Resistance training

Resistance training covers any exercise in which a muscle contracts against a load heavy enough that the effort becomes hard within a limited number of repetitions. The load can come from free weights (barbells, dumbbells, kettlebells), weight machines, elastic bands, everyday objects, or the body itself, as in push-ups, squats, lunges and pull-ups. The muscle responds to the effort required rather than to the type of equipment.

Dynamic training moves a joint through a range of motion, lifting and then lowering a load. Isometric training holds a fixed position against resistance without joint movement, as in a plank or a wall squat (holding a seated position with the back against a wall). Both increase strength, and in trials of blood pressure isometric training produced the largest reductions of any exercise type [4].

Training intensity is commonly described relative to the one-repetition maximum (1RM), the heaviest load a person can lift once with correct technique. Heavy loads are usually defined as more than 60% of 1RM and light loads as 60% or less [6]. Volume refers to the number of hard sets performed per muscle group per week [7].

In the large population studies of resistance training and mortality, participation was almost always self-reported: 13 of the 16 studies in the main review used questionnaires and 3 used interviews [1]. This limits how precisely any weekly amount can be estimated.

The largest summary of the evidence is a 2022 systematic review and meta-analysis (a statistical pooling of several studies) of 16 prospective studies that followed adults without severe illness for at least two years [1]. In the seven studies that compared any muscle-strengthening activity with none (263,058 participants, 42,133 deaths), doing some was associated with a relative risk of 0.85 (95% CI 0.79 to 0.93) for all-cause mortality, meaning about 15% lower risk. Muscle-strengthening activity was also associated with lower risk of cardiovascular disease, total cancer, type 2 diabetes and lung cancer, but not of colon, kidney, bladder or pancreatic cancer.

All included studies accounted for aerobic activity, so the estimates describe an association over and above aerobic exercise. In the three studies that examined both kinds of exercise together (581,194 participants), people who did muscle-strengthening and aerobic activity had 40% lower all-cause mortality than people who did neither (relative risk 0.60, 95% CI 0.54 to 0.67) [1]. The broader evidence on combining exercise types is covered in exercise and longevity.

All the studies were observational. Heterogeneity between them was high (I² of 83% for the all-cause estimate), and the authors rated the certainty of the evidence as generally very low under the GRADE system, mainly because most studies came from the United States [1]. People who lift weights also tend to differ from those who do not in diet, income, smoking and underlying health, and not all of these differences can be measured and adjusted for.

When the 2022 review modelled minutes of muscle-strengthening activity per week, the relationship with mortality was J-shaped rather than a straight line. The lowest relative risk for all-cause mortality, 0.83 (95% CI 0.79 to 0.86), occurred at about 40 minutes a week, and the estimate remained below that of people who did no strength training only up to about 140 minutes a week [1]. Cardiovascular disease and total cancer showed similar J-shaped curves, with the largest reduction (about 10% to 20%) at roughly 30 to 60 minutes a week. Type 2 diabetes was the exception: an L-shaped curve with a large reduction up to 60 minutes a week and no upturn at higher amounts.

The authors concluded that the effect of higher volumes on mortality was unclear, not that higher volumes were harmful [1]. Curves estimated from observational data are least reliable at their extremes, where few participants report very high amounts, and the exposure was self-reported. People reporting very high training volumes may also differ in ways not captured by the models, such as competitive athletes or people in rehabilitation.

The WHO guideline review likewise found no evidence of additional benefit from higher volumes of muscle-strengthening activity [5]. Greater training volumes continue to increase muscle size in trials [7, 3], so the flattening of the mortality curve does not imply that strength and muscle stop improving.

Resistance training increases strength at all adult ages. A 2010 meta-analysis of 47 training studies in adults aged 50 and over (1,079 participants) found average strength gains of 29% on the leg press, 24% on the chest press, 33% on knee extension and 25% on the lat pull-down, equal to 9.8 to 31.6 kg depending on the exercise. Higher-intensity training produced larger gains [2].

A companion meta-analysis of 49 studies in adults aged 50 and over (1,328 participants) found an average gain in lean body mass (body weight minus fat, mostly muscle) of 1.1 kg (95% CI 0.9 to 1.2). Programmes with more sets produced larger gains, and older participants gained less [3].

Load affects strength and muscle size differently. In a meta-analysis of 21 trials lasting at least six weeks, with every set taken to momentary failure, heavy loads (above 60% of 1RM) produced larger gains in 1RM strength than light loads, while gains in muscle size were similar [6]. How muscle size responds to volume and protein intake is described in muscle mass.

Early strength gains come mainly from neural adaptation: in the first weeks of training, motor-unit recruitment, firing rate and coordination between agonist and antagonist muscles improve before measurable hypertrophy occurs. The larger 1RM gains with heavy loads despite similar hypertrophy [6] fit the view that heavy lifting partly trains a task-specific neural skill.

Hypertrophy, the increase in muscle fibre cross-sectional area, follows over weeks to months as repeated mechanical loading raises muscle protein synthesis above breakdown. Protein supplementation adds modestly to this effect in trials [8].

Skeletal muscle is the main site of insulin-stimulated glucose disposal after a meal, and trained muscle takes up glucose more readily. This is one proposed explanation for the steep, L-shaped association between muscle-strengthening activity and type 2 diabetes in the observational data [1]. Mechanical loading of bone by muscle contraction is also a stimulus for bone formation, one reason strength work is included in fall prevention programmes for older adults.

A 2023 network meta-analysis (a method that compares several treatments at once, including through indirect comparisons) of 270 randomised controlled trials with 15,827 participants measured the effect of each exercise type on resting blood pressure [4]. Compared with no exercise, dynamic resistance training lowered systolic and diastolic pressure by 4.55/3.04 mm Hg, aerobic training by 4.49/2.53 mm Hg, combined training by 6.04/2.54 mm Hg, high-intensity interval training by 4.08/2.50 mm Hg and isometric training by 8.24/4.00 mm Hg. Isometric training ranked first for systolic pressure, and the isometric wall squat ranked as the most effective single exercise. These are average changes over trials lasting weeks to months; the outcome was blood pressure, not heart attacks, strokes or deaths.

Evidence on the brain is more limited. In the LISA (Live Active Successful Aging) randomised trial, 309 participants were assigned to heavy resistance training, moderate-intensity training or no exercise, with brain scans at the start and after one and two years. Using brain-age models trained on scans from 2,433 other adults, both training groups showed a reduction in estimated brain age of 1.4 to 2.3 years [9]. The outcome is a surrogate (a machine-learning estimate of brain age from scans), not memory loss or dementia, and the finding comes from a single trial.

The WHO 2020 guidelines on physical activity recommend that all adults do muscle-strengthening activities at moderate or greater intensity that involve all major muscle groups on 2 or more days a week, in addition to 150 to 300 minutes of moderate-intensity aerobic activity a week or an equivalent amount of vigorous activity [5]. This was rated a strong recommendation supported by moderate-certainty evidence.

For older adults, the guidelines add varied multicomponent physical activity that emphasises functional balance and strength training at moderate or greater intensity on 3 or more days a week, to improve functional capacity and prevent falls [5]. Related forms of training are described in balance training and functional fitness.

Two sessions a week that train the legs, hips, back, chest, shoulders and arms fall within the range of about 30 to 60 minutes a week at which observational studies found the lowest mortality [1]. Strength itself is most often measured in population studies by grip strength.