Metabolic Testing & Analysis: Number Crunching

In this week’s training tip, I will discuss how metabolic testing & analysis allows both coach and athlete to evaluate the various metabolic pathways responsible for aerobic energy production. For the competitive endurance athlete, having a clear cut understanding of exactly “where” your energy is coming from and how much of it you are using at certain paces or power outputs, provides you with a number of substantial advantages. With this data in hand, you can effectively steer your training, and, when necessary, your nutritional practices, in order to optimize your performance on race day, thereby gaining that much coveted “edge” on the competition!

In one of my previous training tips, “Determination of Aerobic Profile” I talked at length about the use of metabolic testing to determine both VO₂ max and the percentage of VO₂ max at which an athlete’s maximal lactate steady state (MLSS) occurs. MLSS serves as one of the greatest determining factors in an endurance athlete’s performance; all things being equal, the athlete with the fastest pace at MLSS will typically win the vast majority of endurance related events he or she enters… up to a certain point.

For very long events such as the marathon, half ironman (70.3) triathlon or the grueling Ironman triathlon distance, although velocity/power output at MLSS is of critical importance, so too is the athlete’s ability to conserve much needed glycogen stores for the long haul.

We’ve all heard of the terms “bonking” or “hitting the wall” before, and if you’re a hard core fan of endurance racing, you’ve undoubtedly seen it happen to even the world’s best endurance athletes from time to time. In simplistic terms, when an athlete “bonks” they have exhausted their body’s high octane fuel source: muscle glycogen. Muscle glycogen is a substance that is used by the body to provide energy both aerobically (with oxygen) and anaerobically (without oxygen) depending upon the intensity at which the body is operating at. Generally speaking, maximal/near maximal efforts lasting between .01 and 45 seconds, do not require oxygen whereas efforts lasting longer than 45 seconds do. The longer the effort, the greater the role that O₂ plays in energy production (for a more comprehensive look at the three metabolic pathways that are responsible for energy production, click here). Regardless, glycogen is utilized by the body at just about every intensity level, even at rest; the harder the athlete “pushes” during a race or training session, the more glycogen the athlete’s body will “burn” to keep up with energy demand. As efforts approach and surpass MLSS speed/power output, a very high percentage of the metabolic energy yield is derived from glycogen metabolism. For the endurance athlete, this is where the numbers game begins.

Depending upon many factors including, but not limited to: gender, body weight/muscle mass, training and nutritional practices, etc. our bodies store somewhere between 2,000 – 2,500 calories worth of glycogen; enough energy to “only” run somewhere between 20 – 25 miles…. but don’t despair! A well trained, competitive marathon runner will run within 15 – 30 seconds per mile pace of their MLSS pace on race day. Assuming proper training & nutritional practices, glycogen stores will not be depleted late in the race thanks to the fact that the body will derive a good deal of its aerobic energy from the metabolism of free fatty acids (fats!) in addition to its glycogen stores. As such, the dreaded “bonk” is avoided and performance does not suffer late in the game.

Through metabolic testing, we are able to actively assess what percentage of energy is being derived via fat and glycogen metabolism. By doing so, we can “steer” an athlete’s training and nutritional practices to optimize the percentage of glycogen they burn at specific paces/power outputs. In my next training tip, I’ll provide two theoretical examples of how we would go about using testing data to steer the training practices of 2 runners gearing up for the marathon and discuss how testing data would shed light on the different approaches we’d have to take with both runners to ensure their respective success on race day. Stay tuned!