Aerobic threshold : First ventilatory threshold (LT1)

The intensity at which ventilation starts to rise and lactate slightly exceeds resting level (~2 mmol/L).

The aerobic threshold (SV1 / LT1) marks the lower boundary of fundamental endurance. Below it, the body runs primarily on fat and lactate stays near resting levels. Above it, carbs become the dominant fuel and lactate starts to accumulate. For an Ironman triathlete, SV1 is typically around 70-77% of max HR or ~55-65% of FTP. Most volume training should stay below this threshold to promote aerobic adaptation without accumulating fatigue.