When did it become mandatory to wear hard shell football helmets?

1920s and 1930s,

Explanation:

it's worth reiterating that helmets were not mandatory. That wouldn't happen until 1943. During the 1920s and 1930s, variations of the leather helmet appeared, but in 1939, the game changed –or at least became safer– when John T. Riddel introduced the first plastic helmet

running

dancing

jumprope

walking

streching

jogging

hop scotch

swimming

riding your bike

skating

sit ups

push ups

crunches

sorry there were no options posted so i listed a bunch of workout activiteis