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.
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.
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 musculature8,9,10. 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 alone15,16. 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.
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
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
3. B-Mode video processing
4. Activation ratio calculation
NOTE: An activation ratio is commonly used to determine the degree of muscle thickness change8,9,11. The formula for the activation ratio is contracted thickness (cm)/resting thickness (cm).
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.
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 condition11,17. 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.
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 time22,23. 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.
The authors have nothing to disclose.
None.
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 |