A single number can't describe running fitness. A marathoner with a massive aerobic base and a sprinter who can throw down a 55-second 400m are both "fit" — but in completely different ways. Training that builds one doesn't necessarily build the other. And if you're training for a specific distance, you need to know which dimensions of fitness are strong, which are lagging, and what to do about it.
Sequorr's Running Fitness Score breaks running fitness into four measurable pillars: Base, Endurance, Speed, and Sprint. Each maps to a distinct energy system. Together, they give you a complete profile of where your running fitness stands — not as a single number, but as a shape.
The Four Pillars
Each pillar evaluates a different physiological capacity. They overlap, but they're measured independently because they adapt at different rates and respond to different types of training stimulus.
Base (Aerobic Endurance)
Base measures your aerobic engine — specifically, how fast you can run while keeping your heart rate in Zone 2. This is the foundation of distance running. A strong base means your body is efficient at burning fat for fuel, clearing lactate at moderate intensities, and sustaining effort for hours.
Base is the slowest pillar to build and the slowest to decay. It responds to volume — weeks and months of consistent easy-to-moderate running. You won't see your Base score jump after one workout. You'll see it climb gradually as your aerobic system adapts over a training block.
Endurance (Threshold)
Endurance measures your lactate threshold — the intensity at which your body transitions from primarily aerobic to increasingly anaerobic metabolism. In practical terms, it's roughly the pace you could sustain for 60 minutes in a race. For most runners, this correlates closely with half marathon to 10-mile race pace.
Endurance responds to tempo runs, cruise intervals, and sustained efforts at or near threshold. It's the pillar that improves most directly from structured training — the kind of work where you're running "comfortably hard" for extended periods.
Speed (VO2max)
Speed measures your maximal aerobic power — your VO2max expressed as a running pace. This is the ceiling of your aerobic system, the fastest pace your body can sustain while still relying primarily on oxygen for energy production. In race terms, this roughly corresponds to 3K-5K intensity.
Speed responds to interval training — repeated efforts at 95-100% of max HR with recovery between reps. Classic workouts: 800m repeats, 1K intervals, 3-minute hard efforts. It's the pillar that moves fastest in response to targeted training, but also decays quickly without maintenance.
Sprint (Anaerobic Power)
Sprint measures your peak neuromuscular output — how fast you can run for 30 to 60 seconds. This is pure anaerobic power: the phosphagen and glycolytic energy systems firing at maximum capacity. It's the least "endurance" of the four pillars, but it matters more than most distance runners think.
Sprint matters because it sets the ceiling for everything else. Your threshold pace is a percentage of your max speed. If your max speed improves, the pace associated with every zone shifts up with it. Sprint responds to strides, hill sprints, short reps, and plyometrics.
The Rolling Window
Running Fitness Score uses a 42-day rolling window — the same time constant as CTL. Every qualifying activity from the last six weeks contributes to your scores, with more recent activities weighted more heavily through exponential decay.
This means your Running Fitness Score reflects where your fitness is now, not where it was two months ago. If you stop doing speed work, your Speed score will gradually decline as those interval sessions age out of the window. If you ramp up your long runs, your Base score will climb as those efforts accumulate weight.
The 42-day window was chosen deliberately — it matches the CTL time constant, which means your Running Fitness Score and your fitness trend line (CTL) are looking at roughly the same window of training. They tell complementary stories: CTL tells you how much you've been training. Running Fitness tells you what kind of fitness that training has built.
The Scoring Scale
Each pillar is scored on a 40-100 scale. The scores are normalized against population benchmarks so you can compare across pillars and across time:
- 40-55 — Developing. You're building this capacity. Consistent targeted training will move this number.
- 55-70 — Solid. This is a functional level of fitness in this pillar for a recreational competitive runner.
- 70-85 — Strong. This pillar is a relative strength. You're performing well above average.
- 85-100 — Elite. Top-tier performance in this capacity. Very few runners score here across all four pillars simultaneously.
The scale exists so you can see relative strengths and weaknesses at a glance. If your Base is 74 and your Speed is 52, that tells you something concrete about where to focus your next training block.
The Radar Chart
Inside Sequorr, your Running Fitness Score is displayed as a diamond-shaped radar chart with four axes: Base at the top, Endurance on the right, Speed at the bottom, Sprint on the left. Your scores form a polygon inside the diamond.
The shape of the polygon is the insight. A well-rounded runner has a roughly symmetrical diamond. A marathoner typically shows a tall, wide shape — strong Base and Endurance, moderate Speed and Sprint. A miler shows the opposite — strong Speed and Sprint, less Base. Neither shape is wrong. They reflect the athlete's training and their event.
The chart updates as your 42-day window shifts. Over the course of a training cycle, you can watch the shape change — Base growing during a high-volume block, Speed spiking during an interval phase, everything flattening slightly during a taper as recent training volume drops.
Where the Data Comes From
Running Fitness Score is calculated from three data streams, all of which flow from HealthKit and GPS during tracked running activities:
- Pace-HR pairs — Sampled throughout your run, filtered for steady-state segments (post-warmup, stable HR, relatively flat terrain). These pairs reveal aerobic efficiency — how fast you're moving at a given cardiac output. This is the primary input for Base and Speed.
- Peak pace segments — Rolling best pace over specific durations: 30 seconds, 60 seconds, 5 minutes, 20 minutes, 60 minutes. Your best 30-60 second pace feeds Sprint. Your best 20-60 minute pace feeds Endurance.
- Time-in-zone distribution — How many minutes you spend in each HR zone per activity. This determines which pillar each workout contributes to most heavily. A long Zone 2 run feeds Base. A threshold session feeds Endurance. Intervals feed Speed.
Heart rate zones are calculated using the Karvonen method (Heart Rate Reserve), with max HR and resting HR sourced from HealthKit:
- Zone 1 — 50-60% HRR (recovery)
- Zone 2 — 60-70% HRR (aerobic base)
- Zone 3 — 70-80% HRR (tempo)
- Zone 4 — 80-90% HRR (threshold)
- Zone 5 — 90-100% HRR (VO2max)
Only running activities of 15 minutes or longer are included. Short runs don't produce enough data for reliable pillar scoring. The nightly cron recalculates all four pillars even on rest days — to apply the exponential decay and keep the scores current.
Using Running Fitness to Guide Training
The Running Fitness Score isn't prescriptive — it doesn't tell you what workout to do tomorrow. But it gives you the diagnostic information to make that decision yourself.
Identify your limiter
Look at your radar chart. Which pillar is the smallest? For your target event, is that pillar important? A marathoner with a low Base score has a clear priority. A 5K runner with a low Speed score knows where to focus.
Validate your training block
If you've been doing threshold work for six weeks and your Endurance score hasn't moved, something isn't working — maybe the intensity is wrong, the volume is insufficient, or you need more recovery between sessions. The score gives you feedback on whether the training is actually producing the adaptation you're targeting.
Monitor during taper
During a taper, all four scores will drift slightly downward as recent high-effort sessions age out of the window. This is normal. The key is that they shouldn't crater — a well-executed taper maintains the shape of your fitness while reducing fatigue. If one pillar drops significantly faster than the others, you may be tapering too aggressively for that capacity.
Track long-term development
Over months and seasons, your Running Fitness profile tells the story of how your fitness has evolved. You can see the aerobic base you built in winter, the speed work you layered in spring, and the race-specific sharpening that brought it all together. That long view is more valuable than any individual snapshot.
Open Your Radar Chart
Running Fitness Score is available now in Sequorr. If you've been tracking runs with HealthKit data, your four pillar scores are already being calculated. Open the app, find your Running Fitness card, and look at the shape. That shape is the most honest picture of your running fitness you'll find — four numbers, no hiding.
Then go run. The shape will change. That's the point.