Competitive endurance athletes usually run or cycle very intensely on one day, feel sore on the next day and go easy for that day as they alternate hard-and-easy workout days. Adding strength training to an endurance program markedly increases athletes' chances of injuring themselves unless they learn to recognize the signs of overtraining and back off training when their muscles feel excessively fatigued or sore. If endurance athletes add weights to their training programs, they should do the strength training on the same day that they take their intense running or cycling workouts, and not put stress on their leg muscles on their recovery days.
Muscles are damaged on hard training days, and they heal and get stronger on the following slower recovery day. Lifting weights on a recovery day prevents muscle healing. Extensive data show that lifting weights damages muscle fibers for at least a day afterwards, so that the involved muscles are weaker and would interfere with any attempted intense endurance workouts on the next day (Sports Med, Nov 2017;47(11):2187-2200). For at least a day after lifting weights, athletes are at high risk for tearing muscle fibers if they attempt intense endurance workouts.
Research Supports Strength Training
In one study, 19 well-trained female duathletes were assigned to either:
• run and cycle with an added strength training program, or
• just run and cycle. The strength training program included four lower body exercises, three times each, twice a week for 11 weeks. The added strength training improved their running and cycling performance done after strenuous exercise, but not after resting (Physiol Rep, Mar 2017;5(5)). This is just the latest of many conflicting studies that show either improvement or no improvement when strength training is added to an endurance sport training program.
A review of many scientific articles shows that runners and bicycle racers can run and cycle faster with added strength training (Scand J Med Sci Sports, Oct 2010;20 Suppl 2:39-47) because it makes them stronger (J Strength Cond Res, 2013;27(9): 2433–2443), so that they can run and cycle more efficiently with less effort (Med Sci Sports Exerc, 2008;40(6): 1087–1092). Other research shows that adding strength training to an endurance program can make muscles larger and stronger (Sports Med, Aug 2016;46(8):1029-39). However, the improvement in racing performance with added weightlifting is small, and sometimes nonexistent, because lifting weights does not improve VO2max (the ability to take in and use oxygen). 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 becoming short of breath (Med Sci Sports Exerc, 2002;34(8):1351–1359).
• Non-competitive runners and cyclists should alternate faster and more intense days with slower recovery days. They should do strength training only for their core and upper bodies and not do strength training on their legs. Combining endurance and strength training on the same muscle groups increases risk for injury.
• Most competitive athletes are best off with the same program: leg endurance training and upper body and core strength training.
• Elite runners and cyclists can try to combine leg strength training with leg endurance training, but they should do the strength training not more than twice a week, only on the hard days after intense endurance workouts 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 watch for signs of impending injuries (such as localized pain in just one leg).
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