On a very regular basis, the question of lifting weights comes back to the table. To be honest, this question is still subject to debate in the exercise physiology and exercise science community, to some extent. However, numerous general public articles throw out rehash of old write-ups about weights, some being presented as a panacea to dealing with injuries, or ‘running faster without running more’. They are invariably of poor quality, and as we all know, getting better requires hard work. If it didn’t, we would find something else to do, something that we can be proud of, because we know it took time, dedication, and effort to get there. There, I’m off my high horse now, and we can tackle the weight question. I will make the assumption that people asking ‘should I lift weights’ are asking because they want to swim/bike/run faster, not because they want to look buff. If you intend to look buff, then weights are definitely far superior to running 120mi a week.
I’ll ignore swimming because, first, I have not looked carefully at the literature on weight lifting and swimming, and second, most triathletes are usually very average swimmers, and could just get in the water more to get better. When it comes to cycling and running, there are two main ways to get faster: you can either improve your VO2max (the size of your engine if you will). Or you can improve how efficient you are.
In ‘Effects of concurrent endurance and strength training on running economy and .VO(2) kinetics.’ (GP Millet et al.Med Sci Sports Exerc. 2002 Aug;34(8):1351-9.) demonstrate that ‘additional heavy weight training led to improved maximal strength and running economy with no significant effects on the .VO(2) kinetics pattern in heavy exercise.’ on a population of 15 triathletes. It was particularly interesting since Millet is not only an exercise physiologist, but has also been involved in triathlon coaching for as long as I remember, so he has a truly vested interest that goes beyond publishing yet another paper.
When it comes to cycling, the conclusions of Ronnestad et al in ‘Effect of heavy strength training on thigh muscle cross-sectional area, performance determinants, and performance in well-trained cyclists.’ (Eur J Appl Physiol. 2010 Mar;108(5):965-75. Epub 2009 Dec 4.) are similar, except that they demonstrate an increase in VO2max, while ‘Maximal strength training improves cycling economy in competitive cyclists.’ by Sunde et al. (J Strength Cond Res. 2010 Aug;24(8):2157-65.) demonstrate improved economy from heavy weight training.
However, the population in question are not recreational athletes, and as ‘Effects of a concurrent strength and endurance training on running performance and running economy in recreational marathon runners.’ suggests in J Strength Cond Res. 2010 Oct;24(10):2770-8., there seems to be no impact of heavy weight training on running economy when it comes to average runners.
So, what does this mean? The first studies I presented focus on relatively advanced athletes. Not elite, but what the triathlon world would call very good age groupers. They are often athletes who have been training for a while, and who are relatively talented, so a fair amount of optimization of their training has taken place. In the study pointing to no improvement, we have a group of runners who are just not running enough yet. I am simplifying a bit on purpose.
In essence, unless you are already running and cycling a lot, chances are you will not improve or not much (i.e. no statistical difference in performance) by lifting weights. What this means is that, if you have an hour to spare, you’re better off going for a run, or a ride, if you intend to improve performance. For instance, a run with high intensity hills will help improve your running economy. At the pointy end, and provided you have the time to include weight training and still recover, the studies seem to indicate that lifting could give you that extra edge. One thing to take into consideration though: there is an increase risk of injury with heavy weight training, especially if you cannot do the sessions under the supervision of an experienced trainer, who understands your goals. The conclusion is that from a practical standpoint, for the immense majority of age groupers, weights are not on the to do list. Use your time well. Spend your time honing your skills by swimming, cycling, and running, and work on strength doing sport-specific sessions (paddles+elastic, big gear workouts, hill repeats).