Modelling muscle activity in standing with considerations for bone safety

• Published in: Journal of biomechanics 1997, Vol.30 (1), p.49-56
• Main Authors: Munih, Marko, Kralj, Alojz
• Format: Article
• Language: English
• Subjects: Adult; Bending moment; Biomechanical Phenomena; Bone and Bones - physiology; Electric Stimulation Therapy; Electromyography; Humans; Male; Models, Biological; Muscle forces; Muscle, Skeletal - physiology; Musculo-tendino-skeletal model; Posture; Space life sciences; Standing; Synergism; Tendons - physiology
• ISSN: 0021-9290; 1873-2380
• DOI: 10.1016/S0021-9290(96)00095-4

The functional use of electrical stimulation (FES) for the restoration of movement to paraplegics has been improved in the last decade but questions about the mechanical effect of stimulation on the skeleton have arisen. In intact people, neuromuscular activity not only controls movement, but also minimizes bone and joint tissue loading. Current FES systems do not use feedback and do not even use average natural patterns of muscle activation. FES systems would be safer if muscle activation patterns were synthesized so as to minimize bending in the long bones. By modelling, we have verified that appropriate muscular activity reduces bone bending stresses, an approach we named active unloading of the skeleton. Using this criterion for control is novel. The muscle activation was calculated using measurements from intact people in different postures, and later modelling of the musculoskeletal system. The two-dimensional model of the lower limb includes 23 muscles relevant primarily for movement in the sagittal plane. The muscle model for constraint calculation is divided into first-order activation dynamics and first-order contraction dynamics. Optimization, which includes minimization of net bending moment calculated along the long bones, is static because changes in the observed postures are slow. In the calculated muscle activity patterns, muscle coactivation and cocontraction yield very uniform and low bone loading. Net bending moment values were fairly stable as the posture varies. The moment distribution in the femur was found to be U-shaped, while in the tibia it is sometimes V-shaped. The bones are naturally thicker at the points of peak moment.