No body part grows by beating it everydayâ€”you need to rest to let your arms recover. In the hours after a workout, your muscles lose strength and power as they heal; after 36-48 hours, the muscle actually gets stronger, which is a process called â€œsupercompensationâ€.
And Why?. Most of us train more than one muscle group in a workout, unless we train seven. In the case of arms, the gym wisdom was to train triceps first.