09/01/2026
Leg press does not train the whole thigh, and leg extensions do not train the hip. Twelve weeks of MRI scans showed which muscles each machine left out.
Kinosh*ta and colleagues (Med Sci Sports Exerc 2026) had 17 untrained adults, 11 men and 6 women, train one leg on the leg press and the other on the leg extension machine, five sets of ten at 70 percent of maximum, twice a week for twelve weeks. Because both legs belonged to the same person, diet, sleep and genetics were identical, and MRI measured the volume of 17 hip and thigh muscles before and after.
The re**us femoris, the strip of quadriceps running down the centre of the front thigh, is the one quad muscle that also crosses the hip, where it works as a hip flexor. A leg press extends the hip and the knee together, so a muscle that pulls the hip the other way is fighting the movement and the nervous system turns it down, which the study
confirmed by recording its electrical activity.
Cyclists show the same pattern, with large single-joint thigh muscles and a re**us femoris no bigger than in people who do not train (Ema et al., Scand J Med Sci Sports 2016).
Across all 17 muscles, leg press added 292 cubic centimetres of muscle, a little over a cupful, against 127 for leg extensions, about half a cup. Most of that extra came from the hip: the gluteus maximus grew 15 percent and the adductor magnus on the inner thigh 6 percent on leg press, and neither moved on leg extensions. The three vasti, the bulk of the quadriceps, grew 5 to 7 percent on either machine. The re**us femoris grew 13 percent on leg extensions and 1 percent on leg press, a change indistinguishable from zero. Ten weeks of squats give a similar map: knee extensors up about 5 percent, glutes up, re**us femoris unchanged (Kubo et al., Eur J Appl Physiol 2019).
Seventeen people is a small trial:
whole-quadriceps growth of 7.1 percent on leg extensions against 4.9 on leg press was not a significant difference, so the two may or may not differ there. Legs were assigned by dominance rather than at random. Everyone was untrained, and beginners grow from almost any stimulus, so the gaps could change in experienced lifters. Both exercises used machines at a fixed seat angle, and a free-weight squat or a different backrest setting changes how far the hip bends and could change what the re**us femoris does.
The scans describe a division of labour rather than a winner. Leg press covered the hip, the inner thigh and most of the quadriceps in one movement and added more than twice the total muscle, while leg extensions were the only one of the two to reach the re**us femoris, a muscle that matters for sprinting and kicking. A programme built on one machine alone leaves a measurable muscle untrained, and the scans show which one.
Kinosh*ta et al., Med Sci Sports Exerc 2026;58(7):1566 · PMID 41630124
Ema et al., Scand J Med Sci Sports 2016;26(7):782 · PMID 26148051
Kubo et al., Eur J Appl Physiol 2019;119(9):1933 · PMID 31230110
