Running vs. Cycling: Different Training Focuses for Triathletes
The sport of triathlon has become increasingly popular since its humble beginnings in 1974 and finally became an Olympic sport in 2000. The sport is a challenging combination of three very different endurance disciplines making training for a triathlon very difficult. 
As triathlon is an endurance event, the ‘lactate threshold‘ is an important reference point for triathletes and coaches when designing training sessions. The lactate threshold refers to an exercise intensity that can be maintained for an extended period of time. In other words, it is your physiological potential for endurance exercise. At this point, the lactic acid in your blood (a metabolic waste product which is produced during exercise and associated with fatigue) is being cleared by the body as fast as it is being produced. The lactate threshold has previously been shown to be a good predictor of endurance performance, i.e. those with a higher lactate threshold perform better. The best way to increase the lactate threshold is with high intensity interval training (HIIT) combining short bouts of exercise (30 secs to 4 mins) at high intensities, separated by periods of rest. It is currently unknown whether the lactate threshold is similar to the intensity that triathletes maintain during performance and whether training prescription should be the same across the disciplines. Our recent study conducted at the University of Queensland examined the different demands of 40 km of cycling and 10 km of running (swimming was excluded because controlled laboratory testing is not possible) and determined the importance of the lactate threshold in these two disciplines. We tested the lactate threshold of 10 well-trained male triathletes in cycling and running, as well as a 40 km cycling time trial and a 10 km running time trial. 
The results showed that while the cycle lactate threshold was a similar intensity to what the guys could maintain during the 40 km time trial, the running lactate threshold was much higher than the speed they could maintain during a 10 km time trial. This means that triathletes were successfully cycling at their physiological potential but were unable to match this potential during their 10 km run. These results indicate that for triathletes, the lactate threshold is a good reference point for 40 km cycling intensity, however it is a somewhat unsustainable intensity for running. Though more research would be needed to accurately determine the reasons for this, one potential mechanism could relate to the fact that running is a weight-bearing exercise. As opposed to cycling which is non-weight-bearing, running demands greater energy expenditure and muscle mass involvement. This will likely result in an earlier onset of lactate accumulation in the muscles and earlier fatigue, if running at the LT intensity. As such, an intensity slightly below the LT may be more appropriate during a 10 km running time trial. In regards to training prescription, the intensity selected for HIIT must be above the LT to induce the greatest improvements. In relation to our findings, the LT intensity itself may suffice for the beginning of a running HIIT program, since this is clearly not sustainable for long periods. For cycling HIIT, intensities above the LT would be needed to gain significant improvements. If you are a triathlete please share your training experience in the comments! I will do my best to respond to your questions here. Samantha Fisher Samantha Fisher is a 2013 PhD Student at the School of Human Movement Studies at The University of Queensland. Samantha also works as a Strength and Conditioning Coach at UQ Sport Academy.