To strengthen a muscle, you have to take an intense workout that damages muscle fibers to make them feel sore on the next day. This is called Delayed Onset Muscle Soreness (DOMS). Then you take easy workouts for as many days as it takes for the soreness to lessen before you take your next intense workout. Even if you do not compete in sports, you should use some form of "stress and recover" to strengthen your muscles and keep them strong. You can tell that you are damaging muscles when you exercise vigorously enough to feel burning during exercise and DOMS in those muscles 8-24 hours later (Str & Condit J, October 2013;35(5):16–21). Only when the soreness decreases should you take your next intense workout. The soreness is caused by injury to the muscle fibers themselves and when muscles heal, they are stronger than they were before the soreness occurred (J Strength Cond Res, Feb 2003;17(1):197–208).

Non-Steroidal Pain Medications (NSAIDs)
Some athletes and exercisers use pain medication, such as ibuprofin (Advil, Motrin) and other non-steroidal anti-inflammatory drugs (NSAIDs), because they think it will help them to recover faster from muscle soreness so they will be able to do more training and become better athletes. NSAIDs do not prevent DOMS (Brain Behav Immun, Nov 2006;20(6):578–84). Even if they did prevent DOMS, using them would increase your chance of injuring yourself because you should use DOMS as a signal to tell you when you can safely exercise intensely again (Clin J of Sprt Med, 2002;12(6):373-378). A review of 41 research papers showed a slight association between the use of NSAIDs and reduction of injuries, but the difference was nearly the same as what you would expect from use of placebos (Am J Sports Med, Mar 2017:363546517697957).

Reasons for Not Taking NSAIDs for DOMS
NSAIDs are supposed to reduce pain by blocking inflammation, and inflammation is unlikely to be a cause of DOMS since it does not decrease in any way after years of exercising (Front Physiol, 2015;6:424). Earlier studies have shown that NSAIDs:
• do not significantly prevent DOMS (Brain Behav Immun, Nov 2006;20(6):578–84)
• may not block DOMS at all (J Sports Sci, Mar 1999;17(3):197–203)
• do not help muscles to recover faster (Appl Phys Nutr and Metab, 2008, 33(3): 470-475)
• may delay muscle recovery (J Bone Joint Surg Am, 77:1510-1519, 1995)
• may block some of the muscle growth that makes muscles stronger in the short term (American Journal of Physiology, 2002;282:E551-556; 2011;300:R655-662)
• may cause short-term weakening of bones and tendons (J Bone Joint Surg Am, 77:1510-1519, 1995)

What to Do When You Have DOMS
You can stop exercising when you have DOMS if you want, but you will become stronger if you take easy recovery workouts while your muscles are still sore. Athletes do not usually plan to take off workouts during recovery, even though resting when the muscles feel sore will allow muscles to heal faster than if you exercise at low intensity. Exercising at low intensity during recovery will cause your muscles to become more fibrous and resistant to injury when you stress them in your next intense bout of exercise. If you are a runner, run faster two or three times a week and much more slowly when you feel soreness on the days after running fast. If you are a weightlifter, lift heavy weights once or twice a week and much lighter ones on the following day or days when your muscles feel sore.

What Does Not Decrease DOMS
• cryotherapy, homeopathy, ultrasound or electrical current devices (Sports Med, 2003;33(2):145–64; Phys Ther Sport, May 2012;13(2):101–14)
• glutamine or arginine supplements
• stretching (Cochrane Database Syst Rev, 2011;(7):CD004577)
• Epsom salts
• drinking extra water
• compression garments
• massage therapy (J Orthop Sports Phys Ther, Feb 1997;25(2):107–12)
• vitamin D supplementation, unless a deficiency needs to be corrected (hydroxy vitamin D below 20 ng/ml)

How to Prevent Injuries
Before you start an intense exercise program, you should have exercised regularly for many months, be in good shape and not have any health conditions that might cause exercise to harm you.
• Try to set up your exercise program so that on one day, you take a hard workout that damages your muscles so they feel sore on the next day. Then take easy workouts for as many days as it takes for the soreness to go away, and then take your next hard workout.
• When you are training properly, your muscles will feel sore every morning. If they don't feel better after a 10-minute warmup, take the day off.
• When your muscles start to feel heavy or hurt, stop the workout.
• If you develop a localized pain (one side only) that worsens as you exercise and does not go away when you slow down, stop the workout immediately.

Tips for Recovery from a Hard Workout
• Eat as soon as possible after an intense workout (J of Sports Science, Jan 2004). It doesn't matter what you eat (Am J Clin Nutr, Jan 2017).
• Go to sleep for 1-3 hours as soon as possible after an intense workout. Muscles recover faster when you are asleep.
• Avoid NSAIDs; they may ease pain but will not help you to recover faster (PNAS, June 27, 2017;114(26):6675 – 6684).
See Recovery: The Key to Improvement in Your Sport

CAUTION:Intense exercise can cause a heart attack in a person who has blocked arteries or heart damage. Check with your doctor before you start a new exercise program or make a sudden increase in the intensity of your existing program.