Muscle Burst is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Target creature gets +X/+X until end of turn, where X is 3 plus the number of cards named Muscle Burst in all graveyards.

How it works : Muscle Burst

Original English text — Wizards of the Coast / Scryfall

No additional ruling has been published for this card.

Interactions with other cards

These cards' text mentions Muscle Burst.

Diligent Farmhand

{1}{G}, Sacrifice this creature: Search your library for a basic land card, put that card onto the battlefield tapped, then shuffle. If this card is in a graveyard, effects from spells named Muscle Burst count it as a card named Muscle Burst.