Thursday, July 13, 2017

Harmonising VDOT Pace with Power Based Running : A Case Review

Note : If you're an elite athlete, please take the guidance of your coach. 

The VDOT based system of paces is a tried and tested system for run training designed by Jack Daniels and a colleague of his who worked for NASA. The system, described in JD's book "The Running Bible" appears to be sound at first glance - it is established from metabolic testing and the foundational idea that equally performing runners can be assigned equal aerobic profiles. 

JD has ofcourse outdone himself in both the national and international running stage so we can't argue whether he had his wits about him when he sat down to make VDOT.

I was interested to see how VDOT based paces, when converted to power, compares to the instruction in critical power based running zones. For this I pick an example runner, Tony. 

Converting VDOT Paces to Power

Let's assume that Tony recently raced 3km in 12 minutes for his season's best. Ambient conditions at the race were :

Temp = 95F
Altitude = Sea Level

This equates to a pace of 6:26/mile = 250 meters/min.VDOT is approximately 45 per the VDOT tables. 

The goal is to find an equivalent power for the VDOT pace so that he could implement power based training zones. Jim Vance has defined a simple factor tying speed and watts called Efficiency Index, EI, which is defined as the ratio of speed in meter per minute over watts, both continously captured over a 30 second rolling duration.

If Tony has knowledge that his short races in similar conditions in the past were done with an EI of 0.95 m/min/W, then knowing speed, corresponding power is : 

Equivalent Power for 3K = 250/0.95 = 263W

(According to the metric, as EI decreases, power to produce a given speed increases).

Given that 263W is an equivalent power estimated from race pace and an estimated EI, it must be checked whether this power value can be sustained when the powermeter is applied. 

VDOT Power Validation

1) As a first check, Tony can peer into his power duration (PD) curve in his favorite software of choice. If the PD curve looked something like the one below, which is a multiparameter exponental model fitted to all of Tony's runs 12 minutes and below, the chart seems to suggest a theoretical maximum of 220W for the 12 minute duration. 

Would it be correct to assume that Tony simply cannot sustain this wattage for this time? It might be fair to consider that the software generated PD curve in question just may not have race equivalent data. The model tries to make a trend around what is known, not what is unknown. If most of Tony's runs were from social runs in the park on weekends, the PD curve will underestimate his potential and would probably not be helpful. 

2)  The other bit of useful data from the VDOT system for this runner is the equivalency is terms of 5K, 10K, half and full marathon distances. For example, the equivalent 5K running time for the VDOT of 45 is 21:49 for the same ambient conditions. If you then used the Stryd's critical power model on their website, it suggests a critical power of 236W

By the old definition, critical power is the steady state rate of work output someone can theoretically sustain "indefinitely" although the notion of an indefinite time period is a bit corrupted - no one can exercise indefinitely! As now understood by the scientific community, it is a threshold below which VO2 and heart rate will not not see a rise and is also seen as the greatest point at which energy provision is still wholly oxidative (Poole, Burnley

Knowing that Tony's critical power is 236W, it seems "feasible" that he can pull off a short excursion into his anaerobic energy systems for 12 minutes at 263 Watts, which equates to 111% of his critical power. In this unsteady state, Tony will accumulate fatigue and have a rising heart rate and VO2.

3) Tony can validate this power (i.e know whether he can sustain it for 12 minutes or not) by performing a 3k at race pace in well rested conditions on similar terrain. Perhaps he chooses the same race the next time it's organized. If the average wattage generated by his powermeter is within 3% of 236W, he might be able to conclude that the conversion is roughly equivalent.  His pace to power conversion can also be adjusted by assuming a more generous Efficiency Index if prior data suggests that this is the case. It can also be adjusted by tuning the equivalent 10K pace resulting from the VDOT tables. 

While the VDOT-power conversion hasn't been perfect, it allows Tony to be able to go out and do a trial run or two and make necessary adjustments. Therefore, in so far as VDOTs are generated from race performances and the effect of ambient conditions on race paces are taken into account, I find no flaw with this procedure. 

Comparing VDOT Power Based Zones to Critical Power Based Zones

Under the VDOT guidance, Tony's training zones in terms of paces at a temperature of 95 F would have been  :

Based on a conservative EI estimate of 0.90, these training speeds can be converted to power values by the procedure noted in the earlier section. For example, the easy pace zone for the mile in terms of power would be 188-200Watts. The marathon pace for the mile would be 225W. And so on. 

The table below shows VDOT paces converted to power (watts) at two different EI estimates for the 1 mile distance. An EI = 0.9 for Tony would be conservative estimate for sluggish runs and sub-optimally executed fast races. An EI = 0.95 could be a theoretical maximum for optimal executed races. If evidence suggests the use of a more generous EI, I suppose that would be ok. 

The key question is how these VDOT pace based power zones compare to critical power based running zones prescribed by Stryd. 

Stryd's algorithm for power zones for a critical power of 236 is as follows :

Based on the comparison of the two zone tables, it seems that the choice of EI makes a key difference in interpretation. For example, for EI of 0.90, the VDOT marathon power of 225W would be considered to be threshold power under Stryd's model. For an EI of 0.95, the VDOT marathon power of 213W falls within the "moderate" power zone. But at the upper end, the VDOT repetition power of 261W comes conservative, whereas Stryd's power is a bit more aggressive at 10 extra watts. 

Conclusing Remarks

1) Equivalent running paces from VDOT tables, critical power and Efficiency Index (EI) appear to be key parameters affecting the conversion of VDOT paces to power. In the particular example, using Tony's equivalent 10K pace and an  EI estimate that corresponded to Tony's race data, the VDOT based powers bridge upto Stryd's power zones with minor differences. One might also make the assumption that training at two power values that are within 3-5% of each other elicits similar physiological responses.

2) A unique advantage of VDOT is that it accomodates pace derates for high temperature and altitude. Powermeter based running guidance has it's advantages, but it still has some catching up to do in terms of interpreting application for real world conditions - wind, temperature, humidity and altitude. The practicality of new, cookie cutter running programs based on power must be evaluated critically, particularly by runners living in hot, arid climates where the danger of heat stroke is real.

3) Because training zones are outputs from an algorithm that takes user input, they are estimations at best and it is a given that adjustments will need to be made to tally up RPE and overall whole body feeling to the prescribed guidance. Crucially, it is important that the zones are judged well before jumping straight into applying them in training. No one wants to waste time finding out they can't sustain a prescription. 

4) As far as my experience goes, both the VDOT system and the power based training systems are equally time consuming, although the fruit of added insight for advanced training is a plus. Both systems require frequent data diving, critical evaluation and adjustments. More so, the zones have to be re-evaluated at regular intervals to see how training and racing have modified them, for the better (or for worse!).

5) The interesting and larger implication of my writeup is that a traditional VDOT based system might be nearly equivalent to the power based running prescription, "if used correctly", which means a) it doesn't create a strong case for conversion from one system to the other and b) means that fundamentally both are sound in that they are calibrated to metabolic test data.

6) Additional research is needed to give empirical guidance on how truly different is training by VDOT vs powermeter for different classes of athletes. For example, in cycling research, it has already been established that there is "no empirical evidence" to suggest the superiority of either power based training or HR based training in the implementation of interval training. Both were roughly equivalent in terms of responses seen in recreational cyclists after a multi-week training cycle. 

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