Human hair growth is regulated by a cycle. Each hair follicle goes through growth (anagen), transition (catagen), and rest (telogen) phases. Scalp hair has a long anagen phase of two to seven years, allowing it to reach a decent length if it remains. However, arm and leg hair have a shorter anagen phase lasting only a few months before entering the telogen stage, where the hair falls out and is replaced by new growth.
The length that a specific hair can reach is determined by how long it stays in the anagen phase. Body hair, with its shorter anagen phase, does not reach the lengths of scalp hair.
This article is a response to a question asked by Arianna Carpenter about why arm and leg hairs have a growth limit while scalp hair does not.
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Source: www.sciencefocus.com
