A non-exercise method equation using seismocardiography for estimating V̇O₂peak (SCG V̇O₂peak) has previously been validated in healthy subjects. However, the performance of the SCG V̇O₂peak within a trained population is unknown, and the ability of the model to detect changes over time is not well elucidated. Forty-seven sub-elite football players were tested at the start of pre-season (SPS) and 36 players completed a test after eight weeks at the end of the pre-season (EPS). Testing included an SCG V̇O₂peak estimation at rest and a graded cardiopulmonary exercise test (CPET) on a treadmill for determination of V̇O₂peak. Agreement between SCG V̇O₂peak and CPET V̇O₂peak showed a large underestimation at SPS (bias ± 95% CI: −9.9 ± 1.8, 95% Limits of Agreement: 2.2 to −22.0 mL·min⁻¹ kg⁻¹). At EPS no interaction (p = 0.3590) but a main effect of time (p < 0.0001) and methods (p < 0.0001) was observed between SCG and CPET V̇O₂peak. No correlation in V̇O₂peak changes was observed between SCG and CPET (r = −20.0, p = 0.2484) but a fair agreement in classifying the correct directional change in V̇O₂peak with the SCG method was found (Cohen's κ coefficient = 0.28 ± 0.25). Overall, the SCG V̇O₂peak method lacks accuracy and despite being able to estimate group changes, it was incapable of detecting individual changes in V̇O₂peak following a pre-season period in sub-elite football players. The SCG algorithm needs to be further adjusted and the accuracy and precision improved for the method to be applicable for use within a trained population.