Flexor digitorum brevis flexes the middle phalanges. The ulnar nerve runs between the flexor digitorum superficialis (laterally) and flexor digitorum profundus medially. It passes along the radial side of the tendon of the flexor pollicis longus. The underlying mechanism involves the tendon sheath being too narrow for the flexor tendon. The lumbricals, intrinsic muscles of the hand, attach to the tendon of flexor digitorum profundus. The body maintains the balance between the tone of flexor and extensor muscle groups. The lumbricals attach in part to the flexor digitorum profundus tendons. Flexion is typically instigated by muscle contraction of a flexor. The iliopsoas is the most powerful hip flexor. It lies in the same plane as the flexor digitorum profundus. The flexor pollicis brevis (FPB) has two heads. An important plantar flexor, it is crucial for ballet dancing. Patients generally present with increasing weakness in the facial, neck flexor, and respiratory muscles. The thumb has one long flexor and a short flexor in the thenar muscle group. This allowed for much more surface area for the flexor muscles.