The Moxy SmO₂ Sensor: What It Is, How It Works, and How It Makes You Faster

If you’ve ever wished you could peek inside your muscles during a ride, the Moxy muscle oxygen (SmO₂) sensor is your window. Instead of guessing how hard “hard” is, SmO₂ shows—in real time—how much oxygen is available in the working muscle. Pair it with your head unit (Garmin/Wahoo), watch it alongside power and heart rate, and you’ll train smarter, recover better, and race with more control.

What is SmO₂ (and THb)?

• • SmO₂ = the percentage of oxygenated hemoglobin in a specific muscle (e.g., vastus lateralis). Think of it as “local oxygen balance.”

• • THb (often shown as THb or tHb) = Total Hemoglobin signal, a proxy for local blood volume changes (vasodilation/constriction). It hints at delivery capacity.

Unlike heart rate (systemic and laggy) or power (output), SmO₂ is local and immediate—it reacts quickly when demand outstrips supply.

Why cyclists love it

1. 1. Dial in true endurance
      Keep SmO₂ steady in the aerobic sweet spot (typically ~55–65%). If it drifts downward at “endurance” power, you’re creeping too hard.

1. 1. Find your threshold without a lab
      During a ramp or long step test, watch where SmO₂ takes a sharper downward turn and stops fully recovering between steps. That inflection often lines up with functional threshold.

1. 1. Pace climbs and time trials
      Hold power and keep SmO₂ from crashing early. If SmO₂ nosedives, back off slightly to avoid cooking yourself.

1. 1. Master interval recovery
      Don’t start the next rep on a clock—start when SmO₂ rebounds (e.g., back to ~60%+). That’s personalized recovery.

1. 1. Track adaptations
      Over weeks, you’ll see: higher SmO₂ at the same power, faster post-interval rebound, and less “sag” on long climbs. That’s fitness in pixels.

How to set it up (quickly)

• • Place the sensor on your vastus lateralis (outside quad), lower-to-mid-thigh.

• • Pair it to your head unit as a Muscle Oxygen sensor.

• • Record at least Power, HR, SmO₂, THb (if available), and HRV.

• • Warm up 10–15 min so the signal stabilizes.

How to use SmO₂ in training

1) Aerobic Base & Endurance

• • Ride with SmO₂ ~40-55% and stable.

• • If it keeps trending down at a fixed power, reduce power or increase cadence.

• • Use long steady rides or low-tempo blocks; check that SmO₂ recovers well during easy spins.

2) Tempo / Sweet Spot

• • Expect SmO₂ 25-40%.

• • Watch for a slow drift down across long efforts—if it never stops dropping, the work is too hard/long for today.

3) Threshold

• • SmO₂ 20-30% with small changes in power causing noticeable SmO₂ swings.

• • Between repeats, wait until SmO₂ rises to ~60-75% before the next rep.

4) VO₂ / Anaerobic

• • SmO₂ can fall <20%.

• • Use short reps (e.g., 2–4 min or 30/30s) and full recovery guided by SmO₂ rebound, not a fixed timer.

Interpreting tHb

• • Rising/slightly higher THb at low–moderate work suggests good local blood flow.

• • Flattening or dropping THb at very high intensities may indicate mechanical occlusion (lots of force), limiting delivery—SmO₂ falls faster.

• • Over time, you want more stable THb at given workloads and quicker SmO₂ recovery between reps.

A simple starter workout (SmO₂-guided)

Tempo Blocks, SmO₂-steady

• • 20–30 min warm-up until SmO₂ stabilizes.

• • 3×12 min tempo where SmO₂ sits ~30-40%. If it slips below ~30% and won’t recover mid-block, ease power 5–10 W.

• • Recover until SmO₂ ≥ 60%, then start the next block (not before).

• • Cool down easy until SmO₂ returns to 65–70%+.

Progression: Add time per block first, then a touch of power, always holding the same SmO₂ target.

Common pitfalls (and quick fixes)

• • Chasing power while SmO₂ free-falls → Back off a few watts; cadence up; give it 60–90 s and reassess.

• • Expecting identical SmO₂ day-to-day → Temperature, hydration, carb intake, and placement matter. Use trends, not single points.

• • Starting reps on a strict clock → Let SmO₂ recovery call the shots; you’ll finish more quality work.

Bottom line

Power tells you what you’re putting out. SmO₂ tells you what the muscle can support right now. Use both: set the target with power, pace, and recover with SmO₂. You’ll spend more time in the right physiology, waste less energy, and arrive fresher when it matters.