Muscle mass

Skeletal muscle is made of long cells called fibres, packed with contractile proteins. Muscle protein is broken down and rebuilt continuously, and the amount of muscle depends on the balance between the two. A bout of hard resistance exercise raises the rate of muscle protein synthesis for many hours, and eating protein supplies the amino acids used to build new protein. When synthesis exceeds breakdown repeatedly over weeks, the fibres thicken, a process called hypertrophy.

Continued growth requires progressive overload: the training stimulus has to rise as the muscle adapts, through more weight, more repetitions, more sets, or sets taken closer to failure (the point at which another repetition cannot be completed). Early gains in strength come mostly from the nervous system learning to use existing muscle, as described in resistance training; measurable growth takes weeks to months.

Muscle mass declines from middle age. A 2012 quantitative review found a median rate of loss of 0.47% a year in men and 0.37% a year in women across cross-sectional studies, and faster loss at older ages: longitudinal studies at age 75 reported 0.80% to 0.98% a year in men and 0.64% to 0.70% a year in women [1].

Strength declines faster than mass. At age 75, longitudinal studies reported strength losses of 3% to 4% a year in men and 2.5% to 3% a year in women, and studies that measured both in the same people found strength falling 2 to 5 times faster than mass [1]. The gap reflects falling muscle quality, meaning the force produced per unit of muscle, which depends on fibre properties and on the nerves that drive the muscle.

Ageing muscle also responds less to the same dose of dietary protein, a change known as anabolic resistance [6]. Resistance training still increases lean mass in older adults, although the gain becomes smaller with age [2].

Heavy loads are not required for growth. A meta-analysis of 21 trials compared light loads (60% of the one-repetition maximum or less) with heavy loads (above 60%) over at least six weeks, with every set taken to momentary muscular failure. Gains in muscle size were similar with both; heavy loads produced larger gains only in maximal strength [7]. Because all sets in these trials ended at failure, the results do not show how far short of failure a set can stop and still produce the same growth.

Volume, the number of hard sets per muscle per week, shows a graded relationship with growth. In a meta-regression of 15 studies (34 training groups), each additional weekly set was associated with a 0.37 percentage-point larger gain in muscle size, and higher-volume groups gained 3.9 percentage points more than lower-volume groups within the same studies. A three-level comparison (under 5, 5 to 9, and 10 or more sets per muscle per week) showed only a non-significant trend [8]. The trials were small and short, and muscle size is not itself a health outcome.

In adults aged 50 and over, higher-volume programmes likewise produced larger gains in lean body mass [2].

A 2018 systematic review and meta-analysis of 49 randomised controlled trials (1,863 participants, at least six weeks of resistance training) found that protein supplementation increased gains in one-repetition maximum strength by 2.49 kg (95% CI 0.64 to 4.33) and in fat-free mass by 0.30 kg (95% CI 0.09 to 0.52) beyond training alone [3]. The effect on fat-free mass became smaller with age and was larger in people who were already resistance-trained (0.75 kg).

The same analysis found a ceiling: above a total protein intake of about 1.62 g per kg of body weight per day, extra protein produced no further gain in fat-free mass [3]. One author reported support and honoraria from the US National Dairy Council, which had funded some of the trials reviewed.

For people over 65, the PROT-AGE expert group recommended an average intake of at least 1.0 to 1.2 g of protein per kg per day, at least 1.2 g for those who exercise or are otherwise active, and 1.2 to 1.5 g for most older adults with acute or chronic disease. People with severe kidney disease who are not on dialysis were an exception and may need to limit protein [6]. These figures are expert consensus rather than trial results.

Muscle mass cannot be weighed directly in living people and is estimated with imaging or electrical methods. Dual-energy X-ray absorptiometry (DXA), a low-dose X-ray scan, estimates appendicular skeletal muscle mass, the muscle in the arms and legs. Bioelectrical impedance analysis passes a small electrical current through the body and estimates muscle from the resistance to it. CT and MRI measure muscle cross-sectional area directly and are mostly used in research [9].

Because larger people carry more muscle, results are often divided by height squared. The European consensus definition of sarcopenia treats appendicular muscle mass below 20 kg in men and 15 kg in women, or below 7.0 and 5.5 kg per square metre of height, as low [9]. Lean body mass and fat-free mass, used in many training trials, include organs, bone and water as well as muscle.

In a US study of 3,659 older adults from the NHANES III survey (men aged 55 and over and women aged 65 and over), muscle mass was estimated by bioelectrical impedance and deaths were followed to 2004. After adjustment for waist-to-hip ratio and other factors, people in the highest quarter of muscle mass index (muscle mass divided by height squared) had lower mortality than those in the lowest quarter, with a hazard ratio (the risk in one group relative to another) of 0.80 (95% CI 0.66 to 0.97) [4]. People who were underweight or died in the first two years were excluded to reduce the influence of frailty.

Muscle strength is the more consistent predictor. A meta-analysis of 38 studies with 1,907,580 participants found higher grip strength associated with a hazard ratio of 0.69 (95% CI 0.64 to 0.74) for all-cause mortality and higher knee-extension strength with 0.86 (95% CI 0.80 to 0.93) [5]. A review of ageing muscle concluded that loss of strength is a more consistent risk factor for disability and death than loss of mass [1]. For this reason the current European definition of sarcopenia begins with low strength and uses muscle quantity only to confirm the diagnosis [9]. The measurement of strength is described in grip strength.

Skeletal muscle is one of the largest tissues in the body and takes up a large share of the glucose absorbed after a meal. It is also the body’s main protein reserve, drawn on during illness, injury and bed rest; older adults with acute or chronic disease are advised to eat more protein partly to offset this breakdown [6].

In observational studies, muscle-strengthening activity was associated with 17% lower incidence of type 2 diabetes (relative risk 0.83, 95% CI 0.77 to 0.89), with most of the reduction reached by 60 minutes of training a week [10]. These associations reflect how muscle functions as well as how much of it there is, and they are not proof of cause. Links between muscle, low-grade inflammation and ageing are discussed in sarcopenia.