We must recognize that running off the bike for 13 or 26 miles in a half or full distance triathlon (respectively) is very different than running a stand alone half marathon or marathon. For example, a stand alone endurance runner will start the race is cool or cold conditions, first thing in the morning with a body that is glycogen loaded and well-hydrated. Marathon races are often issued a heat warning if a race is predicted to reach over 75-degrees. The pacing strategy of a marathon is usually one of holding back and increasing the pace as the race continues (negative split). The race course is generally flat and training is based on preparing to hold a specific pace. With all this in mind, a runner goes into a race with little to no added physical stress.
A triathlete couldn't be more different.
A triathlete starts the run in moderately warm or hot conditions. The triathlete has already been exercising for 3-6+ hours when the run begins. The triathlete is now needing to run with muscles in a fatigued state while attempting to avoid extreme dehydration and glycogen depletion. There's also the mental fatigue of racing for 5-15+ hours compared to a stand along marathon that lasts 2-3 hours for the elite athletes.
A few other physiological changes are taking place when running off the bike:
- Oxygen consumption, respiratory frequency, ventilation rate and heart rate increase more when running off the bike vs. stand alone running.
- Metabolic changes - triathletes are running in a state of glycogen depletion, dehydration, ventilatory respiratory muscle fatigue, decreased stroke rate and volume and increased core body temperature.
- Biomechanical changes - triathletes are running 13 or 26 miles with muscle fiber damage and different running economy than that of stand alone running. This includes increased ground contact time, increased vertical oscillation, decreased running economy, altered foot placement, decreased stride length and gait frequency, tight hip flexors, less stored elastic energy in the muscle-tendon complex.
- Neural fatigue - neuromotor patterns are reduced when running off the bike. When running off the bike, triathletes also experience inefficient muscle recruitment patterns, central nervous system fatigue (inability to send signals/recruit muscles to contract), decrease in the voluntary activation of muscles, decrease in the frequency and synchronization of motor neurons, reduced drive from motor cortex.
