Why does my friend build muscle faster than me on the same workout?
The short answer
Roughly half of how you respond to training is genetic, and a single gene called ACTN3 helps decide whether you are built more for power or for endurance.ACTN3 makes a protein in fast-twitch muscle fibres, the ones that fire during sprints and heavy lifts. People with the working version tend to gain explosive strength quickly. People with two non-working copies - common across South Asian populations - lean more towards endurance and recover differently.
This is not an excuse and it is not a ceiling. It is a starting point. If you are a power type doing only long slow cardio, you are training against your wiring. If you are an endurance type chasing one-rep maxes, the same. The fix is not more effort. It is the right kind of effort.
Do this today
Pick one strength session and one endurance session this week. Notice which one leaves you energised and which one wrecks you. That gap is data.
Genes in playACTN3ACEPPARGC1A