About two decades ago, I developed a model showing that runners could benefit from a tailwind on courses where the starting and finishing points were more than 30% apart from each other, relative to the race distance. This model became the basis for the current USA regulation, which defines a road race course as “point-to-point” if the distance between the start and finish is more than 30% of the total race distance. For example, the straight-line distance between the start and finish for a 10 km road course cannot exceed 3 km for the course to be eligible for record purposes. This could mean an out-and-back course of 6.5 km out and 3.5 km back, or even a U-shaped course with the endpoints of the “U” being 3 km apart, while still being classified as a standard course eligible for records.
Recently, the IAAF has adopted a rule for recognizing road records and, rather than drawing from the USA’s past experience with road-record regulations, they decided to make some changes. They have wisely kept the 1 m/km drop limit when defining record-eligible courses. However, they are now allowing up to a 50% distance between the start and finish, prompting me to revisit the technical aspects of how wind affects race times based on different levels of start/finish separation and wind speeds.
The key idea behind this analysis revolves around two different energy expenditures that occur during running. The first is the energy required to run through the air, often referred to as external energy or air resistance. The second category encompasses all other forms of energy required to run, like muscular exertion and energy spent on oxygen transport. This second group can be broadly termed “internal” energy.
Air resistance depends on the square of the wind speed. Although the exact relationship between running speed and internal energy isn’t fully understood, it’s generally thought to increase rapidly as a runner approaches their natural speed limit. For example, no matter how favorable the conditions, it’s unlikely that my muscles could ever handle a speed of 9.78 seconds for 100 meters, even with the help of a significant tailwind. In other words, internal energy expenditure rises sharply near the runner’s speed limit, while for slower speeds, it increases at a more gradual rate.
In this analysis, we don’t need to calculate the precise energy expenditures. Instead, it is enough to estimate the ratio between internal and external energy expenditure (I/E ratio) under calm conditions, where the only air movement comes from the runner’s motion through the air. I tested different I/E ratios on an out-and-back course with varying wind speeds to estimate how much wind affects the pace of a runner capable of running a 30-minute 10K pace (or 180 seconds per kilometer). For example, an I/E ratio of 12 results in an 11-second slow-down when facing a 2 m/s wind and a 65-second slow-down when facing a 5 m/s wind over the course of 10 km.
Experience suggests that a slow-down of about 50 seconds is reasonable for a 5 m/s wind. Anything higher, like 65 seconds, seems excessive, while a slow-down of 38 seconds feels too conservative. Based on this, an I/E ratio of 15 seems appropriate for the rest of the analysis. Luckily, the results are not too sensitive to the exact value of this ratio.
The next step involves calculating the total energy (internal and external) needed to maintain a pace of 30:00 for 10 km, then adjusting the speed for varying wind conditions so that the runner’s energy expenditure remains constant. In other words, with a tailwind, a runner can move faster while using the same amount of energy, and into a headwind, they would move slower while expending the same energy. Once the speeds with tailwind and headwind are known, we can then calculate the overall impact on time as a function of wind speed and the separation between the start and finish lines. Below is a table that shows how race times change based on wind speed and the percentage of start/finish separation for a runner aiming for a 30:00 10K:
| Wind | 10% | 20% | 30% | 40% | 50% | 60% | 70% |
|---|---|---|---|---|---|---|---|
| 1.0 | -0.08 | -0.28 | -0.49 | -0.69 | -0.89 | -1.09 | -1.30 |
| 1.5 | 0.09 | -0.22 | -0.53 | -0.84 | -1.15 | -1.46 | -1.77 |
| 2.0 | 0.35 | -0.05 | -0.46 | -0.87 | -1.28 | -1.69 | -2.10 |
| 2.5 | 0.63 | 0.12 | -0.39 | -0.89 | -1.40 | -1.91 | -2.42 |
| 3.0 | 1.16 | 0.54 | -0.08 | -0.69 | -1.31 | -1.93 | -2.55 |
| 5.0 | 3.97 | 2.90 | 1.83 | 0.77 | -0.30 | -1.37 | -2.43 |
What counts as an “acceptable” speed-up? One consideration is that a net drop of 1 m/km is considered the maximum allowable benefit from elevation changes between the start and finish. This translates to roughly a 1 second per kilometer speed-up. Therefore, wind benefits should not exceed -1.00 sec/km and ideally should be lower since a course with both the maximum allowable drop and the maximum start/finish separation could potentially provide a combined advantage of -2 sec/km.
From the table above, a start/finish separation of 50% can produce benefits as high as -1.4 sec/km, which exceeds the benefit of a 1 m/km elevation drop. These findings are consistent across different runner paces. For a slower runner, with a pace of 33:20 for 10K, the maximum benefit at 50% separation is still -1.54 sec/km, further reinforcing the point that a 50% start/finish separation is not recommended.
There are several reasons to exercise caution when dealing with wind effects. Wind speeds are highly variable over small distances and times. Many road races are held in the morning, a time when wind speeds tend to increase. For instance, on an out-and-back course with a 30% separation, if a runner encounters a headwind of 1 m/s on the outbound leg and a tailwind of 2 m/s on the return, the result would be a speed-up of 15.5 seconds or -1.55 sec/km—exceeding acceptable limits even with a 30% separation.
The same holds true for U-shaped courses, where the wind might benefit runners on one leg without compensating on the others. For example, a 2.5 m/s tailwind on a 3 km stretch of a 10 km U-shaped course could result in a time benefit of -14.1 seconds, which is more than the allowable -1.0 sec/km.
Cross-winds also complicate things. Even though a cross-wind might not directly oppose or assist a runner, it can still create a net effect. For instance, small shifts in wind direction can result in both headwind and tailwind effects, slightly reducing the overall speed benefit. However, in real-world scenarios, the advantage gained from winds often exceeds acceptable limits, especially for courses with high start/finish separation.
In summary, even though a 30% start/finish separation already pushes the limits of fairness in terms of wind benefits, moving to a 50% separation could amplify these effects considerably, especially under ideal wind conditions. The current rules in place, based on the 30% separation, are sufficient to keep the effects of wind within reasonable bounds. Any increase beyond this would likely provide unfair advantages, making it unwise to extend the allowable start/finish separation any further.
This article is a combination of articles appearing in the “Analytical Distance Runner” for weeks 937 and 940, and was written by Ken Young.