Final answer:
The initial growth phase during which the hair follicle actively produces hair is called the anagen phase, which lasts 2 to 7 years and is followed by the catagen and telogen phases.
Step-by-step explanation:
The initial growth phase during which the hair follicle actively produces hair is known as the anagen phase. During this phase, cells divide rapidly at the root of the hair follicle, causing the hair shaft to be pushed upward and outward. Hair grows at a rate of about 0.3 mm per day in the anagen phase. This phase typically lasts between 2 to 7 years, meaning that a single hair strand can grow quite long before transitioning into the subsequent phases. In the catagen phase, which lasts 2 to 3 weeks, the hair follicle begins to transition away from its active growth. Finally, the telogen phase is characterized by a resting hair follicle, during which no new hair growth occurs. This restful state lasts approximately 2 to 4 months, after which the hair cycle may begin anew with the anagen phase restarting the growth process.