Matt,
I am not an expert in this matter, but Tens units can be operated at different frequencies.
Low frequency 3-5 per second and at intensity of 3-5 times perception threshold has an acupuncture like effect.
Tens can also be used at high frequency 100 stimuli per second at 2-3 times perception threshold.
I believe what frequency you choose, and intensity is trial basis what works best for you. Tens stimulation should not cause muscle contraction. I hope this is helpful, for further clarification if needed you could consult with a PT.
Cliff