Weight bearing refers to the act of supporting your body’s mass through your bones, muscles, and joints. This fundamental process allows the skeletal system to counteract gravity, whether you are standing, walking, or running, and is essential for maintaining skeletal strength and overall physical health. In a medical context, particularly after an injury or surgery, weight bearing is precisely controlled to manage the load on a healing limb. This control determines the extent of physical stress applied to recovering tissues and is a major factor in successful rehabilitation.
The Mechanics of Load Bearing
The body’s ability to support its own weight depends on the interplay between the skeletal system, joints, and surrounding musculature. When standing or moving, the force of gravity creates an internal load that is distributed through the bones, which act as the primary load-bearing tissues. Axial loading describes the compression forces traveling along the long axis of a bone, such as the femur or tibia. When a person walks, the force exerted on the ground is met by an equal and opposite ground reaction force that travels up the limb, compressing the bone and joints. This stress distribution is dynamic, changing moment by moment as joints move and muscles contract to maintain balance and propel the body forward.
Muscles and tendons play a significant part by absorbing and transmitting forces across joints, preventing excessive stress on the cartilage and ligaments. The mechanical stress placed on the bones is essential for bone health, a phenomenon known as Wolff’s law, which states that bone adapts to the loads placed upon it. Regular weight bearing exercises stimulate the bones to become denser and stronger, helping to prevent conditions like osteoporosis. Even when standing still, the forces can be equivalent to one times the force of gravity (1g), and this increases to approximately 1.2g of force while walking.
Classifications of Weight Bearing Status
Healthcare professionals use specific classifications to limit the amount of force an injured limb can tolerate during recovery. These restrictions are often described as a percentage of a person’s total body weight. This controlled loading is determined by the nature of the injury or surgery and the stage of tissue healing.
Non-Weight Bearing (NWB) is the most restrictive status, meaning no weight or force can be applied to the limb, which must be held off the floor at all times. This is prescribed when a bone fixation, such as plates or screws, cannot withstand the repetitive stress of movement. The patient must use an assistive device, like crutches or a walker, to ensure the limb bears none of their body weight.
Toe-Touch Weight Bearing (TTWB) or Touch-Down Weight Bearing (TDWB) allows the foot or toes to lightly touch the ground, but only for balance and not for support. This ensures no significant body weight is transferred to the limb. This status is still considered a non-weight bearing restriction, as the goal is to prevent muscle atrophy and maintain proprioception without applying a load.
Partial Weight Bearing (PWB) permits a specific amount of weight to be placed on the injured limb, usually specified as a percentage of body weight, such as 20% or 50%. This status is a transitional phase where a person is allowed to load the limb partially to stimulate healing while using an assistive device to offload the remaining weight. For example, 50% PWB means the person can stand with both feet on the floor, applying half their weight to the affected side.
Weight Bearing As Tolerated (WBAT) is a flexible classification that allows the patient to apply as much weight as they can comfortably manage without increasing pain. Pain serves as the guide for the maximum permissible load, which can range from a gentle touch to full body weight. The patient is encouraged to progress the load as pain allows, transitioning toward a normal gait pattern.
The final stage is Full Weight Bearing (FWB), which signifies that the limb can safely support 100% of the person’s body weight without restrictions. This is the goal of most rehabilitation programs, allowing a return to normal walking and daily activities without the need for an assistive device.
Weight Bearing in Recovery and Physical Therapy
Controlling the degree of weight bearing is an important aspect of recovery following orthopedic injury or surgery. The surgeon determines the initial weight bearing status based on the procedure performed, the stability of internal fixation, and the type of tissue that needs to heal. For instance, a fracture fixed with hardware requires strict adherence to NWB to prevent the hardware from failing or the bone from shifting, which would compromise healing.
Physical therapists are responsible for educating the patient on how to safely maintain the prescribed weight bearing restriction using assistive devices like crutches, walkers, or canes. They may use a dual-scale method to help a patient understand and practice the exact percentage of weight they are permitted to bear. Patient adherence to these instructions is essential, as prematurely increasing the load can lead to complications like nonunion or re-injury.
Controlled loading, such as in PWB, is introduced to stimulate the healing process by applying beneficial mechanical stress to the tissues. This measured stress encourages bone and soft tissue cells to regenerate and strengthen. Weight bearing activities are also beneficial for neurological recovery, especially in patients with conditions like stroke. Putting weight on an affected limb activates proprioceptors—sensors in the muscles and joints—which send signals to the brain, helping to improve muscle activation, balance, and coordination. Physical therapy is a progressive process, moving the patient through the various weight bearing classifications as healing advances, aiming for a complete return to full function.