Let's see. The entire arc of the arm is probably something like 2m (the entire body moves forward as well as the arm going from pointing backward to pointing forward, so 2m is a reasonable guess.
Assuming uniform acceleration (this will not actually be true, but for a back of the envelope calculation it should give us something close to the average acceleration)
We have two equations, one for the velocity, and one for distance travelled that we can solve. We write each in terms of the time it takes to make the pitch, which we call t.
v = a * t = 90mph = 40 m/s
So
(1) a * t = 40
And we know the distance traveled over time t is:
x = 1 /2 a * t^2 = 2
(2) a * t^2 = 4
Now solve the (1) for t and we get
t = 40 / a
Substitute it into (2)
a * 40^2 / a^2 = 4
40^2 / a = 4
40^2 = 4 a
1600 = 4 a
400 = a
So I get 400 m/sec^2
Bob