A Daily Walk and Regular Resistance Training May Maintain Independence of Older Adults

Regular physical activity has numerous health benefits for older adults, including slowing the age-associated loss of muscle strength and aiding in weight maintenance. Despite this, participation declines with age, which increases risk of frailty, disability and disease, worsening clinical outcomes and impairing the ability to perform activities of daily living. A recent study by Kim et al. found that greater adiposity and low grip strength, a marker of overall muscle strength, were associated with mortality [1]. After discussing these conclusions within the wider literature, it could be recommended that older adults should undertake regular physical activity, including both resistance training and moderate aerobic exercise, such as walking or group classes. This should be complemented by a less sedentary lifestyle, avoiding extended periods of sitting, to ensure maximum benefits are obtained. Following this advice may help reduce rate of functional decline, allowing for independence and quality of life to be maintained.  

Biological senescence that occurs with age is frequently associated with functional impairment, impacting mobility, independence and quality of life. The term sarcopenia describes the progressive loss of muscle mass, yet this only explains a small amount of the variance in muscle strength [2], which declines at a greater rate [3], with a loss of 15-20% every decade after 50 years [4]. Dynapenia is a distinct condition that defines this age-associated loss of maximal strength [5]. It is an indicator of functional and nutritional status [6], and is thought to have greater prognostic value than sarcopenia [7] in predicting risk of falls, hospitalisation, time for recovery from illness, disability, frailty and mortality [6]. Dynapenia has also been associated with metabolic abnormalities such as insulin resistance and the metabolic syndrome [5]. However, when dynapenia is combined with abdominal obesity worse outcomes are observed than from either condition alone due to muscle fat infiltration and low grade chronic inflammation [5].

A higher BMI indicates greater amounts of both lean and fat mass, yet visceral obesity is associated with adverse health outcomes [8]. Moreover, muscle strength does not increase proportionally to the increase in muscle mass, which is augmented within obese individuals due to the stimulation of muscle tissue synthesis by the high load placed on muscles. This is such that absolute strength may be higher but strength adjusted for body weight tends to be lower than in non-obese adults [9]. Further to this, obesity is associated with comorbidities including type 2 diabetes (T2D) and cardiovascular disease (CVD), increased risk of arthritis [10], and impaired balance [11]. Consequently, a decline in muscle strength relative to body mass, in combination with additional factors, is likely to limit the ability to perform activities of daily living (ADL).

The increasing prevalence of obesity within the ageing population [12] will place a greater burden on health and social care as a result of a loss of independence, poor clinical outcomes, and high risk of chronic disease. There is a large interindividual variability in the rate of decline in physical function, with fat mass and muscle strength being considered key factors in its progression [13] and modifiable by physical activity. A recently published prospective observational study by Kim et al. investigated the association between mortality and both handgrip strength (GS), which is the most commonly used surrogate measure of overall muscle strength [13], and excess adiposity [1]. This review will discuss their conclusions within wider research to determine whether recommendations for older adults  to slow rate of functional decline with age should focus on weight loss, strength training, or target both components.

Method

Study population

Data was from UK Biobank, a national cohort of over 500,000 adults aged 40-69 years at recruitment, collecting physical measurements, biological samples and assessing sociodemographic, health and lifestyle factors.

Grip strength and adiposity measurement

GS was measured using a dynamometer, with participants squeezing the handle as strongly as possible for 3 seconds. The value from both hands was averaged. BMI was calculated, and waist circumference and body fat percentage measured, with participants then categorised for each. 

Mortality

Data on mortality was obtained from death records and CVD mortality defined by ICD-10 diagnostic criteria.

Statistical analyses

The associations between GS and adiposity with all-cause and CVD mortality were estimated using Cox proportional hazards regression. A number of models were produced, estimating the association between GS and mortality with adjustment for confounders and each of the three adiposity measures separately. Models reflecting adiposity were adjusted for the same covariates and GS. The association with each 5kg increment of GS were also estimated. Joint associations with mortality were investigated by combining sex and age-specific GS quintiles and different adiposity categories.

Sensitivity analyses used maximum GS from either hand, GS normalised for body weight or FFM, excluded mortality in the first 2 years of follow-up and excluded those with COPD or smoking history.

Results

Subject characteristics

403,199 participants were included in the final sample. There were minimal differences in BMI, WC and %BF across GS quintiles.

Results of statistical analysis

Risk of all-cause mortality was lower for men and women in the highest quintiles of GS compared to the lowest after adjustment for confounders and adiposity measures, with a 32% lower hazard for men and 25% for women. An 8% reduced hazard of all-cause mortality was observed per 5kg increase in GS after adjustment for BMI. Similar results were reported from the sensitivity analyses.

The inverse association between GS and CVD mortality were similar and significant for men but not for women, but for both sexes it was significant when measuring a per 5kg increase in GS.

A J-shaped association between BMI and mortality risk was observed and those in the highest BMI and WC categories had increased hazards of all-cause and CVD mortality.

Obese men with low GS had highest risk of all-cause mortality, with an 89% increased hazard compared to normal weight men with highest GS. Normal weight men with low GS had a higher mortality risk than obese men with a higher GS. For women, a 69% increased risk of mortality was observed for those with the highest BMI and lowest GS compared to normal weight women with highest GS. Obese women with higher GS had lower risk of all-cause mortality than non-obese women with lower GS. The associations were similar for CVD mortality.

Discussion

The study by Kim et al. found a lower grip strength to be associated with increased risk of mortality independent of adiposity. This is consistently reported within the literature, with hand grip strength being predictive of post-operative complications [6] and length of hospital stay [14], hazard of death [15], and risk of all-cause mortality independent of comorbidity or inflammatory markers [16]. Moreover, multimorbidity has been suggested to have an additive impact on body strength, with grip strength decreasing linearly with number of chronic diseases [17]. In addition, a relationship between morbidity and handgrip strength is frequently observed, with odds of limitation suggested to be up to 3x greater for those below the cut off value compared to those with normal grip strength [13]. A positive association with physical performance [18] and an inverse association with functional decline [19] have also been observed, as well as an association with hospitalisation [15] and readmission [20], although the latter conclusion may be subject to bias if measurements are taken upon initial hospitalisation as the presenting condition may impact muscle strength. It has also been shown that higher grip strength is related to reduced length of stay on rehabilitation wards, although this could also be influenced by external factors such as the availability of health and social care following discharge [14]. Nonetheless, grip strength is commonly deemed one of frailty criteria [21] therefore, as suggested within the literature, a high grip strength is likely to correlate with reduced vulnerability to stresses and greater physiological reserve.

When analysing the association between grip strength and mortality interindividual differences should be considered. Firstly, women have a shallower slope of strength decline, yet with every kg lost per year they have a greater reduction in survival rate [22]. This may be attributed to their uneven decline in strength, with lower extremities losing strength more rapidly than upper extremities [11]. It has also been discussed that BMI specific cut points for grip strength may improve the sensitivity and specificity of the test [11] as muscle strength adjusted for body weight is significant in functional decline. As this is not generally incorporated into studies it could be that many of the results are slightly attenuated and hand grip strength may be more strongly correlated with morbidity and mortality risk than reported. Results from research using change in grip strength as a variable should also be interpreted with caution as physiological reserves can vary between individuals due to genotype and lifestyle factors. Those with greater physiological reserves are likely to be able to lose a more of their muscle strength prior to onset of disability [16], causing a null association to be observed with physical performance. Finally, inconsistencies in the literature may be a consequence of several characteristics influencing the ability to undertake ADLs, including flexibility, coordination and balance, which tend to be included in short physical performance battery tests but may not correlate with grip strength [16].

As with the observations in the study by Kim et al., obesity is often stated as a risk factor for both mortality [10] and functional limitation [23]. It has also been reported that adiposity, as measured by waist circumference and fat mass, is associated with disability [3]. However, Kim et al. found the highest risk of mortality to be within obese individuals with lowest grip strength, when classifying by BMI [1], a relationship that has mostly been investigated in relation to morbidity rather than survival. Obese individuals with low strength generally have high rates of onset of disability [3], with slower walking speed and lower physical function score compared to non-obese. This effect has been assessed as additive [13]. Inflammation from abdominal adiposity is also likely to augment the functional limitation resulting from dynapenia, with an increase in IL-6 and CRP [24] and muscle fat infiltration [12] impairing the muscular environment and reducing muscle efficiency [25]. Over 5 years of follow-up a greater than 3-fold increase in risk of worsening disability has been observed for those with dynapenia and abdominal obesity compared to those with neither condition. Despite this, Stenholm et al. reported a decrease in mortality risk for obese and overweight subjects compared to normal weight subjects for those over 70 years [10]. This suggests that weight throughout the life course should be taken into account as rapid weight loss, potentially due to energy and protein malnutrition, is likely to accelerate the natural loss of muscle strength leading to poor outcomes. Initiating weight loss in older adults by using restrictive diets may adversely affect physiological reserves and predispose to frailty, and so interventions focusing on increased physical activity could be thought more beneficial.

A less sedentary lifestyle reduces risk of obesity by increasing energy expenditure, aiding in weight maintenance, promoting a negative energy balance, and reducing body fat [26] and waist circumference [27]. It is also said that aerobic exercise may decrease CRP [28], which accelerates dynapenia and is involved in the pathogenesis of inflammatory chronic diseases including CVD and T2D. Nonetheless, there is concern that older adults compensate for vigorous physical activity by less fragmented sedentary behaviour, increasing adiposity, causing muscle disuse [26], and attenuating any effect of exercise interventions.

Moreover, moderate physical activity has been shown to maintain muscle strength [29] or significantly lessen its decline despite loss of lean mass [30]. For this, resistance training is the more commonly studied intervention. An increase in physical and social functioning has been observed following a 12-week systematic strength training programme [31], with greater relative gains occurring in upper limb muscle groups to also improve balance and postural control [32].

The translation from preservation of muscle strength by resistance training to improvement in physical performance has been evidenced, with active subjects having faster gait speed and shorter stair ascension times, as well as a higher aerobic capacity, so reduced exhaustion [33]. However, it may be essential to perform multi-joint exercises [34] and combine strength training with functional training by training individuals to perform particular activities using dynamic tasks and coordinated multi-planar movements [35]. For example, a training programme involving chair standing may reduce time to stand from a chair, or one including balance may improve balance. This approach could have the potential to enhance outcomes in terms of reducing rate of functional decline and maintaining independence. It is important to note that for any type of training undertaken the effect may be influenced by age-related changes to the nervous system which can impair neuromuscular coordination, affecting motor neuron excitation and force production by muscle fibres [32], impairing muscle strength. This may suggest that maintenance of cognitive function is also essential, although physical activity also confers such benefits [36].

Impacts

From discussing the study by Kim et al. within the wider literature, it has been shown that grip strength predicts adverse outcomes, making it an effective marker for overall muscle strength. In addition, there has been agreement with the conclusions that both lower grip strength and greater adiposity increase risk of mortality, and that a combination of dynapenia and obesity augment the effect [1]. A significant association with morbidity and worsening disability has also been observed. It could therefore be concluded that recommendations for older adults that aim to prevent functional decline should target both weight loss and muscle strength preservation, although it would be important to ensure that all individuals do not rapidly lose weight as this can accelerate both sarcopenia and dynapenia, predisposing to frailty.

Physical activity declines with age [33], yet is beneficial for weight maintenance, preservation of lean mass and muscular strength, improves balance and co-ordination, reduces fatigue, and is a modifiable risk factor for metabolic conditions such as T2D. To maximise and diversify its effects, it could thought preferable to perform both aerobic exercise and resistance training. Current UK guidelines for adults over 65 years, which promote 150 minutes of moderate intensity activity each week and physical activity to improve muscle strength on at least 2 days per week [36], may therefore be appropriate advice. However, it would be important to ensure that additional activity is not compensated for by a more sedentary lifestyle, so time spend sat for extended periods should also be minimised.

To provide more specific recommendations, it may be ideal for resistance exercise programmes to involve regular training, with alternating rest days, at an intensity that balances the ability to increase muscle strength with prevention of musculoskeletal injury. It may also be essential to focus on progression, which stimulates the body to exert a greater magnitude of force, enhancing motor performance and increasing exercise tolerance [34]. This could be done by varying frequency, sets, repetitions, duration and exercises. Spending 30 minutes over 5 days walking or cycling, participating in active leisure pursuits such as gardening or dancing, joining group exercise classes, community or gym based activities, or swimming [36] contributes to undertaking 150 minutes of moderate intensity physical activity per week. Such activities are likely to not only maintain functional independence, but also have the potential to enhance psychological and social functioning, improving all domains of health. 

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