FTP Calculator

The Karvonen formula calculates target heart rate using Heart Rate Reserve (HRR):

Target HR = Resting HR + (Max HR − Resting HR) × %Intensity

HRR accounts for individual fitness levels by incorporating resting heart rate, making it more personalised than simple percentage-of-max methods. A lower resting HR (indicating better cardiovascular fitness) shifts all zone boundaries upward in absolute terms.

Polarised training distributes roughly 80% of training time at low intensity (Z1–Z2) and 20% at high intensity (Z4+), with minimal time in the moderate “grey zone” (Z3). Research by Seiler (2010) shows this approach optimises endurance adaptations while managing fatigue and reducing injury risk.

Weekly example (10 hrs):
• 8 hrs easy endurance rides (Z1–Z2)
• 2 hrs intervals & race-pace work (Z4–Z5)
• Avoid chronic Z3 “junk miles”

Blood lactate concentration rises exponentially with intensity. Two key thresholds define training zones:

LT1 (Aerobic Threshold): ~2 mmol/L — the upper limit of easy endurance training. Below this, lactate production and clearance are balanced.

LT2 (Anaerobic Threshold / MLSS): ~4 mmol/L — approximates FTP. Above this, lactate accumulates faster than it can be cleared.

Sweet spot sits at the intersection of high training stress and manageable recovery. At 88–94% of FTP, you accumulate significant TSS per hour while staying below threshold — allowing longer intervals (10–20 min) with moderate fatigue. It is the most time-efficient way to build aerobic power for time-crunched athletes.

Session examples:
• 2 × 20 min @ 90% FTP, 5 min recovery
• 3 × 15 min @ 92% FTP, 4 min recovery
• 1 × 45 min @ 88% FTP (steady state)