Muscles Work in Pairs
While one muscle contracts and shortens the opposing muscle group relaxes and lengthens.
In figure A (top), the bicep contracts while the tricep relaxes) - bicep curl
In figure B (bottom), the tricep contracts while the bicep relaxes (tricep press)
Muscles work together to produce movement. The “agonist” muscle group is the muscle group that is contracting to produce movement and the “antagonist” muscle group is the opposing muscle group that lengthens to allow the movement to take place.
If one muscle group is much stronger or weaker than the opposing group, the resulting muscle imbalance can lead to an increase risk of injury. A common example of this is with sprinters “pulling a hamstring” (muscle strain) during a sprinting activity. Most sprinters have highly developed quadriceps muscles but less developed hamstring muscles and these muscles get overpowered during the sprint and get injured. To reduce this risk, it is important to work muscle groups in pairs.