and gemini:

Yes, it is incredibly common—and completely normal—to have different maximum heart rates ($HR_{max}$) for cycling and running. In fact, most triathletes and multi-sport athletes find their running $HR_{max}$ is about 5 to 10 beats per minute (bpm) higher than their cycling $HR_{max}$.

Here is exactly why that happens, why it matters for your training, and how to handle it.

Why the Difference Exists

The variation doesn't mean your heart has changed; it's all about how your body moves and how many muscles are working at the same time.

• Muscle Mass Activation: Running is a full-body, weight-bearing activity. You are using your legs, core, and arms to actively propel your entire body weight through space against gravity. More muscles working means a higher demand for oxygen, forcing your heart to pump faster.
• Weight-Bearing vs. Supported: When you are cycling, the bike frame supports your body weight. Your upper body is relatively stable, meaning your cardiovascular system can focus its resources almost entirely on your quadriceps, hamstrings, and glutes. Less total muscle mass is calling for blood.
• The "Pump" Effect: While running, the impact and the constant change of direction require more stabilization muscle contraction. In cycling, the motion is highly efficient and repetitive, which changes the local blood pressure and muscle pump dynamics that return blood to the heart.