Detecting mechanical breakpoints during treadmill-based graded exercise test: Relationships to ventilatory thresholds

Abstract

While changes in cardiorespiratory variables during graded exercise tests (GXTs) are well described, less is known about running mechanical alterations. We determined mechanical breakpoints during GXT and compared their temporal location with thresholds in ventilation. Thirty-one recreational male runners completed continuous GXT on an instrumented treadmill, starting at 2.5 m.s with velocity increases of + 0.14 m.s every 30 s. Subsequently, the first and second ventilatory thresholds (VT1 and VT2) were determined from expired gases. Spatio-temporal and antero-posterior force variables, and spring-mass model characteristics were averaged for each stage. Mechanical breakpoints were detected using a linear fit process that partitioned the timeseries into two regions and minimised the error sum of squares. All measurements were normalised to % GXT duration for subsequent comparisons. Fifteen out of 16 mechanical variables (all except leg stiffness) displayed breakpoints occurring between 61.9% and 82.3% of GXT duration; these occurred significantly later than VT1 (46.9 ± 6.4% of GXT duration, < 0.05). Mechanical breakpoints for eight variables (step frequency, aerial time, step length, peak push-off force, braking impulse, peak vertical force, maximal downward vertical displacement and leg compression) occurred at a time point not different to VT2 (75.3 ± 6.2% of GXT duration; all > 0.05). Relationships between mechanical breakpoints and either VT1 or VT2 were weak (all < 0.25). During treadmill GXT, breakpoints can be detected for the vast majority of mechanical variables (except leg stiffness), yet these are not related with ventilatory thresholds.