To preserve muscle and bone size and strength, recreational runners and cyclists should do upper body and core weight lifting and resistance training. Everybody will lose muscle and bone size and strength with aging. Your bones weaken progressively after age 30, and between ages 35 and 50, about 28 percent of North American men and women suffer from bone weakening called osteopenia (J Amer Osteo Assoc, June 2019;119:357-363). By age 65, 25 percent of women and six percent of men are at high risk for breaking their hips or spine with minor falls (CDC/National Center for Health Statistics, August 2015).

The people who lose the most muscle are usually the ones who are most at risk for falls and broken bones. Any prolonged period of inactivity, such as bed rest, injured nerves, wearing a cast or even living in a decreased force of gravity, will cause loss of muscle tissue (Med Hypotheses, 2007;69(2):310-21). If you inactivate a leg by putting it in a cast, you lose a significant amount of muscle size in just four days (Nutrition, Acta Physiol (Oxf), March 2014;210(3):628-41). Just one year of lifting weights can strengthen the bones enough to help protect people from fractures (Bone, Oct 2015;79:203-212).

The hard ground impact of fast running may slow bone loss (BMC Med, Dec 20, 2012;10:168), but cycling does not prevent age-associated osteoporosis. A review of 22 scientific studies shows that adding a resistance program such as lifting weights to endurance sports such as running or cycling can markedly slow this loss of muscle and bone, and actually increase muscle size and strength, with greater benefit from low volume, high-resistance weight lifting than high volume, lower-resistance exercise (Human Movement, July 23, 2020;21(4);18-29).

Strength Training May Not Help Competitive Cyclists or Runners Go Faster
Studies show mixed results on whether runners and bicycle racers can run and cycle faster with added strength training, but most studies show limited benefit of strength training for endurance athletes because strength training does not improve your ability to take in and use oxygen. The limiting factor for how fast an endurance athlete can run or cycle over distance is the time it takes to move oxygen into muscles, and that is improved only by training that involves creating an oxygen debt (becoming short of breath). Lifting weights and doing other resistance exercises do not increase VO2max, the maximal amount of oxygen that a person can take in and use (Med Sci Sports Exerc, 2002;34(8):1351–1359; Sports Med, Aug 2016;46(8):1029-39). Competitive runners and cyclists can do their leg strength training just by running or pedaling their intervals faster (Scand J Med Sci Sports, Oct 2010;20(Suppl 2):11-23 and Nov 2010;38(11):1965-70).

Leg Strength Training Can Cause Injuries in Runners and Cyclists
Most runners and cyclists should do resistance training only for their core and upper body, and use their legs only for running and cycling. A major problem with adding a weight lifting program for the legs to running or cycling is a marked increase in risk for leg injuries. Ideal training for endurance is to go more intensely on one day, damage your Z-lines in your muscle fibers, and feel delayed onset muscle soreness (DOMS) on the next day. (DOMS is necessary for gaining strength.) The day after you do weight training with your legs, your leg muscles will be weaker and this would interfere with any attempted intense endurance workout or resistance exercise (Sports Med, Nov 2017;47(11):2187-2200). You are supposed to exercise slowly and easily for as many days as it takes for the soreness to lessen before you do your next intense workout. Therefore you should not do resistance training for your legs on a day when your legs are recovering from the previous day’s intense workout.

For at least a day after lifting weights, an athlete is at high risk for tearing muscle fibers if he attempts an intense endurance workout. Thus you would need to do your intense endurance workouts and your resistance exercises on the same day, and this increases risk for injuring yourself. If you want to add leg resistance exercises to a leg endurance program, you must learn to recognize the signs of overtraining and back off when your muscles feel excessively fatigued or sore.

My Recommendations
• Non-competitive runners and cyclists should alternate faster and more intense days with slower recovery days, do strength training only for their core and upper body, and not do resistance training on their legs. Combining endurance and strength training on the same muscle groups increases risk for injury. Most competitive athletes should also follow this program.
• Elite runners and cyclists may want to combine leg strength training with leg endurance training, but they should do the strength training not more than twice a week, only on hard days (after an intense endurance workout), and never on recovery days. They should skip the weight workouts when their muscles feel excessively sore or tight, stop the strength training workouts during their competitive season, and hope that they do not suffer injuries.
• If you are not already doing strength-training exercise, first check with your doctor to make sure you do not have any condition that may be harmed by exercise. I have always recommended joining a gym and using the weight training (Nautilus-type) machines there, but until COVID-19 is under control, you will want to find a way to do resistance exercises at home. Resistance exercise can increase muscle size and strength in older people, but with aging you need to work longer to gain the amount of strength that a younger person would get with the same program (Med Sci Sports Exerc, 2011;43 (2):249-58). Since lifting lighter weights many times is less likely to cause injuries, I recommend lifting lighter weights with more repetitions.
• Stop immediately if you feel severe pain or if you have pain that does not go away as soon as you stop lifting the weight. Pain in a muscle or tendon is often the first sign of an impending injury.
• Take the day off if your muscles feel sore or fatigued after a 5-10 minute pre-workout warmup.
• Just using and contracting your muscles in any activity offers health benefits, but you can gain additional benefits by adding intensity with some form of interval training.