Genetics clearly influence sprint performance—but by how much? There are several ways of answering this question. First, we can look for specific genetic variants that have an impact on performance. Second, we can estimate the heritability of speed or speed-related traits like muscular strength, height, and body type with twin and family studies. Finally, we can analyze the demographics of elite and sub-elite sprinters.

Genetic variants

Most of the literature focuses on two specific mutations: the ACTN3 R557R/X and ACE I/D variants.

ACTN3

ACTN3 encodes the α-actinin-3 protein, which is mostly restricted to Type II (fast-twitch) muscle fibers. Humans with the ACTN3 557XX genotype have no α-actinin-3 at all, and in fact appear to have a higher proportion of Type I (slow-twitch) muscle fibers than ACTN3 557RX or ACTN3 557RR humans. The obvious hypothesis, then, is that 577RR athletes should be better sprinters than 577XX athletes because of the existence of α-actinin-3 and a higher proportion of fast-twitch muscle fibers.

Last week, I got my hands on a pair of New Balance’s sprint spikes for 2021, the FuelCell Sigma SD-X. Although not as hyped as Nike’s Air Zoom MaxFly, New Balance’s sprint team has been running some ridiculous times in the SD-X: Trayvon Bromell is the fastest man in the world this year, and Sydney McLaughlin set the 400mh world record wearing New Balances.

Like other super spikes, the SD-X has an extremely stiff carbon fiber plate and a layer of foam in the midsole designed to maximize energy return on the track. The foam layer makes for a taller ride than most spikes—the stack height is comparable to trainers. That said, it’s definitely not a training spike: its spikes are non-removable.

I’ve spent the last couple hours trying to rebuild my mood tracker—although notifications still work, opening the app throws the error “‘mood’ Is No Longer Available”. I think this is because I last built the app a year ago, and apps installed with a paid developer account only last for a year. (Sidenote—it’s ridiculously hard to find definitive information about this. Instead, I had to trawl through the Apple Developer Forums and Reddit to try to figure out what’s going on.)

Just as I’ve settled into a pretty good groove in Atlanta, I spent the past few days driving across the country again, this time to Livingston, Montana where I’ll be spending the next two months! It’s the best time of year to be in Montana, and we absolutely loved it when we visited neighboring Wyoming last fall.

Goal Tracking

• 🟡 Get 1% faster. I raced once in June—a 100 in a rainy 11.05. I only have one more chance this season to set a big PR, at the USATF Masters Outdoor Championships in a few weeks. I have my taper planned out—I’m confident that weather permitting, I’ll have some great races.
• 🔴 50% less discretionary spending. I spent 22% less on discretionary things than my 2020 average. It’s been death by a thousand cuts—I’ve just been spending more on little things. Hopefully I do better in July.
• 🔴 One hour of solitary free time a day. Despite my work-life balance being better than it’s been in a while, I haven’t been spending my extra time very wisely. Mostly it’s been mindless browsing.
• 🟢 Twice-weekly live conversations with friends. Back into the green!

I moved to Atlanta two months ago; here are some disorganized impressions of Atlanta and the Southeast: