The two primary hormones responsible for muscle gain are Testosterone and Growth Hormone.
Healthy males and females reach peak testosterone and growth hormone levels at age 20-25, steadily decrease to 50% by age 40 and level off to 25% of peak levels by age 50. Obviously, higher levels of these hormones with the correct resistance training and nutritional protocol significantly improve your chances of gaining muscle.
Empirical evidence shows that the most muscle mass a 25-year-old "average" male with a year or two of training behind them can expect to gain is roughly 8-12 lbs per year. Gains in the average female are approximately half those seen in males.
Based on the little information available about these hormones in context to muscle gain, around age 40 (male) your muscle gain may be limited to less than 4 lbs a year. And that's if you are healthy, train appropriately, eat correctly, and recover optimally and consistently.
Additionally, while your ability to gain muscle at age 40 is not stellar, it remains adequate. The rate at which you make progress will be steady but slower and will depend greatly on your genetics, age, lifestyle, and eating habits.
Finally, muscle gain is rarely a linear process, and no matter how hard you train, how strict your diet is, or how many muscle building supplements you use, growth will never come at a predictable steady pace.
The information included on this site is for educational purposes only. It is not intended nor implied to be a substitute for professional medical advice. The reader should always consult his or her healthcare provider to determine the appropriateness of the information for their own situation or if they have any questions regarding a medical condition or treatment plan.
read more: Muscle Gain at 40