Quantifying The Relationship Between Contraction Efficiency And Muscle Size Across The Adult Lifespan

Abstract

The neuromuscular system undergoes a natural structural and functional degradation associated with aging. Since the size of a muscle affects its stiffness, the age-related reduction in muscle mass may result in decreased contraction speed and efficiency. PURPOSE: To quantify the relationship between muscle size and contraction efficiency, as measured by electromechanical delay (EMD), across the adult lifespan. METHODS: Seventy-five adults between 18 and 84 yrs. old were included in this study. Panoramic ultrasound images were taken from each participants’ rectus femoris (RF). Muscle cross-sectional area was defined and measured as the area of interest that included as much muscle as possible (mCSA; cm2). In addition, surface electromyographical (EMG) signals were recorded from the RF while ten tendon taps were delivered to the patellar tendon. EMD was calculated as the time (s) between EMG onset and torque onset during the evoked reflex contractions. Bin widths were utilized to condense data, where subjects were grouped based on their age as follows: 18 - 29 yrs. (n = 37), 30 - 39 (n = 7), 40 - 49 (n = 4), 50 - 59 (n = 9), 60 - 69 (n = 5) and > 70 (n = 4). Polynomial regression (2nd order) was performed to fit the apparent curvilinear relationship between EMD and mCSA. RESULTS: As can be appreciated in the figure below, EMD slowed and mCSA decreased with age. However, the relationship between EMD and mCSA was not significant (r = -0.57, p-value = 0.55). CONCLUSION: Surprisingly, EMD was not significantly related to mCSA. It is possible that this may be due to the middle-aged groups being underpowered, which led to an outlier bin (40-49 yrs. old). Additionally, a reflex contraction was used to measure EMD which might differ from the properties of a voluntary contraction. Further analyses might be needed to study the relationship between EMD and mCSA.