Mikro-knemometry: an accurate technique of growth measurement in rats.

A novel, noninvasive technique for accurate measurements is presented which determines the distance between knee and heel of the rear lower leg in the conscious rat (mikro-knemometry). Each measurement consists of initially six, later four subsequent and independent estimations of this distance. During a 14-day training study, the mean standard deviation (technical error) of five (six minus the first estimation) decreased from 196 microns to 101 microns. Measurements at exact 24-h intervals revealed nonlinear increments of rat lower leg growth, with marked infradian variation once every four to six days, similar to "mini growth spurts", described in rabbit and human growth. There was also a significant circadian periodicity of leg length increment (p < 0.01), with a minimum leg increment (after midnight dip) between 2400 h and 0300 h (mean: 4.8% (SEM 2.3%) of the total 24-h increment, p < 0.01), and a maximum increment (early morning spurt) between 0600 h and 0900 h (mean: 34.9% (SEM 2.5%) of the total 24-h increment, p < 0.001). Thus, the technique of mikro-knemometry seems to be a useful tool for the investigation of longitudinal growth in laboratory rats, and may replace conventional techniques of growth measurements such as measuring body weight, nose-tail, or tail length.