The only thing I can think of would be isometrics, where he both flexes his calf and tibialis anterior at the same time as hard as he can with the intention of keeping his ankle immobile. Not that he CAN move it at all.
Again, that's all I've got.
"Generally people can't squat because they're lacking in the 'lower ab' area. As in they need to grow a set." - LtL
"No one ever got big or strong by lifting lightweights" - Carl1174
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Valentin Konstantin Platz