Cross-sectional Properties of the Rat Femur as Influenced by Forced Running Exercise

Abstract

Flattening of limb bones is not infrequent in prehistoric population as well as in modern hunter-gatherers. Platycnemia is well-documented in materials from Japanese prehistoric 'Jomon' period. Interpretation for the reason of this bony feature, however, is still controversial with respect to the mechanical stress due to a hard physical exercise or nutritional elements. This study aims at identifying the factors mainly responsible for the flattening by using experimental animals.
Effects of exercise and diet programs on the cross-sectional properties of the femur were studied in 40 male rats of Sprague-Dawley strain. They were divided into 6 experimental groups, (I) Ordinary Feeding & Non-Exercise Group, (II) Ordinary Feeding & Exercise Group, (III) Restricted Feeding & Non-Exercise Group, (IV) Restricted Feeding & Exercise Group, (V) Free Feeding & Non-Exercise Group, (VI) Ordinary Feeding & Hard Exercise Group (8 rats for groups I-IV and 4 rats for groups V and VI). Each group was raised from 21 days through 110 days age. Restricted feeding groups (III and IV) were fed 60% weight of diet which was ate by ordinary feeding groups (I or II) on the previous day. Free feeding group was fed freely all day long. From 30 days age, the exercise groups were trained to run 20 minutes in a day at a maximum speed of about 40m/min on a newly devised treadmill ('Labotrainer', Clea Japan Inc.) which seemed to elicit least exercise stress in animals. Hard exercise group was trained twice in a day according to the same program. Feeding, running, and light and darkness cycle schedule was arranged so as to minimize the individual difference in food intake and spontaneous activities. It has been reported elsewhere (MATSUMURA, 1982) that the body dimensional development in general suggested an undisturbed physical growth in the present animals. After the training period, the right femur was removed and femoral cross sectional properties as taken at the middle of the femur shaft were examined. While forced running exercise had no significant effect upon the magnitude of the cross sectional area of the femur, it had an unequivocal effect upon the other cross sectional properties; the femur of exercise groups had a smaller cross sectional index, or was significantly flattened, with its major principal axis oriented more antero-posteriorly. This trend was less pronounced in limited-diet groups. Restriction of food intake scarcely had effects upon cross sectional index nor upon major principal axis of the femur, although it exerted a marked negative influence on the cross sectional area.