Using a run-of-the-mill, portable and non-invasive electrical muscle stimulator while doing resistance training can yield greater gains in muscle strength and muscle mass than doing resistance training alone, according to new research.
Conventional resistance training, which is any exercise that causes muscles to contract against external resistance, improves skeletal muscle mass, strength and power. Likewise, using a widely available, non-invasive neuromuscular electrical stimulation (NMES) device to induce involuntary muscle contractions can increase strength and mass in muscles of the upper and lower body.
Now, researchers from the University of Texas at El Paso (UTEP) have led a study to investigate the effect that combining the two – resistance training (RT) and NMES – has on muscle mass and strength.
“To our knowledge, there are no systematic review and meta-analysis studies to date that evaluate the effectiveness of using NMES with RT,” said the researchers. “To address this gap, this systematic review and meta-analysis aimed to assess the effect of superimposed NMES on resistance training-induced increases in muscle strength and muscle mass compared to conventional RT.”
The researchers analyzed 13 randomized controlled trials and intervention studies where a total of 374 participants specifically used a NMES device while they did traditional resistance exercises like bench presses or squats. The included studies were conducted on human participants with no neurological or muscular impairments.
“A meta-analysis provides more comprehensive evidence on studies that explore the same research question,” said Sudip Bajpeyi, PhD, director of the Metabolic, Nutrition, and Exercise Research (MiNER) Laboratory at UTEP’s Department of Kinesiology and the study’s corresponding author. “This approach allows us to move beyond the limitations of individual studies and make them more informed, evidence-based conclusions.”
In the meta-analysis of the 12 studies that compared muscular strength improvement in NMES plus RT groups versus those who did conventional RT, the researchers found that the standardized mean difference (SMD) across all studies was 0.31. Now, before moving on, the relevance of that figure and the use of a SMD requires explanation.
A SMD is used as a summary statistic in a meta-analysis when all studies assess the same outcome but measure it differently. Before results can be compared across studies, they must be standardized into a single, common measurement. Then, once standardized, they can be aggregated in a meta-analysis to produce a single value, the SMD. However, a SMD isn’t tied to a specific unit of measurement. If a SMD was zero, researchers could conclude that those exposed to an intervention were no better off than those not exposed. A SMD above or below zero can be a positive or negative finding, depending on the outcome being measured. If you’d like a more in-depth explanation of SMDs, the video below is helpful.
So, in the present study, a SMD of 0.31 indicates that participants exposed to NMES plus RT showed greater increases in muscle strength compared to those exposed to conventional RT only. A similar effect was observed concerning muscle mass, which produced a SMD of 0.26, suggesting a greater increase in mass associated with the NMES plus RT intervention. The researchers did note that superimposing NMES on RT for eight to 16 weeks resulted in greater increases in muscle mass compared to training for two to eight weeks.
“Therefore, it may be possible that a minimum of 8 weeks of training duration is necessary to see significant improvement in muscle mass between the two modes of exercise,” the researchers said.
To determine if there were factors related to NMES or RT that influenced muscle strength and muscle mass, the researchers performed a sensitivity analysis. RT variables associated with increased muscular strength included the number of sets and repetitions per set. For NMES, stimulation frequencies of 85 Hz or higher were associated with increased muscular strength. Overall, the number of sessions per week, total sessions, and time of training were positively associated with strength. None of these variables were associated with the increased muscle mass observed by the researchers.
“This is the first systematic review and meta-analysis that has investigated whether the additions of NMES to a [sic] RT intervention leads to greater gains in muscle strength compared to RT performed alone,” said the researchers. “The results indicate a significantly greater increase in muscle strength when NMES is superimposed on RT compared to RT performed alone.”
Importantly, none of the studies controlled the participants’ diet. A sufficient intake of protein, particularly, can significantly increase muscle strength and muscle size because it enhances the synthesis of muscle protein and inhibits its breakdown, leading to a net positive muscle protein balance. Because of this lack of control, the impact of diet remains unknown. Further research, preferably using a larger sample size, is needed to investigate this and to obtain more statistically significant data.
Despite the study’s limitations, the researchers say their findings are relevant, especially for people wanting to improve muscle function and strength post-operatively or after a period of inactivity due to illness.
“RT has long been recommended for improving muscle strength and muscle mass,” they said. “NMES is commonly used in therapeutic and rehabilitative settings to prevent the loss of muscle strength and muscle mass during immobilization and physical inactivity. NMES is also practical and convenient to use due to the cost, portability and minimal equipment and effort required to receive the benefits associated with it.”
The study was published in the European Journal of Applied Physiology.
Source: UTEP via EurekAlert!