Dorsal Interossei of the Foot
DescriptionThe dorsal interossei muscles are a group of four bicephalic feather-shaped muscles that fill the space between the metatarsal bones of the foot. Their function lies in spreading the toes apart and in flexing the metatarsophalangeal joints of the second to the fifth toe. To see a 3D model of the dorsal interossei of the foot follow this link.
The interossei dorsales pedis originate on the base of the metatarsal bones I-V. With two heads each, they emanate from the two adjacent bone sides facing each other and integrate into one insertion. Parts of the interossei dorsales originate from the Ligament plantare longum which is situated on the bottom side of the foot.
The musculus (M.) interosseus dorsalis I inserts on the medial side of the base of the second toe's phalanx proximalis. The musculi (Mm.) interossei dorsales II-IV insert on the lateral side of the base of phalanges proximales II-IV.
Innervation comes from the nervus plantaris lateralis (S2-S3); in rare cases also from the nervus tibialis. 
- Flexion of the metatarsophalangeal joints (MTPJ) II-IV, medial adduction of the second toe, and lateral adduction of the third and fourth toe
- Extension of the proximal and distal phalangeal joints II-IV
Together with its synergistic partners (see below) the dorsal interossei of the foot assist in maintaining the balance during the weight shifting in foot roll-over.
- Flexion: M. flexor digitorum longus, M. flexor digitorum brevis, Mm. interossei plantares 1-2 (III-IV), Mm. lumbricales pedis 1-3 (II-IV)
- Abduction: none
- Flexion: M. extensor digitorum longus, M. extensor digitorum brevis
- Abduction: Mm. interossei plantares 1-2 (III-IV)
Although the dorsal interossei are a set of very small muscles, they are important to maintain balance during stance and gait. In case of walking or running barefoot for an unusually long period of time these muscles can sustain damage due to overloading.  This may also happen when standing, walking, or exercising on footwear that does not support the arches of the foot. Metatarsalgia, or sharp pain when putting pressure on the foot (e.g. in standing up or walking), located on the distal instep or in between the metatarsal bones can be an indicator for one or more strained dorsal interossei muscles.
The M. interosseus dorsalis pedis IV is an easily accessible muscle and is likely to be used for routine electromyographic diagnosis of neurological diseases: "Abnormal [spontaneous activity] in [fourth dorsal interosseus pedis] correlates well with the overall neurologic condition, and it may be a useful muscle to include in routine electrodiagnostic evaluation."
An Oxford scale muscle strength assessment can be done as follows for the scales of 0/1 and 2/3. Since the interossei dorsales pedis can rarely be activated willingly, the resistance tests are usually avoided.
- 0/1: supine position, the knee is in supported flexion. The therapist holds the foot in a neutral position and instructs the patient to "spread your toes".
- 2/3: supine position, the knee is in supported flexion. The therapist holds the foot in a neutral position, watching for small movements of the toes, and instructs the patient to "try to spread your toes".
To see a demonstration of the muscle palpation technique for the dorsal interossei of the foot, please go to minute 2:30 of the video.
Trigger points of the interossei dorsales pedis can be located when palpating the muscle belly in between the dorsal metatarsi on the instep. Typical patterns are pain on the base of the toes, in some cases expanding all the way to the tip of the toes, or pain on the instep extending to the anterior of the ankle and lower leg.
When stretching each of the Mm. interossei dorsalis pedis (MIDP) remember that the main function is flexion and abduction in the MTPJ:
- MIDP I: pulling the second toe into extension and lateral abduction
- MIDP II: pulling the second toe into extension and medial abduction
- MIDP III: pulling the third toe into extension and medial abduction
- MIDP IV: pulling the fourth toe into extension and medial abduction
Muscle control & strengthening
Since the interossei dorsales pedis can rarely be willingly controlled and many patients have weak foot muscles in general, the first step is to establish a feeling of muscle activation. This can be approached by toe-spreading exercises, perhaps with pre-positioning of the toes by the therapist and the command to "keep the toes in this position". Further, the patient can be instructed to strengthen the interossei dorsales pedis by spreading the toes without assistance. In general, foot muscle strengthening exercises can be used to train the whole musculature, e.g. walking in sand, picking up tissues with the toes, or holding sticks or pencils between toes.
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