I've used a couple external types of TENS units. The last one from RS Medical helped fatique muscles to cut spasms and it helped ease pain, but mostly helped me increase circulation  and  flexibility.  

The previous unit 20 years ago was a dfferent type from Neurologix and it helped reduce pain, but also was a muscle stimulator and it tightened up my abdominal muscles so my tummy looked a little flatter.  the key was to not over do the charge and cause burns! It could cause me to jump surprisingly. 

I'm a 53 Yr old Para t-4 complete. I have been hit three times by cars while in my wheelchair, but when I broke my leg in July of 2006 it really messed everything up. 

I hope that helpes. PEACE