This protocol uses motion mode ultrasound and surface electromyography simultaneously to measure muscle function of the core. Muscle thickness and activation of the local stabilizers (e.g., transverse abdominis, internal oblique) and global movers (e.g., external oblique) is achievable during specific time points of the side plank and dead bug exercises.
Motion mode (M-mode) ultrasound allows researchers and clinicians to measure the change of muscle thickness across time. Muscle thickness can be measured between fascial borders at a given time point during an exercise. This selected time point produces a one-dimensional image resulting in real-time, live observation of anatomy. Ultrasound used during functional movement can be referred to as dynamic ultrasound; this is feasible and reliable with the use of a linear transducer, elastic belt, and foam block to secure consistent transducer placement. The lateral abdominal wall is commonly investigated using ultrasound due to the overlapping nature of the muscles. Surface electromyography (sEMG) can complement M-mode ultrasound imaging because it measures the electrical representation of muscle activation. There is minimal evidence using M-mode ultrasound and sEMG simultaneously during core exercise. Exercises that challenge the core musculature involve both isometric holds (e.g., side plank), as well as oscillatory extremity movements (e.g., dead bug). In this study, both instruments will be used simultaneously to measure core muscle function during exercise. Ultrasound measurements will be obtained using a linear transducer and ultrasound unit, and sEMG measurements will be acquired from a wireless sEMG system. To make comparisons between participants and exercises, normalization methods using static, exercise starting positions for both instruments will be used. An activation ratio will be used for ultrasound and calculated by dividing the contracted thickness (thickness during a time point of exercise) by the rested (starting position) thickness. Muscle thickness will be measured in centimeters from the superior inferior fascial border to inferior superior fascial border. These methods aim to offer an innovative and practical measurement of muscle function with M-mode ultrasound and sEMG during core endurance exercises.
The lateral abdominal wall is made up of the transverse abdominis, internal oblique, and external oblique1. The lateral abdominal wall contracts concentrically, eccentrically, and isometrically to withstand the forces placed on the body1. The co-contraction of this muscle group provides stabilization of the center of the human body2,3. These muscles are important during the prevention and rehabilitation of lower extremity injuries because poor trunk function is associated with increased hip adduction and knee valgus, which are risk factors for lower extremity injuries4,5. Focusing on strengthening and increasing muscular endurance of the core musculature not only decreases risk factors for the lower extremity but can also decrease low back pain6. Recently, it has been recommended that individuals who are suffering from acute and chronic low back pain should include trunk strengthening, endurance, and specific trunk muscle activation in their rehabilitation6. An example of specific trunk muscle activation is targeting isolated or grouped trunk muscles using co-contraction to restore control or increase coordination of the lumbopelvic-hip region6.
Two ways to objectively measure muscle function are the use of motion mode (M-mode) ultrasound and surface electromyography (sEMG). M-mode ultrasound provides a real-time visualization of muscle and fascia movement during a recorded time which can display onset and extent of the motion7. The distance between the superior inferior fascial border and inferior superior fascial border are measured at a selected time to obtain muscle thickness. Muscle thickness during a specific time point of an exercise can be divided by resting thickness to achieve an activation ratio8. sEMG provides insight into muscle activation and fatigue, as the output can be compared to the muscle's maximum contraction9. These two instruments and methods have been used previously to measure the onset of hip muscle activation during a variety of exercises in healthy and injured individuals10. Exercises that target the trunk, and specifically the lateral abdominal wall, are the side plank and dead bug11,12,13. The side plank is performed in a side lying position with the elbow directly underneath the shoulder and forearm on the ground, the hips are raised off the ground until the spine is in a neutral position. The knees are extended, and the feet are placed on top of one another9 (Supplemental Figure 1). The dead bug is performed in a supine position with both arms straight above and the hips and knees flexed in a 90° angle. The exercise begins when one arm is flexed above the head and the contralateral leg extends. The opposite arm and leg remain in a neutral position and then flex and extend once the original moving arm and leg return to the neutral position13 (Supplemental Figure 2 and Supplemental Figure 3).
The activation of the external oblique has been seen to range from 37% to 62% of maximum voluntary isometric contraction (MVIC) during the side plank11,12,14. During the dead bug, the activation of the external oblique has been recorded between 20% to 30% of MVIC for only five repetitions of the exercise15. The internal oblique and transverse abdominis, the deeper abdominal muscles of the lateral abdominal wall, activate between 22% to 28% of MVIC during the side plank12,14. Due to the overlapping nature of the internal oblique and transverse abdominis, the two muscles have been combined during sEMG collection14. A limitation of sEMG is the crosstalk from adjacent muscles, where the sEMG sensor may be producing an output of a different muscle, resulting in a false understanding of activation16. Muscle thickness measurements obtained with ultrasound can be used to mitigate this limitation, and this measurement is feasible during trunk exercises, such as the isometric holds previously mentioned17.
Muscle thickness of the lateral abdominal wall has been recorded during the side plank as an absolute magnitude of difference between contracted thickness and rested thickness. At the 30 s time point of a side plank, the muscle thickness of the internal oblique and external oblique increased by 0.526 mm and 0.205 mm, respectively17. These measurements were recorded in brightness mode ultrasound at one time point during the side plank. B-mode ultrasound is commonly performed to assess before and after images; however, this method only allows for measurement at two time points18. M-mode ultrasound provides increased advantages compared to B-mode ultrasound, as it can detect onset of muscle activation as well as muscle thickness during the entirety of the exercise with any time point able to be selected for measurement18. Therefore, the overall goal of the current protocol is to provide an innovative and practical measurement of muscle function with M-mode ultrasound and sEMG during core endurance exercises. This is beneficial to researchers and clinicians to understand how muscles function throughout the duration of an exercise, especially of an endurance nature, as opposed to a measurement isolated to a single time point.
All human participants provided informed consent. The protocol was part of a study approved by the Institutional Review Board of the University of Central Florida. Inclusion criteria included ages 18 -45 and physically active according to ACSM guidelines (30 min of moderate to vigorous activity 5 days per week)19. Exclusion criteria included low back pain within the past year, current hip, upper, or lower extremity pain or injury, a year history of low back surgery or lower extremity surgery, self-reported balance disorder, muscular abnormalities, currently pregnant, or having an open wound in the abdominal area (Table 1).
1. Data collection instrument preparation
2. Surface electromyography preparation (see Table of Materials)
Figure 1: Examination location of the lateral abdominal wall. The sEMG sensor is placed 3 cm anterior from the midway point between the lower rib and the iliac crest, parallel to the muscle fibers20. The transducer is located 10 cm lateral to the umbilicus until the lateral abdominal wall is visible on the screen. Please click here to view a larger version of this figure.
3. Ultrasound preparation (see Table of Materials)
Figure 2: Transducer placed through the elastic belt and foam block. Please click here to view a larger version of this figure.
Figure 3: Example of resting image to verify the lateral abdominal wall. Please click here to view a larger version of this figure.
Figure 4: Transducer secured on the lateral abdominal wall by the elastic belt and foam block. Please click here to view a larger version of this figure.
4. Ultrasound static side plank
5. sEMG static side plank
NOTE: Simultaneously, the researcher will also obtain the sEMG output during the static positioning described in step 4.1.
6. Side plank
7. Ultrasound static dead bug
8. sEMG static dead bug
9. Dead bug
10. Ultrasound static measurement
Figure 5: Example of lateral abdominal wall during the side plank static, exercise starting position and measurements of muscles. A = external oblique (0.554 cm), B = internal oblique (0.761 cm), and C = transverse abdominis (0.326 cm). Please click here to view a larger version of this figure.
Figure 6: Example of lateral abdominal wall in the dead bug static, exercise starting position and measurements of muscles. A = external oblique (0.618 cm), B = internal oblique (0.820 cm), and C = transverse abdominis (0.438 cm). Please click here to view a larger version of this figure.
11. Ultrasound dynamic measurements
Figure 7: Example of lateral abdominal wall during the side plank exercise and measurements of muscles in M-mode. A = external oblique (0.968 cm), B = internal oblique (0.937 cm), and C = transverse abdominis (0.714 cm). Please click here to view a larger version of this figure.
Figure 8: Example of lateral abdominal wall during the dead bug exercise and measurements of muscles in M-mode. A = external oblique (0.840 cm), B = internal oblique (0.840 cm), and C = transverse abdominis (0.720 cm). Please click here to view a larger version of this figure.
12. sEMG Measurement
The measurements of both ultrasound and sEMG during the static, exercise starting position are represented in Table 2. These numbers will be used as the denominator when calculating the activation ratio. The thickness values of external oblique, internal oblique, and transverse abdominis during the first 5 s, last 5 s, and total duration (60 s) are in Table 3. These numbers are divided by the numbers in Table 2. The sEMG values normalized to static, exercise starting position during the first 5 s, last 5 s, and peak activity are presented in Table 4.
The activation ratio describes the magnitude of increase in the muscle thickness as a result of the exercise compared to the static, exercise starting position. For example, if the external oblique during the side plank had an activation ratio of 1.73, that means the muscle thickness increased by 73% during the exercise. The activation ratios for the external oblique, internal oblique, and transverse abdominis are summarized in Table 5. Use of the activation ratio allows researchers and clinicians to determine the extent of change in muscle thickness in a variety of exercises and positions26. Collection of ultrasound images with M-mode also allows for synchronization of timing to determine the onset of activation and corresponding thickness at the time of onset7.
Demographics | |
Age | 22.75 ± 4.94 years |
Altezza | 169.25 ± 6.88 cm |
Mass | 67.32 ± 4.94 kg |
Table 1: Patient demographics.
Exercise | First 5 s | Last 5 s | Peak Activity |
TESP | 0.01499725 mV | 0.019264 mV | 0.021207 mV |
Dead bug | 0.02534 mV | 0.021346 mV | 0.02534 mV |
Table 2: sEMG peak activity during first 5 s, last 5 s, and overall peak. EO = external oblique, IO = internal oblique, mV = millivolts, TrA = transverse abdominis.
Exercise | Static, Exercise Starting Thickness | Static, Exercise Starting sEMG | ||
EO | IO | TrA | EO | |
TESP | 0.554 cm | 0.761 cm | 0.326 cm | 0.0059 mV |
Dead bug | 0.618 cm | 0.82 cm | 0.438 cm | 0.0029 mV |
Table 3: Thickness and peak sEMG activity during the static side plank and dead bug exercise starting positions. cm = centimeters, EO = external oblique, IO = internal oblique, TrA = transverse abdominis.
Exercise | First 5 s | Last 5 s | Peak Thickness | ||||||
EO | IO | TrA | EO | IO | TrA | EO | IO | TrA | |
TESP | 0.96 cm | 1 cm | 0.73 cm | 0.91 cm | 0.93 cm | 0.58 cm | 0.98 cm | 1 cm | 0.73 cm |
Dead bug | 0.61 cm | 0.82 cm | 0.43 cm | 0.56 cm | 0.79 cm | 0.38 cm | 0.62 cm | 0.88 cm | 0.5 cm |
Table 4: Muscle thickness during first 5 s, last 5 s, and overall thickest point during the side plank and dead bug exercises. cm = centimeters.
First 5 s | Last 5 s | Peak Thickness | |||||||
Activation Ratio | EO | IO | TrA | EO | IO | TrA | EO | IO | TrA |
Side Plank | 1.73 | 1.31 | 2.24 | 1.64 | 1.22 | 1.78 | 1.77 | 1.31 | 2.24 |
Dead bug | 0.99 | 1.00 | 0.98 | 0.91 | 0.96 | 0.87 | 1.00 | 1.07 | 1.14 |
Table 5: Ultrasound activation ratios during the side plank and dead bug exercises. EO = external oblique, IO = internal oblique, TrA = transverse abdominis.
Supplemental Figure 1: TESP exercise positioning. TESP = torso elevated side support. Please click here to download this File.
Supplemental Figure 2: Dead bug starting position. Please click here to download this File.
Supplemental Figure 3: Dead bug repetition. TESP = torso elevated side support. Please click here to download this File.
M-mode ultrasound provides onset of muscle tissue movement and muscle thickness change during real time observation of anatomy over a selected time21. M-mode ultrasound combined with sEMG provides an overall understanding of muscle function, including electrical representation and visual observation. These instruments can be used in tandem during exercise to provide researchers with a global understanding of muscle function.
Specific training of ultrasound and sEMG techniques is necessary to produce reliable and valid measurements. Normalization methods used with M-mode ultrasound and sEMG need to be similar in order to compare instruments (i.e., static, exercise starting position)26.
A modification may be needed for the foam block used to secure the transducer based on the participant's body size. The tightness of the elastic belt will also need to be modified depending on the participant's body size. The transducer may slightly move from the original positioning during movement before or after exercise. It is important to continue to monitor the image of the lateral abdominal wall on the ultrasound screen throughout the data collection process. Ample amount of ultrasound gel is needed to ensure clear imaging, but too much can interfere with the sEMG sensor adhesive. Modification of the optimal amount of ultrasound gel used is important throughout data collection.
A limitation of both instruments is that they only represent the anatomy directly beneath the sEMG sensor and ultrasound transducer due to the limited broadcast area for each. Assumptions are commonly made that the anatomy directly beneath the sensor still provides an adequate representation of the muscle27,28.
M-mode ultrasound provides an efficient way to detect onset of muscle activation and muscle thickness change throughout the duration of an exercise21. This one-dimensional live image of anatomy over time is helpful to understand change of muscle thickness during a task. M-mode ultrasound can be further complemented with an additional instrument such as sEMG to provide a total understanding of muscle function. Brightness mode should still be used to measure muscle thickness; however, during dynamic exercise, M-mode ultrasound may be alternatively beneficial. The use of M-mode ultrasound can be used during prevention and rehabilitation exercises in healthy and pathological individuals18. Trunk strengthening, increasing endurance, and specific trunk muscle activation are recommended for individuals with acute and chronic low back pain. M-mode ultrasound can be used during exercises that target the aforementioned recommendations to observe onset of activation, muscle thickness at certain exercise time points, and change in thickness.
The correlation between the two instruments was not examined in the current protocol. However, previous studies have noted that comparisons between the two instruments should be used with caution. Ultrasound has been shown to detect onset of muscle activation before electromyography, supporting the idea that these two instruments measure different aspects of muscle function29.
These methods are appropriate for application if the goal is to measure the onset of muscle activation as well as muscular thickness during exercise. As M-mode ultrasound provides a visual representation of the involved muscle, sEMG will compliment this assessment with the electrical representation of the muscle. The % MVIC of an individual during exercise may not change over time or post intervention; in such a scenario, M-mode ultrasound can supplement sEMG to assess the thickness change7,10. Although ultrasound images have been used in both modes to describe muscle movement for many years, this protocol details a more recent dynamic application that can have a direct impact on the use of ultrasound not only in research and highly controlled environments, but importantly in clinical practice with active individuals and in sport.
The authors have nothing to disclose.
None.
Alcohol prep pads | Henry Schein | HS1007 | |
Amazon Basics 1/2- Inch Extra Thick Exercise Yoga Mat | Amazon | YM2001BK | |
Delsys Trigno Sensor Adhesive Interface, 4-Slot | Delsys | SC:F03 | |
Delsys Trigno Wireless System | Delsys | T03-A16014 | |
Galaxy Tablet S5e | Samsung | SM-TS20N | |
GE NextGen Logig e Ultrasound Unit | GE Healthcare | HR48382AR | |
Linear Array Probe | GE Healthcare | H48062AB | |
Trigno Avanti sensors | Delsys | T03-A16014 |