A Protocol for Establishing Critical Power in Running A Working Paper Prepared by The Stryd Team 27 September 2015

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Introduction

One of the primary reasons for considering one’s power consumption during training is to precisely control training intensity, but such control is most valuable with clear power goals in mind. A natural question is, “What power level(s) should I maintain during today’s workout?” Knowing how an individual’s power numbers relate to the transition between (primarily) aerobic and (primarily) anaerobic respiration makes it much easier to determine the best training zone(s) for a workout. Many athletes have used blood lactate tests to determine this transition point; around the transition, blood lactate begins to increase rapidly. However, blood lactate testing is inconvenient, requiring that multiple blood samples be taken during the test. This working paper describes a method of determining the power at which an athlete transitions from aerobic to anaerobic respiration. We will refer to this method as the Critical Power Protocol in the rest of the paper. All the data necessary for this protocol are measured using the Stryd device, and the protocol can be completed in approximately an hour without much impact on an athletes training schedule. We have evaluated the protocol with 20 athletes, using blood lactate threshold for the purpose of comparison and validation. The average error of the proposed Critical Power Protocol when compared to the lactate threshold is 0.41 W/kg (8.9%) with a standard deviation of 0.31 W/kg (6.0%). Work to improve the accuracy and convenience of the protocol is ongoing.

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Why Power?

Runners use a variety of metrics to estimate training intensity, including pace and heart rate. Pace is only an accurate representation of effort on level surfaces. Heart rate is a function of many variables other than training intensity, and it commonly lags intensity by three minutes. Power takes into account inclines and running form and gives almost immediate feedback on energy expenditure in watts.

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The Critical Power Protocol for Running

Table 1 shows the Critical Power Protocol, which was designed with the two goals in mind: 1. it should accurately and consistently determine the critical running power baseline and 1

Table 1: Critical Power Protocol for Running Activity Warm up

Duration (minutes) 10

Maximum distance run

3

Recovery

30

Maximum distance run

9

Cool down

10

Description Start with three minutes of walking or very light jogging to increase muscle and core temperatures. Next, do five 100-meter strides at approximately 80% maximum effort to enhance the blood circulation and ready your muscle for intense use. It is important to run at a consistent pace throughout the three-minute test, but to be nearly exhausted at the end of the test. Therefore, do not start from an all-out sprint, because that would result in an inconsistent pace during the run. The recovery phase is used to bring the heart to nearly the resting rate. Throughout the 30-minute recovery period, the runner should walk or jog slowly. Right before the end of the 30-minute recovery period, do a few light 100 meter strides to prime the body for the second maximum distance effort. As was the case for the three-minute run, it is important to maintain a consistent pace during this run instead of dramatically changing pace (and effort) during the run. The athlete should follow his or her normal post-workout cool-down routine. It is fine to spend more than 10 minutes cooling down if the athlete prefers.

2. it should be easy to follow and interfere minimally with an athlete’s ongoing training schedule. The protocol consists of two maximum pace efforts (three minutes and nine minutes) separated by 30 minutes of recovery.

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Calculating One’s Critical Power

To find critical power, it is necessary to determine the athlete’s average power during both threeminute and nine-minute maximum-effort runs and use those values as input to the following ques-

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Figure 1: PowerCenter being used for calculation of critical power using the Critical Power Protocol.

tion. P9 · 9 min − P3 · 3 min (1) 9 min − 3 min where P3 and P9 are the average power values for the three- and nine-minute running intervals. As shown in Figure 1, Stryd’s PowerCenter can be used to automate this calculation. At this point, the reader has enough information to use the Critical Power Protocol. Next, we explain why this particular method of estimating critical power works. We can restate the goal of determining critical power as follows. Pcrit =

Determine the highest sustainable power output that allows the athlete to continue using (primarily) aerobic respiration instead of transitioning to anaerobic respiration. Maximum three-minute and nine-minute efforts have properties that can be used to determine the athlete’s maximum aerobic power. During the three-minute maximum-effort run, the athlete is relying on anaerobic respiration for a significant portion of the run. More importantly, the durations spent in anaerobic respiration during the three- and nine-minute maximum effort runs are similar. As shown in Equation 1, it is possible to use the energy (power × time) expended during the three-minute run to eliminate the corresponding energy from the anaerobic time interval of the nine-minute run. This leaves only the power expended during the aerobic time interval of the nine-minute run, i.e., the maximum power the athlete can sustain with (primarily) aerobic respiration.

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Comparison of Critical Power Protocol Results and Blood Lactate Test Results

The Critical Power Protocol does not rely on lactate threshold testing. However, we use lactate threshold testing as a means of validating it. Each of the 20 athletes who carried out the Critical 3

Critical Power (watts/kg)

7 6 5 4 3 2 1 0 0

1 2 3 4 5 6 Lactate Threshold (watts/kg)

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Figure 2: A comparison of critical power estimated using blood lactate testing and the Critical Power Protocol for 20 athletes.

Power Protocol also participated in a separate test to determine the relationship between blood lactate level and power. This test, performed in a lab environment on a treadmill set at a 1% incline, consists of multiple four-minute running intervals. After each four-minute interval, blood was drawn and blood lactate was measured using a Lactate Plus meter from the Sports Research Group, Inc. Then, treadmill speed was increased by 0.5 miles per hour and the next running interval commenced. The lactate threshold power was defined as the lowest power at which blood lactate increased by 1.0 mmol/liter or greater relative to the previous treadmill speed. Twenty participants with a variety of athletic conditions participated in the Critical Power Protocol as well as the lactate threshold test. Using lactate threshold power as a reference, the average error of the Critical Power Protocol was 0.41 W/kg (8.9%) with a standard deviation of 0.31 W/kg (6.0%). 65% (13/20) participants had critical power values lower than their lactate threshold powers. Figure 2 compares the critical power values estimated using blood lactate testing and the Critical Power Protocol for all 20 athletes. We are carrying out ongoing studies to identify the sources of error and refine the Critical Power Protocol to reduce them. At present, we can make the following suggestions to users of the Critical Power Protocol. 1. The critical power test should be completed after a rest or recovery day; one should avoid doing the Critical Power Protocol after a workout, either the same day or the following day, because this might lead to a less accurate estimate of critical power. 2. Ideally, weather, temperature, hydration, sleep, and ground surface conditions should match those the athlete expects to encounter during most training. 4

3. The Critical Power Protocol should be reused periodically to track changes to critical power that result from training-induced changes in fitness.

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