Physiological Responses to Commanded Terrain Conditions in a Real-World Video-Based Indoor Cycling System

Background and Purpose: Indoor cycling systems can combine real-world riding footage with resistance commands derived from terrain data. This study characterized cardiovascular, heart rate variability, and metabolic responses to three commanded terrain conditions. Methods: Analyses included three 30 min sessions per participant in 30 healthy men (22 ± 2 years) under low (predominantly downhill), moderate (mixed climbing and descending), and high (predominantly uphill) terrain conditions. All participants completed the three conditions across three study visits in a fixed order: moderate, low, and high. Participants were instructed to maintain 50-60 rpm. Heart rate (HR) and heart rate variability (HRV) were analyzed in 30 participants, and respiratory gas exchange in 29. Condition and measurement period effects were assessed using two-way repeated-measures analysis of variance; all four HRV indices were natural-log-transformed. In exploratory analyses of the moderate condition, participant-level HR, oxygen uptake (VO₂), and carbon dioxide output (VCO₂) slopes were compared between climbing and descending segments defined from an approximately time-aligned reference terrain profile. Results: HR, respiratory frequency, VO₂, VCO₂, and metabolic equivalents increased across conditions, whereas the time-domain HRV indices SDNN and RMSSD decreased (all condition effects, p < 0.01). Exploratory frequency-domain analyses also showed decreases in LF and HF across conditions; interpretation remains limited by exercise nonstationarity, respiration, and unassessed sensitivity to recording quality. In the moderate condition, median HR slopes were +6.34 bpm/min during classified climbing segments and -2.32 bpm/min during classified descending segments (p = 0.005; rank-biserial r_rb = 0.58); the climbing slope exceeded the descending slope in 70% of participants. Corresponding differences were observed for VO₂ (+126 vs. -102 mL·min^-2; p = 0.016) and VCO₂ (+132 vs. -121 mL·min^-2; p = 0.004). Conclusions: Physiological responses differed across the three commanded terrain conditions under the fixed-order protocol, with group-level differences between classified climbing and descending segments in the moderate condition. Delivered mechanical workload was not validated, participant-specific terrain alignment was approximate, and condition effects could not be separated from visit-order effects. These findings characterize responses under the tested protocol and do not establish mechanical load accuracy or precise terrain tracking.