Running Glossary

Apex Run surfaces a lot of data. Here’s what the numbers actually mean.

Performance Metrics

VO2 Max — The maximum volume of oxygen your body can use per minute per kilogram of body weight (mL/kg/min). It’s the gold standard measure of aerobic fitness. Apex Run reads this from Apple Health, where it’s estimated by Apple Watch from your workout data. A rising VO2 Max over months of consistent training is a reliable sign of improving fitness.

Running Power — An estimate of the mechanical power output of your running, in watts. More consistent than pace on varied terrain.

GAP (Grade Adjusted Pace) — Your pace adjusted to remove the effect of hills. Running uphill at 6:00/km takes roughly the same effort as running 5:00/km on flat ground. GAP normalizes this so you can compare efforts across hilly and flat courses. Apex Run shows a GAP chart in the run detail view.

Efficiency Factor (EF) — A measure of how efficiently your cardiopulmonary system converts effort into forward motion.

Formula: EF = GAP (m/s) ÷ Average Heart Rate (bpm)

To remove the effect of hills, EF uses Grade Adjusted Pace rather than raw pace. The result represents how many meters you travel per heartbeat. As fitness improves, you’ll run faster at the same heart rate — reflected as a gradual rise in EF over time.

• Typical range: 1.00 – 2.50 (higher = better aerobic efficiency)
• Long-term trend: A rising EF confirms aerobic development is working
• Watch out for: A significant EF drop at your usual effort may suggest fatigue, dehydration, or heat stress

Because EF depends on your individual max HR and running form, it’s most useful as a personal trend — don’t compare your EF number to other runners.

Pace Stability Index (PSI) — Apex Run’s own metric measuring how consistently you hold your pace across a run. Stable rhythm generally means better energy distribution and lower risk of hitting the wall in long-distance races.

Formula: PSI = (1 − GAP Standard Deviation ÷ GAP Average) × 100%

Note: In interval or fartlek workouts, drastic pace switching is the goal, so a low global PSI is expected and fine. Focus on segment performance instead.

Running Form Metrics

In modern training, pace and heart rate alone don’t tell the full story. Running form metrics — captured by Apple Watch’s motion sensors — quantify how your body moves, revealing technique, energy efficiency, and injury risk.

The core goals of monitoring these metrics:

• Improve Running Economy — run faster and further with less energy
• Prevent Injury — optimize landing mechanics to reduce impact forces

Cadence — Total steps per minute (spm), counting both feet. Cadence is the cornerstone of running dynamics: increasing it is the most direct way to improve form, reduce knee and hip stress, and lower injury risk.

• Target: 170 – 180 spm or above for most runners
• Low cadence often correlates with overstriding (foot landing too far in front of center of mass), which creates a braking effect and raises injury risk

Stride Length — Distance covered between two consecutive ground contacts. Since Speed = Cadence × Stride Length, running faster requires increasing one or both.

Don’t intentionally overstride. Reaching too far forward places your foot ahead of your center of mass, creating a braking force. An optimal stride length is the natural result of a powerful push-off combined with high cadence — let it extend naturally rather than forcing it.

Stride length is highly individual (correlated with height and pace), so there’s no single “best value.” Focus on maintaining cadence above 170 spm and let stride length take care of itself.

Ground Contact Time (GCT) — How long your foot stays on the ground per step, in milliseconds. Shorter GCT means a lighter, more elastic stride. Faster running naturally produces shorter GCT, but shorter is better at any pace.

Vertical Oscillation — How much your torso bounces up and down per step, in centimeters. Running is about moving forward, not upward. Excessive oscillation wastes energy and increases landing impact on the lower limbs.

• Target: Below 10 cm
• Elite runners: Typically 6 – 8 cm

Duty Factor — The percentage of a full gait cycle (one step per foot) that a single foot spends on the ground. It’s the boundary between running and walking: below 50% means there’s a flight phase (both feet off the ground) — that’s running. Above 50% means no flight phase — that’s walking.

Lower values indicate a longer flight phase and a more elastic, bouncy stride, usually reflecting better running economy.

Vertical Ratio — Vertical oscillation divided by stride length, expressed as a percentage. A lower value means more energy is converted into horizontal displacement (forward motion) rather than vertical displacement (bounce). It’s one of the best single-number summaries of running economy.

Training Load

Training Load — A rolling measure of how much stress your training is placing on your body, based on workout intensity and duration over recent weeks.

ACWR (Acute:Chronic Workload Ratio) — The core model Apex Run uses to evaluate whether your training is “healthy.”

• Acute Load (last 7 days) — represents current fatigue
• Chronic Load (last 28 days) — represents long-term fitness baseline
• Ratio = Acute ÷ Chronic

Current version calculates load based on distance. Short high-intensity runs (like intervals) may be slightly underestimated. Heart-rate-based load (TRIMP) is planned for a future version.

Training Load Trend — The direction your overall load is moving. Gradual increases of 10% or less per week are generally safe. Sharp spikes are what ACWR flags.

See also: Training Zones — heart rate zones and pace zones in detail · 80/20 Training Structure — how to balance your training load across intensity levels