Lifting weights makes the muscles grow, and cardio makes us fitter, right? Well, not necessarily.
The muscles getting stronger is a response mechanism after a stress. Working out stresses/damages the muscles, and they adapt in order to be better able to handle that same stressor in the future, if necessary.
A few signaling pathways (Akt/mTOR, MAPK, and Calcium-dependent pathways) are key for muscle anabolism. When activated, they stimulate the body to increase production of hormones that promote muscle growth, like IGF-1, testosterone and growth hormone (PMID: 20847704)
Those pathways respond to exercise. More specifically, they respond to mechanical tension (force + stretch), muscle damage and metabolic stress. So any exercise that creates one or more of these stressors in the muscle, will initiate a hypertrophy process.
So it’s not about what you do, it’s about how you do it. If you lift weights but you don’t create mechanical tension, muscle damage or metabolic stress (perhaps because you’re using weights that are too light, or not doing enough sets), you won’t grow your muscles. But if you’re constantly riding a bike uphill, and your muscles are burning like crazy, guess what, your thigh muscles will get bigger.
This is well exemplified in a study, where the researchers compared acute adaptations after participants worked out using a leg press machine or an ergometer bike (PMID: 29507824). The participants did the same time and effort on both, and guess what?! They had the exact same adaptations, regarding blood lactate, electromyography, Vo2, muscle swelling, etc.
So when we exercise, the factors that promote hypertrophy will be volume and intensity, and not which TYPE of exercise you’re doing. Why does this matter? If someone has a hard time doing traditional strengthening exercises (due to reasons like previous injuries, mobility limitations, lack of weights, etc), “cardio” exercises can be a great alternative to build strong muscles. Cycling sprints, walking uphill, etc. Of course, those adaptations are muscle specific. If you want bigger arms, the rower will be better than the bike, for example.
Willian Alba
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