A Rope to Sit Up By: What a Knotted Rope at the Bed End Asks of the Arms, and Whom It Cannot Help

The trunk and head are taken as 0.35 of body mass. For a patient of 70 kg the gravitational torque about the hips is 72.1 Nm when lying flat and 12.5 Nm at 80 degrees. Under the stated rope geometry, with an effective moment arm of 0.33 m, the arm pull required peaks at 218.5 N for 70 kg when lying flat, which is 0.318 of body weight. The peak is 156.1 N for 50 kg and 280.9 N for 90 kg, at the same fraction of 0.318 of body weight in each case. A person who can pull 0.15 or 0.25 of body weight with both arms therefore cannot sit up by rope alone under these assumptions, at any of the three masses; a person who can pull 0.4 of body weight can. The hardest angle is 0 degrees. With a rope over a smooth bar, a caregiver needs 178.0 N to assist a patient of 70 kg.