Muscles work by contracting, which means they shorten. As a muscle contracts, it pulls on whatever it is attached to. In general, the larger the muscles, the more pulling power it has. Muscles can pull but not push, which is why they work in pairs, acting in opposite directions. When one muscle pulls, its partner muscle relaxes.
Pulling together
All the skeletal muscles work in pairs, in the upper arm, the biceps and triceps muscles work as a team to bend and straighten the arm. The triceps pulls the forearm down, and the biceps pulls it up again.
Triceps
When the triceps muscle contracts, it straightens the arm at the elbow. The biceps, opposite the triceps, is relaxed.
Muscle attachment
The triceps is attached to the shoulder blade at this point.
Firmly fixed
The biceps is attached to the shoulder blade at these two points.
Biceps
When the biceps contracts, it pulls the forearm bones up and bends the arm. The triceps muscle is relaxed.
Tendons
Muscles are firmly attached to bones by tendons.
Picture Credit : Google
