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Resistance training

Also called strength training, weight training, progressive resistance training

Working muscles against a load that rises over time. How research defines it and progressive overload, what it asks of muscle, tendon and bone, what reviews found about dose, and what studies found about injury.

Region
Across the body
Updated
September 30, 2026
Reviewed
September 30, 2026 · how entries are checked

What it is

A broad name for many ways of lifting

A 2009 position statement on youth training from the National Strength and Conditioning Association (NSCA) calls resistance training a specialised way of conditioning1. It involves the progressive use of many kinds of load and many types of training1. The statement adds that people sometimes use strength training and weight training to mean the same thing1. Of the three, it sees resistance training as the broadest, covering more types of training and more goals1. It keeps the word weightlifting for the competitive sport of the snatch and the clean and jerk1.

The American College of Sports Medicine (ACSM) issued a position stand in 20095. It describes strength programmes as including concentric (shortening), eccentric (lengthening) and isometric (holding) muscle work5. It also describes exercises on one or both limbs, moving one joint or several5. Two of these kinds of work have their own pages: eccentric training and isometric training5.

Progressive overload

The ACSM position stand states that progressive programmes are needed to keep driving adaptation toward a given goal5. An anatomy textbook states that lifting the same weight without adding to it does not make a muscle bigger6. A 2022 trial found that a group that added repetitions changed in similar ways to a group that added load7.

A 2022 trial report calls progressive overload a principle of programme design that usually means adding load7. It notes that adding repetitions is another way to raise the demand, and has had little study7. The trial followed 43 people with at least a year of leg training for 8 weeks7. One group added load and the other added repetitions, and the two changed in similar ways7. Strength slightly favoured adding load and front thigh growth modestly favoured adding repetitions, but the gaps were of questionable practical meaning7.

Heavy for strength, light for endurance?

The 2009 ACSM position stand recommends different loads for different goals5. For experienced lifters seeking strength, it recommends 1 to 12 repetition maximum (the heaviest load a person can lift a set number of times)5. Over time the emphasis moves to 1 to 65. For muscle growth it recommends 1 to 12 repetition maximum, stressing 6 to 125. For endurance it recommends light to moderate loads, 40 to 60% of the one-repetition maximum, for more than 15 repetitions5.

A 2021 review calls this the repetition continuum, and says some of its assumptions lack support in present research8. It proposes that muscles can adapt across a wide range of loads8.

What it loads

Muscle

A review of heavy strength training names a larger cross-section of whole muscle and single fibres as the main structural change9. It says this comes from more and larger myofibrils (the contracting strands inside a muscle fibre)9. Another review names mechanical signals as the prime candidates for what starts muscle growth10. The muscle page covers this growth in more depth9.

Tendon

A meta-analysis of 61 studies looked mostly at the Achilles and patellar tendons, and mostly at resistance training11. Loading raised tendon stiffness (the force it takes to stretch it by a given amount) moderately11. It raised modulus (how stiff the material is in itself) a lot and cross-sectional area a little11. Resistance training raised stiffness a lot and jump training a little11. Two groups doing concurrent training (resistance and aerobic training in one programme) showed no change in stiffness11.

  • How far the tendon stretched may matter. Programmes stretching it about 5% gave large stiffness gains, and those near 3% gave no significant gain11. Only three studies reported how far the tendon stretched11.
  • The type of muscle work made no clear difference. In high-intensity programmes, concentric, eccentric, combined and isometric work all raised stiffness, with no significant differences between them11.
  • The evidence is narrow. It rests mostly on untrained or recreationally active people and the Achilles, patellar and quadriceps tendons11. Three studies of well-trained athletes found no change in patellar tendon stiffness11.

Bone

In a meta-analysis of 14 randomised trials in adults aged 65 and over, resistance training raised hip bone density by about 2.8%12. It did not raise density in the lumbar (lower back) spine, and certainty was lower for bone than for strength12. A review of 18 trials in adults aged 50 or over compared high and lower intensity13. Density at the spine and femoral neck (below the ball of the hip) did not differ significantly between them13.

One trial followed 101 postmenopausal women with low bone mass14. High-intensity resistance and impact training raised their spine density by 2.9%14. Low-intensity home exercise saw a 1.2% fall14.

How the body adapts

Strength and size rise with almost any plan

One network meta-analysis of randomised trials in healthy adults compared plans that differed in load, sets and weekly sessions2. Across 178 studies, every plan beat no exercise for strength, and loads above 80% of the one-repetition maximum gave the most2. Across 119 studies, every plan beat no exercise for muscle size, and all built muscle to a comparable degree2. Another network meta-analysis of 28 studies found greater hypertrophy (muscle growth) in untrained people15.

Nerves adapt early, muscle follows

A 2021 review states that early strength gains are thought to come mainly from neural changes, though where these happen is unclear3. A meta-analysis of 30 randomised trials found subtle changes, in the brain’s outer layer and below it, that raise the drive to muscles16. Its authors concluded these likely add to the rise in strength16. A review of heavy strength training adds that muscle growth processes also begin at the start9. The nerve page covers this mix in more depth9.

One review proposes that gains in muscle size over about the first four sessions are mostly swelling from muscle damage17. It puts modest growth after about 10 sessions, and true growth after about 1817. One review notes that adult muscle can change after 3 to 4 weeks of heavy training, but no such quick tendon changes have been reported18. In 20 studies, soreness and creatine kinase (a blood marker of muscle damage) were higher after a first bout of damaging exercise than a second19.

What reviews found about dose

  • Load. In 28 studies of adults training to failure, growth did not differ between low, moderate and high loads15. Strength rose more with high and moderate loads than low ones15. A second review of 21 studies also found more one-repetition-maximum strength with heavier loads, and similar changes in size20.
  • Weekly sets. In 15 studies, each extra weekly set was linked to a 0.37% larger gain in muscle size21. In 9 studies, strength also rose with more weekly sets22. In 67 studies of mostly young men, returns shrank as sets rose, much more so for strength23.
  • Sets per exercise. In 8 studies, several sets were linked to 40% larger growth effects than one set24.
  • Sessions a week, for size. With total sets matched, a 2016 meta-analysis found two sessions a week built more muscle than one25. A 2019 meta-analysis of 25 studies found no significant difference with sets matched26. A third analysis, of 67 studies, found an effect on size compatible with a negligible one23.
  • Sessions a week, for strength. In those 67 studies, more sessions went with more strength, with shrinking returns23. A review of 22 studies, mostly in untrained people, found no significant effect once total sets were matched27.
  • Training to failure. In 15 studies of young adults, ending sets at failure made no significant difference to strength or size overall28. In the trained people among them, failure did show a significant effect on growth28.
  • Rest between sets. In 23 studies, strength rose well with rests under 60 seconds, but trained people seemed to need over 2 minutes for the most strength29. In 9 studies, rests over 60 seconds seemed to give a small growth benefit, with no appreciable difference beyond 9030.

In 43 studies of healthy adults, adding aerobic training did not reduce gains in muscle size or top strength31. Gains in explosive strength may be smaller, more so when both were done in one session31.

In older adults and young people

A Cochrane review of 121 randomised trials in 6,700 older people found a large effect on strength32. It found a small gain in physical ability and a moderate to large effect on rising from a chair32. In 25 trials in adults averaging 65 or older, strength rose substantially but muscle size only a little33. In 18 trials of adults aged 50 or over, higher intensity gave more leg press and leg extension strength13. Falls and adverse events did not differ significantly between the two intensities13.

In a pooled analysis of healthy people under 18, training raised strength, and the ability to gain it seemed to grow with age and maturity34. In 43 studies of young athletes aged 6 to 18, gains were moderate for strength and jumping35. Gains were small for sprinting, agility and sport-specific performance35.

A 2016 overview states that gains in children rely partly on better nerve-muscle control and partly on changes in muscle and tendon36. It adds that they adapt much as adults do, but more subtly36. A 2026 meta-analysis of 23 studies in 1443 people under 18 found training gave a small rise in muscle size against untrained controls37. An older view, from a 2008 Canadian position paper, credited children’s strength gains mainly to the nervous system38.

Injury in sport and in the gym

An analysis of 25 trials of exercise programmes (26,610 participants) grouped them by type4. For the strength-training trials, the relative risk of injury was 0.315, and stretching showed no benefit4. In 6 trials with 7,738 participants aged 12 to 40, the strength-training groups had a relative risk of 0.33839. In those trials, higher volume and intensity went with lower injury risk39.

Injuries also happen during resistance training. Two reviews of weight-training sports, drawing mostly on backward-looking or low-quality studies, reported these rates40,41:

SportInjuries per 1,000 hours
Bodybuilding0.24 to 140
Powerlifting1.0 to 4.441
Olympic weightlifting2.4 to 3.341
Strongman4.5 to 6.140
Highland Games7.540

These sports appear to have fairly low injury rates next to common team sports40. Shoulders, lower backs, knees, elbows and wrists or hands were hurt most often40. Strains, sprains and tendinitis (pain and swelling of a tendon) were the commonest types40. US emergency department data estimated 970,801 weight training injuries from 1990 to 200742. In 65.5%, weights dropped on the person, and 90.4% involved free weights42.

Connections

The ACSM position stand describes resistance programmes as using both single- and multi-joint exercises5. Related exercise pages: back squat, deadlift, bench press, overhead press and barbell row. More exercise pages: biceps curl, triceps extension, calf raise, single-leg Romanian deadlift, shrug, farmer’s carry and wrist curl. Related method pages: free weights and machines, elastic resistance training, kettlebell training and blood flow restriction training.

It loads muscle, tendon, bone and the nerve pathways that drive muscle9,11,12,16. In one meta-analysis, jump training gave a small rise in tendon stiffness and resistance training a large one; see plyometric training11.

Resistance training loads

Resistance training is used in

Resistance training is related to

Resistance training is compared with

Sources

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Every source the library cites

Changes

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