Most runners start training for events high on ambition and succumb to the pitfall of always training at the same moderate intensity. Knowing the physiology behind different running intensities can help guide training, allowing for optimal fitness gains without burnout.
by Kristina Pattison, DPT, OCS, CSCS
Aerobic Training
At low intensities, energy is primarily procured from the burning of fat which requires copious oxygen from your respiratory system. Stress is low on the body, effort is easy, breathing can usually occur in and out through the nose, and generally your body could go on forever on stored fuel. The “Aerobic Threshold Test” determines your heart rate and speed at the top end of this low intensity. This ensures you keep your easy runs easy enough by staying below this “aerobic threshold” (zone 1 or 2).
Over time, training below aerobic threshold allows the body to become more efficient at processing oxygen and stored energy. Increased density of capillaries and mitochonria–energy burning organelles within muscle–make it possible to run faster at the same low intensity. At the same time, the musculoskeletal system develops improved neuromuscular coordination for running efficiently at lower cardiovascular intensities. This is especially important for events that occur over long duration such as 50km-100 miles. Retesting this threshold on a regular basis allows for appropriate pacing as fitness improves.
Aerobic Threshold: at the top end of low intensity, muscles demand more energy and oxygen from the cardiorespiratory system to keep up the pace. The muscles begin burning more of a faster burning energy source–glycogen stored in muscle–to support fat metabolism, entering into moderate intensity (or zone 3).
Anaerobic Training
As intensity increases further, energy and oxygen demand increases and the body burns higher percentages of glycogen to keep going. At the top end of moderate intensity, the body is using mostly glycogen for metabolism which can be burned without oxygen. This is known as the Anaerobic Threshold or second ventilatory threshold, and is the beginning of high intensity work (zone 4+). But glycogen stores in the body are limited, and the time to exhaustion above anaerobic threshold is about an hour or less. At the same time, metabolic byproducts, such as lactate, build up rapidly, decreasing time to exhaustion further.
With experience, training above anaerobic threshold allows muscles to become better at processing and storing glycogen. Processing of metabolic byproducts also improves, including lactate, allowing the body to maintain faster speeds for longer. And, running at higher speeds regularly improves biomechanical efficiency at these paces. This is especially important for events occurring over shorter distances such as half-marathon and less. But anaerobic training comes at a price. Higher intensities require increased recovery time. Usually only 5-20% of a typical training program is spent at or above the Anaerobic Threshold (AnT). The AnT test is to make sure your hard runs stay hard enough by staying near or above anaerobic threshold.
Anaerobic Threshold: (AnT) when intensity is high enough, the muscles keep up the pace by getting the majority of their energy from anaerobic metabolism–glycolysis.
When easy runs are truly easy, and hard runs are appropriately intense, athletes can make progressive gains in fitness without a dreaded plateau. This is especially important for endurance athletes who have a tendency toward overtraining, injury, and burnout.
Tips:
For easy runs (zone 1-2), use your heart rate as your guide. Otherwise, keep breathing in and out through the nose to ensure you’re keeping effort low. You may have to slow your pace to keep the effort low.
For tempo runs or cruise/tempo intervals (zone 3) you can use heart rate (above aerobic & below anaerobic thresholds) or pace (when on flat ground).
For repetition workouts: use your perceived effort as a guide for intensity. Usually when working in zone 4 and above, your reps may be short enough that your heart rate doesn’t respond fast enough during the rep to use this as a guide. Generally they will be at an effort of 8-10/10.
About the Author:
Kristina Pattison is a Missoula, Montana, based runner focused on mountain, ultra, sky and trail. Locally, she works as a physical therapist for Inspire Physical Therapy and Wellness. She is a certified ultrarunning coach, a board certified specialist in orthopedic physical therapy, and a certified specialist in strength and conditioning. Formerly, Kristina worked as a wildland firefighter and Missoula Smokejumper. She is passionate about helping athletes dream big and achieve their goals. Contact Kristina today about coaching through Flight Phase Coaching.