What is a Muscle Stimulator?
Also known as an electrical nerve stimulator, or TENS machine, these are devices which stimulate low frequency electrical pulses through electrodes placed on the skin, inducing short and low intensity muscle contraction. The most common brand-named devices used in the functional fitness world are Compex and PowerDot, however it is worth noting that if you don’t have a spare £500, you can buy an almost identical TENS machine for £20 from Amazon.
How are they meant to work?
TENS machine are predominantly used as a form of pain relief, for example in chronic pain, arthritis or during labour. Although there is not a vast amount of evidence for their success, they are recommended by the NHS as some patients find them extremely helpful, and they are considered cheap and safe with minimal side effects. (1)
The question is, how would they improve recovery from training?
When a person exercises, they become fatigued, ‘a decline in a person’s ability to exert force’. (2) This fatigue consists of pain (DOMS), reduced range of motion of joints, and reduced muscle strength. The underlying cause of this fatigue is not completely well understood. Lactic acid build up is one contributing factor, but does not explain it alone, as blood lactate levels return to normal well within the average time between training sessions. Other changes also occur, muscle damage, glycogen (energy store) depletion and other metabolic disturbances such as raised creatine kinase. It is likely that fatigue is caused by a combination of these factors. (3)
There are 2 widely accepted theories for how electrical stimulation may work to enhance recovery. Firstly, by increasing blood flow to the muscle, thus increasing the washout of these substances causing fatigue. (4) Secondly, by modulation of neurotransmitter release, causing short term pain relief. (5)
Do they really work?
Essentially, the evidence is extremely variable. Neric et al. compared 3 recovery strategies on blood lactate levels, passive (i.e doing nothing), active (low intensity swimming), and TENS following a 200yard front crawl sprint. Active recovery was the most efficient method of lactate removal. TENS was shown to improve lactate removal compared to passive recovery. (6)
Lattier et al. tested TENS after high-intensity intermittent uphill running. There was a small improvement in performance during an all-out running test performed 80 min after the initial bout following TENS compared to passive recovery. (7)
Heyman et al. however showed no improvement in climbing performance during a second bout after using TENS. Active recovery resulted in improvement back to initial ability. (8)
Denegar and Perrin have shown that TENS was a valuable technique for reducing muscle pain. High-frequency TENS appeared as effective as cold or cold combined with TENS when compared to control and placebo conditions. These treatments also had positive effects for DOMS associated joint range of motion recovery. However, the reduction in pain was not accompanied by a faster restoration of muscle strength. (9)
Craig et al. found no improvement in pain or function in patients with experimentally induced DOMS treated with low or high frequency TENS. (10)
In conclusion, it is apparent that TENS does improve blood flow to muscles and does reduce lactate levels when used immediately after exercise, although it is not as effective as active recovery. There is some evidence that TENS alters pain signalling within the body. The evidence that this results in a perceivable reduction in pain is extremely variable and is likely to vary from person to person. There seems to be minimal evidence currently that TENS has any effect on enhancing restoration of muscle strength or range of motion following high intensity training. (11)
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