A Practical Look at the First Week

The first week of testing a new protocol for measuring VO₂ max on forest trails forced a handful of real tradeoffs. This post walks through what those were and why they mattered.

We set up a portable ergospirometry unit on a 2.4 km loop with a 6% average gradient. The surface was loose topsoil over compacted clay — not ideal for consistent readings, but representative of what most trail runners face. The main constraint was battery life: the unit lasted about 90 minutes, which meant we could only collect data on two full loops per session.

The first decision was whether to prioritize steady-state segments or include a short sprint section. We chose steady-state because the runner’s heart rate plateaued around 168 bpm on the second loop, and we wanted to see how oxygen uptake behaved under sustained load rather than a spike. The data showed a 4.2% drop in V̇O₂ between the first and second loop, likely due to accumulated micro-decelerations on uneven footing.

Another tradeoff involved the mask seal. The standard silicone mask shifted on two of the five runs, introducing a leak that affected the gas exchange readings. We switched to a neoprene strap with a chin cup for the remaining sessions, which reduced leakage but added 12 seconds to the donning process. That extra time meant the runner started the second loop slightly warmer, which may have skewed the baseline.

On the shoe side, we tested three midsole compounds: standard EVA, a TPU blend, and a polyolefin foam. The TPU blend returned 87% of the energy on a force plate test, but on the trail it felt stiffer at low cadence. The polyolefin foam absorbed more vibration on descents, but the runner reported a “mushy” push-off on short climbs. No clear winner — just a set of tradeoffs that depend on the runner’s weight, cadence, and preferred terrain.

The practical takeaway from the first week is that lab-grade precision is hard to replicate outdoors, but the tradeoffs are worth documenting. A runner who knows their V̇O₂ drops 4% on loose soil can adjust their pacing strategy for a race. A coach who sees a 12-second mask delay can decide whether to accept the small baseline shift or redesign the warm-up protocol. That kind of concrete detail is what makes the data useful.