How Muscles Contract

The muscles of the human body are over 70% water. About 22% of muscle tissue is protein mostly consisting of millions of strands of Actin, a thin protein and Myosin, a thick protein.

Muscles contract when the actin and myosin protein strands slide over each other causing the length of the cell to shorten. Calcium, Magnesium and two other proteins aid contraction by helping to form cross-bridges as more of the strands pull together.

Below is a simple diagram of a bicep muscle cell illustrating how the cell contracts. Notice how the overall length of the cell shortens on contraction.

Length       of       muscle        cell       before        contraction

*    =   cross bridges

Length of muscle cell when contracted

Inside a muscle cell   Actin protein filament          * * * * * * * * * * * Myosin protein filament

We can see this happening in our muscles. Straighten your arm and roughly measure  the length of the biceps muscle, then curl the hand towards the shoulder and measure again. The bicep muscle will be shorter in length. You can see this happen in all muscles; the calf muscle when we tip toe or the thigh muscle when we straighten the leg.

In a muscle's fully contracted position illustrated in the second diagram notice there are more cross-bridges formed (more stars in diagram). This is where a muscle is in its strongest position. For example, imagine a child trying to pull down his dad's arm from the shoulder, if dad holds his arm closer to his shoulder the biceps will be stronger because there will be more cross-bridges formed between the sliding filaments.

How muscles grow

Muscles grow as a result of demands placed on cells by the intensity of contractions. When under intense stress the actin and myosin protein filaments within muscle cells shatter and breakdown. During rest (between workouts) these filaments are repaired back to their pre-workout state, however the stress also created a need for cells to lay down additional protein so they become better prepared for similar stress levels. The extra protein filaments make the cells stronger as there are more protein strands working to pull a load. The additional protein also causes the cell to hold more water, thus they thicken and enlarge!

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