According to the American Council on Exercise, strength training is defined as “exercising with progressively heavier resistance for the purpose of strengthening the muscular skeletal system.” Equipment used includes weight machines, free weights or resistance bands with each providing increasing resistance as the muscles develop. In theory, progressively increasing resistance could build muscle mass and strength infinitely as muscles overloaded keep getting larger. Conventional strength training usually targets select major muscle groups.
Yoga on the other hand provides a more balanced approach to working muscles and muscle groups, however, the muscles will only get to a certain size based on bodyweight because of the lack of “progressively heavier resistance”. But because yoga uses bodyweight, it will develop muscles and strength up to a point, especially if you are first starting out.
While the question is “Can yoga replace strength training?” the real question should be “Should yoga replace strength training?” in regard to just how much muscle mass does one need. Yoga provides more useful full-body toning as most poses don’t focus on just one muscle group, such as a bicep curl, but multiple muscles large and small of a targeted group.
While strength training might allow you to carry bags of groceries easier, yoga would allow you to carry the same bags of groceries, but also allow you to better walk, twist and bend while carrying the groceries and putting them away.
So in the end, it comes down to goals – what goal do you hope to gain from strength training? If it is to get huge biceps, then strength training is the best means to accomplish that goal. However if your goal is to tone up and define full body muscles, and maybe add just a little mass, then yoga could be for you. Plus yoga teaches breathing control and stress reduction through meditation instead the singular goal of just building muscle as is the case with pure strength training.
Many experts recommend a combined approach to get the maximum benefit from each. Do yoga but occasionally mix in some conventional strength training. Why? Because the two disciplines work in exact opposites. While strength training use concentric contraction – the shortening of muscles to do the movement – yoga uses eccentric contraction – the elongation of the muscles to do the pose. By doing both, muscles are better defined while at the same time strengthened.