What Causes Painful Lumps On The Lower Legs Post Strenuous Exercise?

Hard lumps apprear on my husband s leg when he gets rigorous excersize. The lumps are colorless and not in a perfect circular formation - but form a raised area about the size of a 1/4 high and can be as large in diameter as an orange. The lumps are extremely painful for him enough to make him stop his physical activity. They usually appear mid-to-low calf area and on his feet...any suggestions for what might cause this?