Muscle strength development: time to go electric?

SPB looks at new research on electrical muscle stimulation and the potential benefits it can offer

Most athletes who strength train will be aware muscular contractions can only occur when the brain sends an appropriate electrical signal to muscles in question. What is less appreciated however is that by applying the right kind of electrical signal directly to the muscles using an external device – a so-called electrical muscle stimulation or EMS device – muscles can be made to contract involuntarily (ie without any effort to do so). This has led to an increasing interest in electrical muscle stimulation (also known as EMS), a procedure where a carefully controlled electric current is sent through muscles in order to make them contract. These EMS devices produce an electrical current, which is delivered to small electrically conducting pads. These pads are attached to the skin at specific positions in order to target and stimulate certain muscles.

EMS uses

Traditionally, EMS has been used for pain relief and for the purposes of injury rehabilitation. When muscles contract as a result of EMS, patients typically experience reduced pain and tension, and a reduction in swelling⁽¹⁾. EMS is therefore useful for those who are working to recover from an injury, because it can actually speed up the recovery process. However, a growing body of evidence also suggests that EMS devices could be used to either as a substitute or an adjunct to strength training in order to increase strength and performance in athletes and non-athletes alike^(2,3).

What kind of strength and conditioning gains be achieved by adding EMS to a training program? Because there are so many types of devices, electrical stimulation protocols, and ways of assessing strength and performance gains, there are no hard and fast answers to this question. However, we can get a feel for what’s possible by looking at some of the findings from previous studies.

In one 2012 study, researchers publishing in the Journal of Strength and Conditioning Research carried out a review study (a study that pools findings from a number of previous studies) on EMS⁽⁴⁾. The researchers identified the type of EMS used, and the most relevant parameters for improvement for high performance sport - eg speed, strength, power and jumping and sprinting ability. The researchers also identified three types of users – non-athletic populations, trained subjects and crucially elite performers (they placed a specific focus on the elite). Overall, they concluded that EMS is effective for developing physical performance. In particular, they found that after a stimulation period of 3-6 weeks, significant gains were shown in maximal strength (isometric force up by 58.8%; dynamic force up by 79.5%). In terms of specific sports improvement, they found the following:

• Vertical jump height increased by 25%
• Squat jump height increased by 21.4%
• Countermovement jump height increased by 19.2%
• Drop jump height increased by 12%
• Sprint speed was increased by up to 4.8% in trained and elite athletes.

The team concluded that not only was EMS a promising alternative to traditional strength training for enhancing strength parameters, it also offered clear-cut advantages in time management.

Recent data

A very recent study published earlier this year (ie also covering data for the intervening nine years) has looked at all the data on EMS for improving body composition and strength. Interestingly, the researchers included studies on whole body EMS – ie where EMS was used to train all the major muscle groups in the body, rather as you would when strength training using conventional weights/resistance⁽⁵⁾. In the study, a number of different ‘training’ interventions were considered (remember that in EMS, there is no conscious effort required by the subject):

• Trials with at least one exercise and one control group.
• Trials where EMS was used as the primary physical intervention.
• Trials where EMS was used with at least six electrodes covering most muscle groups (whole body).
• Trials with non-athletic cohorts.

In summary, 16 studies with 19 individual WB-EMS groups involving 897 participants of various ages and levels of body fat were included. The electrical stimulation impulse protocols (more later) used were roughly comparable, but training frequency (1-5 sessions per week) and intervention length (6 to 54 weeks) differed between the studies. An analysis of the data showed that using EMS, both for individual muscles and for whole body training resulted in significant and large-sized gains in muscle mass and strength parameters. Regarding body fat and composition, there was a small overall improvement (ie drop in body fat %), but this improvement was not large enough to be statistically meaningful – ie to draw any definitive conclusions.

EMS in sport performance

The results of the above study are very encouraging. However, a legitimate question is how relevant they are to other sportsmen and women who are in regular training and who may well already be performing regular strength work? The answer appears to be very relevant. For example, other studies carried out (mostly) over the past five years have shown that:

• A whole-body EMS program improves strength, sprinting, jumping, and kicking capacity in elite soccer players⁽⁶⁾.
• Adding a whole-body EMS program to a conventional strength program augments strength adaptations and type II muscle fiber growth in soccer players during a competitive season⁽⁷⁾.
• Using whole-body EMS-training improves the performance of ice hockey players of different competitive statuses⁽⁸⁾.
• The use of whole-body EMS improves key performance-related parameters in runners⁽⁹⁾.
• The use of leg muscle EMS improves function endurance performance in the trained, untrained and those with patients with functional impairments (see figure 1)⁽¹⁰⁾.
• Leg muscle EMS improves muscle strength and jumping ability in basketball players⁽¹¹⁾.
• Leg muscle EMS improves the maximal quadriceps strength, vertical jump height, and shuttle sprint time in competitive tennis players⁽¹²⁾.

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Figure 1: Improvements in leg muscle endurance following EMS⁽¹⁰⁾

Percent improvements in functional endurance after high- and low-frequency neuromuscular electrical stimulation (NMES) interventions in healthy subjects (gray histograms) and patient populations (black histograms).

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How much, how often?

The evidence suggests that using EMS is indeed effective for a range of sportsmen and women. But what kind of EMS protocol is likely to work? To answer that, we can again look to the research. For example, a study published in the Journal of Strength and Conditioning Research looked at the practical application of EMS in trained athletes to enhance sprint performance⁽¹³⁾. This protocol consisted of the following:

• Ten EMS cycles up to the maximal tolerated intensity applied every other day to the foot flexion muscles (sole and calf muscles).
• Each stimulation periods was for 10 seconds, with recovery periods in between of 50 seconds.
• In total, ten sessions were carried out over a 3-week period.

This protocol improved standing start 10-metre performance by 5.3% and flying 10-metre times by 4.7%, adding credence to the idea that quick gains can be made from EMS.

A brand new study published just last month also provides a useful insight into the kind of protocol that works, but this time the researchers looked at recreationally active sportsmen and women rather than highly trained athletes⁽¹⁴⁾. Published in the International Journal for Environmental Research and Public Health, this study by Polish scientists compared the effects of three weeks of EMS and a traditional method of strength training (isometric training) in young, fit recreationally active adults who were training on average for 3-4 hours per week. The participants were randomly divided into two groups:

• The study group consisted of ten participants (7 women and 3 men), who underwent a 3-week training program (10 sessions) using EMS.
• The control group consisted of nine participants (4 women and 5 men), who performed a training program at home consisting of 10 sessions.

RSQ1 therapy

The EMS group underwent a version of EMS known as ‘RSQ1 therapy’ lasting three weeks and consisting of ten sessions. In RSQ1 therapy, two types of currents overlap: a low-frequency current of 40–500 Hz, and a medium-frequency current of 1000–10,000 Hz (see figure 2). Due to this, the properties of each current are combined. Indications for the RSQ1 treatment include the need for the increase of muscle mass and strength (it can be used as an alternative to strength training), restoration of function and the appropriate range of mobility in the joints. RSQ1 therapy is also used for the promotion of blood circulation, reduction in swelling, prevention of thrombosis.

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Figure 2: The application of electrode pads during RSQ1 EMS training⁽¹⁴⁾

Placement of electrodes of the thigh and the RSQ1 device: (A) front view and (B) from above view.

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In the first week, each subject participated in four treatment sessions, and in the second and third weeks, three sessions. As the current flowed through the RSQ1 device, the participants also performed simple, gentle and repetitive exercises to enhance the effects of the EMS; in the first to the fourth sessions, they performed a standing up and sitting down exercise using a chair (to target the quadriceps of the frontal thigh). These movements were performed five times in the first two sessions, and ten times in the next two sessions. In the fifth to the tenth sessions, the subjects performed static wall squats, to help target the gluteals of the buttocks and the hamstrings. These were of 30, 40 and 50 seconds’ duration in the5/6^th, 7th and 8th sessions respectively.

In the control group meanwhile, the subjects performed a training program of ten sessions for three weeks at home. Exercises during the first four sessions (first week) were the same as in the EMS group (sitting/standing), but without EMS. The task of the participants was to slowly perform this exercise 20 times. From the fifth to the tenth sessions (2nd and 3rd weeks), the control participants performed static wall squats but without EMS. All these exercises were held for durations of 60 seconds. Overall, the exercises performed in the control group far exceeded the demands of the EMS group, both in terms of time required and intensity.

The findings

Before the starting and after ending the training program, measurements of the circumference of the thigh were performed in both study groups. In addition, an assessment of the peak torque of the flexor and extensor muscles in both knee joints was made before and after the intervention using a BiodexSystem3 measuring system. The key finding was that (as expected) both groups made significant gains in peak torque and in thigh circumference. When the results of the two groups were compared, no statistically significant differences were found in the strength parameters and circumferences between the two groups. In other words, the EMS plus very light exercise had produced equally impressive gains as the more strenuous traditional strength training approach. The researchers concluded that using EMS plus light exercises could therefore be an excellent way for athletes to maintain or develop strength when injured or during the early stages of recovery following injury, when conventional (higher-loading) training is not possible.

Practical applications for athletes

The evidence is clear – EMS is a proven way of stimulating muscles for functional strength gains, and can therefore be considered a practical method of supplementing an existing strength program, or as a means of maintaining/developing muscle function when injury or during the early stages of rehabilitation.

In terms of practical application, there’s a paucity of data comparing different EMS protocols for their effectiveness in terms of developing muscle strength and function. However, a number of coaches advocate the 10 seconds on/50 seconds off methodology (as used in the studies above) with around three sessions per week performed⁽¹⁵⁾. In any sport that involves running (most field and team sports and well as runners) or leg muscle power (eg cyclists, rowers, skaters etc), the key muscle groups that should be targeted are as follows:

• Lower back
• Abdominals
• Quads
• Hamstrings
• Gluteals (buttocks)
• Feet

When an EMS session is included in a program, it should be the final session of the day and should be preceded by a basic warm-up involving dynamic movements. Since EMS involuntarily creates muscle contractions, there is no equivalent ‘fatigue drain’ on the central nervous system; this means that it can be used to maintain strength during the competitive phase of the season without the same risk of generating neural fatigue. As with any new training method, EMS needs to be introduced carefully and progressively, and it is important for athletes and coaches to monitor performance to gauge which protocols might work best. Also, there should be regular breaks from EMS use within a periodized training plan.

References

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