Sunday, October 16, 2011

Pig Physics

No kidding!  While looking for biomechanical studies on the motion of hogs, I found the following interesting article written for orthopedic surgeons.  On our farm, we regularly wrestle pigs to the ground in order to weigh them, and watch our 700 lb boar ruin perfectly good 1/4 inch steel rods in fencing with a playful bite.  This study shows how physics and the tools of engineering (e.g., strain gages) can help us learn more about the science of pig bones and their abilities and design. The full article is here.
Jaw muscles and the skull in mammals: the biomechanics
of mastication
Susan W. Herring , Katherine L. Rafferty, Zi Jun Liu, Christopher D. MarshallDepartment of Orthodontics, Uni ersity of Washington, Box 357446 Seattle, WA 98195-7446, USA

Among non-mammalian vertebrates, rigid skulls with tight sutural junctions are associated with high levels of cranial loading. The rigid skulls of mammals presumably act to resist the stresses of mastication. The pig,  Sus scrofa, is a generalized ungulate with a diet rich in resistant foods. This report synthesizes previous work using strain gages bonded to the bones and sutures of the braincase, zygomatic arch, jaw joint, and mandible with new studies on the maxilla. Strains were recorded during unrestrained mastication andor in anesthetized pigs during muscle stimulation. Bone strains were 100
- 1000 , except in the braincase, but sutural strains were higher, regardless of region. Strain regimes
were specific to different regions, indicating that theoretical treatment of the skull as a unitary structure is probably incorrect. Muscle contraction, especially the masseter, caused strain patterns by four mechanisms:   
1. direct loading of muscle attachment areas; 
2. a compressive reaction force at the jaw joint;
3. bite force loading on the snout and mandible; and 
4. movement causing new points of contact between mandible and cranium. Some expected patterns of loading were not seen. Most notably, strains did not differ for right and left chewing, perhaps because pigs have bilateral occlusion and masseter activity.