It is expected that the world’s proportion of elderly people above the age of 60 years will almost double within the next 35 years [1]. At this time, approximately one fourth of the Danish population is above the age of 65 years [2]. Due to a longer life expectancy than seen in the past, in 2012 the Danish Parliament decided to increase retirement age [3], thereby increasing the number of elderly workers. In practice, this means that individuals born after 1966 can expect to be at least 69 years old at the time of retirement. The increased life expectancy was one of the central arguments; however, this may be problematic as increased lifespan is caused by other factors than delayed onset of the aging processes, such as healthier nutrition, better living standards and education, and public-health efforts [4]. Hence, elderly people working in a physically demanding environment (eg, building and constructions sites) may be rendered unable to retain the ability to adequately perform the physical requirements of their jobs, due to age-related decreases in physical performance. In healthy elderly people, increased longevity may lead to a postponed retirement age, but that is far from certain for people with chronic conditions such as osteoarthritis and osteoporosis. Therefore, increasing the retirement age may not necessarily lead to the goal of keeping everybody in the labor market for a longer time [5]. Knowledge about the variations in physical performance of the elderly workforce when physically challenged is limited [6-8]. Accordingly, the effects of an increased retirement age among elderly manual workers are difficult to predict.

Aging is associated with a gradual loss of muscle mass, strength, and power [9-11], in addition to reductions in maximal rate of oxygen consumption (VO_2max) [12], lung capacity [13,14], and impairment of postural control [15]. Reductions in VO_2max have been observed to be approximately 10% per decade after the age of 30 (a process which seems to accelerate further after the age of 70 [16-18]), whereas forced expiratory volume in 1 second (FEV₁) declines approximately 20 mL annually from the age of 25 years [19]. This annual decline increases to 38 mL after the age of 65 years [14], which means that on average a person at the age of 70 has lost 1 L in FEV₁ over the preceding 45 years. Concomitant to these changes, forced vital capacity (FVC) is reduced at a slightly slower speed [19]. Furthermore, the age-related reductions in muscle strength affects both inspiratory and expiratory muscles [20], possibly inhibiting ventilation during physical activity.

Contrary to the age-related loss of respiratory capacity, skeletal muscle strength is relatively well preserved until the age of 50 years. From this age, the annual decline in strength is approximately 1-2%, whereas muscle power is lost at an even faster rate [11,21]. Regarding rate of force development (RFD), a previous study demonstrated that a significant loss of rapid force capacity would be evident after the fifth decade, while loss of maximal strength would be more pronounced after the sixth decade of life [22]. Some authors argue that RFD is of more functional significance in elderly people compared to maximal strength because rapid force production is more related to tasks such as postural control and grabbing a rail to regain balance and prevent falls [23]. Conversely, daily activities that involve the manipulation of objects rarely require maximal force or explosive force, but rely mostly on submaximal force production. Hence, measurements of submaximal force production in both static and dynamic conditions (eg, force tracking) may provide additional information about force steadiness and coordination [24], which is needed during activities of daily living at work and during leisure.

Sarcopenia, also known as the age-related loss of muscle mass, typically starts from the end of the fifth decade of life [9,25], and may sometimes be accompanied by an increase in fat mass [26]. Thus, body mass may remain somewhat stable in the elderly [27]; however, a negative change in body composition may have detrimental consequences [28]. Given that people with physically demanding occupations are required to be physically active throughout a workday, a decrease in muscle mass together with an increase in fat mass would be especially problematic in this population. Another challenge for elderly individuals is that comorbidity (ie, having more than one chronic condition) increases with age [29]. Additionally, some conditions may interact, thereby exacerbating the negative impact of a disease on quality of life and work ability. For instance, chronic obstructive pulmonary disease is usually accompanied by skeletal muscle weakness, musculoskeletal disorders, osteoporosis, and chronic inflammation [30,31]. Moreover, the prevalence of hip and knee osteoarthritis increases with age, while physically demanding work may accelerate the development of this pathology [32]. Several risk factors have been proposed to explain this interaction, including age and physical inactivity. The exact pathogenesis contributing to these comorbidities, together with the treatment strategies, are not clearly established. Interestingly, systemic inflammation, which is typically elevated in patients with chronic obstructive pulmonary disease, is not only associated with an age-related decline in lung function [33], but is also negatively associated with muscle mass in elderly people [34]. These findings call for research initiatives assessing both the respiratory and the musculoskeletal system in relation to occupational exposures, and other risk factors such as body composition and inflammatory status.

Balance is typically compromised in elderly people. Although postural control is largely related to muscle strength and the ability to produce rapid force, it also requires an integration of the information from the visual, vestibular, and somatosensory systems to generate the appropriate motor response to maintain static and dynamic balance [15,35]. Thus, a combination of the alterations in the musculoskeletal system and the afferent information from sensory systems are responsible for the reduced ability to maintain motor coordination (including balance) among the elderly [35]. Paradoxically, although visual acuity is progressively compromised with increasing age, the reliance on the visual system to maintain postural control increases with age, especially when balance is challenged [35]. These changes may therefore result in impairments in physical performance and, as a result, the ability to sustain the required level of efficiency in a physically demanding job. Hence, the physiological changes in relation to aging may challenge the ability of elderly manual workers to maintain an adequate level of performance in their workplace. Moreover, these changes may result in elderly manual workers working closer to their maximal capacity on a daily basis, which may result in an increased risk of developing musculoskeletal disorders [36,37].

To date, we know too little about how physically demanding work affects physical performance and how declining physical performance affects the ability to perform physically demanding work in elderly populations. Specifically, more information is needed about how different work exposures, health statuses, and physical activity levels relate to different measures of physical performance. Although some of the reductions in physical performance might begin as early as the third decade of life, the most pronounced impairments occur starting from the fifth decade. Thus, information regarding the variations in physical performance among elderly manual workers in the last two decades of working life is of importance, so that recommendations related to their work environment can be made. Lastly, chronological age (ie, time since birth) may well be an important risk factor for several adverse outcomes; however, all people age differently. The term biological age has been used to describe a person’s health status, thereby giving a better prediction of physical capacity later in life than chronological age [38]. Although a single marker of biological age has not been discovered, a combination of several putative biomarkers (including handgrip strength, standing balance, muscle mass, inflammatory status, and lung function) have been suggested to create a model for determining biological age [39,40]. Hence, the present study will enable us to not only investigate the effect of chronological age on physical performance, but may also give insight into the biological age of elderly manual workers.

The present paper describes the study protocol for a study that aims to investigate variations in physical performance among elderly Danish manual workers aged 50 to 70 years. Our primary outcomes include handgrip strength, FEV₁, and FVC. Secondary outcomes include force steadiness, reaction time, aerobic capacity, motor coordination, balance, body composition, back mobility, and inflammatory status. Tertiary outcomes are self-reported levels of physical activity, health status, and different work exposures. This is the first study to combine measurements of respiratory and musculoskeletal systems in elderly manual workers, and the collected data will therefore increase our knowledge regarding the elderly workforce. The main research questions in the study are:

Based on the available literature, we hypothesize that physical performance is negatively associated with age, and that physical work magnifies this negative relationship when compared to the general population. Thus, we expect work environment and physical activity to be predictive of respiratory and musculoskeletal function in this cohort.

Subjects are currently being contracted via telephone and email. It is expected that data collection will be completed by March 2018.

In several jobs, the physical demands for elderly workers are at the same level as for younger workers [37,61,62]. Due to a potential decrease in working capacity, the resulting workload may change from an acceptable load into daily physical overload, which might result in negative long-term health effects, such as chronic musculoskeletal symptoms [63,64], work absenteeism, and early pension retirement. Physical overload is often related to the positive adaptive responses of endurance and resistance training. Most studies, however, find no training effect of prolonged exposure to heavy manual labor [65-68]. Therefore, a more detailed understanding of the effects of age on physical performance in elderly manual workers will provide additional knowledge regarding issues related to changes in physical performance, confounding factors (eg, smoking, work environment, health status), and the balance between physical workloads and physical work capacity. Some studies have investigated the effect of age on physical performance among elderly workers; however, these studies have either been restricted by testing relatively young (<60 years) workers [65,67,69,70] or by assessing only a limited number of parameters [68,71]. Therefore, an extensive test-battery with direct measures of respiratory capacity and musculoskeletal function in the oldest of workers, along with health profiles and work exposures, will enable us to identify associations between age and physical performance in this specific population. Moreover, this study might shed light on the potentially problematic decision to increase retirement age.