The 4 Numbers, explained.
Four research-backed measures of how you'll move, live, and stay independent — each testable in your living room in minutes, each trainable at any age. Here's exactly what they are, how we test them, and what your scores mean.
Strength
Can you carry your own life?
Carrying and grip strength are among the most-studied predictors of healthy aging — because they summarize everything at once: muscle, bone, nerve, and will. Strength is what lifts the suitcase into the overhead bin, moves the furniture, and catches you when the ground surprises you.
It responds to training at every age. In study after study, adults in their 60s, 70s, and beyond build meaningful strength within weeks of starting progressive resistance work.
How we test it
- The Carry Hold — hold a fixed, personal load (set on Day 1) standing tall, timed until grip or posture breaks. Your number: seconds at that exact load.
- The Goblet Ladder — max quality reps at a fixed weight to a depth standard. The scorecard converts reps to an estimated strength number in pounds, so there's no ceiling to outgrow.
| Band | Meaning |
|---|---|
| 0–39 | Priority zone — fastest gains available here |
| 40–69 | Building — on the path, keep stacking |
| 70–100 | Load-bearing — maintain and advance rank |
Muscle
Your armor against aging
Lean mass is the body's reserve currency. It protects joints and bones, steadies metabolism, and is what you draw on to recover from every setback life throws — an illness, an injury, a hard season. Untrained, it declines by roughly 3–8% per decade after 30. Trained, it rebuilds.
Muscle is the slowest number to move — which is why we measure it honestly, on a 90-day clock, not a 30-day one.
How we test it
- Standardized tape measures — flexed upper arm and mid-thigh at fixed landmarks, non-dominant side, average of three pulls, recorded to the millimeter.
- Optional upgrades — smart-scale lean-mass trend (7-day average), or DEXA for members who want lab-grade tracking.
Your first Muscle verdict lands at your second Benchmark — real muscle change takes a quarter, and we won't pretend otherwise.
Engine
Enough capacity to live on
Your engine is your cardiovascular floor: the capacity behind stairs, hills, long walks, hard days, and travel. Aerobic capacity is one of the strongest overall predictors of long-term health in the research literature.
Here's where we differ from most of the industry: we treat the engine as a floor to secure, not a finish line to chase. We build enough capacity to live well — and put the bulk of training time where it compounds for decades: under load.
How we test it
- The 12-Minute Engine Test — cover as much distance as you can in exactly 12 minutes on a flat route, walking or rucking. Distance is your raw number; the scorecard converts it to an estimated capacity score.
- Wearable overlay — if you wear a Garmin, Apple Watch, or Whoop, your 7-day resting heart rate trend corroborates the test.
Balance
The independence number
Falls are the leading cause of injury in adults over 65 — and balance is trainable now, decades before it's tested for real. This is the number that decides whether a slip on the ice is a story you tell or a winter you lose.
Fair warning: the eyes-closed stand humbles almost everyone the first time. That's the point. It moves fast with training, and watching it climb is one of the most satisfying curves in the whole system.
How we test it
- The Eyes-Closed Stand — single-leg stand, eyes closed, near a counter for safety. Your score is your worse leg.
- The No-Hands Get-Up — down to the floor and back to standing without using your hands. Scored by assist level, then by time.
Scored against your age band. Ranked by improvement.
Every raw result converts to a 0–100 score against norms for your age band and sex — so a 61-year-old beginner and a 44-year-old ex-athlete can stand on the same leaderboard, ranked by how much they improved, not where they started.
Every 90 days, the whole community re-tests in the same week: the Gauntlet. Ranks are earned there — Bronze, Silver, Iron, Titanium — and nowhere else.
Get your baseline this weekend.
Take the 2-minute quiz, run the tests at home, and get your personal 4 Numbers scorecard with the exact place to start.
Get Your 4 Numbers