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 o
f this distance. During a 14-day training study, the mean standard dev
iation (technical error) of five (six minus the first estimation) decr
eased from 196 mu m to 101 mu m. Measurements at exact 24-h intervals
revealed nonlinear increments of rat lower leg growth, with marked inf
radian variation once every four to six days, similar to ''mini growth
spurts'', described in rabbit and human growth. There was also a sign
ificant 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 (me
an: 34.9% (SEM 2.5%) of the total 24-h increment, p < 0.001). Thus, th
e technique of mikro-knemometry seems to be a useful tool for the inve
stigation of longitudinal growth in laboratory rats, and may replace c
onventional techniques of growth measurements such as measuring body w
eight, nose-tail, or tail length.