Although TENS and EMS units are e-stim machines that operate on the same principle, they have different functions and purposes. TENS units focus on sensory nerve endings to stop pain signals from getting to your brain. On the other hand, EMS devices initiate muscle contraction for muscle building and rehabilitation. Both can help with back pain, injury recovery, muscle spasms, and arthritis. But of the two, only the EMS unit can build muscle, and only the TENs unit can help labor and anxiety.