We developed an eccentric contraction induced electrical stimulation (ES) training system. The purpose of this study was to investigate whether the eccentric contraction induced ES enhance the knee extension torque compared with typical ES. Twenty-two young untrained men (age: 23 ± 3 years) in the acute response trial (single training) and seven untrained men in the long period training trial (for 6 weeks) were studied. We measured muscle thickness and knee extension torque evoked by ES with eccentric contraction training system (ES + ECC) or ES alone for the quadriceps muscle of men. The levels of pain and discomfort were evaluated using numeric rating scale (NRS) and heart rate variability. The knee extension torque of ES + ECC was higher than that of ES alone in the acute response trial. There were no significant differences in the levels of pain and discomfort between ES and ES + ECC. Additionally, ES + ECC training for 6 weeks was effective on the quadriceps muscle thickness and knee extension torque. In contrast, the ES alone training failed to increase muscle thickness and knee extension torque. These results suggest that eccentric contraction induced ES would have the potential to become an effective intervention to promote muscle strengthening.
Resistance exercise can be effective for muscle strengthening [
It has been well established that electrical stimulation (ES) can be effective to induce muscle strengthening [
In contrast, ES causes pain and discomfort [
This study recruited twenty-two young untrained men (mean age ± SD: 23 ± 3 years, height: 176 ± 7 cm, mass: 67 ± 7 kg, respectively) who responded to an invitation to participate in the acute response trial (Experiment 1) and seven young untrained men (mean age ± SD: 23 ± 8 years, height: 175 ± 9 cm, mass: 67 ± 3 kg, respectively) who responded to an additional invitation to participate in 6-weeks training trial (Experiment 2). In the acute response trial, the subjects were measured in the left limb. In the long period training trial, the subjects were trained the both limbs. The subjects were free from known cardiovascular, neurological, or orthopedic problems, volunteered to participate in the study. The subjects were asked to avoid stimulants (e.g. alcohol, caffeine, chocolate) and exercise on the test day, and did not perform any intense exercise 2 days prior to the tests. The subjects were informed of all the procedures, purposes, benefits, and risks of the study and signed an informed consent form, which was approved by the Medical Ethical Committee of Kobe University in accordance with the Declaration of Helsinki. We measured Experiment 1 from February in 2015 to April in 2015 and Experiment 2 from July in 2015 to September in 2015.
Our eccentric contraction induced electrical stimulation system consists of two parts: 1) a continuous passive movement (CPM) device for the knee joint; and 2) a ES device with controller. The CPM device includes an actuator (EASM6, oriental motor, Tokyo, Japan) to generate knee movements with a set velocity which can set freely and an exoskeleton to fix the limb. The exoskeleton was designed to allow the knee joint ROM from 5˚ (fully extended) to 100˚ (flexed). The ES device (ES-360, Ito, Tokyo, Japan) was used to stimulate the quadriceps femoris muscle focus on vastus intermedius (VI) muscle only while the knee joint was flexing; thus, VI muscle could perform eccentric contractions without voluntary contraction. A controller was used to link the CPM and the ES device, controlling the knee joint movement using the current intensity modulation function and triggering the knee joint was in flexion only while (
The effects in the acute response trial compared between before and after exercise session (a single bout training) in Experiment 1. In addition, the long period training trial compared between before the first training and after 48 h from the last training day in Experiment 2. One burst of electrical stimulation was delivered every 3 sec (time on: 1 sec and time off: 2 sec) for 1 min, followed by 5 min of rest. Exercise which included six consecutive stimulation sessions was performed.
Eccentric contractions were induced at an angular velocity of 30˚/sec as described previously [
At first, isometric knee extension torque was recorded at MVC using Cybex (CYBEX NORM, CYBEX Division of LUMEX, New York, USA) set at 0˚/sec angular velocity as the subjects sat strapped a chair. Subjects completed 3 maximal isometric repetitions of the dominant limb for 10 sec at 5˚ of knee flexion (full knee extension, 0˚) to match the knee flexion angle of start position. Each maximal isometric repetition was followed by a 3 min rest interval. During voluntary contractions, participants were encouraged verbally and received visual feedback during each repetition. The greatest peak torque achieved was determined as the maximal voluntary contraction torque. After determined MVC force, the current intensity determined. Current intensity was increased gradually and was determined as the subject’s maximum tolerance current level, but no more than 80 mA, with the system start position; the mean value was 49.5 ± 5.3 mA. Maximal tolerated intensity was identified as the intensity of stimulation received when the subject said that he could no longer tolerate an increase in intensity. After the intolerance current level set, we set the current intensity induced 30% MVC force considered comfortable and safety in the present study. The quadriceps muscle torque of maximum voluntary contraction was shown 97 ± 9 N∙m. The quadriceps muscle torque of the ES was shown 37 ± 4 N∙m, and confirmed that the intensity of ES was set 30% MVC force approximately.
While subjects reclined on the training system for the assigned posture with start position, the thickness of the VI muscle was measured with an ultrasound image device with 9 MHz linear transducer (EUB-415, HITACHI medico, Tokyo, Japan) at rest (REST), at MVC, and at stimulated electrically (ES) respectively. Seven healthy untrained men were recruited for the reliability analysis. The intraclass correlation coefficients (ICC) for the test-retest reliability of the muscle thickness measurements were 0.991 (95% CI 0.971 - 0.996) for the vastus intermedius; these results indicated a high degree of reproducibility in measuring muscle thickness of these muscles.
In order to evaluate acute response with the developed training system, a dynamometer (GT-30, OG giken, Okayama, Japan), which was incorporated in developed training system as to adhere the front part of the ankle, was used to measure at MVC, during peak flexion torque at with (ES + ECC) and without training system (ES).
To evaluation of pain during using training system, NRS (Numeric Rating Scale) scores was compared between rest condition (REST), ES with (ES + ECC), and without training system (ES). Additionally, to evaluate the subjects intolerance current, NRS was compared between the current intensity was 10% down from 30% MVC force (20% MVC), 30% MVC (30% MVC), and 10% up (40% MVC). For the NRS, the pain intensity was rated on a numerical scale from 0 to 10 (0 = no pain and 10 = worst pain imaginable). The electrocardiogram (ECG) signals were obtained from a portable ECG recorder (Check My Heart, Daily Care BioMedical, Chungli, Taiwan) and transferred to a computer loaded with heart rate variability (HRV) analysis software. HRV sampling frequency is 250 samples/sec and measured for 5 min. The two components of power of the R-R Interval (RRI: ms∙ms), low frequency (LF: 0.04 - 0.15 Hz) and high frequency (HF: 0.15 - 0.4 Hz), were calculated. The participants were allowed to set supine position comfortably on a training system in a quiet environment for 5 min, as a rest condition. Then, the record of the ECG signal for HRV analysis started. LF/HF ratio was measured at rest (REST), at ES, and at stimulated electrically (ES + ECC) respectively. To measure the change of HRV during training, we set the last training period for 5 min.
The previous studies have suggested that it was necessary to induce muscle strengthening at least 50% MVC [
Data were presented as mean ± SD. In the acute response trial, the thickness of the VI muscle and quadriceps muscle torque, LF/HF measures were obtained for subjects with one-way repeated measures analysis of variance (ANOVA). When a significant difference was found post hoc comparisons were performed using a Bonferroni correction. In the long period training trial of isometric knee extension torque compared pre and post training, differences were assessed by two-way analysis of variance (ANOVA). The Tukey-Kramer post-hoc test was performed if the two-way ANOVA indicated a significant difference. Student’s t-test was performed to compare the VI muscle thickness compared pre and post training. Statistical significance was set at P < 0.05. To achieve a significant difference at α = 0.05 and with 80% power, the necessary and sufficient n was calculated using the mean and SD from a pilot study involving similar experimental groups and from a previous study on the effects of muscle thickness [
The thickness of the VI muscle was thicker in the MVC (P < 0.05) and the ES (P < 0.05) than in the REST condition (
The quadriceps muscle torque of MVC was higher than in the ES and the ES + ECC (
The NRS was higher in the 30% MVC force trial than in the 10% down trial in which 10% current down from the current of 30% MVC force was used, and lower than the 10% current up trial in which 10% current up from the current 30% MVC force was used (
The LF/HF ratio of HRV in the ES was higher than that in the REST (
The NRS score in the ES and the ES + ECC was higher than that in the REST (
The change value of VI muscle thickness in the ES + ECC after 6 weeks training was higher than in the ES (
extension torque between Pre-ES and Pre-ES + ECC (
The main finding of the present study is the promotional effects eccentric contraction training using the training system synchronized ES on the enhancement of loaded muscle torque without enhancing the pain and discomfort induced ES. In addition, eccentric contraction induced ES for 6 weeks training was effective on muscle strengthening. In contrast, the only ES training failed on muscle strengthening. Therefore, our findings suggest that eccentric contraction induced ES might be not only more effective training for muscle strengthening than only ES training, but also available to avoid the increases of pain and discomfort induced by high intensity electrical stimulations which are usually selected to cause strong muscle contraction.
The present study demonstrated the increases of thickness of VI during ES as well as that during MVC in Experiment 1. Recently, ES with middle frequency burst-modulated alternating current has also been used to stimulate skeletal muscles, as well as low frequency direct current [
Our results showed that the muscle torque of the ES + ECC was shown approximately 69% MVC force although that of ES alone was shown 30% MVC force in the experiment 1 In the present study, we have developed the ES with eccentric contraction system from two points of view. First point was to enhance the promotional effects of muscle strengthening by using eccentric contraction system. The principle of overload is generally recognized as fundamental to the strengthening process, meaning that when the target muscle was loaded with resistance training, the muscle will adapt to become able to enhance the effects of training involving physiological changes e.g. muscle hypertrophy or neural adaptations following increased muscle loading [
Our results (
The results of present study showed that ES with middle frequency could be induced effective muscle contraction on deep muscle, and promotional effect by ES with eccentric contraction system was found in the VI muscle thickness after 6 weeks. In contrast, the only ES training failed on muscle strengthening. The results showed that although ES was induced muscle loading insufficiently for muscle strengthening, ES with eccentric contraction system was induced the muscle loading sufficiently for muscle strengthening at the same time as suppressing increased current intensity. Therefore, it has been suggested that eccentric contraction induced ES would lead to muscle strengthening without sever pain and discomfort even if using only ES induced insufficient muscle loading for muscle strengthening.
The present study has been conducted with limitations. First, the present study was conducted with the healthy men. Therefore, it is unclear that the results of the present study apply the neuromuscular patients, disuse atrophy of the lower limbs of patients, and loss of skeletal muscle mass induced during aging (sarcopenia). Second, the protocol such as current intensity and angular velocity is unknown in effective therapy using eccentric contraction for various patients. Therefore, we plan to perform further studies to answer to the question.
Eccentric contraction induced ES enhanced muscle torque in the quadriceps femoris muscle in comparison to ES alone. Additionally, eccentric contraction induced ES did not increase pain and discomfort. Moreover, eccentric contraction induced ES for 6 weeks training trial showed to be effective for muscle strengthening. These results suggest that eccentric contraction induced ES would have the potential to become an effective intervention to promote muscle strengthening.
This study was supported by Grants-in-Aid for Scientific Research from the Japanese Ministry of Education, Culture, Sports, Science and Technology, and Japanese Society of Physical Therapy.
The authors declare that there is no conflict of interest regarding the publication of this article.
Tanaka, M., Nakanishi, R., Maeshige, N. and Fujino, H. (2017) Effects of Eccentric Contractions Induced Electrical Stimulation Training System on Quadriceps Femoris Muscle. International Journal of Clinical Medicine, 8, 519-533. https://doi.org/10.4236/ijcm.2017.89049