Flexor digitorum superficialis

Flexor Digitorum Profundus (FDS)

The FDS is a thickened muscle which runs along the backside of your thigh from the hip bone to your shin bone. Its main function is to provide support for the knee joint and to prevent excessive motion of the leg during running or jumping movements. When it becomes irritated, it causes pain in one or both legs, especially when running or jumping activities.

It is not uncommon for someone with a history of repetitive stress injury (RSI) to develop FDS. The most common cause is overuse injuries such as running or sports related activities. Other possible causes include:

Over stretching of the muscles, ligaments and tendons.

Trauma to the area, usually due to falls or other trauma.

Injury to the same muscle group multiple times resulting in scar tissue formation.


Athletes may have a predisposition towards developing FDS. These athletes are often involved in high impact activities such as football, basketball, soccer and wrestling. They also run or jump frequently and at high speeds.

Their bodies do not have the ability to heal itself properly if they sustain repeated strains or tears of their muscles and tendons due to these activities.

FDS also develops due to multiple surgeries on the same area of the body, especially if the surgeries involve incisions that cross the knee joint. It may take several months for patients to experience pain after an operation or injury, so it is possible that you may have FDS without knowing about it.

FDS can also develop as a result of age-related degeneration or wear and tear of the muscle. It is extremely common for people over the age of 40 to show signs of FDS.

Flexor digitorum superficialis pain

The pain associated with FDS is usually located in the back of your knee and can range from a dull ache to a sharp, stabbing pain. Pain may also be felt along the back of your lower leg and heel, though this is less common.

Flexor digitorum superficialis signs and symptoms

The signs and symptoms of FDS usually include one or more of the following:

Pain in your back of the knee or along the back of your lower leg or heel. The pain is usually felt during or after physical activities.

Stiffness in the back of your knee that prevents you from bending your leg.

Strenuous physical activities lead to an increase in pain.

Swelling of your back of the knee.

Tight muscles in your lower leg and heel.

Rest, ice, elevation and over the counter pain medications can help relieve the pain and reduce swelling. Anti-inflammatory medications such as Ibuprofen can also help relieve pain and reduce inflammation. In more serious cases, your doctor may prescribe a corticosteroid injection to reduce inflammation in the area and provide long term relief.

Treatment for FDS is aimed at alleviating pain and preventing further damage. If you are experiencing pain, rest is extremely important so as to allow your body to heal itself from the inside out. If you experience a sharp, stabbing pain in your knee, thigh or lower leg, you should stop the physical activity that caused it immediately.

Continuing to stress an injured area can cause further tearing and damage to the muscles.

In cases where stress and overuse have caused the development of scar tissue, your doctor may suggest surgery to repair the damaged tissue. In extreme cases, such as when a patient has undergone several knee surgeries or has a degenerative condition that hinders proper healing ability, your doctor may suggest surgery to remove the damaged tissue. This allows the healthy tissue to heal properly, relieving pressure and stress on the knee joint.

If you suffer from FDS, it is best to consult with a doctor or physical therapist in order to treat and prevent further damage.

Sources & references used in this article:

Effect of grip span on maximal grip force and fatigue of flexor digitorum superficialis by JR Blackwell, KW Kornatz, EM Heath – Applied ergonomics, 1999 – Elsevier

Variations of the flexor digitorum superficialis of the small finger by GJ Austin, BM Leslie, LK Ruby – The Journal of hand surgery, 1989 – Elsevier

Tensions of the flexor digitorum superficialis are higher than a current model predicts by JT Dennerlein, E Diao, CD Mote Jr, DM Rempel – Journal of biomechanics, 1998 – Elsevier

Platelet rich plasma (PRP) enhances anabolic gene expression patterns in flexor digitorum superficialis tendons by LV Schnabel, HO Mohammed, BJ Miller… – Journal of …, 2007 – Wiley Online Library

Telemetered electromyography of flexor digitorum profundus and flexor digitorum superficialis in Pan troglodytes and implications for interpretation of the OH 7 hand by RL Susman, JT Stern Jr – American Journal of Physical …, 1979 – Wiley Online Library

… stem cells and insulin‐like growth factor‐I gene‐enhanced mesenchymal stem cells improve structural aspects of healing in equine flexor digitorum superficialis … by LV Schnabel, ME Lynch… – Journal of …, 2009 – Wiley Online Library

Selective recruitment of single motor units in human flexor digitorum superficialis muscle during flexion of individual fingers by TJ Butler, SL Kilbreath, RB Gorman… – The Journal of …, 2005 – Wiley Online Library