Claire Smith, Jocarol Shields, Shawn Reese, Marcel Dos Santos and Jason DeFreitas

Oklahoma State University, Stillwater, OK

Recent evidence has shown that strength training may increase a nerve’s motor conduction velocity. Considering the very strong relationship between an axon’s conduction velocity and its diameter, this increase in velocity may be indicative of axonal growth within motor neurons. Furthermore, given the potentially detrimental effects of age-related motor axon degeneration and its role in sarcopenia, as well as the functional benefits of stimulating axonal growth, it is important to find non-invasive techniques that could estimate the axonal size of motor neurons. PURPOSE: The purpose of this study was to determine if training-induced changes in motor neuron size could be identified at the nerve level via ultrasound. METHODS: Sixteen healthy subjects (18-73 yrs.) participated in and have completed the study thus far. The subjects were split into two groups, young (n=11, 18-46 yrs.) and older (n=5, >60 yrs.). Both groups underwent a 4-week handgrip training intervention designed to increase conduction velocity of the median nerve. Cross-sectional area (CSA) measures of the median nerve from both arms were taken pre- and post-training using ultrasound imaging and were analyzed using the NIH’s open source software, ImageJ. RESULTS: The intervention was successful at improving nerve conduction velocity, indicating that the axons of the motor neurons likely grew in diameter. A 2-way (2×2) mixed factorial ANOVA showed a significant Group × Time (p=.004) interaction, with both groups showing an improvement in nerve CSA across time (p=.001). The mean increases in nerve CSA (averaged across both limbs) were 10.2% and 12.5% for the young and older groups, respectively. CONCLUSION: This preliminary data suggests that there is an increase in nerve size after a four-week training intervention in both young and older adults. More importantly, this study shows that nerve CSA, as measured by ultrasound, may be sensitive enough to identify large-scale changes in the size of motor neuron axons. These findings provide clinicians and researchers with a potential technique for tracking both beneficial and detrimental changes to the axons of motor neurons using a widely available tool, ultrasonography.

FUNDING: Supported by the Central States ACSM Doctoral Student Research Grant

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