REF-1327 · Swain & Franklin, 2002 · Medicine & Science in Sports & Exercise · Paper · PubMed

Is very easy riding enough to get fitter?

Across 18 studies, below about 45% of VO2 reserve did not raise VO2max in groups starting above 40 mL/kg/min; below 40, even about 30% worked. A soft threshold, from 2002 (C+).

Main caveat: Abstract only, 2002. The 18 studies, their sizes, durations, frequencies and modes, and the conversion equations could not be checked.
thin evidenceMeasured on non-athletes and recreational people

Swain and Franklin set out to express the minimum training intensity that improves cardiorespiratory fitness in % of VO2 reserve (VO2R, the percentage of the difference between maximal and resting VO2), at a time when the American College of Sports Medicine gave a minimum of 50% of VO2max for most healthy adults and 40% for people of very low fitness.

They identified 18 earlier training studies in healthy subjects that trained at low to moderate intensities (≤ 60% VO2max) and translated each study's intensity, reported variously as % VO2max, % heart-rate reserve, % HRmax or a heart rate, into % VO2R.

In studies whose subjects started with a mean VO2max above 40 mL/kg/min, intensities below about 45% VO2R consistently failed to raise VO2max. In studies whose subjects started below 40 mL/kg/min, no intensity was ineffective; the lowest examined were about 30% VO2R.

The authors conclude that, "although evidence for a threshold intensity was not strong", 45% VO2R is a reasonable minimal effective intensity for fitter subjects and 30% VO2R for less fit ones.

Our reading: the direction — the floor rises with fitness — fits other evidence in the base (in an earlier, 1997 meta-analysis of lactate and ventilatory thresholds, sedentary people improved at every training intensity while conditioned people showed non-significant gains, REF-1005); the exact numbers are soft, and because the analysis looks only at intensity, a "failed" low-intensity study may simply have had too little total work.

Protocol

Question: the threshold intensity for improving VO2max, expressed in % VO2R units.
Studies: previous studies in healthy subjects (n = 18 studies) that evaluated training at low to moderate intensities (≤ 60% VO2max).
Conversion: each study's intensity — % VO2max, % heart-rate reserve, % HRmax or a specific heart rate — translated into % VO2R by the authors.
Grouping: by the study's mean initial VO2max, above or below 40 mL/kg/min.
Not in the abstract: how the studies were found, how many subjects, training duration, frequency and session length, modes, conversion equations, statistics.
Funding and interests: not stated in the abstract or the PubMed record; Crossref lists no funder.

Key Results

Mean starting VO2max of the study groupLowest intensity examinedWhat happened
> 40 mL/kg/minbelow about 45% VO2Rconsistently ineffective at raising VO2max
< 40 mL/kg/minabout 30% VO2Rno intensity was found ineffective
Authors' conclusion: 45% VO2R as a minimal effective training intensity for higher-fit subjects and 30% VO2R for lower-fit subjects, while "evidence for a threshold intensity was not strong".
Context given by the authors: the ACSM minimum at the time was 50% VO2max for most healthy adults and 40% VO2max for those with very low initial fitness.
What these floors mean in % VO2max (our arithmetic, resting VO2 taken as 3.5 mL/kg/min): 45% VO2R is about 49% of VO2max at a VO2max of 50 and about 48% at 60; 30% VO2R is about 37% of VO2max at a VO2max of 35.

Mechanisms

Measured: nothing physiological; VO2max change in the original studies, by intensity and starting fitness.
Authors' interpretation: the minimal intensity that works rises with initial fitness (45% VO2R for higher-fit, 30% VO2R for lower-fit subjects); why % VO2R was chosen as the unit is not argued in the abstract beyond its recent introduction for prescription.
Inference: a training stimulus has to exceed what the body already copes with daily, which is why the floor rises with fitness; but the analysis cannot separate intensity from volume — a low-intensity study with short or few sessions may have failed for lack of total work, not because the intensity was below a threshold (compare REF-1328, where minutes per week set the gain at a fixed low intensity in sedentary women).

Practical Application

Is very easy riding enough to get fitter? Across 18 studies, below about 45% of VO2 reserve did not raise VO2max in groups starting above 40 mL/kg/min; below 40, even about 30% worked. A soft threshold, from 2002 (C+).
Beginners can start very easy and still gain: for someone starting below about 40 mL/kg/min no intensity tested was too low, which agrees with sedentary people improving at every intensity (REF-1005, A-) and with a graded gain at 50% of VO2peak (REF-1328). Habit and minutes come first (SYNTH-75).
For a fit amateur, very easy riding may hold rather than raise VO2max (inference): with VO2max well above 40, the floor is about 45% VO2R, roughly half of VO2max (our arithmetic). In 50 amateur cyclists 55% of VO2peak already sat as low as 38% of ramp peak power, the level of 72% HRmax (REF-1324), so a ride at half of FTP probably sits near this floor, not far above it (our arithmetic chain across studies; FTP was measured in neither).
VO2max is not the only reason to ride easy: this analysis says nothing about durability, fat oxidation, recovery or the volume that easy riding allows; those carry their own entries (SYNTH-51).
Not from this paper; the base's own practice: a recovery spin is capped at about 55% of FTP, 30-60 min (SYNTH-70); that such a ride does not build VO2max in a trained rider is our inference from the floor above, not a guide line.

Limitations

Abstract only, 2002. The 18 studies, their sizes, durations, frequencies and modes, and the conversion equations could not be checked.
Intensity only. The abstract classifies studies by intensity only and reports no control for duration, frequency or total volume, so the floor is probably confounded with dose (inference).
Group means and a crude split. Studies are classed by the mean starting VO2max of their subjects (above or below 40 mL/kg/min), not by individuals; no measure of individual response.
Converted intensities. Most studies did not measure VO2R; their % HRmax, % HRR or heart-rate targets were translated, which adds error the abstract does not quantify; the authors' own wording is that the evidence for a threshold "was not strong".
Outcome is VO2max only. Lactate threshold, economy, durability and performance are not covered.
Funding and interests: not stated in the abstract or PubMed record; no funder on Crossref.
How this entry was checked and changed
Why this level

by the rubric a narrative review without a described systematic method (the abstract says only that previous studies "were identified"), re-expressing 18 training studies in % VO2R without pooling: how the studies were found, the conversion equations, the sample sizes and the training doses are not in the abstract, the outcome is VO2max only, and the authors themselves call the evidence for a threshold "not strong". Pre-2010, kept because it is still the only quantitative floor for an effective aerobic intensity by fitness the base holds. Read from the abstract (closed access; the repository copy listed by OpenAlex sits behind a bot-check page, which was not bypassed)