If reading detailed research isn’t your cup of tea, here’s the executive summary: If you strength train with very heavy resistance (>85% of your one rep max – (1RM)) along with your endurance training, you can improve your long-term endurance without any increase in muscle mass. Yes, I know that may be contrary to intuition, but there you have it. Other studies show that weight training with <80% of your 1RM just doesn’t cut it. So, you have to lift heavy! However, please don’t sue me when you herniate a disc trying to squat 532 pounds. Utilize a personal trainer if you want to go >85% of your 1RM – that’s what they’re there for.
OK, here are the details…
This study just came out in March of this year: Effects of resistance training on endurance capacity and muscle fiber composition in young top-level cyclists. The gist of the relatively small study was that they had two groups of highly trained cyclists – one group continued with Endurance training alone (E group) while the other group added high resistance Strength training to their Endurance routine (SE group). For the SE group, they weight trained 2-3X/week and progressively increased the weight. They progressed over a period of a couple weeks to 4-6 reps of ≥85% of their 1RM. After 16 weeks of training, both the E and the SE groups improved their short term (5 min) cycling performance, but when it came to a 45 minute cycling, the endurance capacity increased only in the SE group (by 8%). Interestingly, the muscle fiber area and capillarization (# of blood vessels) was no different.
Take home message…
Combining strength training with endurance has shown equivocal results in different studies. However, when you dissect the studies, it turns out that long term endurance performance has not shown gains in the SE group when the subjects did low volume (<8 weeks duration) and/or low-intensity (<80% 1RM) strength training (found here, here and here). However, when the volume is high and the intensity is high (≥85% 1RM) there is significant improvements in long-term endurance capacity (found here, here, here and here)
Pretty much all of these studies concur that there is no change in muscle size because the normal increase in size that you would see is blunted because of the continued endurance training which is catabolic.
OK, so if there is no change in Vo2max, muscle size or capillarization, why do they get better at endurance capacity? The answer there is still unknown, but the theories are numerous. The one that makes the most sense to me is a change in economy of movement because of improved neuromuscular recruitment. Simply put, when you lift heavy, you learn how to utilize more muscle fibers and you become more coordinated in firing the muscles. Therefore, if you run or cycle at 70% of your Vo2 max, but your are recruiting more power with the same effort, you will go faster and use less oxygen.