Why Does Leg Hair Only Grow So Long?

Humans have hair all over their bodies. Some hair, such as the hair on the head, grows very long if left alone. Other hair, such as the hair on the arms and the legs, grows only an inch or two before it appears to stop growing. Eyebrow hair and eyelash hair grow to even shorter lengths before they appear to stop growing. These differences are attributed to the hair-growth cycle.

Hair Growth

Hair growth occurs in a cyclic pattern of growth, transition and rest called the anagen, catagen and telogen phases. In humans, individual hair follicles do not transition through the hair-growth cycles at the same time, meaning a resting follicle could be surrounded by actively growing hair. Approximately 10 percent of follicles are resting or not growing hair at any given time, according to the International Society of Hair Restoration Surgery.

Anagen

Hair actively grows approximately 1 cm every four weeks during the anagen phase. During this phase, cells deep within the follicle divide and die at a very rapid pace. Dead cells form the hair shaft. Continued activity beneath the hair shaft -- more dividing and dying -- results in hair growth.

Catagen

Approximately 3 percent of follicles are in the catagen, or transitional, stage at any given time. During the catagen phase, hair growth halts and the follicle dies and becomes more superficially rooted in the skin. The hair detaches from the hair-growing structures and forms a club at the root of the hair. This phase lasts approximately two to three weeks.

Telogen

The follicle rests during the telogen phase. The hair, held in only by the club-shaped root, can shed at any time during this phase. Approximately 25 to 100 hairs are shed daily, according to Dutasteride.com. The rate of shedding can be affected by seasonal changes, pregnancy, surgery, rapid weight loss, certain medications, fever and severe emotional or physical shock.

Cycle Speed

All hairs on the body cycle through the same three cycles and grow at the same rate; however, the rate at which hair cycles varies according to where the hair is located. In the case of leg and arm hair, hair actively grows for only a couple of months before entering the catagen phase. In sharp contrast, hair on the head grows for as long as eight years. This means that hair located on the legs and arms does not have time to grow very long. In addition, follicles on the arms, legs and eyebrows rest longer than those on the head. If left to grow uninterrupted, leg hair would grow very long, but short growing periods followed by long resting periods prevent it.