Responsa for Bava Metzia 210:5

אמרי דבי ר' ינאי לתפלה ולתפילין ד' קבין

over [loose] stones that can be moved, if he travels upon a man or beast of feeble strength, he is unclean.<span class="x" onmousemove="('comment',' The person who actually walks on this field becomes unclean, even if it contains no loose stones. But if one rides upon a man or beast, without himself coming into contact with the field, he becomes unclean only if he causes loose stones to be moved. Hence two conditions are necessary for his defilement: (i) that the field shall contain loose stones; (ii) that the man or beast ridden upon shall be weak and bowed down by the weight of the rider, so that he disturbs the stones more than he would otherwise have done. But if the bearers are so strong that the rider makes no difference to their gait, the latter is clean. ');">⁵</span>

Teshuvot Maharam