Human hair growth is regulated by cycles. Each hair follicle goes through a growth (anagen), transition (catagen) and rest (telogen) phase. The hair on your scalp has an anagen phase that’s quite long, ranging from two to seven years, which allows it to grow to a decent length if left (though this varies between people). But for your arm and leg hair, this anagen phase only lasts a few months. After this, it enters the catagen and then the telogen phases, where the hair eventually falls out and is replaced with new growth. How long a particular hair can grow is simply the length it can reach in the anagen phase. So, with such a short anagen phase, body hair can never reach the length of scalp hair – which is probably just as well!
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