In English exercise doesn't change if it's used transitively or intransitively, but that's not the case for the Latin word it comes from. As Lewis and Short (and I'm sure other dictionaries) say, exerceo in the active means 'to exercise something,' whereas in the passive and used intransitively it means 'to exercise, practice, or train [oneself].'
Therefore if you had exercet, you would expect an object. See e.g. the first Cicero passage listed:
me adolescentem multos annos...[Hortensius] exercuit, Cic. Brut. 64, 230
Hortensius trained adolescent me for many years...
In your sentence, exerceo doesn't have an object or an agent, so you can assume it's being used in this intransitive sense, which is often called the 'middle voice.' Otherwise you might see se as an object of exercet, 'to exercise [oneself].'