When people who don't exercise regularly try a new type of strength workout that focuses on lengthening muscles, their upper body and back muscles get much weaker and more sore than their leg muscles. This shows that different parts of the body are more sensitive to the stress of unfamiliar exercise.

Doing a new, intense workout that focuses on lengthening muscles under tension can cause temporary muscle damage and soreness. This shows up as a noticeable drop in strength, higher pain levels, and a spike in a specific muscle enzyme in the blood for up to four days.

If you do a tough, unfamiliar full-body workout and then repeat the exact same workout two weeks later, your body will experience much less muscle soreness, strength loss, and tissue damage. This shows that your muscles quickly adapt to the same type of stress, protecting you from injury the second time around.

If you're new to lifting weights, doing exercises where you lower the weight slowly (eccentric) might make your biceps grow bigger than exercises where you lift the weight up (concentric), after 8 weeks of training.

Lifting weights slowly (eccentric) or quickly (concentric) both make your quads stronger in about 10 weeks — and you don’t need to be sore or damaged to get stronger.

If young, healthy guys do strength training for 12 weeks using different types of muscle contractions—whether slow or fast—they’ll get significantly stronger in their thigh muscles, no matter which style they pick.