RESEARCH ARTICLE
Inferior G Protection with an Electrical Muscle Stimulation Suit Compared to a Standard G-Suit Ulf I. Balldin and John A. Gibbons BALLDIN UI, GIBBONS JA. Inferior G protection with an electrical muscle stimulation suit compared to a standard G-suit. Aviat Space Environ Med 2014; 85:1071–7. Background: At +1 Gz, electrical muscle stimulation (EMS) has been shown to increase systemic blood pressure similarly to a standard G-suit or lower body muscle straining. It was hypothesized that EMS might improve G protection at increased G levels. Methods: An EMS suit was developed with electrodes over the calves, thighs, gluteal, and abdominal muscles. Using nine subjects, the EMS suit was compared to a standard five-bladder G-suit during various G profiles up to +9 Gz in a human-rated centrifuge with EMS activated by electrical muscle stimulators at G levels at or above +4 Gz. The optimal EMS stimulation for a solid muscle contraction was determined for each muscle group in each subject prior to the G exposures. Results: The mean maximal G level attained in the standard suit was 1.1 G higher during a relaxed gradual onset profile, 1.5 G higher during a relaxed rapid onset profile, and 2.0 G higher during a straining rapid onset profile when compared to the EMS suit. During a simulated aerial combat maneuver (SACM) ride, duration was 46 s longer with the standard suit compared to the EMS. During the SACM, the average heart rate was 23 bpm lower with the standard suit compared to EMS. All of the above differences were statistically significant. Finally, there were four G-LOCs with the EMS and none with the standard suit. Conclusion: The tested EMS suit did not give sufficient G protection at high Gs for pilots, nor substitute for a standard G-suit, as indicated by lower G protection and the episodes of G-LOC. Keywords: electrical muscle stimulation, G-suit, centrifuge, acceleration, G tolerance.
T
HE ANTI-G STRAINING maneuver (AGSM) for G protection consists of both isometric muscle contractions of the lower body and abdominal muscles as well as a cyclic breathing maneuver (breathing attempt against a closed glottis as in a Valsalva maneuver) with increased intrathoracic pressure. The voluntary contractions of the peripheral muscles together with the respiratory straining maneuver will increase heart-level blood pressure (9). If performed properly the AGSM can provide up to 3 to 4 G protection above the aircrew’s resting tolerance. The increase of blood pressure at heart level may also be accompanied by an increase in cerebral blood pressure to avoid blackout and G-induced loss of consciousness during high G loads. However, this technique is extremely fatiguing and requires the pilot to anticipate the change in acceleration and forcefully engage their skeletal muscles in an effort to prevent blood from pooling in his/her lower extremities (3). The AGSM may be accompanied by the use of an anti-G suit, which acts by compressing the lower extremities and abdomen to reduce blood pooling to the lower body and reflexively to induce a peripheral vasoconstriction (8,9). This will also reduce blood pressure,
lowering the effect of the increased G load at heart and brain level. The anti-G suits’ bladders are currently inflated through an anti-G valve supplied by compressed gas. It can take up to 3-4 s to fully inflate the bladders of the anti-G suit, which is a relatively long time during rapid-onset high G loads. In addition, the bladders tend to be bulky when inflated and can cause thermal stress. In seeking for new principles of G protection, a literature search showed that electrical muscle stimulation (EMS) has been used in rehabilitation after surgery (e.g., knee surgery), in rehabilitation after spinal cord injury and strokes, and in muscle strength training of athletes (e.g., 12–14). Both animal and human studies have shown an improving circulatory response to EMS in, e.g., wound healing (6). EMS was found to increase blood flow in the human calf muscle (10). The muscle torque generated by electrical muscle stimulation may be similar to voluntary maximal muscle contractions (17). Sparse information from the literature also indicated that arterial systolic blood pressure may be increased in animal models and in humans by EMS. In one study, systolic blood pressure increased by 30% and diastolic pressure by 50% during percutaneous electrical stimulation of the thighs in human subjects (11). EMS has been used after G exposures to test if anti-G straining maneuvers are mainly restricted by central or peripheral fatigue (1). Electrical muscle stimulation appeared to reduce the central fatigue of the muscle contractions. In an earlier study we found that, when using an elastic suit with cloth electrodes sewn into the garment over the calves, thighs, gluteal, and abdominal muscles, the electrical muscle stimulation created systolic blood pressures similar to those produced with an inflated standard G-suit and with lower body muscle straining at normal gravity acceleration (2). The average blood pressure at 1 G increased from about 125 6 15 mmHg at rest From Wyle Integrated Science and Engineering, Brooks City-Base, TX, and the 711th Human Performance Wing/RHD, Joint Base San Antonio, Fort Sam Houston, TX. This manuscript was received for review in June 2014. It was accepted for publication in August 2014. Address correspondence and reprint requests to: Ulf I. Balldin, M.D., Ph.D., Wyle Integrated Science and Engineering, 2485 Gillingham Dr., Brooks City-Base, TX 78235-5104; [email protected]. Reprint & Copyright © by the Aerospace Medical Association, Alexandria, VA. DOI: 10.3357/ASEM.4082.2014
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EMS SUIT VERSUS A G-SUIT—BALLDIN & GIBBONS to about 143 6 15, 146 6 13, and 150 6 12 with standard G-suit inflation, an anti-G straining maneuver involving contraction of lower body muscles, and EMS, respectively. Blood pressure was maintained at the higher level longer with EMS than with the inflated G-suit and voluntary lower body muscle straining. Heart rate, on the other hand, was higher during voluntary muscle straining than during use of the inflated G-suit and EMS. Our hypothesis for this present study was that electrical stimulation of lower body muscles would increase the arterial blood pressure to the extent that it might improve G tolerance during high G exposures, similar to the improvement seen with an anti-G suit and voluntary muscle straining maneuvers. We also hypothesized that the fatigue caused by voluntary muscle contractions during muscle straining maneuvers, which can be substantial with repeated straining maneuvers at high G, could be reduced by using electrical muscle stimulation instead. METHODS The study protocol was approved in advance by the USAF Air Force Research Laboratory Institutional Review Board. Each subject provided written informed consent before participating. For electrical muscle stimulation we used two FDA-approved battery-operated (9 VDC) Vectra Genisys electrotherapy system devices (Chattanooga Group, DJO Global, Vista, CA) with four channels each of individually adjustable stimulation outputs. The two four-channel battery-powered stimulators were used to ensure human subject safety and isolation from AC line noise. The electrical muscle stimulation used biphasic stimulation waveforms of square pulsed-in pulse trains producing a maximum stimulation voltage of 300 volts peak-to-peak. The electrical stimulation could be controlled via a USB data link through centrifuge slip-rings from the centrifuge operating room. Four standard vertical sternal and horizontal biaxillary electrodes were used for ECG, safety, and heart rate monitoring. The abdominal muscle stimulation, but not stimulations from the other muscle stimulation locations,
showed disturbances in the biaxillary, but not in the sternal ECG recordings. Modification of the location and length of the abdominal muscle stimulator electrodes, and moving the biaxillary ECG electrodes adjacent to the sternocostal junctions of the 3rd to 4th intercostal space on both sides, made the ECG recording mostly acceptable for ECG interpretation. The vertical sternal ECG recording was still normal and satisfactory. Prizm Medical, Inc., Duluth, GA, completed 12 different electrical muscle stimulation elastic suits (Under Armorw Performance Apparel Pants and Heat Gear T-shirt manufactured with 64% nylon, 21% polyester, and 15% elastane) in four different sizes (small, medium, large, and extra-large) with sewn in electrodes and connectors. Prizm Medical, Inc., also made the EMS electrodes with elastic knitted Electro-Meshw highly conductive stimulation electrodes using Intelligent Textiles for Medicinew with silver treated nylon fibers blended with Dacron. The locations of the seven pairs (positive and negative) of electrodes were circumferential over the upper and lower calves on both legs, circumferential over the upper and lower thighs on both legs, over the left and right gluteal muscle, and over the abdominal muscles (Fig. 1). Before each centrifuge test, the electrodes of the electrical muscle stimulation suit were pretreated with Thera-Creamw (Prizm Medical, Inc.) and the subjects had the gel applied to the skin under the electrodes. We tested the required voltage for muscle stimulation with and without Thera-Creamw application to the skin or the electrodes. The Thera-Creamw required up to 60 V (peak-to-peak) stimulation for a solid muscle contraction with very little or no prickling sensation and only a tingling sensation. Later in the data runs, we changed to another electrode gel with a highly conductive multipurpose electrolyte (Signa Gelw, Parker Laboratories, Fairfield, NJ). This gel has been and is used for ECG electrodes in the centrifuge and seemed to require less voltage for EMS. It was concluded that Signa Gelw created a solid muscle contraction with very little or no prickling sensation and only a tingling sensation. Signa Gel w appeared even better for muscle stimulation
Fig. 1. Electrical muscle stimulation suit seen (inside out) from the front inside (left) and from the back inside (right) with the positive and negative mesh electrodes sewn into the elastic suit covering the calf, thigh, abdominal, and gluteal muscles.
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EMS SUIT VERSUS A G-SUIT—BALLDIN & GIBBONS because the measured resistance was lower, allowing solid muscle contractions with less than 10 V peak-topeak stimulation. It was noted that the skin conductivity or resistance could vary widely between different subjects. During the EMS in the centrifuge, a stimulation frequency of 70 Hz was selected with low amperage distributed over a large surface area in bursts of about 140 ms duration for every 1 s of stimulation. This stimulation intensity is commonly used in muscle training and muscle rehabilitation programs (17) and was found in earlier EMS tests at 1 G to be the most efficient (2). A previous review of the literature reports frequencies from 40 to 100 Hz used to elicit muscle contractions, with 60 or 70 Hz being preferred, and is discussed in our earlier study (2). A possible explanation for this frequency preference was sought using a computer simulation. The NEURON program was used to model the stimuli and motor neuron responses to sinusoidal and rectangular waveforms from 40 to 100 Hertz (7). Rectangular and sinusoidal waveforms required similar amplitudes at 40 Hz to elicit action potentials. As the frequency was increased to 70 and 100 Hz, larger amplitude was required to elicit action potentials with a rectangular waveform than with a sinusoidal waveform. The technical integration of the muscle stimulators in the centrifuge gondola and the computer-controlled remote control (from the centrifuge control room to the centrifuge gondola) of the muscle stimulator were adapted in the centrifuge and in the centrifuge operating room. Facilities and Human Subjects The study was done at the human-rated centrifuge facilities at Wyle Science, Technology and Engineering, Brooks City-Base, TX (formerly belonged to the USAF). The centrifuge arm is 19.5 ft to the center of the gondola. It is capable of increasing the G level by 6 G · s21 up to +9 G. Originally, 15 volunteer male subjects were to participate in the study. However, because of ethical concerns (see data analysis section), only nine subjects were allowed to complete the study. All were men as no female subjects volunteered for the study. Subject mean age was 30 yr (range 24-39), mean weight was 81 kg (range 70-91), and mean height was 175 cm (range 168183). After a preliminary medical screening and a limited centrifuge orientation ride, a more detailed follow-on medical screening based on USAF Flying Class II/III standards was accomplished, followed by in-depth centrifuge training up to +9 Gz according to USAF Research Laboratory Operating Instructions. The training sessions also included an F-16 tracking task both at +1 Gz and up to +9 Gz. During the computer-controlled tracking task, the subject moved a crosshair with his side mounted control stick to chase an aircraft on the videoscreen. The root mean square of the deviations from the ideal distance to the target and the time on target was computed continuously and stored in the computer. Both root mean square and time on target were later retrieved from the computer at the maximal
common duration time of the simulated aerial combat maneuver (SACM) G exposures for both the standard G-suit and the EMS suit condition for proper comparisons between the different conditions. The subject’s activity, food, and fluid intake the day prior to each test were ad libitum, except for alcohol, which was forbidden. Experimental Procedures For each subject, G tolerance and performance were measured under various G profiles while wearing the EMS suit and were measured again on a separate day while wearing standard anti-G suit equipment. The two different conditions were conducted with at least 22 h between tests in a balanced order. Before donning the EMS suit, the electrodes were carefully covered with Thera-Creamw or later during the data runs Signa Gelw and the cream was meticulously rubbed into the electrodes. The subject also applied the cream to the skin surface later covered by the electrodes. After that procedure, the subjects donned the EMS suit and shoes. In the control experiment the USAF basic G-protection garments with the CSU-13B/P Anti-G Suit were donned. In the centrifuge, the F-16 seat configuration with 30° seat back angle was used for the G-tolerance assessments. Before starting the experiment, skin resistance was measured with a multimeter over each of the seven muscle groups stimulated. If it was more than 10 KΩ at any muscle group, more Thera-Creamw or Signa Gelw was added to the electrode. After that, each muscle group was tested, gradually increasing voltage from the muscle stimulator for maximal muscle contraction without causing pain. On some occasions, the subject would develop a muscle cramp in the stimulated muscle, typically occurring in the calf muscles. When that happened, it was found that voluntary muscle contraction during electrical stimulation reduced the subjective discomfort and permitted a higher stimulation voltage to be used. This procedure was used for some subjects and repeated up to two times to get the optimal muscle stimulation. The skin resistance varied from 0.1 to 9.5 KΩ at all the skin regions in all the subjects tested. The voltage during the testing of maximal tolerable contraction without pain induced by the electrical muscle stimulator varied from 4 to 32 V in all subjects for all muscle groups tested. The final maximal stimulator value was saved in the stimulator for each muscle group and the same values were used during the automatically induced muscle stimulation of all muscle groups simultaneously above +4 G in the centrifuge. For the test conditions with electrical muscle stimulation, the stimulation was automatically activated at +4 Gz to the predetermined stimulation level at G levels above that, up to the maximal G level of +9 Gz. When wearing the basic USAF G-protection garment (standard), the anti-G suit was inflated according to the standard aircraft schedule, which started at +2 Gz, increased linearly with the G level, and had a maximum inflation pressure of about 525 mmHg at +9 Gz.
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EMS SUIT VERSUS A G-SUIT—BALLDIN & GIBBONS G-Profiles Used in the Centrifuge For all test conditions, the following G profiles were used to estimate relaxed gradual and rapid onset tolerances. Relaxed gradual onset (0.1 G · s21) run (GOR): The subjects were asked to be totally relaxed during both the standard and EMS runs with no straining of the muscles of the lower body or doing any respiratory straining Valsalva maneuvers. When the subject reported 100% loss of peripheral vision and/or 50% loss of central vision as determined by peripheral green lights at a 60° angle from centerline and a central red light, he released the handheld brake to stop the centrifuge and started executing the muscular and respiratory straining maneuvers until he regained vision and full mental alertness. Relaxed rapid onset (6 G · s21) runs (RORrel): After a 5-min rest period, a series of relaxed rapid onset (6 G · s21) runs (ROR) were accomplished, starting at +4 Gz and increasing by +1 Gz per run to a maximum of +9 Gz. Each G exposure lasted 15 s or until vision endpoint criteria as described above were reached. If the endpoint criteria were reached, a 0.5 G lower G level was tested and, if successful, was recorded as the maximal G level. Subjects had a 2-min rest period after each exposure. Straining rapid onset (6 G · s21) runs (RORstr): After completing the relaxed rapid onset runs, the subject continued the G exposures at one G level above the earlier maximal successful level, but with the execution of necessary straining maneuvers of the lower body muscles (not including abdominal and upper body muscles and without respiratory straining Valsalva maneuvers). This process was continued in steps increased by 1 G until a maximum of +9 Gz or vision endpoint criteria were reached. If endpoint criteria were reached, a 0.5 G lower G level was tested and, if successful, recorded as the maximal G level. SACM: After a 10-min rest period the research subject performed a simulated aerial combat maneuver G profile consisting of 10-s periods varying between +5 Gz and +9 Gz. He was then allowed to do both lower and upper body muscle straining maneuvers as needed, including the full anti-G straining maneuvers with respiratory Valsalva maneuvers, during both the standard and EMS conditions. Simultaneously he also performed an F-16 tracking task by moving a control stick to chase a target displayed on the video screen in front of the subject. This G exposure continued for a maximum of four peaks at +9 Gz or until light loss, exhaustion, fatigue, or non-intentional G-induced loss of consciousness (G-LOC). The subjects also reported their estimates of light loss and subjective levels of effort and discomfort after each ROR exposure and after the SACM by using designated effort and discomfort level scales with units from 0 to maximal 11 (0 nothing at all and 11 maximal), as modified from the Borg scale (4).
clear that the EMS was not providing added G protection and might even be having a negative impact on G tolerance. Therefore, we decided it was ethically appropriate to halt data collection and avoid further subject exposures until interim statistical analyses could be performed to determine the value of continuing with the study. For the interim analyses of the nine subjects, descriptive statistics were computed for each outcome measure and Student’s paired t-tests were performed to compare the effects of the electrical muscle stimulation suit with the effects of the standard G-suit. RESULTS The mean maximal G levels reached during the relaxed GOR were 7.1 and 6.0 G for the standard G-suit (standard) and EMS suit, a statistical difference (P 5 0.031) favoring the standard suit (see Table I). During the RORrel runs the maximal G levels were 6.0 and 4.5 G (P 5 0.002), and during the RORstr runs they were 8.3 and 6.3 G (P , 0.001) for the standard and EMS suits, respectively. Both differences were statistically significant, favoring the standard suit. With the straining SACM, the duration averaged 80 s (maximum was limited to 87 s) for the standard, and 34 s for the EMS; a statistical significance (P , 0.001) in favor of the standard suit (see Table II). During SACMs, at a common duration for both conditions, the average heart rate was 126 bpm with the standard suit and 149 bpm with the EMS suit, a statistically significance difference (P 5 0.018). During relaxed GOR, RORrel, and RORstr, average heart rates were somewhat higher for the EMS suit, but the differences only approached statistical significance for the first and third conditions (P 5 0.083, P 5 0.346, and P 5 0.077, respectively, see Table III). The subjective effort level appeared to be lower with the standard suit compared to the EMS suit (average 6.2 units versus 7.7), but
TABLE I. THE MAXIMAL G-LEVELS REACHED DURING RELAXED, GRADUAL ONSET RUNS (GOR-rel) AND RELAXED AND STRAINING, RAPID ONSET RUNS (ROR) WITH THE STANDARD G-SUIT AND ELECTRICAL MUSCLE STIMULATION (EMS) SUIT.
Subject
GOR (Relaxed) (G)
ROR (Relaxed) (G)
ROR (Straining) (G)
std
EMS
std
EMS
std
6.7 8.5 5.8 5.7 5.1 5.1 5.2 4.6 6.9 6.0 1.2
6.0 7.0 7.5 5.0 5.5 6.5 5.5 5.0 6.0 6.0 0.9
4.5 5.0 4.5 5.0 4.5 4.0 3.5 3.5 6.0 4.5 0.8
8.0 7.0 9.0 8.5 9.0 7.0* 8.0 6.0 7.5 5.5 9.0 5.5 6.5 4.5 9.0 6.5† 9.0 6.5 8.3 6.3 0.9 1.1 ,0.001
Data Analyses
1 2 3 4 5 6 7 8 9 Mean SD P
Originally, the protocol called for 15 subjects. However, by the completion of the ninth subject it was becoming
* G-LOC at 8 G. † Almost loss of consciousness at 7 G. P is the P-value for the paired t-test.
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6.4 8.9 8.7 5.3 5.3 7.2 8.4 5.6 8.2 7.1 1.5
EMS
0.031
0.002
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EMS SUIT VERSUS A G-SUIT—BALLDIN & GIBBONS TABLE II. THE MAXIMAL SIMULATED AERIAL COMBAT MANEUVER (SACM) G-PROFILE G-EXPOSURE DURATION WITH MUSCLE STRAINING DURING THE CENTRIFUGE RUNS WITH THE STANDARD G-SUIT AND ELECTRICAL MUSCLE STIMULATION (EMS) SUIT. SACM (Straining) Duration (s)
Heart Rate (bpm)
Subject
std
EMS
std
EMS
1 2 3 4 5 6 7 8 9 Mean SD P
68 87 87 87 87 87 40 87 87 80 16
60 43* 24 23 43* 23 23 23* 46 34 14
149 113 146 138 135 75 150 105 125 126 25
170 142* 168 142 135* 148 154 145* 137 149 13 0.018
,0.001
DISCUSSION
* These runs ended with G-LOC (G-induced loss of consciousness). P is the P-value for the paired t-test.
only approached statistical significance (P 5 0.069). The discomfort level at a common G level during relaxed ROR was significantly lower with the standard suit compared to the EMS suit (0.6 units vs. 1.9 units, P 5 0.007). The only statistically significant difference in light loss during the centrifuge runs was during relaxed ROR, where the peripheral light loss was less with the standard suit compared to the EMS suit (19% 6 20 compared to 66% 6 26, P 5 0.004). While not statistically significant, all other average values of peripheral and central light loss during straining ROR and SACM tended to be lower with the standard suit compared to the EMS. For the tracking task data collected during the SACM (root mean
TABLE III. HEART RATES (HR) REACHED DURING THE COMMON MAXIMAL G-LEVEL WITH RELAXED, GRADUAL ONSET RUNS (GOR) AND RELAXED AND STRAINING RAPID ONSET RUNS (ROR) WITH THE STANDARD (std) G-SUIT AND ELECTRICAL MUSCLE STIMULATION (EMS) SUIT. GOR (relaxed) (bpm) Subject 1 2 3 4 5 6 7 8 9 Mean SD P
ROR (Relaxed) (bpm)
ROR (Straining) (bpm)
std
EMS
std
EMS
std
EMS
98 131 131 100 77 78 110 100 115 104 20
108 132 128 115 100 109 112 100 100 113 11
80 99 133 97 78 68 103 92 85 93 16
87 97 98 118 112 90 100 98 91 99 10
121 118 147 116 97 100 124 115 129 118 14
130 128 135* 125 125 135† 128† 125 119 128 5
0.083
0.346
square and time on target), no statistical differences were detected between the standard suit and the EMS. Spontaneous comments from the subjects after both the standard and EMS suit suggested that there was less support for G with the EMS suit. Some subjects felt they could not fine-tune the muscle contraction with EMS, and even with solid EMS muscle contractions EMS did not give the support needed at higher G levels. Other comments were that: it was not possible to increase the muscle contraction when they were already contracted by the EMS suit, the standard G-suit offered better contraction than the EMS suit, and the squeezing of the legs felt more effective with the standard suit. Two subjects also complained about muscle soreness of the electrically stimulated muscles 1 to 2 d after the muscle stimulation experiments.
0.077
* G-induced loss of consciousness (G-LOC). † The HR with the EMS suit was at a 1-G lower level than with the standard G-suit. P is the P-value for the paired t-test.
The results with only nine subjects (out of the planned 15) clearly showed that G protection given by the EMS suit during increased G with relaxed GOR and relaxed and straining ROR exposures and for the duration of the SACM was inferior to that given by the standard G-suit. The three G-LOCs with the SACM and one G-LOC and one almost loss of conciousness during the testing of the straining ROR G levels also supported that. The lower heart rates during SACM and relaxed GOR with the standard suit may also be indicators of less protection with the EMS suit. During the relaxed GOR and ROR runs, even with solid electrical contractions of the lower body muscles, the EMS apparently did not maintain sufficient circulatory vasoconstriction to protect against the G-induced blood pressure decrease in the head region. The standard G-suit has pneumatic bladders that compress the larger muscles of the lower extremities, increasing arterial compression and decreasing blood pooling in the superficial veins. Taut lacing of the lower extremity portions compresses the deep veins to a lesser extent. The muscle tensing portion of the AGSM provides some arterial compression, up to 300 mmHg, and decreases blood pooling in the deep veins. The combination of the standard G-suit and the AGSM increases arterial resistance and reduces blood pooling in the superficial and deep veins. Even during lower body muscle straining ROR, the EMS suit was not giving the same protection as the standard G-suit itself. This means that the muscular contraction of the lower extremities induced by EMS was not as efficient as the combination of compression by the standard G-suit plus voluntary muscle contraction. Furthermore, when the lower body muscle contractions were used in combination with full Valsalva respiratory straining maneuvers in the SACM runs both with the EMS and standard suits, the duration at increased G was less with the EMS suit and, furthermore, three G-LOCs occurred. This means that even with all the help of full muscle and respiratory straining maneuvers the EMS suit did not give the same G protection as the standard G-suit.
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EMS SUIT VERSUS A G-SUIT—BALLDIN & GIBBONS Therefore, the hypothesis that electrical stimulation of lower body muscles would increase the arterial blood pressure seen at 1 G to the extent that it may improve G tolerance during high G exposures similar to the effects of an anti-G suit could not be verified. It was not anticipated that the EMS suit would be inferior to the standard G-suit. The electrical muscle stimulation at 1 G showed that a similar blood pressure response occurred with both the EMS and the standard G-suit and lower body voluntary muscular contraction (2). Also, the studies by Davies (5), Hultman et al. (11), and Sampson et al. (15) showing EMS causing a dose-dependent increase in blood pressure for possible use in hypotension support the idea of EMS increasing blood pressure at 1 G. Furthermore, a single case study indicated that electrical stimulation of abdominal muscles could be used to elevate blood pressure in a tetraplegic patient (16). However, at higher G, in contrast to at +1 G, the EMSinduced blood pressure response apparently was not sufficient enough to secure the systemic, as well as the cerebral, blood pressure to maintain the ability to withstand the increased G levels and avoid G-LOCs. The standard pneumatic G-suit compresses the legs and abdomen, with a linear increase in G-suit pressure from 2 G to a maximum of about 525 mmHg at +9 Gz. This forces the venous blood upwards to the thorax, increasing cardiac preload, and increasing the aortic valve blood pressure by increasing the peripheral resistance and reducing venous capacitance. The abdominal compression by the abdominal air bladder of the standard G-suit also elevates the heart and, thereby, decreases the hydrostatic distance between the heart and head region, which will further improve cerebral blood pressure. The EMS suit was activated at and above +4 Gz and, although it contracts the calf and thigh muscles, may not sufficiently compress the lower extremity blood vessels, particularly the superficial veins. This: 1) may create less peripheral resistance in the lower body arteries than that of the standard G-suit with more circumferential compression of the legs, in turn causing less arterial vasoconstriction, leading to reduced systemic and cerebral blood pressure protection; and 2) may not reduce venous capacitance enough to prevent blood pooling in the lower extremities. The higher G-onset level for activation of the EMS was chosen to avoid EMS discomfort effects at lower G levels between 2-4 G, where the pneumatic standard G-suit administers only very low pressure (at +3 G the standard G-suit pressure is just above 50 mmHg, compared to 525 mmHg at +9 G. The abdominal EMS contracts the rectus abdominis muscles, but may not give the same external compression effect on the abdomen compared to the abdominal bladder of a standard G-suit, thereby not pushing the heart upwards and not decreasing the vertical hydrostatic distance between the heart and the brain. That may also cause less cerebral blood pressure protection by the EMS than the standard G-suit during high G. The gluteal muscles are contracted with the EMS suit and may not be contracted as much as by 1076
voluntary contractions with the standard G-suit, but with both suits, the gluteal muscles are compressed by sitting in the seat. This muscle compression effect will increase linearly with the increased G level. Therefore, the lack of voluntary compression of the gluteal muscles with the standard G-suit will likely not make enough difference compared to the EMS suit to affect G tolerance. However, the EMS suit may possibly be useful for training of new fighter pilots. Anecdotal information indicates that many pilots, when training on the Brooks centrifuge, only perform a marginally adequate AGSM. EMS together with simultaneous EMG recordings may help to demonstrate a more effective, coordinated lower body muscle tensing in the training of these G-protecting straining maneuvers. Further research may help to validate such an effect. In conclusion, the EMS suit, as designed for this study, with selected electrode placement and selected muscle stimulation frequency and intensity, showed inferior G protection compared to a standard G-suit in terms of relaxed GOR and ROR and straining ROR G exposures as well as during full straining (both lower body muscles and respiratory Valsalva maneuver) SACM G exposures. Furthermore, while using the EMS suit, a total of four G-LOCs occurred compared to no G-LOCs with the standard G-suit. ACKNOWLEDGMENT This study was funded by the U.S. Air Force Surgeon General’s Office (contract #BAA FA7014-09-C-0034). The views expressed in this paper are those of the authors and do not necessarily reflect the policy or position of the U.S. Air Force. The authors want to thank Terry Guess, Wyle, Houston, for his careful selection and technical integration of the electrical muscle stimulators for the study and with all help during the data collection. Thanks also go to Joseph Fischer, Wyle, Brooks City-Base, San Antonio, for the statistical analyses of the results of this study and to Daren Chauvin, Wyle, Brooks City-Base, San Antonio, for all help with technical issues of both muscle stimulator equipment, data collection, and for providing the tracking task data from the computer. Furthermore, thanks go to Mac Baker, Wyle, Brooks City-Base, San Antonio, for the time-consuming centrifuge training of the research subjects. Authors and affiliations: Ulf I. Balldin, M.D., Ph.D., Wyle Integrated Science and Engineering, Brooks City-Base, TX, and John A. Gibbons, D.O., M.P.H., 711th Human Performance Wing/RHD, Joint Base San Antonio, Fort Sam Houston, TX. REFERENCES 1. Bain B, Jacobs I, Buick F. Is there central fatigue during simulated air combat maneuvering? Aviat Space Environ Med 1995; 66: 1–5. 2. Balldin U, Annicelli L, Gibbons J, Kisner J. An electrical muscle stimulation suit for increasing blood pressure. Aviat Space Environ Med 2008; 79:914–8. 3. Balldin UI, Werchan PM, French J, Self B. Endurance and performance during multiple intense high +Gz exposures with efficient anti-G protection. Aviat Space Environ Med 2003; 74:303–8. 4. Borg GA. Psychophysical bases of perceived exertion. Med Sci Sports Exerc 1982; 14:377–81. 5. Davies CT, Starkie DW. The pressor response to voluntary and electrical evoked isometric contractions in man. Eur J Appl Physiol Occup Physiol 1985; 53:359–63. 6. Gentzkow GD. Electrical stimulation to heal dermal wounds. J Derm Surg Onc 1993; 19:753–8. 7. Gilles A, Sterratt D. NEURON tutorial. Accessed 31 Jan 2012 from http://www.anc.ed.ac.uk/school/neuron/.
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EMS SUIT VERSUS A G-SUIT—BALLDIN & GIBBONS 8. Glaister DH, Prior RJ. The effects of long duration acceleration. In: Ernsting J, Nicholson AN, Rainford DJ, eds. Aviation medicine, third ed. Oxford, England: Butterworth and Heineman; 1999: 128-47. 9. Green NDC. Protection against long duration acceleration. In: Ernsting J, Nicholson AN, Rainford DJ, eds. Aviation medicine, third ed. Oxford, England: Butterworth and Heineman; 1999: 148-56. 10. Heath ME, Gibbs SB. High voltage pulsed galvanic stimulation: effect of frequency of current on blood flow in the human calf muscle. Clin Sci 1992; 82:607–13. 11. Hultman E, Sjoholm H. Blood pressure and heart rate response to voluntary and nonvoluntary static exercise in man. Acta Physiol Scand 1982; 115:499–501. 12. Milner-Brown HS, Miller RG. Muscle strengthening through electric stimulation combined with low-resistance weights in patients with neuromuscular disorders. Arch Phys Med Rehabil 1988; 69:20–4.
13. Ratkevicius A, Skurvydas A, Povilonis E, Quistorff B, Lexell J. Effects of contraction duration on low-frequency fatigue in voluntary and electrical induced exercise of quadriceps muscle in humans. Eur J Appl Physiol Occup Physiol 1998; 77: 462–8. 14. Russ DW, Vanderborne K, Binder-Macleod SA. Factors in fatigue during intermittent electrical stimulation of human skeletal muscle. J Appl Physiol 2002; 93:469–78. 15. Sampson EE, Burnham RS, Andrews BJ. Functional electrical stimulation effect on orthostatic hypotension after spinal cord injury. Arch Phys Med Rehabil 2000; 81:139–43. 16. Taylor PN, Tromans AM, Harris KR, Swain ID. Electrical stimulation of abdominal muscles for control of blood pressure and augmentation of cough in a C3/5 level tetraplegic. Spinal Cord 2002; 40:34–6. 17. Wong RA. High voltage versus low voltage electrical stimulation. Force of induced muscle stimulation and perceived discomfort in healthy subjects. Phys Ther 1986; 66:1209–14.
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Inferior G Protection with an Electrical Muscle Stimulation Suit Compared to a Standard G-Suit Ulf I. Balldin and John A. Gibbons BALLDIN UI, GIBBONS JA. Inferior G protection with an electrical muscle stimulation suit compared to a standard G-suit. Aviat Space Environ Med 2014; 85:1071–7. Background: At +1 Gz, electrical muscle stimulation (EMS) has been shown to increase systemic blood pressure similarly to a standard G-suit or lower body muscle straining. It was hypothesized that EMS might improve G protection at increased G levels. Methods: An EMS suit was developed with electrodes over the calves, thighs, gluteal, and abdominal muscles. Using nine subjects, the EMS suit was compared to a standard five-bladder G-suit during various G profiles up to +9 Gz in a human-rated centrifuge with EMS activated by electrical muscle stimulators at G levels at or above +4 Gz. The optimal EMS stimulation for a solid muscle contraction was determined for each muscle group in each subject prior to the G exposures. Results: The mean maximal G level attained in the standard suit was 1.1 G higher during a relaxed gradual onset profile, 1.5 G higher during a relaxed rapid onset profile, and 2.0 G higher during a straining rapid onset profile when compared to the EMS suit. During a simulated aerial combat maneuver (SACM) ride, duration was 46 s longer with the standard suit compared to the EMS. During the SACM, the average heart rate was 23 bpm lower with the standard suit compared to EMS. All of the above differences were statistically significant. Finally, there were four G-LOCs with the EMS and none with the standard suit. Conclusion: The tested EMS suit did not give sufficient G protection at high Gs for pilots, nor substitute for a standard G-suit, as indicated by lower G protection and the episodes of G-LOC. Keywords: electrical muscle stimulation, G-suit, centrifuge, acceleration, G tolerance.
T
HE ANTI-G STRAINING maneuver (AGSM) for G protection consists of both isometric muscle contractions of the lower body and abdominal muscles as well as a cyclic breathing maneuver (breathing attempt against a closed glottis as in a Valsalva maneuver) with increased intrathoracic pressure. The voluntary contractions of the peripheral muscles together with the respiratory straining maneuver will increase heart-level blood pressure (9). If performed properly the AGSM can provide up to 3 to 4 G protection above the aircrew’s resting tolerance. The increase of blood pressure at heart level may also be accompanied by an increase in cerebral blood pressure to avoid blackout and G-induced loss of consciousness during high G loads. However, this technique is extremely fatiguing and requires the pilot to anticipate the change in acceleration and forcefully engage their skeletal muscles in an effort to prevent blood from pooling in his/her lower extremities (3). The AGSM may be accompanied by the use of an anti-G suit, which acts by compressing the lower extremities and abdomen to reduce blood pooling to the lower body and reflexively to induce a peripheral vasoconstriction (8,9). This will also reduce blood pressure,
lowering the effect of the increased G load at heart and brain level. The anti-G suits’ bladders are currently inflated through an anti-G valve supplied by compressed gas. It can take up to 3-4 s to fully inflate the bladders of the anti-G suit, which is a relatively long time during rapid-onset high G loads. In addition, the bladders tend to be bulky when inflated and can cause thermal stress. In seeking for new principles of G protection, a literature search showed that electrical muscle stimulation (EMS) has been used in rehabilitation after surgery (e.g., knee surgery), in rehabilitation after spinal cord injury and strokes, and in muscle strength training of athletes (e.g., 12–14). Both animal and human studies have shown an improving circulatory response to EMS in, e.g., wound healing (6). EMS was found to increase blood flow in the human calf muscle (10). The muscle torque generated by electrical muscle stimulation may be similar to voluntary maximal muscle contractions (17). Sparse information from the literature also indicated that arterial systolic blood pressure may be increased in animal models and in humans by EMS. In one study, systolic blood pressure increased by 30% and diastolic pressure by 50% during percutaneous electrical stimulation of the thighs in human subjects (11). EMS has been used after G exposures to test if anti-G straining maneuvers are mainly restricted by central or peripheral fatigue (1). Electrical muscle stimulation appeared to reduce the central fatigue of the muscle contractions. In an earlier study we found that, when using an elastic suit with cloth electrodes sewn into the garment over the calves, thighs, gluteal, and abdominal muscles, the electrical muscle stimulation created systolic blood pressures similar to those produced with an inflated standard G-suit and with lower body muscle straining at normal gravity acceleration (2). The average blood pressure at 1 G increased from about 125 6 15 mmHg at rest From Wyle Integrated Science and Engineering, Brooks City-Base, TX, and the 711th Human Performance Wing/RHD, Joint Base San Antonio, Fort Sam Houston, TX. This manuscript was received for review in June 2014. It was accepted for publication in August 2014. Address correspondence and reprint requests to: Ulf I. Balldin, M.D., Ph.D., Wyle Integrated Science and Engineering, 2485 Gillingham Dr., Brooks City-Base, TX 78235-5104; [email protected]. Reprint & Copyright © by the Aerospace Medical Association, Alexandria, VA. DOI: 10.3357/ASEM.4082.2014
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EMS SUIT VERSUS A G-SUIT—BALLDIN & GIBBONS to about 143 6 15, 146 6 13, and 150 6 12 with standard G-suit inflation, an anti-G straining maneuver involving contraction of lower body muscles, and EMS, respectively. Blood pressure was maintained at the higher level longer with EMS than with the inflated G-suit and voluntary lower body muscle straining. Heart rate, on the other hand, was higher during voluntary muscle straining than during use of the inflated G-suit and EMS. Our hypothesis for this present study was that electrical stimulation of lower body muscles would increase the arterial blood pressure to the extent that it might improve G tolerance during high G exposures, similar to the improvement seen with an anti-G suit and voluntary muscle straining maneuvers. We also hypothesized that the fatigue caused by voluntary muscle contractions during muscle straining maneuvers, which can be substantial with repeated straining maneuvers at high G, could be reduced by using electrical muscle stimulation instead. METHODS The study protocol was approved in advance by the USAF Air Force Research Laboratory Institutional Review Board. Each subject provided written informed consent before participating. For electrical muscle stimulation we used two FDA-approved battery-operated (9 VDC) Vectra Genisys electrotherapy system devices (Chattanooga Group, DJO Global, Vista, CA) with four channels each of individually adjustable stimulation outputs. The two four-channel battery-powered stimulators were used to ensure human subject safety and isolation from AC line noise. The electrical muscle stimulation used biphasic stimulation waveforms of square pulsed-in pulse trains producing a maximum stimulation voltage of 300 volts peak-to-peak. The electrical stimulation could be controlled via a USB data link through centrifuge slip-rings from the centrifuge operating room. Four standard vertical sternal and horizontal biaxillary electrodes were used for ECG, safety, and heart rate monitoring. The abdominal muscle stimulation, but not stimulations from the other muscle stimulation locations,
showed disturbances in the biaxillary, but not in the sternal ECG recordings. Modification of the location and length of the abdominal muscle stimulator electrodes, and moving the biaxillary ECG electrodes adjacent to the sternocostal junctions of the 3rd to 4th intercostal space on both sides, made the ECG recording mostly acceptable for ECG interpretation. The vertical sternal ECG recording was still normal and satisfactory. Prizm Medical, Inc., Duluth, GA, completed 12 different electrical muscle stimulation elastic suits (Under Armorw Performance Apparel Pants and Heat Gear T-shirt manufactured with 64% nylon, 21% polyester, and 15% elastane) in four different sizes (small, medium, large, and extra-large) with sewn in electrodes and connectors. Prizm Medical, Inc., also made the EMS electrodes with elastic knitted Electro-Meshw highly conductive stimulation electrodes using Intelligent Textiles for Medicinew with silver treated nylon fibers blended with Dacron. The locations of the seven pairs (positive and negative) of electrodes were circumferential over the upper and lower calves on both legs, circumferential over the upper and lower thighs on both legs, over the left and right gluteal muscle, and over the abdominal muscles (Fig. 1). Before each centrifuge test, the electrodes of the electrical muscle stimulation suit were pretreated with Thera-Creamw (Prizm Medical, Inc.) and the subjects had the gel applied to the skin under the electrodes. We tested the required voltage for muscle stimulation with and without Thera-Creamw application to the skin or the electrodes. The Thera-Creamw required up to 60 V (peak-to-peak) stimulation for a solid muscle contraction with very little or no prickling sensation and only a tingling sensation. Later in the data runs, we changed to another electrode gel with a highly conductive multipurpose electrolyte (Signa Gelw, Parker Laboratories, Fairfield, NJ). This gel has been and is used for ECG electrodes in the centrifuge and seemed to require less voltage for EMS. It was concluded that Signa Gelw created a solid muscle contraction with very little or no prickling sensation and only a tingling sensation. Signa Gel w appeared even better for muscle stimulation
Fig. 1. Electrical muscle stimulation suit seen (inside out) from the front inside (left) and from the back inside (right) with the positive and negative mesh electrodes sewn into the elastic suit covering the calf, thigh, abdominal, and gluteal muscles.
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EMS SUIT VERSUS A G-SUIT—BALLDIN & GIBBONS because the measured resistance was lower, allowing solid muscle contractions with less than 10 V peak-topeak stimulation. It was noted that the skin conductivity or resistance could vary widely between different subjects. During the EMS in the centrifuge, a stimulation frequency of 70 Hz was selected with low amperage distributed over a large surface area in bursts of about 140 ms duration for every 1 s of stimulation. This stimulation intensity is commonly used in muscle training and muscle rehabilitation programs (17) and was found in earlier EMS tests at 1 G to be the most efficient (2). A previous review of the literature reports frequencies from 40 to 100 Hz used to elicit muscle contractions, with 60 or 70 Hz being preferred, and is discussed in our earlier study (2). A possible explanation for this frequency preference was sought using a computer simulation. The NEURON program was used to model the stimuli and motor neuron responses to sinusoidal and rectangular waveforms from 40 to 100 Hertz (7). Rectangular and sinusoidal waveforms required similar amplitudes at 40 Hz to elicit action potentials. As the frequency was increased to 70 and 100 Hz, larger amplitude was required to elicit action potentials with a rectangular waveform than with a sinusoidal waveform. The technical integration of the muscle stimulators in the centrifuge gondola and the computer-controlled remote control (from the centrifuge control room to the centrifuge gondola) of the muscle stimulator were adapted in the centrifuge and in the centrifuge operating room. Facilities and Human Subjects The study was done at the human-rated centrifuge facilities at Wyle Science, Technology and Engineering, Brooks City-Base, TX (formerly belonged to the USAF). The centrifuge arm is 19.5 ft to the center of the gondola. It is capable of increasing the G level by 6 G · s21 up to +9 G. Originally, 15 volunteer male subjects were to participate in the study. However, because of ethical concerns (see data analysis section), only nine subjects were allowed to complete the study. All were men as no female subjects volunteered for the study. Subject mean age was 30 yr (range 24-39), mean weight was 81 kg (range 70-91), and mean height was 175 cm (range 168183). After a preliminary medical screening and a limited centrifuge orientation ride, a more detailed follow-on medical screening based on USAF Flying Class II/III standards was accomplished, followed by in-depth centrifuge training up to +9 Gz according to USAF Research Laboratory Operating Instructions. The training sessions also included an F-16 tracking task both at +1 Gz and up to +9 Gz. During the computer-controlled tracking task, the subject moved a crosshair with his side mounted control stick to chase an aircraft on the videoscreen. The root mean square of the deviations from the ideal distance to the target and the time on target was computed continuously and stored in the computer. Both root mean square and time on target were later retrieved from the computer at the maximal
common duration time of the simulated aerial combat maneuver (SACM) G exposures for both the standard G-suit and the EMS suit condition for proper comparisons between the different conditions. The subject’s activity, food, and fluid intake the day prior to each test were ad libitum, except for alcohol, which was forbidden. Experimental Procedures For each subject, G tolerance and performance were measured under various G profiles while wearing the EMS suit and were measured again on a separate day while wearing standard anti-G suit equipment. The two different conditions were conducted with at least 22 h between tests in a balanced order. Before donning the EMS suit, the electrodes were carefully covered with Thera-Creamw or later during the data runs Signa Gelw and the cream was meticulously rubbed into the electrodes. The subject also applied the cream to the skin surface later covered by the electrodes. After that procedure, the subjects donned the EMS suit and shoes. In the control experiment the USAF basic G-protection garments with the CSU-13B/P Anti-G Suit were donned. In the centrifuge, the F-16 seat configuration with 30° seat back angle was used for the G-tolerance assessments. Before starting the experiment, skin resistance was measured with a multimeter over each of the seven muscle groups stimulated. If it was more than 10 KΩ at any muscle group, more Thera-Creamw or Signa Gelw was added to the electrode. After that, each muscle group was tested, gradually increasing voltage from the muscle stimulator for maximal muscle contraction without causing pain. On some occasions, the subject would develop a muscle cramp in the stimulated muscle, typically occurring in the calf muscles. When that happened, it was found that voluntary muscle contraction during electrical stimulation reduced the subjective discomfort and permitted a higher stimulation voltage to be used. This procedure was used for some subjects and repeated up to two times to get the optimal muscle stimulation. The skin resistance varied from 0.1 to 9.5 KΩ at all the skin regions in all the subjects tested. The voltage during the testing of maximal tolerable contraction without pain induced by the electrical muscle stimulator varied from 4 to 32 V in all subjects for all muscle groups tested. The final maximal stimulator value was saved in the stimulator for each muscle group and the same values were used during the automatically induced muscle stimulation of all muscle groups simultaneously above +4 G in the centrifuge. For the test conditions with electrical muscle stimulation, the stimulation was automatically activated at +4 Gz to the predetermined stimulation level at G levels above that, up to the maximal G level of +9 Gz. When wearing the basic USAF G-protection garment (standard), the anti-G suit was inflated according to the standard aircraft schedule, which started at +2 Gz, increased linearly with the G level, and had a maximum inflation pressure of about 525 mmHg at +9 Gz.
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EMS SUIT VERSUS A G-SUIT—BALLDIN & GIBBONS G-Profiles Used in the Centrifuge For all test conditions, the following G profiles were used to estimate relaxed gradual and rapid onset tolerances. Relaxed gradual onset (0.1 G · s21) run (GOR): The subjects were asked to be totally relaxed during both the standard and EMS runs with no straining of the muscles of the lower body or doing any respiratory straining Valsalva maneuvers. When the subject reported 100% loss of peripheral vision and/or 50% loss of central vision as determined by peripheral green lights at a 60° angle from centerline and a central red light, he released the handheld brake to stop the centrifuge and started executing the muscular and respiratory straining maneuvers until he regained vision and full mental alertness. Relaxed rapid onset (6 G · s21) runs (RORrel): After a 5-min rest period, a series of relaxed rapid onset (6 G · s21) runs (ROR) were accomplished, starting at +4 Gz and increasing by +1 Gz per run to a maximum of +9 Gz. Each G exposure lasted 15 s or until vision endpoint criteria as described above were reached. If the endpoint criteria were reached, a 0.5 G lower G level was tested and, if successful, was recorded as the maximal G level. Subjects had a 2-min rest period after each exposure. Straining rapid onset (6 G · s21) runs (RORstr): After completing the relaxed rapid onset runs, the subject continued the G exposures at one G level above the earlier maximal successful level, but with the execution of necessary straining maneuvers of the lower body muscles (not including abdominal and upper body muscles and without respiratory straining Valsalva maneuvers). This process was continued in steps increased by 1 G until a maximum of +9 Gz or vision endpoint criteria were reached. If endpoint criteria were reached, a 0.5 G lower G level was tested and, if successful, recorded as the maximal G level. SACM: After a 10-min rest period the research subject performed a simulated aerial combat maneuver G profile consisting of 10-s periods varying between +5 Gz and +9 Gz. He was then allowed to do both lower and upper body muscle straining maneuvers as needed, including the full anti-G straining maneuvers with respiratory Valsalva maneuvers, during both the standard and EMS conditions. Simultaneously he also performed an F-16 tracking task by moving a control stick to chase a target displayed on the video screen in front of the subject. This G exposure continued for a maximum of four peaks at +9 Gz or until light loss, exhaustion, fatigue, or non-intentional G-induced loss of consciousness (G-LOC). The subjects also reported their estimates of light loss and subjective levels of effort and discomfort after each ROR exposure and after the SACM by using designated effort and discomfort level scales with units from 0 to maximal 11 (0 nothing at all and 11 maximal), as modified from the Borg scale (4).
clear that the EMS was not providing added G protection and might even be having a negative impact on G tolerance. Therefore, we decided it was ethically appropriate to halt data collection and avoid further subject exposures until interim statistical analyses could be performed to determine the value of continuing with the study. For the interim analyses of the nine subjects, descriptive statistics were computed for each outcome measure and Student’s paired t-tests were performed to compare the effects of the electrical muscle stimulation suit with the effects of the standard G-suit. RESULTS The mean maximal G levels reached during the relaxed GOR were 7.1 and 6.0 G for the standard G-suit (standard) and EMS suit, a statistical difference (P 5 0.031) favoring the standard suit (see Table I). During the RORrel runs the maximal G levels were 6.0 and 4.5 G (P 5 0.002), and during the RORstr runs they were 8.3 and 6.3 G (P , 0.001) for the standard and EMS suits, respectively. Both differences were statistically significant, favoring the standard suit. With the straining SACM, the duration averaged 80 s (maximum was limited to 87 s) for the standard, and 34 s for the EMS; a statistical significance (P , 0.001) in favor of the standard suit (see Table II). During SACMs, at a common duration for both conditions, the average heart rate was 126 bpm with the standard suit and 149 bpm with the EMS suit, a statistically significance difference (P 5 0.018). During relaxed GOR, RORrel, and RORstr, average heart rates were somewhat higher for the EMS suit, but the differences only approached statistical significance for the first and third conditions (P 5 0.083, P 5 0.346, and P 5 0.077, respectively, see Table III). The subjective effort level appeared to be lower with the standard suit compared to the EMS suit (average 6.2 units versus 7.7), but
TABLE I. THE MAXIMAL G-LEVELS REACHED DURING RELAXED, GRADUAL ONSET RUNS (GOR-rel) AND RELAXED AND STRAINING, RAPID ONSET RUNS (ROR) WITH THE STANDARD G-SUIT AND ELECTRICAL MUSCLE STIMULATION (EMS) SUIT.
Subject
GOR (Relaxed) (G)
ROR (Relaxed) (G)
ROR (Straining) (G)
std
EMS
std
EMS
std
6.7 8.5 5.8 5.7 5.1 5.1 5.2 4.6 6.9 6.0 1.2
6.0 7.0 7.5 5.0 5.5 6.5 5.5 5.0 6.0 6.0 0.9
4.5 5.0 4.5 5.0 4.5 4.0 3.5 3.5 6.0 4.5 0.8
8.0 7.0 9.0 8.5 9.0 7.0* 8.0 6.0 7.5 5.5 9.0 5.5 6.5 4.5 9.0 6.5† 9.0 6.5 8.3 6.3 0.9 1.1 ,0.001
Data Analyses
1 2 3 4 5 6 7 8 9 Mean SD P
Originally, the protocol called for 15 subjects. However, by the completion of the ninth subject it was becoming
* G-LOC at 8 G. † Almost loss of consciousness at 7 G. P is the P-value for the paired t-test.
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6.4 8.9 8.7 5.3 5.3 7.2 8.4 5.6 8.2 7.1 1.5
EMS
0.031
0.002
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EMS SUIT VERSUS A G-SUIT—BALLDIN & GIBBONS TABLE II. THE MAXIMAL SIMULATED AERIAL COMBAT MANEUVER (SACM) G-PROFILE G-EXPOSURE DURATION WITH MUSCLE STRAINING DURING THE CENTRIFUGE RUNS WITH THE STANDARD G-SUIT AND ELECTRICAL MUSCLE STIMULATION (EMS) SUIT. SACM (Straining) Duration (s)
Heart Rate (bpm)
Subject
std
EMS
std
EMS
1 2 3 4 5 6 7 8 9 Mean SD P
68 87 87 87 87 87 40 87 87 80 16
60 43* 24 23 43* 23 23 23* 46 34 14
149 113 146 138 135 75 150 105 125 126 25
170 142* 168 142 135* 148 154 145* 137 149 13 0.018
,0.001
DISCUSSION
* These runs ended with G-LOC (G-induced loss of consciousness). P is the P-value for the paired t-test.
only approached statistical significance (P 5 0.069). The discomfort level at a common G level during relaxed ROR was significantly lower with the standard suit compared to the EMS suit (0.6 units vs. 1.9 units, P 5 0.007). The only statistically significant difference in light loss during the centrifuge runs was during relaxed ROR, where the peripheral light loss was less with the standard suit compared to the EMS suit (19% 6 20 compared to 66% 6 26, P 5 0.004). While not statistically significant, all other average values of peripheral and central light loss during straining ROR and SACM tended to be lower with the standard suit compared to the EMS. For the tracking task data collected during the SACM (root mean
TABLE III. HEART RATES (HR) REACHED DURING THE COMMON MAXIMAL G-LEVEL WITH RELAXED, GRADUAL ONSET RUNS (GOR) AND RELAXED AND STRAINING RAPID ONSET RUNS (ROR) WITH THE STANDARD (std) G-SUIT AND ELECTRICAL MUSCLE STIMULATION (EMS) SUIT. GOR (relaxed) (bpm) Subject 1 2 3 4 5 6 7 8 9 Mean SD P
ROR (Relaxed) (bpm)
ROR (Straining) (bpm)
std
EMS
std
EMS
std
EMS
98 131 131 100 77 78 110 100 115 104 20
108 132 128 115 100 109 112 100 100 113 11
80 99 133 97 78 68 103 92 85 93 16
87 97 98 118 112 90 100 98 91 99 10
121 118 147 116 97 100 124 115 129 118 14
130 128 135* 125 125 135† 128† 125 119 128 5
0.083
0.346
square and time on target), no statistical differences were detected between the standard suit and the EMS. Spontaneous comments from the subjects after both the standard and EMS suit suggested that there was less support for G with the EMS suit. Some subjects felt they could not fine-tune the muscle contraction with EMS, and even with solid EMS muscle contractions EMS did not give the support needed at higher G levels. Other comments were that: it was not possible to increase the muscle contraction when they were already contracted by the EMS suit, the standard G-suit offered better contraction than the EMS suit, and the squeezing of the legs felt more effective with the standard suit. Two subjects also complained about muscle soreness of the electrically stimulated muscles 1 to 2 d after the muscle stimulation experiments.
0.077
* G-induced loss of consciousness (G-LOC). † The HR with the EMS suit was at a 1-G lower level than with the standard G-suit. P is the P-value for the paired t-test.
The results with only nine subjects (out of the planned 15) clearly showed that G protection given by the EMS suit during increased G with relaxed GOR and relaxed and straining ROR exposures and for the duration of the SACM was inferior to that given by the standard G-suit. The three G-LOCs with the SACM and one G-LOC and one almost loss of conciousness during the testing of the straining ROR G levels also supported that. The lower heart rates during SACM and relaxed GOR with the standard suit may also be indicators of less protection with the EMS suit. During the relaxed GOR and ROR runs, even with solid electrical contractions of the lower body muscles, the EMS apparently did not maintain sufficient circulatory vasoconstriction to protect against the G-induced blood pressure decrease in the head region. The standard G-suit has pneumatic bladders that compress the larger muscles of the lower extremities, increasing arterial compression and decreasing blood pooling in the superficial veins. Taut lacing of the lower extremity portions compresses the deep veins to a lesser extent. The muscle tensing portion of the AGSM provides some arterial compression, up to 300 mmHg, and decreases blood pooling in the deep veins. The combination of the standard G-suit and the AGSM increases arterial resistance and reduces blood pooling in the superficial and deep veins. Even during lower body muscle straining ROR, the EMS suit was not giving the same protection as the standard G-suit itself. This means that the muscular contraction of the lower extremities induced by EMS was not as efficient as the combination of compression by the standard G-suit plus voluntary muscle contraction. Furthermore, when the lower body muscle contractions were used in combination with full Valsalva respiratory straining maneuvers in the SACM runs both with the EMS and standard suits, the duration at increased G was less with the EMS suit and, furthermore, three G-LOCs occurred. This means that even with all the help of full muscle and respiratory straining maneuvers the EMS suit did not give the same G protection as the standard G-suit.
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EMS SUIT VERSUS A G-SUIT—BALLDIN & GIBBONS Therefore, the hypothesis that electrical stimulation of lower body muscles would increase the arterial blood pressure seen at 1 G to the extent that it may improve G tolerance during high G exposures similar to the effects of an anti-G suit could not be verified. It was not anticipated that the EMS suit would be inferior to the standard G-suit. The electrical muscle stimulation at 1 G showed that a similar blood pressure response occurred with both the EMS and the standard G-suit and lower body voluntary muscular contraction (2). Also, the studies by Davies (5), Hultman et al. (11), and Sampson et al. (15) showing EMS causing a dose-dependent increase in blood pressure for possible use in hypotension support the idea of EMS increasing blood pressure at 1 G. Furthermore, a single case study indicated that electrical stimulation of abdominal muscles could be used to elevate blood pressure in a tetraplegic patient (16). However, at higher G, in contrast to at +1 G, the EMSinduced blood pressure response apparently was not sufficient enough to secure the systemic, as well as the cerebral, blood pressure to maintain the ability to withstand the increased G levels and avoid G-LOCs. The standard pneumatic G-suit compresses the legs and abdomen, with a linear increase in G-suit pressure from 2 G to a maximum of about 525 mmHg at +9 Gz. This forces the venous blood upwards to the thorax, increasing cardiac preload, and increasing the aortic valve blood pressure by increasing the peripheral resistance and reducing venous capacitance. The abdominal compression by the abdominal air bladder of the standard G-suit also elevates the heart and, thereby, decreases the hydrostatic distance between the heart and head region, which will further improve cerebral blood pressure. The EMS suit was activated at and above +4 Gz and, although it contracts the calf and thigh muscles, may not sufficiently compress the lower extremity blood vessels, particularly the superficial veins. This: 1) may create less peripheral resistance in the lower body arteries than that of the standard G-suit with more circumferential compression of the legs, in turn causing less arterial vasoconstriction, leading to reduced systemic and cerebral blood pressure protection; and 2) may not reduce venous capacitance enough to prevent blood pooling in the lower extremities. The higher G-onset level for activation of the EMS was chosen to avoid EMS discomfort effects at lower G levels between 2-4 G, where the pneumatic standard G-suit administers only very low pressure (at +3 G the standard G-suit pressure is just above 50 mmHg, compared to 525 mmHg at +9 G. The abdominal EMS contracts the rectus abdominis muscles, but may not give the same external compression effect on the abdomen compared to the abdominal bladder of a standard G-suit, thereby not pushing the heart upwards and not decreasing the vertical hydrostatic distance between the heart and the brain. That may also cause less cerebral blood pressure protection by the EMS than the standard G-suit during high G. The gluteal muscles are contracted with the EMS suit and may not be contracted as much as by 1076
voluntary contractions with the standard G-suit, but with both suits, the gluteal muscles are compressed by sitting in the seat. This muscle compression effect will increase linearly with the increased G level. Therefore, the lack of voluntary compression of the gluteal muscles with the standard G-suit will likely not make enough difference compared to the EMS suit to affect G tolerance. However, the EMS suit may possibly be useful for training of new fighter pilots. Anecdotal information indicates that many pilots, when training on the Brooks centrifuge, only perform a marginally adequate AGSM. EMS together with simultaneous EMG recordings may help to demonstrate a more effective, coordinated lower body muscle tensing in the training of these G-protecting straining maneuvers. Further research may help to validate such an effect. In conclusion, the EMS suit, as designed for this study, with selected electrode placement and selected muscle stimulation frequency and intensity, showed inferior G protection compared to a standard G-suit in terms of relaxed GOR and ROR and straining ROR G exposures as well as during full straining (both lower body muscles and respiratory Valsalva maneuver) SACM G exposures. Furthermore, while using the EMS suit, a total of four G-LOCs occurred compared to no G-LOCs with the standard G-suit. ACKNOWLEDGMENT This study was funded by the U.S. Air Force Surgeon General’s Office (contract #BAA FA7014-09-C-0034). The views expressed in this paper are those of the authors and do not necessarily reflect the policy or position of the U.S. Air Force. The authors want to thank Terry Guess, Wyle, Houston, for his careful selection and technical integration of the electrical muscle stimulators for the study and with all help during the data collection. Thanks also go to Joseph Fischer, Wyle, Brooks City-Base, San Antonio, for the statistical analyses of the results of this study and to Daren Chauvin, Wyle, Brooks City-Base, San Antonio, for all help with technical issues of both muscle stimulator equipment, data collection, and for providing the tracking task data from the computer. Furthermore, thanks go to Mac Baker, Wyle, Brooks City-Base, San Antonio, for the time-consuming centrifuge training of the research subjects. Authors and affiliations: Ulf I. Balldin, M.D., Ph.D., Wyle Integrated Science and Engineering, Brooks City-Base, TX, and John A. Gibbons, D.O., M.P.H., 711th Human Performance Wing/RHD, Joint Base San Antonio, Fort Sam Houston, TX. REFERENCES 1. Bain B, Jacobs I, Buick F. Is there central fatigue during simulated air combat maneuvering? Aviat Space Environ Med 1995; 66: 1–5. 2. Balldin U, Annicelli L, Gibbons J, Kisner J. An electrical muscle stimulation suit for increasing blood pressure. Aviat Space Environ Med 2008; 79:914–8. 3. Balldin UI, Werchan PM, French J, Self B. Endurance and performance during multiple intense high +Gz exposures with efficient anti-G protection. Aviat Space Environ Med 2003; 74:303–8. 4. Borg GA. Psychophysical bases of perceived exertion. Med Sci Sports Exerc 1982; 14:377–81. 5. Davies CT, Starkie DW. The pressor response to voluntary and electrical evoked isometric contractions in man. Eur J Appl Physiol Occup Physiol 1985; 53:359–63. 6. Gentzkow GD. Electrical stimulation to heal dermal wounds. J Derm Surg Onc 1993; 19:753–8. 7. Gilles A, Sterratt D. NEURON tutorial. Accessed 31 Jan 2012 from http://www.anc.ed.ac.uk/school/neuron/.
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EMS SUIT VERSUS A G-SUIT—BALLDIN & GIBBONS 8. Glaister DH, Prior RJ. The effects of long duration acceleration. In: Ernsting J, Nicholson AN, Rainford DJ, eds. Aviation medicine, third ed. Oxford, England: Butterworth and Heineman; 1999: 128-47. 9. Green NDC. Protection against long duration acceleration. In: Ernsting J, Nicholson AN, Rainford DJ, eds. Aviation medicine, third ed. Oxford, England: Butterworth and Heineman; 1999: 148-56. 10. Heath ME, Gibbs SB. High voltage pulsed galvanic stimulation: effect of frequency of current on blood flow in the human calf muscle. Clin Sci 1992; 82:607–13. 11. Hultman E, Sjoholm H. Blood pressure and heart rate response to voluntary and nonvoluntary static exercise in man. Acta Physiol Scand 1982; 115:499–501. 12. Milner-Brown HS, Miller RG. Muscle strengthening through electric stimulation combined with low-resistance weights in patients with neuromuscular disorders. Arch Phys Med Rehabil 1988; 69:20–4.
13. Ratkevicius A, Skurvydas A, Povilonis E, Quistorff B, Lexell J. Effects of contraction duration on low-frequency fatigue in voluntary and electrical induced exercise of quadriceps muscle in humans. Eur J Appl Physiol Occup Physiol 1998; 77: 462–8. 14. Russ DW, Vanderborne K, Binder-Macleod SA. Factors in fatigue during intermittent electrical stimulation of human skeletal muscle. J Appl Physiol 2002; 93:469–78. 15. Sampson EE, Burnham RS, Andrews BJ. Functional electrical stimulation effect on orthostatic hypotension after spinal cord injury. Arch Phys Med Rehabil 2000; 81:139–43. 16. Taylor PN, Tromans AM, Harris KR, Swain ID. Electrical stimulation of abdominal muscles for control of blood pressure and augmentation of cough in a C3/5 level tetraplegic. Spinal Cord 2002; 40:34–6. 17. Wong RA. High voltage versus low voltage electrical stimulation. Force of induced muscle stimulation and perceived discomfort in healthy subjects. Phys Ther 1986; 66:1209–14.
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