Tonic vs Phasic Muscles

Tonic and phasic muscles are two types of muscles in the body that serve different roles. Tonic muscles are mainly responsible for maintaining posture. They work all the time and don’t get tired easily because they are made of slow-twitch fibres. These muscles are good at endurance and tend to become tight or overactive when the body is out of balance or under stress. They often need to be stretched or relaxed during rehabilitation.

Phasic muscles, on the other hand, are designed for quick and powerful movements. They are made of fast-twitch fibres, which means they are strong but get tired quickly. These muscles usually become weak or inactive when something goes wrong, like poor posture or injury. In rehab or training, they often need to be strengthened and reactivated.

For example, the hamstrings and upper back muscles are mostly tonic, while the glutes and lower shoulder muscles are mostly phasic. When the body isn’t working properly, tonic muscles get tight and phasic muscles get weak, which creates imbalances and poor movement. Correcting these issues usually involves relaxing the tonic muscles and strengthening the phasic ones.