What is Flexor Digitorum Longus?

 Flexor Digitorum Longus Muscle

Introduction

The Flexor Digitorum Longus (FDL) is a long, slender muscle situated in the deep posterior compartment of the leg. It plays a crucial role in lower limb function, particularly in flexing the lateral four toes and aiding in balance and gait. Although smaller and less robust compared to other leg muscles, the FDL's contribution to fine motor control and stabilization during dynamic movement is indispensable.

Anatomy

Origin

• The FDL originates from the posterior surface of the tibia, specifically from the middle third, just below the soleal line.

Insertion

• The muscle travels downward, transitioning into a tendon that passes behind the medial malleolus. It then enters the sole of the foot, where it divides into four tendinous slips, each inserting on the base of the distal phalanges of the second to fifth toes.

Nerve Supply

• Tibial nerve (a branch of the sciatic nerve), primarily arising from the nerve roots L5 to S2.

Blood Supply

• Posterior tibial artery.

Function

• Primary Action: Flexes the distal phalanges of the lateral four digits.

• Secondary Actions: Assists in plantarflexion of the foot at the ankle joint and helps maintain the longitudinal arch of the foot during walking or standing.

Palpation

Palpating the FDL directly is challenging due to its deep location. However, indirect assessment can be performed:

• Position the patient in prone with the foot overhanging the table.

• Ask the patient to curl their toes while the examiner resists toe flexion.

• Deep palpation posteromedially on the tibia may give a sense of activity in the FDL muscle belly.

Muscle Power Testing

• Patient Position: Supine or sitting.

• Procedure: Ask the patient to flex their toes against resistance provided by the examiner.

• Grading follows the MRC scale (0–5), assessing the strength of toe flexion.

Length Testing

• The FDL is tested for tightness by passively extending the toes while dorsiflexing the ankle.

• A restricted range or discomfort indicates tightness or dysfunction in the muscle.

Therapeutic Exercises

1. Toe Towel Curls

• Position: Seated with foot flat on the floor.

• Procedure: Place a towel under the foot and instruct the patient to scrunch the towel toward themselves using the toes.

2. Resistance Band Toe Flexion

• Position: Long sitting with a resistance band looped around the toes.

• Procedure: The patient flexes the toes against the resistance of the band, strengthening FDL activation.

3. Marble Pickup

• Position: Seated.

• Procedure: Pick up marbles using toes and place them into a container. Enhances fine motor coordination.

Trigger Points

Trigger points in the FDL may refer pain to the arch of the foot or the plantar surface of the toes. These points are usually caused by overuse, faulty biomechanics, or prolonged standing. Dry needling, myofascial release, and stretching can help alleviate these symptoms.

Common Injuries and Clinical Relevance

1. Tendinitis/Tendinopathy:

• Overuse of the FDL, especially in athletes, can cause inflammation and pain along the posterior tibia and plantar surface.

2. Tarsal Tunnel Syndrome:

• The FDL tendon runs through the tarsal tunnel; compression in this area can cause neuropathic symptoms.

3. Flat Feet (Pes Planus):

• The FDL plays a role in maintaining the arch. Dysfunction can contribute to collapse of the medial longitudinal arch.

4. Posterior Tibial Tendon Dysfunction:

• Though not directly related to FDL, compensatory overuse of FDL may occur when the posterior tibialis is weakened.

Conclusion

The Flexor Digitorum Longus is a vital yet often overlooked muscle in the posterior leg compartment. Its contributions to toe flexion, arch support, and locomotion underline the importance of understanding its anatomy and clinical relevance. Proper assessment and strengthening of the FDL are key components in lower limb rehabilitation programs.

Frequently Asked Questions (FAQs)

Q1: What is the primary function of the Flexor Digitorum Longus muscle?
A1: The Flexor Digitorum Longus (FDL) primarily functions to flex the lateral four toes (digits 2–5) and assists in plantarflexion of the foot at the ankle. It also helps maintain balance during standing and walking.

Q2: How can I palpate the Flexor Digitorum Longus?
A2: The FDL is deep and challenging to palpate directly. However, its tendon can be felt behind the medial malleolus of the ankle. Flexing the toes against resistance may help in locating it indirectly.

Q3: What are some common injuries involving the FDL?
A3: Common issues include tendonitis due to overuse (especially in athletes), strain during excessive plantarflexion, and entrapment in tarsal tunnel syndrome.

Q4: What exercises can strengthen the Flexor Digitorum Longus?
A4: Toe curls using a towel, marble pickups, resisted toe flexion using bands, and balance exercises that challenge the toe flexors are excellent ways to strengthen the FDL.

Q5: What nerve supplies the Flexor Digitorum Longus?
A5: The FDL is innervated by the tibial nerve, specifically nerve roots L5–S2.

Q6: Is the Flexor Digitorum Longus involved in any postural or gait problems?
A6: Yes. Weakness or dysfunction in the FDL can contribute to flatfoot deformity, claw toe posture, and impaired toe-off during gait.

References

1. Neumann DA. Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation. 3rd ed. Elsevier; 2017.

2. Standring S. Gray's Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. Elsevier; 2020.

3. Travell JG, Simons DG. Myofascial Pain and Dysfunction: The Trigger Point Manual. Vol. 1. Lippincott Williams & Wilkins; 1999.

4. Kendall FP, McCreary EK, Provance PG. Muscles: Testing and Function, with Posture and Pain. 5th ed. Lippincott Williams & Wilkins; 2005.

5. Clarkson HM. Musculoskeletal Assessment: Joint Range of Motion and Manual Muscle Strength. 3rd ed. Lippincott Williams & Wilkins; 2012.