Based on searchers for Belastbarkeit-EKG, Belastbarkeit here seems to refer to exercise or effort or stress, in the context of how the heart reacts to these.

The most common translation of Belastbarkeit in the context of an ECG seems to be "after effort" which would presumably include exercise, but I would like to propose the use of the word "stress" here as a more general solution.

I do not completely disagree with the first suggestion, but I wanted to suggest another translation, which can frequently be found in papers and articles about various cardiac diseases and cardiac failure.
"Exercise intolerance" can already mean difficulties to climb the stairs and does not necessarily imply a more intense physical activity. Also, tread-mill or bicycle exercise tests are frequently done in patients with cardiac disease, so depending on whether this patient underwent such a test, this might be a more appropriate term to use.

"Exercise intolerance, defined as an impairment in the capacity to perform physical activities (PA) accompanied by symptoms of significant dyspnea and/or fatigue, is a hallmark of chronic heart failure (HF) and is associated with reduced quality of life (QoL) and increased mortality" (cf. reference JACC)

Compare with this:
"Stamina describes a person's ability to sustain physical and mental activity. People with low mental stamina may find it difficult to focus on tasks for long periods and become distracted easily. People with low physical stamina may tire when walking up a flight of stairs, for example." https://www.medicalnewstoday.com/articles/324904

This is just another option (see my "agrees")

Yet another option.