J Spinal Cord Med. 2012 May;35(3):140-7.

A biomechanical analysis of exercise in standing, supine, and seated positions: Implications for individuals with spinal cord injury.

McHenry CL, Shields RK.

Source
Physical Therapy and Rehabilitation Science, Carver College of Medicine, University of Iowa, Iowa City, IA 52242, USA.

Abstract
CONTEXT/OBJECTIVE:
The distal femur is the primary fracture site in patients with osteoporosis after spinal cord injury (SCI).
OBJECTIVE:
To mathematically compare the compression and shear forces at the distal femur during quadriceps stimulation in the standing, supine, and seated positions. A force analysis across these positions may be a consideration for people with SCI during neuromuscular electrical stimulation of the quadriceps.
DESIGN:
A biomechanical model.
SETTING:
Research laboratory.
OUTCOME MEASURES:
Compression and shear forces from the standing, supine, and seated biomechanical models at the distal femur during constant loads generated by the quadriceps muscles.
RESULTS:
The standing model estimated the highest compressive force at 240% body weight and the lowest shear force of 24% body weight at the distal femur compared with the supine and seated models. The supine model yielded a compressive force of 191% body weight with a shear force of 62% body weight at the distal femur. The seated model yielded the lowest compressive force of 139% body weight and the highest shear force of 215% body weight.
CONCLUSIONS:
When inducing a range of forces in the quadriceps muscles, the seated position yields the highest shear forces and lowest compressive forces when compared with the supine and standing positions. Standing with isometric contractions generates the highest compressive loads and lowest shear forces. Early active resistive standing may provide the most effective means to prevent bone loss after SCI.

http://www.ncbi.nlm.nih.gov/pubmed/22507023