electrical stimulation | preventing detraining in inactive muscles

OK, so you’ve broken a limb, and the damn thing is in a cast. Or maybe you’ve strained your anterior cruciate ligament, and your sports doctor says the best thing is to keep your leg immobilised for a little while. What can you do to keep the muscles in your inactive appendage from becoming detrained – from shrinking down to puny proportions and from losing their ability to perform your athletic movements with their usual force?

A little juice will help. That is, high-intensity electrical stimulation is a proven way to maintain size – and even function – in muscles which must temporarily take a complete break from training (1). The idea may sound a little shocking, but study after study has confirmed that the right kinds of electrical stimulation can keep muscles relatively sound, even when they are not being stimulated by the nervous system or engaging in any real activity.

In one of the earliest published studies on the effects of high-intensity electrical stimulation on the maintenance of size and strength in immobilised muscles, researchers electrically stimulated the quads and hamstrings on a daily basis for three weeks in the immobilised leg of an athlete wearing a lower-extremity cast as a result of Grade-II medial-collateral and anterior-cruciate ligament sprains in his knee (2). On the day the cast was removed, the girth of the athletes thigh was increased, suggesting that muscle hypertrophy had occurred, instead of the usual cast-associated atrophy. In addition, single-leg, vertical-leap height was 92% as great in the immobilised leg following cast removal, compared with the uninjured leg, and the athlete was able to immediately return to competition.

No difference in strength

Of course, studies like these lack controls, but subsequent research has confirmed that neuromuscular electrical stimulation can be a valid therapeutic modality for injured athletes (3). Basically, several investigations have documented increases in isometric strength in muscles treated with neuromuscular electrical stimulation (NMES) when NMES-stimulated subjects are compared with unexercised controls; some of these studies have found no difference in strength between NMES-treated subjects and voluntary-exercise groups.

The use of NMES to prevent muscle atrophy as a result of prolonged knee immobilisation following either injury or knee-ligament reconstructive surgery has been very intensely studied. Basically, this research has shown that NMES is effective in preventing decreases in muscle strength, muscle size, and even the oxygen-consumption capabilities of thigh muscles after knee immobilisation. In all but one of the studies in this area which have been published in scientific journals, NMES has been shown to be better in preventing negative changes in leg and knee-joint function, compared with no exercise, isometric exercise of the quadriceps-femoris muscles, and even isometric co-contractions of the quads and hamstrings.

Why does it work?

There is also a fair amount of evidence that NMES can enhance functional performance in a number of different strength-related tasks. How can NMES produce effects similar to those associated with physical training? One theory is simply that NMES produces high-intensity muscle contractions which are similar to those occurring during standard, low-rep, high-resistance strength training, and that as a result muscles respond to NMES in ways which are similar to the adaptations which occur during normal training. As French researchers M. Vanderthommen and J. M. Crielaard of the University of Liege put it, NMES imposes specific patterns of muscle recruitment and a particular “metabolic solicitation” which forces muscle cells to respond in a significant way (1).

However, there may be other factors at work. It is known, for example, that NMES produces what is called a “reversal of voluntary recruitment order”. Here’s how this works: At the beginning of many volitional sporting activities, the central nervous system ordinarily first activates the smallest “alpha motoneurons” (nerve cells which originate in the spinal cord and have relatively thin branches which run out to muscle cells; alpha motoneurons can stimulate muscle fibres to become active). As exercise continues and more force production by muscles is required, increasingly larger alpha motoneurons (i.e., muscle-stimulating nerve cells which have larger-diameter branches) become active. This order of activation – from smaller to larger motor-nerve cells – has been termed the ‘size principle’ of muscle-cell recruitment (4).

Reversing the recruitment order

What is even more interesting is that the size of the alpha motoneuron is closely related to the type of muscle cell which is actually innervated. Slow-oxidative (Type-I) muscle fibres are usually recruited first, by the small alpha motoneurons, whereas fast-glycolytic (Type-II) muscle cells are ordinarily much more difficult to recruit and generally depend on the biggest alpha motoneurons for stimulation. Incidentally, this helps explain why someone who feels completely exhausted during prolonged endurance exercise will suddenly feel much better – paradoxically enough – if he/she actually forces himself/herself to exercise much more intensely. In such cases, non-recruited, non-fatigued, fast-glycolytic muscle cells can be brought into the fray, providing a big boost to exercise capacity.

At any rate, during electrical stimulation of muscles the order of muscle-fibre recruitment is often reversed, with the fast-glycolytic fibres stimulated first rather than last and the slow-oxidative fibres recruited later. In theory, because type-II muscle fibres have a higher specific force than type-I muscle cells, selective augmentation of type-II fibres (via electrical stimulation) may increase the overall strength of a muscle or group of muscles.

And healthy athletes?

Understandably, there has been a keen interest in whether NMES might work for healthy athletes, in addition to injured ones. In fact, there was considerable excitement generated by the early work of Y. Kots in the former Soviet Union, which suggested that in certain cases NMES could be significantly more effective than exercise training itself in strengthening the muscles of elite athletes (5). If Kots’ findings were valid, athletes could improve their power while sleeping, simply by placing the right electrodes over the key muscles involved in their sport! Subsequently, electrical-muscle-stimulation devices have been marketed to athletes and the general public, with the advertising for the devices claiming that they can improve muscle strength, decrease body weight and body fat, and upgrade muscle firmness and overall tone. Sales of the NMES contraptions appear to be red-hot, with a large number of people buying the concept that they can build rock-hard buttocks while watching TV, drinking a beer, or smoking a cigarette.

Recent, well-controlled scientific research has not been so kind to such ideas, however. In a new study carried out at the University of Wisconsin, scientists assigned 27 college-age volunteers into either a NMES group (16 subjects) or a control group (11 individuals). The NMES group underwent electrical stimulation three times a week, following manufacturer’s recommendations, while the control group underwent concurrent sham-stimulation sessions (4). The muscles stimulated included the biceps femoris, quadriceps femoris, biceps brachii, triceps brachii, and abdominals (rectus abdominis and obliques). The electrical-stimulation unit utilised was the ‘Bodyshapers” model BM1012BI, considered to be representative of the quality and price of over-the-counter electrical-stimulation units marketed to general consumers (this device can be readily purchased over the internet).

The results

As it turned out, NMES had no significant effect on body weight, body fatness, fat weight, lean body weight, arm girths, waist girths, thigh girths, isometric strength, isokinetic strength, or even the appearances of the subjects, compared with the sham treatments. Why the bad results? In order for muscles to bother with improving their strength, they must be stimulated beyond a critical threshold. This threshold might be as low as 30% of maximal-voluntary-contraction strength in sedentary slugs but probably needs to be as high as 60% of max-voluntary-contraction strength in well-trained athletes (6). In addition, when electrical stimulation is utilised, the minimum threshold probably has to be at least 60% of max (7). Unfortunately, the over-the-counter device tested in this Wisconsin study produced a force equal to less than 20% of max-voluntary contraction. Importantly, too, the over-the-counter machine produced muscle-stimulation frequencies of 90 to 151 pulses per second, whereas 50 to 75 pulses are considered optimal (overly high frequencies may induce too-early muscle fatigue). In addition, the ‘on-off ratio” (the ratio of time stimulated to recovery time) was only 1:3.5, even though about 1:5 is considered optimal (considerable recovery is needed between bouts of stimulation to allow muscle cells to overcome fatigue).

The bottom line? If a health professional wants to treat your injured limb with high-quality NMES of sufficient intensity, appropriate frequency, and proper on-off ratio, you may accept such a suggestion graciously and assume that the electrical treatments will have a salubrious effect on your inactive muscles. Generally, health-care providers purchase medical-grade electrical stimulators which have been approved by national testing laboratories; approval of the devices by these unbiased laboratories can assure you that an electrical stimulator has passed a series of rigid tests. Don’t try to ‘train” – or treat your injury – using over-the counter electrical-stimulation units, unless they have the approval of your health professional. Rather than firming your bottom, such devices may prove to be a pain in the bum.

Owen Anderson

References

‘Muscle Electric Stimulation in Sports Medicine,‘ Rev Med Liege, Vol. 56(5), pp. 391-395, 2001
‘High Intensity Electrical Stimulation Effect on Thigh Musculature during Immobilisation for Knee Sprain. A Case Report,‘ Physical Therapy, Vol. 67(2), pp. 219-222, 1987
‘Neuromuscular Electrical Stimulation. An Overview and Its Application in the Treatment of Sports Injuries,‘ Sports Medicine, Vol. 13(5), pp. 320-336, 1992
‘Effects of Electrical Muscle Stimulation on Body Composition, Muscle Strength, and Physical Appearance,‘ Journal of Strength and Conditioning Research, Vol. 16(2), pp. 165-172, 2002
‘Electrostimulation. Symposium on Electrostimulation of Skeletal Muscles.‘ Canadian-Soviet Exchange Symposium, Concordia University, December 6-10, 1977
‘Training Muscle Strength,‘ Ergonomics, Vol. 2, pp. 216-222, 1959
‘Augmenting Voluntary Torque of Healthy Muscle by Optimization of Electrical Stimulation,‘ Physical Therapy, Vol. 68, pp. 333-337, 1988