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A Diffusion Tensor Imaging approach to investigate the effects of exercise on quadricep muscle fiber lengths

Reese A. Dunne^1,2, Garry E. Gold², and Valentina Mazzoli²
¹Department of Mechanical Engineering, Mississippi State University, Mississippi State, MS, United States, ²Department of Radiology, Stanford University, Stanford, CA, United States

Synopsis

There is currently no established method to monitor muscle architecture throughout exercise intervention. This work investigates the value of DTI and fiber tractography to detect changes in fiber length in the quadriceps caused by resistance training. Five human subjects participated in a three-month training regimen, and DTI scans of their quadriceps were acquired. Through DTI tractography, fiber lengths were measured and compared between quadricep muscle components. There were no statistically significant changes in fiber length after three months of exercise, showing further optimization is needed to utilize DTI tractography for monitoring fiber length changes due to exercise.

Introduction

Muscles are a highly plastic tissue that can adapt due to exercise intervention and resistance training. Athletes, particularly those engaging in high-caliber training, are prone to muscle injuries that could have physical, mental, and financial consequences³. To minimize the risk of muscular injuries, preventative strategies (e.g., targeted exercises) are used with the primary focus of increasing muscle fiber length, as longer muscle fibers are less prone to tearing or long-term damage^2,5,13. In order to improve muscle adaptation in training regimens and monitor the effects of preventative strategies and rehabilitation over time, it is crucial to not only measure global changes in volume, but also changes in muscle architecture. While ultrasound imaging has investigated changes in superficial muscles throughout preventative strategies, the modality is limited to two-dimensional measurements and a narrow field-of-view that prevents analysis of deeper muscles^6-7. A three-dimensional method is needed to easily track the effectiveness of preventative strategies. Diffusion Tensor Imaging (DTI), combined with deterministic fiber tractography, was shown to detect changes in muscle fiber length in the lower leg due to passive stretching⁸. This study aims to investigate the value of DTI and fiber tractography to detect changes in quadricep fiber length caused by resistance training.

Methods

Five healthy participants performed 3x15 repetitions of single-leg decline squats twice daily, four to five days a week for three months. The DTI acquisition was performed in two stacks with the following parameters: single-shot EPI readout, b=0 s/mm² (3 volumes) and b=400 s/mm² (15 volumes), matrix size=256x256x26, voxel size=1.56x1.56x6 mm³, TR=3000ms, TE=43ms, 8min15s. Post-processing of DTI data included denoising, registration to anatomical scan, and tensor fitting. Dixon water datasets were also acquired from each subject for anatomical reference, manual muscle segmentation at baseline, and subsequent transfer to the other time points using imaging registration. This process was repeated for five time points, as shown in Fig 1. Then, a deterministic fiber tractography algorithm used these DTI scans to reconstruct the muscle fibers in the quadricep muscles of all five subjects, as seen in Fig 2.

The Vastus Lateralis, Vastus Medialis, and Rectus Femoris muscles in the left and right leg were manually segmented for each subject and used to place seed points for initiating fiber tracking. Fiber tract density was derived from the number of tracts crossing each voxel in a whole leg tractography. Tracked fibers were terminated upon reaching a certain fiber density threshold or a dilated version of the segmented volume. The fiber density method allows for semiautomatic segmentation and exclusion of tendons and fascia that have higher tract density values compared to muscle tissue⁸. The segmentation volumes were slightly dilated to account for small errors in the registration between time points. Tracking parameters included a step length of five and a curvature threshold of 20°. Fiber lengths in the left and right muscle groups were averaged together with a minimum threshold of 30 mm. Differences in average muscle fiber lengths were compared for all subjects between the five time points using the Single Factor ANOVA statistical analysis with a significance level of 0.05.

Results and Discussion

Figure 3 shows three-dimensional renderings of a representative subject’s quadricep muscle fibers for all five time points. Figure 4 quantifies the change in average fiber length between each muscle group throughout the experiment. There were slight changes in average fiber length between the beginning of exercise intervention (Month 0) and the conclusion (Month 3), including a 0.8 mm decrease, 2.0 mm increase, and 12.1 mm increase in the Vastus Lateralis, Vastus Medialis, and Rectus Femoris groups, respectively. However, these differences in average muscle fiber length are not statistically significant (p > 0.05).

The lack of significant differences in fiber length could be associated with the specific exercises performed by the subjects. While the single-leg decline squats primarily targeted the human quadricep muscles, implementing exercises that are specifically aimed at increasing muscle fiber length (e.g., Nordic Training) may yield more significant results¹⁰. Nevertheless, the DTI muscle tractography approach successfully calculated quadricep muscle fiber lengths that agree with values found in literature, as represented in Table 1^1,12. Similar experiments used ultrasound and magnetic resonance imaging and observed an approximate 10-11% increase in quadricep muscle fiber length over time due to resistance training^9,11. While no statistically significant changes were determined using the DTI tractography method, the Rectus Femoris muscle increased by about 12% from Month 0 to Month 3, which corresponds well with literature. DTI tractography provides short and easy muscle fiber acquisition and could potentially be used to conduct three-dimensional assessments of fiber length changes due to exercise. However, optimized studies with more specific fiber-lengthening exercises and larger sample sizes should be explored in the future.

Conclusion

This work used DTI and deterministic fiber tractography to investigate the effects of resistance training on fiber length in the quadricep muscles. After the three-month resistance training regimen, no statistically significant differences in fiber length were observed. However, DTI and muscle tractography calculated quadricep fiber lengths that agree with values found in literature. Taken together, these results suggest that, with further optimization, DTI fiber tractography could be a useful tool to monitor changes in quadricep muscle fiber length due to exercise.

Acknowledgements

No acknowledgement found.

References

1. Chow, John W., Warren G. Darling, and James C. Ehrhardt. "Determining the force-length-velocity relations of the quadriceps muscles: II. Maximum muscle stress." Journal of Applied Biomechanics 15.2 (1999): 191-199.

2. Davis, J. F., et al. "The mechanisms of adaptation for muscle fascicle length changes with exercise: Implications for spastic muscle." Medical Hypotheses 144 (2020): 110199.

3. Edouard, Pascal, and Kevin R. Ford. "Great challenges toward sports injury prevention and rehabilitation." Frontiers in sports and active living 2 (2020): 80.

4. Evans, Tracey. “The Quadriceps.” The Islander magazine (n.d.).

5. Kawakami, Yasuo. "The effects of strength training on muscle architecture in humans." International Journal of Sport and Health Science 3.Special_Issue_2005 (2005): 208-217.

6. Klimstra, Marc, et al. "The effect of ultrasound probe orientation on muscle architecture measurement." Journal of Electromyography and Kinesiology 17.4 (2007): 504-514.

7. Kwah, Li Khim, et al. "Reliability and validity of ultrasound measurements of muscle fascicle length and pennation in humans: a systematic review." Journal of applied physiology 114.6 (2013): 761-769.

8. Oudeman, Jos, et al. "A novel diffusion‐tensor MRI approach for skeletal muscle fascicle length measurements." Physiological reports 4.24 (2016): e13012.

9. Reeves, Neil D., Marco V. Narici, and Constantinos N. Maganaris. "Effect of resistance training on skeletal muscle-specific force in elderly humans." Journal of applied physiology 96.3 (2004): 885-892.

10. Richards, Jim, et al. "The effect of different decline angles on the biomechanics of double limb squats and the implications to clinical and training practice." Journal of human kinetics 52 (2016): 125.

11. Seynnes, Olivier Roger, Maarten de Boer, and Marco Vincenzo Narici. "Early skeletal muscle hypertrophy and architectural changes in response to high-intensity resistance training." Journal of applied physiology 102.1 (2007): 368-373.

12. Ward, Samuel R., et al. "Are current measurements of lower extremity muscle architecture accurate?." Clinical orthopaedics and related research 467.4 (2009): 1074-1082.

13. Zhao, Heng, et al. "Changes of calf muscle-tendon biomechanical properties induced by passive-stretching and active-movement training in children with cerebral palsy." Journal of applied physiology 111.2 (2011): 435-442.

Figures

Figure 1: Timeline of DTI scans generated from each of the five subjects throughout the experiment. An initial (Baseline) DTI scan of each subject was taken between one to three months before the resistance training regimen. Then, each subject had another scan taken the day before starting the training regimen (Month 0). Subsequent DTI scans were taken after one month of exercise (Month 1), two months of exercise (Month 2), and at the conclusion of the three-month resistance training regimen (Month 3).

Figure 2: Flowchart of DTI and muscle tractography approach to track quadricep muscle fibers. (a) Each of the five human subjects underwent a DTI scan on the left and right quadricep muscles¹². (b) DTI scans of the subjects were generated, and individual muscle components of the quadriceps were segmented, indicated by the colors. (c) A muscle tractography algorithm used the DTI scan to track and reconstruct muscle fibers in the quadricep muscles.

Figure 3: Three-dimensional quadricep fiber renderings for each time point. As expected, the general shape, size, and location of fiber lengths in each muscle group remained consistent throughout the subject’s three-month training regimen. Each muscle fiber length from these three-dimensional reconstructions was extracted and subsequently averaged for quantitative analysis and comparison between muscle groups.

Figure 4: Average quadricep muscle fiber lengths for all subjects. Each cluster of bars represents fiber lengths from an individual quadricep muscle group, and the color of the bars represents different time points studied in the experiment. Single Factor ANOVA tests were performed for each muscle group, showing no statistical differences in average fiber length over time for all muscle groups.

Table 1: Calculated quadricep muscle fiber length versus fiber length found in literature.

Proc. Intl. Soc. Mag. Reson. Med. 30 (2022)

1022

DOI: https://doi.org/10.58530/2022/1022