A velocity curve is a type of growth curve, in which the increments of growth (the amounts added in specific time intervals) are plotted against time or chronological age. Such a curve shows the variation in the rate of growth with time, and is therefore known as a “velocity curve”. The curve tends to zero as growth ceases. The pattern created by changing rates of growth can be more precisely witnessed as the rate of change of size with time, i.e., ‘growth velocity’ or particularly ‘height velocity’, term coined by Tanner, based on the writings of 0’Arcy Wentworth Thompson (1860-1948). Thompson wrote in his landmark biology text, Growth and Form. that while the distance curve “showed a continuous succession of varying magnitudes”, the curve of the rate of change of height with time “shows a succession of varying velocities”. It is often called the curve offirst differences or simply a velocity curve. The velocity of growth experienced by De .Montbeillard’s son is displayed in The Y-axis record the height gain in cm/yr; and the X-axis record the chronological age in years. It shows clearly that in general the velocity of growth in height decreases from birth onwards. Through the curve, we observe two distinctive points where the growth rate is markedly accelerated, which are termed as ‘growth spurts’, becau e of their rapid increase in rate with comparison to the regular growth pace. The first marked increase is between 6-8 years called the juvenile or mid-growth spurt, while the second increase is noticed between 11-18 years of age called the adolescent growth spurt.
There i , in fact, another growth spurt that occurs prior to birth and thus, termed prenatal spurt. The peak velocity of length of the foetus is reached at about 4 months’ of gestation, similarly, increase in weight also experiences prenatal spurt at 30-40 weeks of gestation. The prenatal spurt and juvenile growth spurt may vary in magnitude; they seem to occur at roughly the same age, both within and between the sexes. However, the adolescent growth spurt varies both in magnitude and timing within and between the sexes. Males usually enter their adolescent spurt two years later Le. 13-15 years than females (11-13 years) and have slightly greater magnitude towards height gaining. Thus, they have extra two years of growth prior to adolescent age that adds to their increase in adult heights. Other skeletal changes occurring in the body also add towards the wider shoulders in males, besides addition of wider hips in females. Males demonstrate rapid increase in muscle mass and females accumulate greater fat amounts. The fat distribution in females is usually observed in gluteo-femoral region or in a ‘gynoid’ pattern, while males characteristically demonstrate -a centralised distribution or an ‘android’ pattern. Physiologically males develop greater strength and lung capacity. These differences bring about the morphologically distinct characters noticeable at the end of adolescence between the two sexes. Thus, velocity curve provides an opportunity to have actual information about the pattern of growth of an individual and, reveals differences in the timing, intensity and rate of growth between different individuals. As the growth velocity denotes innate capacity of a child to grow and develop, so this type of curve helps to understand the influence of genetic and environmental factors on the growth dynamics of children.