Concentric Phase

At the muscular level, the concentric phase occurs when the force produced by the muscle exceeds the external resistance, causing the fibers to shorten. The actin and myosin filaments slide over each other, bringing the muscle ends closer together. It's the contraction that feels most natural: you push, you pull, you lift. Every rep in the gym has at least one concentric phase, and it's the one most people instinctively focus on.

There's an important fact to know: in the concentric phase you produce less force than in the eccentric and isometric phases. The muscle is biomechanically weaker while shortening. That's why the maximum load you can lift (your concentric 1RM) is lower than the maximum load you can control on the way down. This has direct programming implications: techniques like negatives or isometrics exploit precisely this strength gap.

The speed of the concentric phase shapes the type of stimulus you give the muscle. An explosive concentric, with the intent to move the weight fast, recruits more fast-twitch fibers. A slow, controlled concentric increases time under tension. There's no single rule: it depends on your goal. For strength and power, push with maximal intent. For hypertrophy, maintain control without artificially slowing down. In both cases, the concentric phase is where the real mechanical work gets done.