When strengthening muscles, the principle of overload is key. To increase overload, you may use specific muscle movements by changing the intensity, frequency, time and types of movement. There are specific muscle contractions including isometric, concentric and eccentric that can be performed.
ISOMETRIC muscle contractions are held for a period of time. There is no change in movement. Think of these types of contractions as holding contractions that are part of the active or PNF stretching regimens.
CONCENTRIC muscle contractions are when the muscle shortens to move the bones. When you pull your hand towards your shoulder in a bicep curl, the muscle shortens to bring your hand upward.
ECCENTRIC muscle contractions are when the muscle lengthens to move the bones. When you slowly lower your arm in a bicep curl while controlling a heavy weight against gravity, the muscle lengthens to bring your hand down. Although the tricep can extend the arm, gravity forces the bicep to control the motion, or your arm would extend quickly.
Understanding that stronger muscles are needed for more flexibility is key. Tight muscles are typically weak muscles, which means overloading to increase muscle strength will allow a greater range of motion.