You must specify ‘conditioning’ and ‘work capacity’ relative to the breakdown I provided a few posts ago.
For example, for an athlete whose anaerobic threshold is 170bpm, exercising between 120-140bpm is quite comfortable; however, this intensity range is effective for developing cardiac output.
Alternatively, for that same athlete, in order to promote aerobic development he/she would have to train around 160-173bpm which is obviously more challenging.