Arnold has been taking a substance that is not an anabolic steroid but has a reputation as a performance-enhancing drug. although he notes an increase in lean muscle mass, he does not experience improved strength or endurance. what substance is arnold taking?

Human growth hormone also abbreviated as HGH

Explanation:

People who may have an insufficient HGH hormone secretion may get benefits from medically introduction of HGH into their body.

Also people who exercises and may want to boost their muscle build up may use HGH.

It has this following benefits

Increase exercise capacity

Increase bone density

Increase muscle mass

Decrease body fat

What can human growth hormone do for healthy adults?

Research have shown that human growth hormone supplement has the potential to increase your muscles growth and also decrease the fat content in your body however it has not been seen to have an increase effect on one's strength.

yes he did writing dots to make the answer