What is strength training? Definition, effects and guidelines

Strength training is a form of exercise in which skeletal muscles contract against a resistance in order to become stronger, larger or more resistant to fatigue. The resistance can be external, such as a barbell, dumbbell, kettlebell, cable or machine, or it can be your own body weight. Resistance training and weight training are common synonyms. The WHO physical activity guidelines use the term muscle-strengthening activities and specify that these should be performed at moderate or greater intensity and involve all major muscle groups ^[1].

A muscle can produce force in three ways: while shortening (concentric), while lengthening under control (eccentric) and while holding a position without moving (isometric). Well-designed programmes use all three, and combine exercises that move one joint, such as a biceps curl, with exercises that move several joints at once, such as a squat. To keep producing adaptations, the programme has to become harder over time, a principle known as progressive overload ^[2]. The resistance, the number of repetitions or the number of sets increases gradually as the body adapts.

Strength training can be done with free weights, machines and cables, body weight, resistance bands or kettlebells. For health purposes the guidelines do not prescribe a specific tool; what they ask for is moderate or greater intensity across all major muscle groups ^[1]. Free weights and machines allow small, measurable increases in load. Bodyweight exercises and bands are practical at home, but become harder to progress in small steps once the basic movements feel easy. An overview of strength exercises for every muscle group lists the individual movements per category.

Activities such as Pilates, yoga and circuit classes contain strength elements. Whether they count as strength training depends on how much resistance they provide and whether that resistance increases over time. A separate page examines whether Pilates counts as strength training and how it compares with training with weights.

Strength training first changes how the nervous system controls the muscles, and only later the muscles themselves. The strength gains of the first weeks are thought to come mainly from neural adaptations, including changes in the activity of motor units: a motor nerve cell together with the muscle fibres it controls ^[3]. Coordination in the trained movement improves at the same time.

With continued training the muscle fibres grow thicker, a process called hypertrophy. A review of resistance training and health reports that ten weeks of training may increase lean mass by about 1.4 kg (3 lb), reduce fat mass by about 1.8 kg (4 lb) and raise resting metabolic rate by about 7% ^[4]. These are average values; individual responses vary.

The effects extend beyond muscle. The same review describes improved insulin sensitivity, lower HbA1c (a measure of average blood glucose over several months), less visceral fat, lower resting blood pressure, lower LDL cholesterol and triglycerides, higher HDL cholesterol and increases in bone mineral density of 1–3% ^[4]. Resistance training may also reduce low back pain and ease discomfort associated with arthritis and fibromyalgia ^[4].

Strength training also uses energy during the session itself. How many calories strength training burns depends on body weight, the duration of the session and how intensely you train.

Muscle that is not used declines with age. Inactive adults lose an estimated 3–8% of their muscle mass per decade, accompanied by a fall in resting metabolic rate and an increase in body fat ^[4]. The loss of strength and muscle reduces physiological resilience and increases vulnerability to serious health events ^[5].

The National Strength and Conditioning Association describes resistance training as a powerful intervention against the loss of muscle strength and muscle mass in older adults, with benefits for physical functioning, mobility, independence, chronic disease management, psychological well-being and quality of life ^[5]. In a meta-analysis of randomised trials in people aged 65 and over, progressive resistance training increased leg strength and raised bone mineral density at the hip and femur by about 2.8%. The change at the lumbar spine was not statistically significant ^[6].

For this reason the WHO recommends that adults aged 65 and over, as part of their weekly physical activity, do varied multicomponent activity that emphasises functional balance and strength training at moderate or greater intensity on three or more days a week, to enhance functional capacity and prevent falls ^[1].

Strength training is healthy: regular muscle-strengthening activity is associated with a lower risk of early death and chronic disease, independent of aerobic exercise. A 2022 meta-analysis of 16 prospective cohort studies found that muscle-strengthening activities were associated with a 10–17% lower risk of all-cause mortality, cardiovascular disease, total cancer, diabetes and lung cancer ^[7]. No association was found for some specific cancers, such as colon and kidney cancer.

The relationship with dose was not linear. For all-cause mortality, cardiovascular disease and total cancer, the largest risk reduction of about 10–20% occurred at roughly 30–60 minutes of muscle-strengthening activity per week, and the benefit of much larger volumes was unclear. For diabetes, the risk kept falling up to about 60 minutes per week. People who combined strength training with aerobic activity had a lower risk of all-cause, cardiovascular and cancer mortality than people who did neither ^[7]. Cohort studies show associations; they cannot prove that strength training itself causes the lower risk.

The WHO classifies its recommendation for muscle-strengthening activities as strong, based on moderate-certainty evidence, and found no evidence of a dose-response relationship with higher volumes ^[1]. As with any form of exercise, load and volume should be increased gradually.

Adults should do muscle-strengthening activities at moderate or greater intensity that involve all major muscle groups on two or more days a week, in addition to 150–300 minutes of moderate-intensity aerobic activity ^[1]. Children and adolescents should include activities that strengthen muscle and bone on at least three days a week ^[1].

Two sessions a week that each train the legs, hips, back, chest, shoulders, arms and trunk meet this guideline. A full-body session built around a squat, a hip hinge, a push, a pull and a trunk exercise is one way to do that; spreading muscle groups over more days is another. For people whose goal is muscle growth rather than health, how often to train each muscle group is a separate question from the health minimum.

Women and men adapt to strength training through the same mechanisms, so the same principles of load, volume and progression apply. A 2020 meta-analysis that compared men and women on identical programmes found no difference in relative muscle growth or in relative lower-body strength gains, while relative upper-body strength gains were larger in women ^[8]. The included studies were relatively short and involved young to middle-aged adults, most of them untrained.

The concern that strength training makes women bulky is not supported by these data. Relative gains are comparable, but women on average start with less muscle mass, so the same percentage increase corresponds to fewer kilograms. Muscle growth is also gradual: the average gain reported after ten weeks of training is about 1.4 kg of lean mass, together with a loss of fat mass ^[4]. A markedly muscular build requires years of training and eating specifically for that goal.

Strength training is also part of the recommendations during pregnancy and after childbirth. The WHO advises pregnant and postpartum women without contraindications to include a variety of aerobic and muscle-strengthening activities ^[1]. Anyone who is pregnant or has a medical condition should consult a doctor or physiotherapist before starting or changing a training programme.

The training goal determines the load, the number of repetitions and the rest between sets. For maximal strength, the American College of Sports Medicine (ACSM) recommends that trained individuals eventually emphasise heavy loads of 1–6 RM (repetition maximum: the heaviest weight that can be lifted for that number of repetitions) with 3–5 minutes of rest between sets. For local muscular endurance it recommends light to moderate loads of 40–60% of the one-repetition maximum (1RM) for more than 15 repetitions, with rest periods shorter than 90 seconds ^[2].

Muscle growth depends less on the load than strength does. A meta-analysis of 21 studies in which all sets were taken to muscular failure found greater 1RM strength gains with heavy loads (above 60% of 1RM), but similar hypertrophy with light and heavy loads ^[9]. The rep range for muscle growth is therefore wide, provided the sets are taken close to failure.

Volume also matters for hypertrophy. The ACSM recommends higher-volume, multiple-set programmes to maximise muscle growth, with an emphasis on 6–12 RM and 1–2 minutes of rest between sets ^[2]. The number of sets per muscle group per week that produces the most growth has been studied separately.

Beginners make steady progress with a simple full-body programme and a gradual increase in load. For people without training experience, the ACSM recommends loads that allow 8–12 repetitions per set, a frequency of two to three days a week, and an increase in load of 2–10% once one or two repetitions more than the target number can be completed ^[2]. Within a session, it advises training large muscle groups before small ones and multi-joint exercises before single-joint exercises ^[2].

A beginner programme needs only a few basic movements: a squat or leg press, a hip hinge such as the Romanian deadlift, a push such as the push-up or bench press, a pull such as the row or lat pulldown, and a trunk exercise. Learning the technique with a light load first, and writing down weights and repetitions, makes progression measurable. A step-by-step guide on how to start strength training covers the first weeks in more detail.

Training provides the stimulus; the adaptation happens between sessions. Rest days and sufficient food are therefore part of the programme. A separate page explains what to eat before and after strength training.