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
- 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.
- Dial in true endurance
- 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.
- Find your threshold without a lab
- Pace climbs and time trials
Hold power and keep SmO₂ from crashing early. If SmO₂ nosedives, back off slightly to avoid cooking yourself.
- Pace climbs and time trials
- 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.
- Master interval recovery
- 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.
- Track adaptations
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.