I found this post, but no matter how many times I try, either in radians or in degrees, the results are simply weird.
My "correctest" result was -179533.
The dimensions are 100mm of length, 20mm of diameter and a pressure of 15 MPa, assuming a contraction of 20% and a increase in diameter of 40% that is normally observed in McKibben muscles.
Well, I even tried to calculate the force applied to the walls of the cylinder for a simplified model. Imagining it as a cylinder with 20mm of diameter, its walls would have a circumference of 6.28319cm, so if I add the 10cm of height, accordingly to area calculators I would have 62.83cm². Which would be around a circle of 8.9cm of diameter, which would apply a force of 96.1001 kg.
So, even if a single fiber where linearly and laterally attached to the muscle, it would only suffer 100kg of force. Assuming that the force is equally distributed every linear fiber, of 5mm of diameter each one would only receive 12.56kg.