Heel pain experienced when lying flat on the back (the supine position) is a localized symptom that often confuses people because it occurs during rest, not activity. This discomfort is distinct from common heel pain felt when standing or walking, which involves weight-bearing structures. When the body is fully supported by a mattress, the pain is usually caused by the mechanical interaction between the bony anatomy and the resting surface, rather than an injury. Understanding this positional pain involves examining direct pressure on the heel bone, the sensitivity of local nerves, and any underlying tension in the leg’s soft tissues.
Direct Pressure on the Calcaneus
The most immediate cause of heel pain when lying flat is the sustained, localized pressure placed directly upon the calcaneus, the largest bone in the foot. The calcaneus forms a distinct bony prominence at the posterior base of the foot. This structure is not designed to absorb static, horizontal force like it handles vertical force during walking. When the leg is relaxed and straight, the weight of the entire limb concentrates onto this small area of bone pressed against the mattress.
This sustained pressure can irritate the periosteum, the thin, sensitive membrane covering the bone surface. The heel is also protected by a specialized fat pad, a shock-absorbing structure. While standing, this fat pad spreads out to cushion the foot, but when compressed horizontally against a hard surface, its protective function is compromised. This compression restricts blood flow and causes localized pain and tenderness, often described as a deep ache or a bruised feeling, which subsides only when the position is changed.
Positional Nerve Irritation
The supine position can also create tension or compression along neural pathways running down the leg. The medial calcaneal nerve, a sensory branch of the posterior tibial nerve, is particularly vulnerable in this area. This nerve supplies sensation to the skin and heel pad, making it a frequent source of localized discomfort.
When the foot relaxes naturally while lying on the back, it often falls into a slight plantar-flexed position (toes pointing away from the body). This position can inadvertently create tension along the nerve pathway as it curves toward the heel. Localized pressure from the mattress may then compress the nerve against the calcaneus or surrounding tissues, causing sharp, shooting, or burning pain. Since this discomfort is nerve-related, it is highly sensitive to slight positional shifts. Merely turning the foot or slightly bending the knee can provide immediate relief. The nerve can also be compressed near the heel bone, potentially entrapped by the fascia of the abductor hallucis muscle.
Underlying Musculoskeletal Factors
Positional heel pain is often intensified by pre-existing conditions that sensitize the musculoskeletal structures surrounding the calcaneus. Tightness in the calf muscles, specifically the gastrocnemius and soleus, is a common predisposing factor. These muscles connect to the heel bone via the Achilles tendon, and when tight, they maintain a constant pulling force on the calcaneus.
This underlying tension persists even at rest, making the heel structure more rigid and less tolerant of external pressure. The resulting strain on the Achilles tendon insertion point makes the heel area more sensitive to static compression from the mattress. Low-grade inflammation, such as a mild case of plantar fasciitis or retrocalcaneal bursitis, can also be aggravated by sustained pressure. Although these conditions may not cause pain during the day, localized compression at night restricts fluid movement and blood flow to the inflamed tissue. This combination of tissue tension and localized pressure easily irritates the heel, manifesting as pain during periods of rest.
Practical Strategies for Relief
Immediate relief from supine heel pain requires modifying the pressure dynamics between the heel and the resting surface. The most straightforward solution is to use a pillow, rolled towel, or specialized suspension device to completely offload the heel. The padding should be placed beneath the calf or lower leg, ensuring the heel is suspended in the air and not touching the mattress. This eliminates direct compression on the calcaneus and the sensitive heel fat pad.
Incorporating a gentle stretching routine for the calf muscles and Achilles tendon before bed can help reduce the baseline tension contributing to the pain. Simple stretches, such as pulling the toes toward the body while the leg is straight, help lengthen these posterior structures. If sleeping on the back is preferred, placing a small pillow beneath the knees introduces a slight bend in the leg. This naturally relaxes the posterior chain muscles and reduces tension on the nerve pathways, diminishing the intensity and frequency of nighttime heel discomfort.