Apex Run surfaces a variety of advanced running performance metrics to evaluate your cardiovascular output and pacing consistency. Here is a breakdown of what the numbers mean and how to use them.
1. VO2 Max#
- Definition: The maximum volume of oxygen your body can use per minute per kilogram of body weight (mL/kg/min). It is the gold standard measure of aerobic fitness.
- How it is read: Apex Run reads this directly from Apple Health, where it is estimated by Apple Watch from your workout data.
- Trend: A rising VO2 Max over months of consistent training is a reliable sign of improving cardiovascular fitness.
2. Running Power#
- Definition: An estimate of the mechanical power output of your running, expressed in watts (W).
- Value: Power is more consistent than pace on varied terrain, helping you regulate effort on hills or windy days.
3. GAP (Grade Adjusted Pace)#
- Definition: Your running pace adjusted to remove the effect of gradients. Running uphill at 6:00/km takes roughly the same effort as running 5:00/km on flat ground.
- Value: GAP normalizes this gradient impact so you can compare efforts accurately across hilly and flat courses. Apex Run Pro shows a continuous GAP chart and split analysis for every run.
- Deep Dive Guide: Learn the physiological science and Minetti formula behind hill pacing in our Grade Adjusted Pace (GAP) Guide.
4. Efficiency Factor (The Running Efficiency Formula)#
- Definition: A core metric of aerobic efficiency popularized by legendary endurance coach Joe Friel. It measures how effectively your cardiopulmonary system converts internal physiological effort (heart rate) into external mechanical output (forward running pace).
- Formula:
Efficiency Factor (EF) = [GAP (m/s) × 60] ÷ Average Heart Rate (bpm)
(Expressed as meters of flat-equivalent distance per heartbeat) - How to read: To remove the distorting effect of hills, EF uses Grade Adjusted Pace rather than raw pace. The resulting number represents how many meters of flat-equivalent distance you travel per heartbeat. As your aerobic fitness improves, you will run faster at the exact same heart rate—reflected as a steady rise in EF over time.
- Typical range: 1.10 – 1.60+ (higher = superior aerobic efficiency)
- Long-term trend: A rising EF confirms your aerobic base training is successfully expanding your capillary and mitochondrial density.
- Alerts: A sudden drop in EF at your normal easy pace suggests accumulated fatigue, dehydration, impending illness, or heat stress.
🏃 The Mechanics Connection: What drives a higher Efficiency Factor? Beyond cardiovascular conditioning, your Running Form Metrics are the primary lever. By decreasing vertical bounce (lowering your vertical ratio) and shortening ground contact time, you waste less mechanical energy, allowing every single heartbeat to propel you further down the road.
5. Pace Stability Index (PSI)#
- Definition: Apex Run’s custom metric measuring how consistently you hold your pace across a run. A 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%
| PSI Range | Interpretation |
|---|---|
| > 90% | Very steady rhythm, excellent energy distribution |
| 85 – 90% | Good rhythm, handles terrain changes well |
| < 85% | Large pace swings — try focusing on cadence to find a steadier rhythm |
In interval or fartlek workouts, drastic pace switching is the goal, so a low global PSI is expected and normal. Focus on segment performance instead.
Frequently Asked Questions#
What is the Running Efficiency Formula?#
The standard running efficiency formula is Efficiency Factor (EF) = GAP (m/s) ÷ Average Heart Rate (bpm). Popularized by Joe Friel, it quantifies how many meters you travel per heartbeat. A higher EF indicates that you produce more speed with less cardiovascular strain.
What is Grade Adjusted Pace (GAP)?#
Grade Adjusted Pace calculates what your pace would be if running on flat ground at the same effort. It removes the impact of uphill climbs and downhill descents. Read our complete Grade Adjusted Pace guide for formulas and slope impact tables.
What is Efficiency Factor (EF) in running?#
Efficiency Factor divides Grade Adjusted Pace (m/s) by average heart rate (bpm). A rising EF trend over months shows improved aerobic cardiovascular efficiency.
What is Pace Stability Index (PSI)?#
Pace Stability Index measures how consistently you maintain pace throughout a run. A score above 90% indicates steady, well-distributed pacing.
Related Tools & Guides#
- Running Efficiency Calculator — Calculate your Vertical Ratio, Duty Factor, and Efficiency Factor (EF) interactively.
- Grade Adjusted Pace Guide — Minetti equation, hill pacing strategy, and downhill biomechanics.
- Heart Rate Zones — 5-zone HR model, LTHR, and calculation methods.
- 80/20 Training Structure — How to balance your training load across intensity levels.
- Running Form Metrics — Cadence, stride length, ground contact time, and other technique measures.
- Training Load & ACWR — Evaluating fatigue and fitness progression.
References & Further Reading#
- Minetti, A. E., et al. (2002). “Energy cost of walking and running at extreme uphill and downhill slopes.” Journal of Applied Physiology, 93(3), 1039-1046. PubMed ID: 12183501
The foundational biomechanical study behind Grade-Adjusted Pace (GAP) energy cost algorithms. - Bassett, D. R., Jr., & Howley, E. T. (2000). “Limiting factors for maximum oxygen uptake and determinants of endurance performance.” Medicine and Science in Sports and Exercise, 32(1), 70-84. PubMed ID: 10647532
The primary review paper on VO2 Max, lactate threshold, and running efficiency parameters.