Preventing ischial pressure ulcers: II. Biomechanics

Background: Pressure ulcers (PUs) are common and debilitating wounds that arise when immobilized patients cannot shift their weight. Neuromuscular Electrical Stimulation (NMES) has been investigated for Pressure Ulcer Prevention (PUP) for over 20 years. Historically gluteus maximus (GM) has been considered an important actuator in attempting to redistribute seated pressures through NMES. Methods: Analysis of skeletal biomechanics to quantify the value of GM relative to hamstring hip extensors (HS), using muscle moment models based on torques and rigid body mass estimates from the literature. Surface stimulation experiments (n = 10 + 1, non-paralyzed) to validate model and identify promising stimulation sites and treatment strategies that would approximate healthy biomechanics. Results: Literature values and Rigid Body Analysis estimate: similar to 63 Nm extensor torque requirement calculated for complete ipsilateral unloading of the buttocks. Muscle Moment Analysis: GM can provide 70% of total hip extensor torque when walking vs. 18% when seated. HS can provide 100 Nm hip extension torque when seated, exceeding 63 Nm requirement. Surface Stimulation: ipsilateral seated interface pressure mean -26% during HS stimulation vs. + 16% with GM; peak pressure area -94% HS vs. + 213% GM. Conclusions: GM activation reduces disuse atrophy and improves circulation, but appears neither required, nor desired, for unloading when seated. HS stimulation alone should be capable of sufficient unloading. This new proposed approach is explored clinically in companion paper III.