Metatarsalgia is a bad term for a foot problem that is common in athlete. It is a phrase that unfortunately gets used often but is quite a meaningless term and diagnosis. The term is like, for example, saying you have a sore hip. That sore hip could be due to any one of many different things. Metatarsalgia means pain in the metatarsals or ball of the forefoot, so it can be due to multiple different things. That is why the term is a bad term.
The cause of the pain could be a corn on the skin under the ball of the foot; it could be a pinched nerve that causes radiating type pains; it could be an atrophy of the fat pad that causes problems only when weight bearing; or it could be osteoarthritis type symptoms inside the metatarsophalangeal joints in the ball of the foot. Each of these possible reasons for metatarsalgia are extremely different to each other with different set of symptoms and different causes which just serves to illustrate just what a bad term it is to use.
Probably the most frequent cause for pain under the ball of the foot is a condition called a plantar plate tear. This is a sprain of the ligament under the joints in the ball of the foot that can progress to a minor tear. The symptoms are typically over the joint on the bottom of the foot and just in front of the joint. Typically it is the second metatarsophalangeal joint, but it can affect all or any of the joints in the ball of the foot. It is more common in those who exercise more and are more active. It starts as a small ache which is often ignored, but it can progress unless there is a therapy started since it does not heal on its own.
Usually the treatment of a plantar plate tear is to make use of strapping to hold the toe in a plantarflexed or pointing downward position to prevent it from bending up so the strain is given a rest. A rocker sole footwear or a shoe that is more rigid can also help do this. If this helps, it can take a long time and patience is needed to get better. If this approach not help, then some plantar plate tears need a surgical treatment.