Training Fast and Slow: How Thresholds Guide Training

For a long time, it has been a standard training practice in the elite endurance world to use thresholds to guide workout intensity. Most elite athletes know the exact numbers to use in each session with regards to heart rate, power and/or pace. However, the proper distribution of intensity is not often talked about and is, in many cases, misunderstood. Today, the majority of training hours in an elite endurance athlete’s schedule are spent at low intensity. More specifically, a majority of training time is spent at or below what is called the first ventilatory threshold (VT1).

This level of effort is closely related to the first lactate threshold (LT1) when measured by a lactate test, and is also known as the “talking speed/effort” (1*). When training at this effort, athletes can spend a longer amount of time in sessions without getting fatigued. Slower sessions below VT1 contribute to enhanced mitochondrial function by promoting fatty acid utilization over carbohydrate metabolism. When the training load is low, the body has an affinity towards fatty acids as a fuel source, as they are more efficient in terms of carbon dioxide expulsion per unit of energy produced. 

However, as fatty acids are a slower fuel source, the body will have to upregulate mitochondrial density for better utilization. This will increase mitochondrial density within a muscle cell and promote the formation of slow twitch muscle fibers over the more carbohydrate-dependent fast twitch fibers. This increased number of mitochondria in the working muscles contributes to a higher ability to burn fat and, interestingly, absorb pyruvate from converted lactate under load (4,5*). Counter intuitively, training slowly and spending the majority of your time below VT1 will give you a considerable boost to your endurance abilities and your performance at higher workloads.

Despite these benefits, low intensity training alone is not enough to provide the stimulus to be able to perform at a high level. Some training time must also be spent at higher intensities. The ratio of this training distribution is somewhat close to 80/20 in favor of easy training sessions, although the exact distribution varies between individuals. Nevertheless, using 80/20 as a baseline would be highly beneficial for athletes of all activity levels. 

In a study performed by Thomas Stöggl and Billy Sperlich and published in Frontiers in physiology in 2014, the training benefits of four different methods were tested in a pool of 48 runners, cyclists, triathletes, and cross-country skiers. Out of the four methods, classified as high volume, high intensity interval, threshold training and polarized training, the polarized approach reaped the highest benefits over the 9-week test period. In this study, a polarized approach meant spending an average of 68% of the participant’s time below VT1, 6% between VT1 and VT2, and 24% of the time above VT2 (3*). As with low intensity training below VT1, high intensity training also increases a person’s VO2 max and overall mitochondrial respiration and density. (6*)

For a runner doing five sessions per week, the optimal structure might be to use Tuesdays and Saturdays as hard interval days, with easy sessions below VT1 on Wednesdays, Thursdays and Sundays. As a general rule of thumb, including two intense sessions in a week is a good strategy, with all other sessions being easy. Another strategy, perhaps more in line with people who are not bound by weekly cycles, is to use a four-day approach. This means alternating easy, long and hard sessions so they always loop around in the same rhythm. This could be set up as “easy - hard - easy - long”. The easy sessions would then always be under VT1, the hard sessions would feature interval training, and the longer session only differing from the easy ones in their duration. 

Naturally, the most important aspect before creating a training plan is to know how your thresholds are distributed, and then to  train accurately based on those values. As a great coach once said: “Training at the incorrect intensities is time wasted, at best”.



Siggi Ragnarsson

Hi! I’m Siggi, and I’m originally from Iceland. I got my start in Triathlon in 2015 and became a pro in Ironman 70.3's in 2018. Prior to competing in triathlon, I was a competitive swimmer. As both an athlete and engineer - my formal education includes a Bachelor and Masters in Mechanical Engineering from the University of Iceland - I’m extremely interested in optimizing the human body for exercise. After successfully transforming myself from an amateur to a pro triathlete, I wanted to use my insights to help others achieve their own health and fitness goals. I started a performance testing service and personal coaching business in Reykjavik where we offer blood analysis, metabolic testing, and personalized coaching for athletes of all abilities. I’m excited to share my research and thoughts with you and hope you will find it both interesting and useful in your own athletic journey!