The process of phagocytosis (a macrophage consuming a pathogen) in macrophages appears to be associated with an acute reduction in creatine phosphate stores (45%)[436] and an increase in creatine kinase activity,[290] although prolonged stimulation is met with an increase in creatine phosphate (20%).[290] The creatine kinase activity does not appear to be altered based on creatine availability,[290] but since ATP seems to be preserved in these instances,[436][290] the increase in phosphocreatine may be explained by an overall creatine pool paralleling that found in medium.[290]
Early infancy androgen effects are the least understood. In the first weeks of life for male infants, testosterone levels rise. The levels remain in a pubertal range for a few months, but usually reach the barely detectable levels of childhood by 4–7 months of age.[15][16] The function of this rise in humans is unknown. It has been theorized that brain masculinization is occurring since no significant changes have been identified in other parts of the body.[17] The male brain is masculinized by the aromatization of testosterone into estrogen, which crosses the blood–brain barrier and enters the male brain, whereas female fetuses have α-fetoprotein, which binds the estrogen so that female brains are not affected.[18]
A: If your goal is the largest accrual of muscle mass possible then there may be some benefit to ingesting nutrients with a period of 30-60 minutes after your workout. Does this have to be a protein shake? No, but ideally it should be a meal lower in fat to enhance the digest rate of nutrients within the gastrointestinal tract. However, if you have just eaten a mixed macronutrient meal pre-workout then you should keep in mind that that meal is still likely digesting so there’s no need to throw down the weights after your last set and rush to your locker to slam a protein shake.
In regard to practical interventions, concurrent glycogen loading has been noted to increase creatine stores by 37-46% regardless of whether the tissue was exercised prior to loading phase.[176] It is important to note, however, that creatine levels in response to the creatine loading protocol were compared in one glycogen-depleted leg to the contralateral control leg, which was not exercised.[176] This does not rule out a possible systemic exercise-driven increase in creatine uptake, and the increase in creatine noted above[176] was larger than typically seen with a loading protocol (usually in the 20-25% range). Consistent with an exercise-effect, others have reported that exercise itself increases creatine uptake into muscle, reporting 68% greater creatine uptake in an exercised limb, relative to 14% without exercise.[153]
There's good news, though: These temporary muscle pumps are critical to improving muscle hypertrophy, or muscle growth, according to 2014 research in the Strength and Conditioning Journal. So you can think of your weight-room pump as a preview of the muscle results that are to come. Speaking of which, here's an expert-endorsed timeline to reach your muscle-building goals.

Although research is underway, doctors do not know the long-term health effects of taking creatine supplements, especially in children who are still growing. Because of these unknown risks, children and adolescents younger than 18 years and pregnant or nursing women should never take creatine supplements. People with kidney problems also should never take creatine supplements.


Testosterone was first used as a clinical drug as early as 1937, but with little understanding of its mechanisms. The hormone is now widely prescribed to men whose bodies naturally produce low levels. But the levels at which testosterone deficiency become medically relevant still aren’t well understood. Normal testosterone production varies widely in men, so it’s difficult to know what levels have medical significance. The hormone’s mechanisms of action are also unclear. 

Testosterone is a steroid from the androstane class containing a keto and hydroxyl groups at the three and seventeen positions respectively. It is biosynthesized in several steps from cholesterol and is converted in the liver to inactive metabolites.[5] It exerts its action through binding to and activation of the androgen receptor.[5] In humans and most other vertebrates, testosterone is secreted primarily by the testicles of males and, to a lesser extent, the ovaries of females. On average, in adult males, levels of testosterone are about 7 to 8 times as great as in adult females.[6] As the metabolism of testosterone in males is greater, the daily production is about 20 times greater in men.[7][8] Females are also more sensitive to the hormone.[9]
That soreness you feel post-strength session may seem like a setback. Yet over time, you’ll come to acknowledge that it signifies you’re getting stronger. “You think, ‘I’ve done something worthwhile. My body is telling me I’ve had a workout.’ You look forward to the fatigue and interpret it in a positive way,” says John Spence, PhD, professor of physical education and recreation at the University of Alberta in Canada, who wrote a review on the effect of exercise on self-esteem. (Wondering how sore is too sore to work out? Here’s your answer.)

Having a strong butt will get you far—literally. Our glutes are responsible for powering us through everything from long runs to tough strength workouts to a simple jaunt up a flight of stairs. Strong glutes that can take on the brunt of the work can help us avoid overcompensating with smaller muscles during lower-body exercises. Plus, beyond just helping us move, the glutes play an important role in "stabilizing our entire lumbo-pelvic-hip complex," says Cori Lefkowith, NASM-certified personal trainer and owner of Redefining Strength in Costa Mesa, California. That translates to better form, more efficient movement, and a reduced risk of straining your lower back and hips.
×