It’s a complicated question to answer (although I can tell you the plantar fascitis is not known to result in numbness in the foot). It is most likely due to your prior condition.

It might be your posture while you are running which may change if you try to go faster (most people will flex their spine more with more resistance) and this mechanical alteration could result in increased numbness.

Back brace really won’t prevent you from obtaining bad postures, just give you a tactile cue when you are doing it.

The other consideration is if your foot is going numb with increased effort and stays that way only during class is do you have any history of poor arterial circulation. If you have a family hisotry of cardiovascular disease, if your feet are regularly cold, if you smoke, you may have some early vascular claudication (not too common at 40, but another thing to think about).

Finally, it may just be an inflammatory response…if it doesn’t get any worse, it might actually work itself out if you continue to expose your body this stress…or it might be a very stable thing…happening each time you do it, but never results in any lingering symptoms. In which case, taking a low dose prophylactic NSAID like ibuprofen or alleve prior to class may be of benefit, but you should discuss this with your physician.

If it does progress and lingers longer and longer after this amount of effort, you might be doing more harm than good in which case other things can be done to increase your heart rate…free style swimming, more use of the upper extremeties, etc.

Good luck and keep active.

I am not a physical therapist but have friends who have issues with their back one of them ended up needing to use a brace to help their problem.

The numbness in the foot could also be caused by something else like plantar fasciitis (an inflammation of the plantar fascia-tissue at the front of the foot but this is an example only). I would recommend getting an opinion from a podiatrist (a doctor that diagnoses and treats diseases and disorders of the foot and ankle such as nerve problems in that area) to see what they think could be going on.