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Chapter 1

            Increased life expectancy

            Human life expectancy is increasing. Advances in medicine, hygiene and safety make
            people  survive  longer  than  ever  before.  Life  expectancy  in  the  Netherlands  is
            projected to reach 86 years in the year 2040 [1]. Such an increase in lifespan is not
            only a great achievement of humankind, but the additional years of life also create
            new  opportunities,  such  as  starting  further  education  or  even  a  new  career.  The
            extent to which additional years can be used to pursue such opportunities is, for a
            large part, dependent on health and independence [2]. Older adults report a desire
            to stay socially active, mentally active and physically active during ageing [3]. To be
            able to succeed in these domains, older adults should maintain an adequate level of
            physical  performance  [4].  However,  physical  performance  in  older  adults  is  often
            compromised [5].

            The ageing muscle

            During ageing, muscle mass decreases. After the age of 30, people lose on average
            three to eight per cent of muscle mass every decade [6, 7]. Such loss implicates that
            if a person has 35 kg of muscle mass at the age of 30, this person might end up with
            between 23 kg and 30 kg muscle mass remaining at the age of 80. One of the reasons
            for this age-related loss in muscle mass is a decreased exposure to anabolic stimuli
            later in life, such as physical activity [8], protein intake [9] and anabolic hormones
            [10]. Besides, the anabolic response to these stimuli is blunted in older people, a
            phenomenon that is termed anabolic resistance [11].

            Underneath  this  visible  muscle  mass  decline,  a  change  that  is  less  obvious  but
            arguably  as  important  is  happening:  a  declining  muscle  strength.  During  ageing,
            muscle strength is decreasing at a  much faster rate than muscle  mass does. The
            estimated loss in muscle strength is between 10 and 20% per decade up to the age
            of 70 [12], and between 20% and 30% in the decades thereafter [13]. The loss in
            muscle strength is considered a more important risk factor for functional decline than
            the loss in muscle mass [14]. Muscle strength might, therefore, be more relevant than
            muscle mass for quality of life of older adults. This notion is indeed confirmed by the
            work of Balogun et al. [15] They compared community-dwelling older adults in the
            lowest  20%  of  handgrip  strength  or  lower-limb  muscle  strength  with  those  with
            normal  strength  levels.  They  found  a  significant,  and  clinically  meaningful,  lower
            health-related quality of life over 10 years of follow-up for those with low strength.
            The associations between low appendicular mean mass and health-related quality of

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