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TENS machines work by sending tiny electrical currents, at variable rates and intensity, to the nerves in the pain area. The resulting reactions send messages to your brain to block or dull the pain. You can read about the science at the link above.
A TENS machine usually has setting for EMS - Electrical Muscle Stimulation. Using a similar principle, muscles are contracted at a particular rhythm and rate with the purpose of toning or recvering muscle function. EMS machines have been around for decades. Bruce Lee (Kung Fu hero) was using one in the 60s, as illustrated in this movie clip.
Not all devices have been given their own health certificate so watch out for the FDA approval (US) and CE clearance (EU). ONe FDA cleared device is the Slendertone Flex Max Abdominal Training System and a smaller version is the Neurotrac Sports EMS Muscle Stimulator but there are lots of others. The main problem with Chinese imports is that the instructions are very poor. I'll do my best to find out what the different settings actually mean!
EMS Technical specifications
Some examples of EMS Technical specifications:
Technical Specification Power
2 alkaline batteries 1.5v AA (LR6)
Max 100 mApp across 1000 Ohm load (using 200uS pulse-width)
From 50uS to 400uS adjustable in steps of 50uS
From 1Hz to 110Hz
Mono-phasic alternated Treatment Timer
Continuous 10min 20min 30min 45min 60min 90min
From 1 to 5sec, adjustable in steps of 1 sec.
138mm x 68mm x 28mm
160g inc batteries
EMS instructions and guidance
Some useful information is here.
EMS works best as the last training element of the day, separated from other work by at least two hours. This is usually done at night before bed, as it can be done at home and the supra-maximal stimulus it provides is excellent for promoting the release of growth hormone during sleep.
Use a hot shower as warm up preparation, being careful to remove any oils or creams from the areas to be treated to ensure proper conductivity (oils left on the skin can cause the current to jump around the skin surface causing considerable discomfort). The increased blood flow in the muscles after the shower heats the muscle motor neurons, lowers electrical resistance, and makes them more receptive to EMS.
Start the EMS session with a gentle pulsing mode for three to five minutes to complete the warm-up before starting the maximal contractions. Warm down using the same pulsing method.
Contractions - Timing
Each muscle group is stimulated maximally for ten reps of ten seconds duration with a fifty second rest period between contractions. It's critical to maintain the rest periods as prescribed as this is the absolute minimum recovery time needed to maintain a maximal contraction on the next rep. A shortened rest period may, in fact, change the nature of the exercise so that it enhances the wrong fiber type.
Sprinters use the full ten second contraction time, though shot putters and linemen find that six seconds is about the longest they can maintain a maximal contraction. In either case, the same fifty second rest period must be maintained.
TENS product examples
Size : 53mm x 30mm x 10mm (HT329L1) Size : 54mm dia x 10mm (HT329L2) Size : 53mm x 35mm x 10mm(HT329L3) Size : 54mm x 40mm x 10mm(HT329L4) * Channel : single channel output * Power supply : 3V CR2025 battery 1pcs * Output : 0 ~ 90 mA P to P into 500Ω load * Density : change of 16 steps pules amplitude * Timer : 30 minutes backward to auto turn off * Wave form : dual phase square wave * Display : LCD display with timer/mode/density * Mode : Part A P1~P5 are 5 massage program Part B P6~P0 are 5 EMS fitness program Rate: 2 Hz~86 Hz Contraction(on time) 2 sec. or 4 sec. Relaxation(off time) 2 sec. or 4 sec. * Pulse wave form