Background: Biomechanical stiffness has been linked to risk of injury and found to be a measureable characteristic in musculoskeletal disorders. Specific identification of stiffness may clarify who is most likely to benefit from the trigger point dry needling (TDN). The purpose of this study is to investigate the reliability and concurrent validity of the MyotonPRO® to the criterion of shear wave ultrasound elastography for the measurement of biomechanical stiffness in the infraspinatus, erector spinae, and gastrocnemius of healthy subjects over increasing muscle contraction. Second purpose is to investigate the biomechanical effects of TDN to latent myofascial trigger points (MTrPs) in the infraspinatus, erector spinae, or gastrocnemius. Research Design and Method: The first phase of the study investigated 30 subjects who completed three levels of muscle contraction in standardized test positions for the infraspinatus, erector spinae and gastrocnemius. Biomechanical stiffness measures were collected using shear wave elastography and MyotonPRO®. The second phase of the study investigated 60 new subjects who were categorized into infraspinatus, erector spinae, or gastrocnemius group based on an identified latent MTrP. These subjects underwent TDN while monitoring biomechanical stiffness at baseline, immediately post TDN, and 24 hours later. Analysis: Discriminate ability, reliability, and correlations were calculated for measured stiffness variable across the three conditions of contraction in the first phase of the study. Differences between stiffness at baseline and after TDN were calculated in the second phase of the study. Results: Correlation of the two measurement methods in the three muscle regions was significant and strongest in the gastrocnemius. MyotonPRO reliability was excellent, and demonstrated ability to discriminate between the three levels of muscle contraction. In the second phase, immediate decreased stiffness was observed in the MTrP following TDN treatment. Significant decreased stiffness was found in in the erector spinae and gastrocnemius group who also demonstrated a localized twitch response during TDN. Stiffness returned to near baseline values after 24 hours. Discussion: The MyotonPRO® stiffness measurement was found to be reliable and discriminate across predefined muscle contraction intensities. TDN may cause an immediate change in stiffness but this change was not observed at 24 hours. It is not known whether these effects are present in a symptomatic population or related to improvements in other clinical outcomes. Future studies are necessary to determine if a decrease in biomechanical stiffness is an indication of patient improvement in pain and function
