Providing Visual Biofeedback Using Brightness Mode Ultrasound During a Golf Swing

Summary

Brightness mode ultrasound can be used to provide visual biofeedback of the muscles of the lateral abdominal wall during a golf swing. Post-swing visual and verbal instruction can increase the muscle activation and timing of the external and internal obliques.

Abstract

Using ultrasound biofeedback in conjunction with verbal cueing can increase muscle thickness more than verbal cueing alone and may augment traditional rehabilitation techniques in an athletic, physically active population. Brightness mode (B-mode) ultrasound can be applied using frame-by-frame analysis synchronized with video to understand muscle thickness changes during these dynamic tasks. Visual biofeedback with ultrasound has been established in static positions for the muscles of the lateral abdominal wall. However, by securing the transducer to the abdomen using an elastic belt and foam block, biofeedback can be applied during more specific tasks prevalent in lifetime sports, such as golf. To analyze muscle activity during a golf swing, muscle thickness changes can be compared. The thickness must increase throughout the task, indicating that the muscle is more active. This methodology allows clinicians to immediately replay ultrasound videos for patients as a visual tool to instruct proper activity of the muscles of interest. For example, ultrasound can be used to target the external and internal obliques, which play an important role in swinging a golf club or any other rotational sport or activity. This methodology aims to increase oblique muscle thickness during the golf swing. Additionally, the timing of muscle contraction can be targeted by instructing the patient to contract the abdominal muscles at specific time points, such as the beginning of the downswing, with the goal of improving muscle firing patterns during tasks.

Introduction

The muscles of the lateral abdominal wall include the external oblique, internal oblique, and transverse abdominis. The external obliques perform lateral flexion and contralateral trunk rotation, while the internal obliques perform ipsilateral trunk rotation. The transverse abdominis is the deepest layer of the abdominal musculature, and it functions to increase intra-abdominal pressure and increase the segmental stability of the spine¹. The proper function of these muscles is important to reduce low back pain risk and improve athletic performance as core stability allows for increased strength and power through the extremities².

During sports with an emphasis on trunk rotation, such as golf, tennis, baseball, or softball, there is a high demand for the core muscles. For example, during a golf swing, the obliques on the trail side of the body peak at 64% of the maximal voluntary isometric contraction (MVIC) when measured using surface electromyography, while the lead obliques peak at 54% MVIC³. Trunk rotation is a key contributor to the distance and accuracy of golf shots⁴. The stresses of the golf swing and the high demand for core muscle activity may contribute to low back pain, which is the most common injury in golf⁵. Additionally, in elite golfers with low back pain, the timing of external oblique activity is delayed during the golf swing compared to healthy individuals⁶. Another study using electromyography found golfers with low back pain have an earlier onset of the erector spinae than golfers without low back pain⁷, suggesting a focus on anterolateral muscles may be beneficial. Therefore, measuring the extent and timing of abdominal muscle activity during a golf swing is important to improve performance and reduce the risk of low back pain.

Rehabilitative ultrasound is commonly used to assess lateral abdominal wall muscles due to the layered nature of this musculature^8,9,10. There is no difference in transverse abdominis activation in college golfers with and without low back pain in the supine position or in a more functional golf swing setup position¹¹. However, transverse abdominis activity is only one component of a golf swing, and rotation may be more important for this population. Previous literature has used an elastic belt and foam block to secure the ultrasound transducer to the abdomen, allowing for ultrasound assessment of the core musculature during dynamic movement such as a single leg squat or gait⁸. Applying ultrasound during dynamic movements has been shown to have acceptable to excellent reliability¹². This technique can be applied to measure thickness changes in the lateral abdominal wall during a golf swing or other sport-specific task. While surface electromyography is commonly used to measure the electrical activity of muscles, this is less feasible in the abdominal region. The layered anatomy leads to cross-talk between the muscles and does not allow for a visual representation of the individual muscle layers of the core¹³. Ultrasound provides an advantage over alternatives like surface electromyography for the core musculature because it allows for a representation of each individual muscle while also giving an image for feedback¹⁴.

Since ultrasound provides an image of the muscles of interest in real time, it can also be used as a tool for visual biofeedback. Ultrasound biofeedback has improved the ability to increase the muscle thickness of the transverse abdominis and lumbar multifidus compared to verbal cueing alone^15,16. Additionally, in golfers with and without low back pain, real-time ultrasound biofeedback increases transverse abdominis thickness in supine and in the golf-setup position¹¹. Biofeedback training in supine also translates to upright loaded tasks¹⁷. More research is needed to determine the required frequency and duration of biofeedback training, as most studies are single-session or short-term training protocols¹⁵. Since ultrasound has been applied during functional tasks and there is evidence that golfers can increase deep muscle pre-activation in the setup position, research should next investigate the use of ultrasound biofeedback to increase oblique muscle thickness during the golf swing.

Therefore, this methodology aims to use ultrasound as a feedback mechanism to improve the activation and timing of the abdominal obliques during the golf swing.

Protocol

The present protocol was part of a study approved by the Institutional Review Board at the University of Central Florida. Informed consent was received from all human participants for the present study. To be included in the study, participants had to be between 18 years and 75 years of age, play golf at least once per month for the past year or once per week for the past 2 months, have played golf for at least 2 years, and have had at least two episodes of low back pain in the past 12 months. The exclusion criteria were balance disorders, current pregnancy, surgery to the low back or lower extremities in the past year, or an open wound in the abdominal area where the transducer must be placed.

1. Ultrasound setup and data collection

1. Open and turn on the ultrasound device (see Table of Materials) using the On/Off button.
2. Press the Patient button on the keyboard to add a new patient and select New Patient on the left side of the screen.
   1. Enter the desired patient ID number, ensure MSK is selected as the exam type, and click on Register. Exit to begin the exam and enter B-mode.
      NOTE: Ultrasound settings in MSK Abdominal Preset, B-mode: B color = Tint Map D, Write Zoom Height = 4, Write Zoom Width = 4, Thermal Index = Tls, ATO Level = Low, Focus Number = 2, Focus Number CrossXBeam = 2, Focus Depth = 50, Depth (cm) = 3, Compression = 1, Focus Width = 1, Focus Width CrossXBeam = 1, Line Density = 3, Line Density CrossXBeam = 3, Suppression = 0, Frame Average = 4, From Average CrossXBeam = 2, CrossXBeam 2, CrossXBeam # = Low, CrossXBeam Type = Mean, Edge Enhance = 3, B Steer = 0, Gray Scale Map = Gray Map C, Gain = 34, Dynamic Range = 69, Rejection = 0, Frequency (MHz) = 12.
3. Position the linear array transducer in the elastic belt by placing the head of the linear transducer through a horizontal slit in the middle of the belt (see Table of Materials). Next, apply one to two foam blocks to secure the transducer in its place.
   1. Apply ultrasound gel (see Table of Materials) to the linear transducer. Position the transducer on the lateral abdominal wall, approximately 10 cm lateral to the umbilicus⁸.
   2. Secure the belt to the participant using the hook and loop fasteners. Ensure the belt is tight enough so that the transducer is secured perpendicular to the lateral abdominal wall⁸.
   3. Adjust the depth and gain (brightness) of the ultrasound if necessary to obtain a clear image of the fascial borders and muscle thickness of the external oblique, internal oblique, and transverse abdominis. Ensure that the fascial border of the transverse abdominis is visible on the edge of the screen in this longitudinal view (Figure 1)⁸.
      NOTE: The depth will depend on the size of the individual being measured, but ensure that the deep fascial border of the transverse abdominis is visible and the image is clear.
   4. Once the image is clear, position the patient in a manner that mirrors the task they will complete. For example, if they perform a golf swing, have them stand in their setup position. Press Freeze, then Store to capture a static image, save it to the patient's exam, and measure in real time or access it later to measure the resting muscle thickness.
      NOTE: See Figure 1 for an example of a clear image of the lateral abdominal wall using B-mode ultrasound.
4. Select Freeze again to return to live imaging. Press Store to begin recording in B-mode video. On the bottom-right corner of the screen, check that a timer begins and is highlighted in bright green, indicating that a video is being captured. Instruct the participant to take a full golf swing once the recording has begun.
   1. Press Store again to end the video and save it to their exam.
      NOTE: Saved images and videos will appear below the active image, with up to five visible per row. Select the cursor to scroll through or select previous images.
5. Repeat step 1.4 as many times as necessary for the study. Adjust the positioning of the ultrasound probe within the foam block as needed if the image becomes blurry.
   NOTE: Typical protocols comparing biofeedback and non-biofeedback conditions use between 3 and 10 repetitions per condition^11,17.
   1. After each trial, review the video for image clarity. If the image becomes anechoic (entirely black) at any point, it indicates that the ultrasound probe moved during the swing. In such a case, exclude the trial and re-measure.
6. Following each swing, have the patient positioned where they can view the ultrasound screen. Open the most recent trial.
   1. Provide a brief overview and description of the muscles of interest (for a rotational task, this will include the external and internal obliques). Describe how the muscles should get thicker during the movement and show them their trial, providing education based on each attempt.
   2. For example, if the obliques stay relatively the same size throughout the task, instruct them to focus during the next attempt to contract their core muscles forcefully throughout the movement instead of passively rotating the trunk.
      ​NOTE: Emphasis on trunk rotation as a power source during a golf swing is a common trait of higher-skilled golfers¹⁸. During rotational movements, it is generally not feasible to provide biofeedback during the task. Therefore, providing feedback following each trial is thought to be acceptable¹⁹.
7. Repeat the biofeedback trials for as many attempts as desired. Provide the ultrasound images as well as verbal cueing or instruction following each attempt, adjusting the cueing as needed.

Figure 1: Image of the right lateral abdominal wall during quiet standing. (A) External oblique. (B) Internal oblique. (C) Transverse abdominis. Please click here to view a larger version of this figure.

2. Resting image processing

1. Open the first resting image to measure. Using the cursor, hover over the desired image from the library below the active image and click on Enter.
   1. Click on Measure once to open the measurement tool. Click on Enter when the cursor is over the superior fascial border of the muscle of interest.
      NOTE: For the present study, the external and internal obliques were measured as they are the muscles of interest, but one can also measure the transverse abdominis in this view.
   2. Click on Enter again once the cursor is over the inferior fascial border.
      NOTE: The length of the measured segment will appear on the bottom-left portion of the screen, measured in centimeters. Record this length in a spreadsheet organized to include the participant or patient name/number and resting muscle thickness.
2. Repeat steps 2.1.1-2.1.2 for all the images needed to measure for both the external and internal obliques.

3. B-Mode video processing

1. Once the desired number of videos have been captured, open the first video to be processed. Using the cursor, hover over the desired image from the library below the active image and click on the Enter button.
2. Adjust the viewing frame within the B-mode video until the first frame is reached. Determine the desired sampling rate.
   NOTE: Previous literature has used 10% intervals of the total frames from 0%-100% for analysis, but one may wish to use more or less depending on the aims²⁰.
3. Once there is the desired frame open, use the Measure tool. Click the button once to open the tool, place the cursor on the superior fascial border of the muscle of interest, and click on Enter to place the first end of the measurement.
4. Drag the measurement line to the inferior fascial border of the muscle of interest and click on Enter again to complete the measurement.
   NOTE: The distance measured will appear on the bottom-left portion of the screen.
5. Press Store to save the measured image.
6. Record the measurement in a spreadsheet organized to include participant or patient name/number, frame number, and thickness measurements. Return to the original B-mode video frame (step 3.2).
7. Scroll to the next frame to be analyzed. For example, if one is sampling 11 time points (10% intervals from 0%-100%) of a swing that captured 100 frames, one will now use frame 10 (i.e., 10/100 corresponds to frame 10 out of 100 total frames).
   1. Repeat steps 3.3-3.4 to measure the next frame.
8. Repeat step 3.7 until all the desired frames are measured.
9. Move to the next video and repeat steps 3.1-3.8 until all the videos are measured.
   ​NOTE: See Figure 2 for a compilation of the processed frames throughout phases of a golf swing.

4. Activation ratio calculation

NOTE: An activation ratio is commonly used to determine the degree of muscle thickness change^8,9,11. The formula for the activation ratio is contracted thickness (cm)/resting thickness (cm).

1. To determine the activation ratio at a specific time point, measure the thickness by following step 3. Divide this value by the resting image thickness obtained from step 2.

Figure 2: Frame-by-frame analysis of a B-mode video on the trail side (right) lateral abdominal wall of a right-handed golfer. EO = external oblique; IO = internal oblique. Please click here to view a larger version of this figure.

Representative Results

	Non-feedback			Biofeedback
Swing Duration	External Oblique Thickness (cm)	Internal Oblique Thickness (cm)	Combined Oblique Thickness (cm)	External Oblique Thickness (cm)	Internal Oblique Thickness (cm)	Combined Oblique Thickness (cm)
0.0%	0.630	0.978	1.608	0.446	1.109	1.555
12.5%	0.609	1.043	1.652	0.565	1.446	2.011
25.0%	0.870	1.533	2.403	0.598	1.370	1.968
37.5%	0.620	0.696	1.316	0.674	1.174	1.848
50.0%	0.859	0.826	1.685	0.587	1.152	1.739
62.5%	0.652	1.022	1.674	0.880	1.326	2.206
75.0%	0.837	1.022	1.859	0.761	1.511	2.272
87.5%	0.717	0.859	1.576	0.772	0.761	1.533
100.0%	0.685	0.859	1.544	0.598	1.304	1.902

Table 1: Comparison of the combined oblique thickness throughout a golf swing between the non-feedback and ultrasound biofeedback conditions.

For the desired result, the measures of oblique thickness must be larger during biofeedback trials compared to traditional, non-feedback swings. Ideally, this would occur throughout all phases of the golf swing. Multiple trials can be used, and each trial is discussed before the next attempt. Typical protocols comparing biofeedback and non-biofeedback conditions use between 3 and 10 repetitions per condition^11,17. For example, based on the trial in Table 1, the clinician's next instructions to the patient must be for them to target the obliques and increase trunk rotational power during the backswing. This participant's combined thickness of the obliques decreased at the 25% time point during the biofeedback trial, which corresponds with approximately the middle of the backswing. At most other points, the obliques were larger during the biofeedback trial, which suggests the patient understands how to use their obliques during the golf swing effectively.

In the present example, 5% of the images were used for sampling. There were 180 frames captured during the swing, and 9 were measured to represent the phases of the golf swing, as seen in Figure 2. If desired, the sampling rate could be increased to represent a larger percentage of the ultrasound video. This would simply require analysis of more images as described in the "B-Mode video processing" section of the protocol.

To calculate whether the biofeedback trials were effective, one can compare the activation ratios at different time points of the swing. An activation ratio takes the contracted thickness, or thickness during the swing, and divides it by the resting (or setup) thickness. A ratio above 1.0 indicates the muscle increased in thickness and is active. For example, the 0% swing duration in Table 1 can be used as the resting thickness, as this is the thickness of the external and internal obliques before the swing is initiated. To determine whether the combined oblique thickness increased at the 50% time point when biofeedback was provided, activation ratios can be calculated as follows:

Non-biofeedback: 1.685 cm/1.608 cm = 1.048

Biofeedback: 1.739 cm/1.555 cm = 1.118

Based on this individual sample, it can be inferred that biofeedback led to a small increase in oblique muscle activation at the 50% duration of the golf swing.

Video 1: Full B-mode ultrasound video of the right lateral abdominal wall during a golf swing of a right-handed golfer. Please click here to download this Video.

Discussion

Providing ultrasound biofeedback following a rotation-based sports movement such as a golf swing can be used to increase the muscle thickness of the lateral abdominal wall. As shown in the representative results, a single trial of ultrasound biofeedback can lead to short-term increases in oblique muscle activity throughout a golf swing.

Previous research has also used B-mode ultrasound secured with an elastic belt during dynamic tasks²⁰. This was measured while individuals walked on a treadmill. Similar to the present methods, the authors reduced the B-mode video to a smaller percentage of still images for measurement to represent the walking gait cycle. This makes data processing and analysis more feasible, as they are sampled at 10% intervals throughout a gait cycle. The current protocol builds upon this methodology by applying B-mode ultrasound as a biofeedback tool during a complex, rotation-based movement. However, since gait is cyclical, direct comparison to this protocol must be cautioned. A golf swing can be broken down into smaller portions, such as the address, backswing, downswing, and follow-through²¹. However, it can be challenging to synchronize B-mode ultrasound video to the specific phases of the golf swing because the backswing of elite players averages under 1 s, and the downswing averages less than 0.25 s²¹. To directly compare swing phases, the clinician can view the changes in muscle activity by synchronizing a swing video and the B-mode ultrasound video. The swing and ultrasound images can then be compared frame-by-frame to determine the muscle activity at various points in the swing. Synchronizing the B-mode video to a stopwatch with lapped times at the start and end of the swing can also provide a rough estimate of swing phases. As the backswing begins, one can expect the trail side (right side in a right-handed golfer) internal oblique to increase in thickness as they rotate the trunk toward the right side during the backswing. Similarly, the lead side (left in right-handed golfers) external oblique should increase in thickness during the backswing, as there is contralateral trunk rotation during this phase of the swing²¹. See Figure 2 for a frame-by-frame comparison of oblique activity throughout the phases of the golf swing.

A critical step is to ensure consistent skin contact with the transducer to obtain accurate, clear images during rotational movements. This may require troubleshooting or modifications in the foam blocks used. For example, larger or smaller individuals may need different amounts of padding, so it is advised to confirm image clarity prior to data collection. Instruct the individual to perform a few practice swings, which allows them to familiarize themselves with the positioning while the clinician observes the ultrasound screen to ensure image clarity. Since the transducer will be secured with an elastic belt wrapped tightly around the abdomen, this familiarization period is important. It allows the individual to become comfortable with the equipment and swing as normally as possible. The belt may alter their mechanics minimally, but it will allow for a consistently clear image.

Additional limitations of this methodology include the potential for unclear images. While the risk for unclear images can be mitigated with proper positioning and familiarization periods, some frames throughout the B-mode video may be unclear. When choosing the sampling rate or determining what percentage of frames will be measured, some adjustments may need to be made to ensure clear images are analyzed. For example, if every 10th frame is measured, starting with frame 1, the investigator and/or clinician might have to choose a frame slightly out of sequence if the desired image is out of focus-such as using the 12th frame rather than the 11th. Maintaining consistent contact between the skin and transducer is essential to reduce the risk of poor image quality. Video 1 shows an example of a full B-mode video. When paused at different time points, the viewer can see how, even when proper procedures are followed, there are occasional blurry frames. Despite these challenges, dynamic ultrasound assessments are reliable measures¹².

In addition to the current methodology for image processing performed on the ultrasound device, other processing software can measure thickness changes¹². For example, saving the videos to an external drive and opening them on a computer with freely available measurement software allows later processing without needing the ultrasound device. A similar procedure is used with this type of software, where screenshots of the desired frames are taken from the video and then analyzed using a measurement tool. If using this method, ensure to set the scale of images, using the depth measurements on the right side of the ultrasound image as a reference line.

Training participants with ultrasound biofeedback requires a sound understanding of anatomy, familiarization with the ultrasound device, and the ability to explain the image or video to a participant in real time^22,23. This method of ultrasound biofeedback is considered augmented feedback with knowledge of performance, where the patient is given additional information (video of their muscle activity) that shows what they did while completing the movement. In injured populations, augmented feedback can provide missing information due to changes in sensory pathways¹⁹. Augmented feedback can enhance learning and decrease the amount of practice needed to learn a complex task¹⁹. The timing of biofeedback has been debated, and one limitation of this methodology is that concurrent feedback seems to be most effective¹⁹. However, providing feedback during a swing is not feasible due to the nature of ultrasound imaging and the golf swing. Therefore, providing post-swing feedback is a reasonable alternative. Lastly, the ideal frequency of biofeedback training has not been established²⁴, and future studies should compare long-term feedback training to determine the most effective strategy.

While the current protocol focuses on the golf swing, a similar methodology could be followed for other sports focusing on trunk rotation. These include tennis, baseball, and softball, among others. Since these also involve a rotational component, the lateral abdominal wall can be targeted similarly, and ultrasound can be used for biofeedback. This protocol focuses on the golf swing due to the high prevalence of low back pain in golfers, commonly attributed to the rapid rotation of the trunk combined with lateral shear and compressive forces⁴. Future research should investigate whether training with ultrasound biofeedback can improve muscle activity patterns and reduce the risk of low back pain in golf and other rotation-based sports.

Materials

Name	Company	Catalog Number	Comments
Aquasonic 100	Parker	BT-025-0037L	Ultrasound gel
GE NextGen Logig e Ultrasound Unit	GE Healthcare	HR48382AR
Linear Array Probe	GE Healthcare	H48062AB
Velcro straps	VELCRO		Fasteners for the elastic belt used to secure the ultrasound transducer