared with the single-joint test. This phenomenon is true for tests at different velocities. Moreover, it is found that RPD multi-joint is more associated with squat jump height compared with RPD single-joint. Therefore, research and clinical practice should consider focusing on the initial phase of rapid power development and multi-joint testing for the detection of functional disability during aging. Chapter 3 investigates the effect of aging on power production at different accelerations and aims to gain a better insight in the underlying mechanisms of the age-related decline in muscular function in terms of neural activation and in vivo fascicle behavior. In article 4, it is shown that RPD largely declines with aging with the steepest decline at the highest acceleration. Neuromuscular activation declines more at higher compared to lower acceleration for rectus femoris (RF) and vastus lateralis (VL) muscles. The age-related changes at high relative to lower acceleration in RF and VL activity are associated with the age-related decline in RPD across the sample. These findings emphasize the magnitude of the age-related decline in leg extensor RPD in response to abrupt changes in movement velocity across the adult life span and its association with impairments in neuromuscular activity. In article 5, it is shown that VL fascicle shortening and shortening velocity increase significantly with aging with no difference between accelerations. This age-related change is mainly due to a higher shortening during the phase of electromechanical delay. VL fascicle shortening and shortening velocity are positively associated with RPD. These findings demonstrate that the age-related decline in RPD in fast actions is accompanied by an increase in fascicle shortening and shortening velocity. While in vivo fiber shortening velocity seems to be no limiter, specific fiber force may have a bigger influence on rapid power production at advancing age than its capacity to shorten fast. Increased VL fascicle shortening might result in less optimal fiber lengths, which may be due to decreased stiffness of the series elastic element with aging. All together, the findings of the different papers highlight that time-dependent measures of muscular function could be considered as potential identifiers of muscular aging in research and clinical practice, especially in dynamic, multi-joint testing at high accelerations. Preventive exercise interventions should not only focus on improving muscle mass and maximal strength, but also neural activation, fiber properties and tendon stiffness.