The theory of progressive overload - A MUST read!
This is a very well written article which nails the explanation of progressive overload. If you want to see real gains in resistance training then this is a must. Especially understanding the rest times between sets, depending on your particular training goal. If unsure ask any of our trainers what your work to rest ratio should be for your particular goal. This principle also applies to people wanting to increase cardiovascular fitness. This is the secret - that shouldn't be a secret! https://www.bodybuilding.com/content/progressive-overload-the-concept-you-must-know-to-grow.html