Several studies have found that endurance athletes can greatly increase their strength by adding sport specific weight training to their program. However, these strength gains did not result in improved endurance or, more importantly, improved performance.
One 1993 study was designed to determine whether adding sport specific strength training to would improve sprint-swim performance. The results showed that strength training did not improve sprint swim performance, even though their strength improved by 25-35%. The conclusion of the researchers? The extra strength gained from the resistance training program did not result in improved stroke mechanics.
A similar result was found in a study of rowers in 1989. In addition to their regular training, one group performed 18-22 high-velocity, low-resistance repetitions, while another group did low-velocity, high-resistance repetitions (6-8 reps). The result here showed training effects were specific to the resistance training mode and did not transfer to the more complex action of rowing. Resistance training programs may actually restrict the volume of beneficial, sports specific training that can be achieved because of increased levels of fatigue.
One group of National level cross-country skiers supplemented their normal aerobic workouts with "explosive" strength sessions of plyometric exercise and heavy resistance (80% of 1 RM) squats improved their jumping height and time to reach maximal isometric force production significantly. However there was no differences in VO2max or measures of the aerobic and anaerobic "thresholds" between the two groups after the different training regimens.
Professional cyclists added 3 sets of 6-8 maximal repetitions of leg press, quadriceps extensions and hamstring curls to the regular training. This resulted in strength gains of about 25%. However, this still did not result in improved cycling performance. The exact opposite effect actually occurred. Their 40 km times slowed from 58.8 minutes to 61.9 minutes after weight training. The cyclists also complained of feeling "tired and heavy" while riding and even reduced their weekly training distance by about 20% during the study. Other studies have found beneficial effects of strength training on both short and long-term endurance capacity.
Hickson et al. (1988), found that 10 weeks of a three-times-a-week strength training did not change the VO2max of moderately-trained runners and cyclists. But a short-term (4-8 minutes) endurance test was improved by 12% for both running and cycling, while long-term endurance improved from 70 to 85 minutes for cycling. Marcinik et al. (1991) showed that strength training had positive effects of endurance cycling capacity. Eighteen males performed 12 weeks of strength training three times a week. The strength training consisted of 8-12 repetitions of upper body exercise (bench press, push-ups, lat pull-downs, arm curls) and 15-20 repetitions on lower body exercises (knee extensions, hip flexion's, parallel squats) with a 30-second rest between exercises. The strength training program had no effect on the subjects VO2max. However, 1 RM for knee extension and hip flexion improved by 30% and 52% respectively. More important, cycle time to exhaustion at 75% of VO2max improved a massive 33% from 26.3 minutes before strength training to 35.1 minutes after training. The conclusion: "strength training improves cycle endurance performance independently of changes in VO2max... and that this improvement appears to be related to increase in leg strength."
What's the Answer?
If you look at the subjects in the different studies, it becomes evident the those who benefit from strength training are individuals who are relatively new to the sport and not highly trained. These untrained individuals will benefit from any increase in general fitness, be it an improvement in strength or endurance. This explains why the greater muscle strength increases endurance performance in these individuals.
For highly-trained athletes who already posess a great deal of sport specific strength, further strength gains are less important to enhanced endurance. At the highest level of competition, increases in strength and power are not as critical as the development of correct technique. For elite athletes, the concept of specificity of training and the principles of conditioning still apply.
So, if you are already an elite athlete, is may be more important for you to practice skills and general sport technique. If, however, you are new to a sport or still find you fatigue more easily than then your peers, you should focus on sport specific weight training routines.