While there may be some exceptions to this rule ie

While there may be some exceptions to this rule (i.e. sports such as
skateboarding, skiing, and snowboarding, where athletes may land on
unstable surfaces), true functional balance training for athletes would rely
on interventions on stable surfaces to best simulate the demands of the
athletic endeavor.
Muscle Recruitment Patterns and UST
The question of functional carryover also extends to the concern for
altered muscular recruitment patterns and reduced force output under
unstable conditions. Behm and colleagues (2002) compared knee
extensor and plantarflexor activity under stable (seated in a chair) and
unstable (seated on a stability ball) conditions.

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The investigators found that force production under stable
conditions was significantly greater than under unstable conditions for
both the knee extensor and plantarflexor tests. Stable knee extensor and
plantarflexor forces were 70.5% and 20.2% greater than under unstable
conditions. Mean quadriceps activation under unstable conditions was
44.3% less than with stable conditions, whereas the plantarflexor deficit
was only 2.9% (not statistically significant). EMG analyses of the
antagonist muscles revealed that hamstring and tibialis anterior activity
increased by 29.1% and 30.3% under unstable conditions as compared to
stable conditions (7).
More concisely, there was less force and activation of the prime
movers and increased activation of the antagonists. The researchers
noted that the change in muscular recruitment patterns was likely due to
"excess stress associated with the increased postural demands" and "the
dispersion of concentration (neural drive) in attempting to control 2 limbs
with differing responsibilities (balance and force)" (7).
While increased antagonist co-contraction may assist in maintaining
joint stability and coordinating movement, it is generally perceived as
counterproductive, especially in strength tasks.
You shouldn’t be doing leg extensions, but if for some
reason you must, your hamstrings shouldn’t be going crazy.

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Torque developed by the antagonists decreases net torque in the
desired direction of movement, and – through the process of reciprocal
inhibition - may also impair an individual’s ability to completely activate the
agonists (62). Therefore, it can be argued that unstable surface training
creates a hesitant athlete for whom stability is gained at the expense of
mobility and force production.
Sale (2003) noted that two muscular activation alteration patterns
have emerged in longitudinal resistance training studies. In some studies,
there has been “a decrease in absolute antagonist activation in conjunction
with either an increase or no change in agonist activation” (62). In other
studies, there was no change in “absolute antagonist activation but
increased agonist activation, decreasing the antagonist/agonist ratio” (62).
The acute muscular activation patterns observed with unstable surface
training work contrary to both of these outcomes, which were observed in