Pendulum Leg Extension
If an athlete trains on a bicycle ergometer and significantly reduces the force generating capacity of his or her musculature, you probably wouldn't be surprised that it would also significantly affect the athlete's ability to maintain the same level of precision on a balance testing device that they were capable of before they began exercising.
By the same token if an athlete trained utilizing leg extensions and leg curls to fatigue, you might readily accept that this too would effect the ability to perform at the same level on a balance test as the athlete would have in an unfatigued state.
The ability to maintain appropriate balance in a fatigued state is compromised, because muscular fatigue induces a delay in the activation of both the quadriceps and hamstring muscles in response to rapid destabilizing or what is called perturbations. Perturbations are disruptions in function and can be due to external mechanisms outside the body or internal mechanisms within the body itself.
Pendulum Neck Machine
What if your legs remained strong and you dramatically reduced the strength of just your neck extensors? It has clearly been shown that cervical muscle fatigue in itself effects balance. Certainly a reminder that keeping the entire system strong is a balancing act the athlete must follow to excel at the highest level.