Next time you hold some interesting creature in front of you, against the force of gravity, take a moment to consider what you have done. This type of muscle contraction is called isometric because the muscle length stays the same (“iso” = “same”). To prevent itself from either shortening or lengthening, the muscle must produce an internal force (tension) that perfectly balances the gravitational force. It does this by activating just the right number of muscle cells to balance the load. Within each active cell, millions of proteins are engaged like soldiers in a desperate tug-of-war against gravity. Recruiting the right number of “regiments” (muscle cells) ensures that the army neither wins, nor loses.

On a microscopic level, your muscles look like a “honeycomb” of flattened hexagons. A stack of of these hexagons is called a sarcomere. The outer frame of each hexagon is made of actin, and surrounds an inner core made of myosin. During muscle contraction, it is the myosin that performs the “tug-of-war”, pulling on the actin. If it is able to bring the two sides of each hexagon together, this will shorten the sarcomeres, and thereby shorten the entire muscle. Just like a real tug-of-war, each myosin goes through a cycle of reaching out, gripping the actin, pulling it, and then releasing again to grip at a different point.

In an isometric contraction the tension is equal to the load, so the tug-of-war reaches an impasse and the sarcomeres are frozen in position. Each myosin molecule is able to pull its patch of actin a little bit, but its grip may slip due to the external force, and also, when it releases its grip to pull at another patch, the external force tends to pull the actin back the other way. It’s analogous to someone who is trying to climb a rope but keeps slipping back to where they started. Our myosin foot-soldier finds itself in a stalemate – but does not realize that it is all part of a carefully orchestrated plan. Your brain commands many isometric contractions throughout the day, for example in the muscles that maintain posture.

And of course there are other ways to use your muscles. When you first picked up that creature, your muscle was able to “win” against gravity because your brain recruited enough cells for the task, with a large enough army of myosin molecules to make progress on their rope climb. This is called a concentric muscle contraction because the muscle’s ends move “toward the center” and the muscle shortens. Conversely, placing that creature gently back on the ground means allowing your muscles to lengthen in a controlled manner (eccentric contraction = “away from the center”). In this case the muscle “loses” to gravity – the myosin molecules slide right down to the bottom of their rope, while maintaining some grip to slow their descent.