Lifting weights makes the muscles grow and cardio makes us fitter, right? Well, not necessarily
Bigger muscles come as a response after muscular stress. Working out stresses/damages the muscles, and they adapt in order to better handle that 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 (1).
Those pathways are activated as a 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.
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 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 a stationary bike (2). The participants exercised for the same duration and the same level of 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 the TYPE of exercise you’re doing. Why does this matter? Because knowing this gives us more tools to chase muscle growth.
If someone has a hard time doing traditional strengthening exercises (due to reasons like previous injuries, mobility limitations, lack of equipment, etc), “cardio” exercises can be a great alternative, as long as we properly adjust the duration and the load. For example, sprinting or biking uphill for 30s, resting for 2min between sets, would be a good work to rest ratio for muscle building. As always, high intensity (relatively to each individual’s fitness level) is key for good results. And of course, the adaptations are muscle specific. Biking can build strong thighs, but a rower is a much better alternative for the arms.
Willian Alba
1. PMID: 20847704
2. PMID: 29507824
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