At a Glance: Friction is the rubbing of skin against a surface, and it damages the superficial layers you can see. Shear happens beneath the skin when the outer layer stays put while the underlying tissue and bone slide in the opposite direction, stretching and tearing blood vessels. Pressure gets most of the attention, but shear and friction often decide how fast a wound forms and how deep it goes. Managing both takes a combination of safe transfers, good positioning, and a support surface built to reduce these forces.
Pressure injuries are usually described as a pressure problem. Too much weight sits on one spot for too long, blood flow drops, and the tissue underneath starts to die. That description is accurate, and it is why repositioning schedules and pressure redistribution surfaces sit at the center of almost every prevention plan. It also leaves out a good part of what is actually happening to the skin.
Two other forces go to work every time a patient is moved. Friction is the rubbing that happens when skin drags across a sheet. Shear is what happens underneath, when the skin holds still against the surface while the bone and the tissue attached to it slide the other way. The National Pressure Injury Advisory Panel writes shear directly into its definition of a pressure injury for that reason. For patients, caregivers, and clinicians managing care at home, these two forces explain why a wound can show up even when the repositioning schedule is being followed and why the way a patient gets moved matters as much as how often.
Why Shear and Friction Matter
Pressure injuries rarely come from pressure alone. The National Pressure Injury Advisory Panel defines a pressure injury as localized damage to the skin and underlying soft tissue, usually over a bony prominence or related to a medical device, resulting from intense or prolonged pressure or pressure in combination with shear.
For patients, caregivers, and clinicians managing care at home, understanding pressure and shear forces explains why a wound can appear even when a repositioning schedule is being followed and why the way a patient is moved matters as much as how often.
Friction: Damage You Can See
Friction is the resistance between the skin and the surface it rubs against. Dragging a heel across a sheet or pulling a patient up in bed without lifting both creates friction.
That rubbing generates heat and strips away the top layer of skin. The result is a friction injury: an abrasion, a raw patch, or a blister on the skin surface. Common sites include the heels, elbows, and the tailbone.
Friction rarely causes deep wounds on its own, but it does two things that raise pressure injury risk:
- It removes the skin’s protective outer layer, leaving tissue exposed
- It anchors the skin to the surface, which is what allows shear to develop underneath
Shear: Damage You Cannot See
Shear is the more dangerous of the two because it does its work below the surface.
Picture a patient sitting up in bed with the head of the bed raised. Gravity pulls the body downward. The skin along the tailbone stays stuck to the sheets through friction, but the skeleton and the deeper tissues attached to it keep sliding down. Both layers move in opposite directions.
This opposing movement of the outer layer of skin and the underlying tissues causes the capillaries to stretch and tear, which then causes decreased blood flow and oxygenation of the surrounding tissues. Blood vessels that pass through the subcutaneous tissue get pulled apart. The tissue below loses its blood supply, a state called tissue ischemia, and cells begin to die.
What makes shear so difficult is that the damage starts deep and works its way up. The skin can look intact while tissue damage is already underway beneath it. By the time discoloration or an open wound appears, deeper tissues may have been injured for some time. This is part of why deep tissue injuries can seem to appear out of nowhere.
Friction vs. Shear
- Friction acts on the skin surface, damaging superficial layers, and shows up as visible scrapes or blisters
- Shear acts beneath the skin, stretching and tearing blood vessels in the subcutaneous tissue, and often shows no early warning
- The two usually happen together, since friction holds the skin in place while the body slides
Where These Forces Come From
Shear and friction are built into everyday care routines. The most common sources at home include:
- A Raised Head of the Bed: Anything above about 30 degrees encourages the body to slide down, which is the classic setup for shear at the tailbone.
- Sliding Rather than Lifting: Pulling a patient up in bed by dragging them across the sheets creates both forces at once.
- Transfers: Moving between a bed and a wheelchair without a lift or slide sheet drags skin across surfaces.
- Sitting and Sliding Forward: Patients who slump in a chair experience the same opposing movement at the tailbone and hips.
- Medical Devices: Tubing, braces, and equipment pressed against the skin create friction and constant pressure at the contact point.
Who Is Most at Risk?
Some patients are far more vulnerable to shear and friction than others.
Patients With Limited Mobility
Anyone who cannot reposition or lift themselves relies on caregivers for every movement, which means more transfers and more opportunities for skin to drag. Patients with a spinal cord injury are a common example, since reduced sensation means they may not feel the damage happening.
Older Patients
Aging skin is thinner, less elastic, and more loosely attached to the tissue beneath it. That combination makes it easier for elderly patients to have skin that separates and tears more easily compared to younger patients. Elderly patients in home care and intensive care settings both face elevated risk.
Patients With Fragile Skin or Poor Perfusion
Reduced blood flow, poor nutrition, and existing wounds all lower the skin’s ability to withstand mechanical stress. Immobility, poor perfusion, undernutrition, and reduced sensation are each an established risk factor for pressure injury. Nutrition plays a supporting role here: adequate protein, vitamin C, and hydration support immune function and the formation of healthy granulation tissue during wound healing.
How to Reduce Shear and Friction at Home
Prevention comes down to how a patient is moved and what they rest on.
Change how you move the patient
- Lift rather than drag. Use a draw sheet, slide sheet, or mechanical lift so skin never scrapes across a surface.
- Keep the head of the bed at or below 30 degrees when the care plan allows. Use the knee break to stop the body from sliding.
- Reposition on a schedule, and check that the patient has not slid down between turns.
- Keep skin clean and dry, since damp skin grips surfaces harder and tears more easily.
- Protect vulnerable areas. Heels, elbows, and the tailbone benefit from offloading, and a healthcare provider may recommend foam dressings over high-risk sites.
Change what the patient rests on
The right support surface reduces shear by allowing the body to sink in rather than slide across, and it lowers friction at the skin surface. Static surfaces, such as a standard foam mattress, do neither well, which is why they fall short for at-risk patients.
The mattress overlays and replacements available through Ethos are designed for pressure redistribution and reduced shear at the same time. For patients with advanced wounds, air fluidized therapy beds let the body float on a fluid-like surface, which minimizes both shear and friction because there is very little rigid surface for skin to catch against.
Working With Your Care Team
Any risk assessment should account for shear and friction, not pressure alone. The Braden Scale, used widely across the United States, includes a friction and shear category that scores how much help a patient needs to move and whether they slide during transfers. Ask your healthcare professional what that score is and what it means for the care plan.
Report early signs promptly. Redness that does not fade, a scrape near a bony area, or new discoloration all deserve attention before they progress. The Agency for Healthcare Research and Quality offers prevention guidance that combines safe handling with the right equipment, and the National Pressure Injury Advisory Panel publishes the staging definitions clinicians use to classify damage. Ethos Clinical Services works directly with patients and care teams to match a surface to the patient’s current wound status.
Protect At-Risk Patients with Ethos
Shear and friction do damage that constant pressure alone cannot explain. Friction wears away the surface. Shear tears blood vessels in the tissue underneath, often long before anything shows on the skin. Preventing both takes safe transfers, thoughtful positioning, and a surface that works with the body instead of against it.
As the largest supplier of in-home air fluidized therapy beds and support surfaces in the United States, Ethos Therapy Solutions pairs every surface with clinical, advocacy, and technical services that support patients from day one. Explore the full range of therapy solutions, browse the Mattress Replacements & Overlays collection, or contact the Ethos team to talk through which option fits the patient’s needs.