A recent review of 22 scientific studies shows that adding a resistance program such as lifting weights to endurance sports such as running or cycling can 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). It showed little, if any, benefit for competitive endurance events because resistance exercise does 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).
Other 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 because runners can do their strength training just by running their intervals faster and cyclists by pedaling harder (Scand J Med Sci Sports, 2010 Oct;20 Suppl 2:11-23).
The limiting factor for how fast an endurance athlete can run or cycle is the time it takes to move oxygen into muscles, and that is improved only by training that involves shortness of breath. Some studies show that endurance exercise interferes with muscle growth by activation of AMPK, by inhibiting protein-synthesis, or by limiting the mTOR pathway (Med Sci Sports Exerc, 2006 Nov;38(11):1965-70).
Strength Training for Core and Upper Body, Not Legs
Most runners and cyclists should do resistance training only for their core and upper body, and use their legs during their endurance exercise. 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 involved muscles are weaker and would interfere with any attempted intense endurance workout or resistance exercise on the next day (Sports Med, 2017 Nov;47(11):2187-2200). You are supposed to exercise slowly and easily for as many days as it takes for the soreness to lessen. Therefore you cannot 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.
• 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.