Leeftijdsgerelateerde veranderingen in spierpower van de onderste ledematen - Functionele metingen ter evaluatie van het fysiek prestatievermogen op volwassen leeftijd

Due to improvements in lifestyle and healthcare facilities worldwide, people are living longer. Unfortunately, these extra life years are not spent in good health for most people. Healthy ageing strategies should aim to prevent or delay the onset of disabilities, as minimizing disability and maintaining functional ability are key factors to achieve a high quality of life in older adults. Therefore, it is crucial to identify people who are at risk for the development of functional disability as early as possible. Preferably before any functional problems are already present so that interventions can focus on the preservation of functionality and the prevention of functional deficits that come with increasing age. Lower-limb muscle power, the product of muscle force and velocity, deteriorates faster with ageing compared to muscle force and is highly related to physical functioning. Lower-limb muscle power is needed to perform functional activities such as walking, balance maintenance, standing up from a chair and stair climbing, which makes it an important parameter for independent living. Therefore, tracing muscle power loss at an early stage can be seen as a key element in the prevention of functional decline with ageing. Traditionally, muscle power of the lower limbs is measured with complex and expensive equipment. However, the use of simple functional tests like a stair climbing or sit-to-stand test seems better suited for frequent assessments in large-scale practice. Inertial measurement units can be a helpful tool, as they can be used to estimate the vertical power required to elevate the body's center of mass during a sit-to-stand transfer or stairclimbing. The use of inertial measurement units have still some practical and economical disadvantages, as there is a hardware component and accompanying software needed. Alternative methodologies that allow lower-limb power measurements on large-scale basis exist, such as duration-based equations to estimate mean sit-to-stand or stairclimbing power. These more simplified methodologies only generate a mean power value and therefore they could miss important information about a person's movement strategy. The main objective of this doctoral thesis is twofold: 1) to explore the ability of various field tests to detect age-related differences in performance across the full adult life span, to contribute to the early detection of functional problems that come with ageing (chapter 1); and 2) to investigate clinically feasible methodologies for measuring lower-limb muscle power, in particular during stairclimbing (chapter 2) and sit-to-stand (chapter 3). In the context of prevention of functional problems that come with ageing, early signs of functional deterioration should be detected as soon as possible. In chapter 1 (article 1) the ability of various field tests to detect age-related differences in performance across the full adult lifespan (145 men, 159 women; 19-85 years) is examined. The following tests were assessed: timed up and go, maximal gait speed, handgrip strength, sit-to-stand, stairclimbing and a countermovement jump. These functional tests were all selected and included because they are already widely used in clinical practice. Additionally for these tests standardized protocols and reference values already exist. However, the question rises whether these tests are responsive enough to differentiate between well-functioning adults as these were mainly developed for older adults aged ≥70 years. In young and middle-aged people (before the age of 60 years), the stairclimbing and countermovement test showed high responsiveness to observe age-related differences in physical and muscle function. In contrast the timed up and go and sit-to stand test showed less responsiveness, and the handgrip strength and maximal gait speed test were not responsive to detect age-related differences before the age of 60 years. After the age of 60, all abovementioned tests - except the sit-to-stand test - seemed sufficiently responsive to detect age-related declines in physical and muscle function. The declines are most pronounced in the stairclimbing and countermovement test. After the focus on detecting age-related functional deterioration in chapter 1, chapter 2 focuses on stairclimbing power in particular, as stairclimbing was seen as the most responsive test to detect age-related declines in well-functioning adults. The aim of chapter 2 was threefold, namely 1) to examine test-retest reliability of stair climbing power parameters measured with an inertial measurement unit (article 2); 2) To compare stairclimbing performance on different stair models with a different number of steps (i.e., 3, 6 and 12 steps), as a full flight of stairs is often not available in clinical practice (article 2); and 3) To explore potential differences between stairclimbing power measured with an inertial measurement unit and estimated with a mathematic equation across the full adult lifespan (article 3). Article 2 focused on community-dwelling men and women aged ≥65 years (59 men, 57 women). Our study showed that different power parameters during stairclimbing can be measured reliably and that stairclimbing performance can be distinguished into three phases; an acceleration phase where power is build up, a phase of maximal performance or highest power and a phase of power loss (only measurable in the 12-step staircase). Furthermore, this article showed that a 3-step stair model can be used for measuring SC power reliably in settings where a regular staircase is not available. Article 3 explored potential differences between stairclimbing power measured with an inertial measurement unit and estimated with a mathematic equation in adults across full lifespan (156 men, 162 women; 19-93 years). Equation-derived power showed lower values compared to sensor-derived power and the differences between both methodologies enlarged in subjects with better performance (i.e., men and younger subjects). Furthermore, equation-derived power seemed less sensitive to detect age-related changes in physical functioning compared to sensor-derived power. Not all settings have access to a staircase, therefore chapter 3 focusses on sit-to-stand power as this test is the most feasible alternative when limited space is available. The aim of chapter 3 was to examine whether age, functional limitations and sex affect the movement strategy and power production during fast sit-to-stand transitions across full adult lifespan (352 men, 297 women with an age range of 19-93 years old; 18 men and 28 women aged ≥65 years were limited functioning) (article 4). Our article showed that age, sex and functional level have an influence on the movement strategy when standing up from a chair. A sit-to-stand transition can be divided into 4 phases: a sit-to-stand, stand, stand-to-sit and sit phase. Weaker subjects spend proportionally less time in the power-generating sit-to-stand phase and proportionally more time in the eccentric stand-to-sit phase. Additionally, limited-functioning older adults (≥65 years) spend proportionally more time in the static sit and stand phases compared to well-functioning older adults, resulting in proportionally less time in the dynamic sit-to-stand and stand-to-sit phase. Lastly, trunk flexion during the sit-to-stand phase increase with increasing age and were higher in women compared to men. We propose the use of stair climbing power tests to detect changes in lower-limb muscle power prior to the onset of mobility limitations, as these tests appear to be more responsive to detect age-related declines compared to other commonly used field tests. However, not all settings have access to a staircase and not all people can climb stairs. Therefore sit-to-stand tests may be considered as a feasible alternative. Both stairclimbing and sit-to-stand muscle power can be measured reliable with an inertial measurement unit and can be estimated with a duration-based mathematic equation. Inertial measurement units measure continuous and give insight in the different sub-phases of the movement. This information is missing when estimating mean power with a duration-based equation. Our results showed that both methodologies give significantly different power values in well-functioning adults. This indicates that power estimated with an equation can be mainly of value in older and less-functioning age groups, making it less interesting from a screening perspective.