Background: Muscle thickness (MT) measurement using ultrasound image is emerging as a useful method to assess muscle mass during stretching. Traditionally, range of motion (ROM) measurements assessed by digital inclinometer are considered reliable and valid parameter for monitoring muscle flexibility. The ultrasound-based measurement for MT to monitor muscle flexibility has yet to be validated. Objects: This study aimed to determine whether ultrasound measurement can serve as a valid alternative to a digital inclinometer for assessing muscle flexibility following stretching interventions. Methods: A randomized crossover design study was conducted with 20 healthy young participants. The stretching exercise program was carried out passive static and hold-relax proprioceptive neuromuscular facilitation stretching exercises applied on gastrocnemius-soleus muscle group with 3 times a week for 2 weeks. Data were collected 3 times for pre- and post-tests. Dorsiflexion ROM was measured by a digital inclinometer and the MT of gastrocnemius muscle by the ultrasound. Intraclass correlation coefficient (ICC) analysis for test-retest reliability, Wilcoxon sign rank test for stretching effects, and Spearman’s correlation analysis for determining the concurrent validity of two measurements. Results: ICC values of pre- and post-tests in both ROM and MT measurements were showed good to excellent intra rater reliabilities ranging from 0.782 to 0.968 (p < 0.001). A significant increase in the ROM measurements was observed following the stretching program (p < 0.01), whereas no significant change in the MT measurement was showed. Spearman’s correlations revealed weak relationships between the ROM and MT measurements (r = –0.217 for pre-test, r = –0.259 for post-test, r = –0.282, p < 0.05 for changes). Conclusion: These findings suggest that both the MT and ROM measurements were not compatible for monitoring muscle flexibility following stretching exercises regardless of their good reliabilities. Further investigations may be required to the MT measurement alone to be an alternative method.

INTRODUCTION
MATERIALS AND METHODS
    1. Participants
    2. Procedures
    3. Stretching Interventions
    4. Instrumentations
    5. Data Analysis
RESULTS
    1. Comparison of the Reliability of DFROM andGastrocnemius MT Measurements
    2. Comparison of Changes in the DFROM andGastrocnemius MT
    3. Analysis of Concurrent Validity Between the DFROMand Gastrocnemius MT
DISCUSSION
CONCLUSIONS
FUNDING
ACKNOWLEDGEMENTS
CONFLICTS OF INTEREST
AUTHOR CONTRIBUTION
ORCID
REFERENCES

• Bong-sam Choi(Department of Physical Therapy, College of Health and Welfare, Woosong University, Daejeon, Korea)
• Jin-hyun Yang(Department of Physical Therapy, College of Health and Welfare, Woosong University, Daejeon, Korea)
• Chang-ho Kim(Department of Physical Therapy, College of Health and Welfare, Woosong University, Daejeon, Korea)
• Sookyoung Park(Department of Physical Therapy, College of Health and Welfare, Woosong University, Daejeon, Korea) Corresponding author