Influence of muscle activity on musculotendinous stiffness quantification in stunted, prepubertal children

The quick-release technique to estimate musculotendinous (MT) stiffness has been extensively used over the last years, in both animals and humans, to gain insights in the adaptive process of the series elastic component (SEC). Recently, MT stiffness quantification, i.e., SEC behavior, has been revisited for subjects not able to fully activate their muscles (effects of long-term spaceflight or non-mature muscles). Such a phenomenon can also be encountered in stunted children. So, the aim of the present study was to analyze the effect of stunting on MT stiffness taking into account possible defect in muscle activation. For this study, 20 eutrophic children (EU) with an average age of 9years±4months were compared to 11age matched stunted children (S) evaluated by the height-to-age index. The MT stiffness index was obtained with regard to stiffness-torque and stiffness-soleus EMG relationships. The children of the S group presented a significantly lower Maximal Voluntary Contraction (MVC) in plantar flexion in comparison with children of the EU group (-37.8%). The significantly lower MT stiffness index for S children (-42.6%) was evidenced only when quantified with regard to the stiffness-soleus EMG relationship (66.5±42.8 vs. 38.2±19.9 Nmrad(-1)%(-1)). Possible delay in fiber type differentiation or tendinous structure maturation can account for the lower MT stiffness index in S children. In conclusion, stunting during early childhood delays the differentiation and maturation processes of musculotendinous structures as shown by the lower MT stiffness quantified with regards to muscle activity, also altered for stunted prepubertal children.

Influence of overweight on the active and the passive fraction of the plantar flexors series elastic component in prepubertal children

The influence of overweight, as a precursor to obesity, was analyzed on the elastic properties of the triceps surae. Based on body mass index (BMI), children (9 years ± 4 mo) were classified as control (CON; n = 23; BMI -1SD>Z score<1SD) or overweight (OW; n = 21, BMI 1SD>Z score<3SD) with regard to reference data from the World Health Organization. Musculotendinous (MT) stiffness of the series elastic component (SEC) was determined using quick-release tests to obtain 1) the MT stiffness index from the slope of either linear stiffness-torque (SI(MT-Torque)) or stiffness-EMG (SI(MT-EMG)) relationships and 2) passive stiffness from the intercept point with the ordinate. Finally, the SEC active (α(0)) and passive fractions (C(passive)) were separated as described by Morgan (Am J Physiol, 1977), using alpha-torque (α(0-Torque,) C(passive-Torque)) or alpha-EMG (α(0-EMG,) C(passive-EMG)) relationships. No significant differences in SI(MT-Torque) or α(0-Torque) were observed between OW and CON. SI(MT-EMG) or α(0-EMG) values were significantly different between OW and CON, which indicate an increase in MT stiffness. In all cases, passive stiffness (K(p), C(passive-torque), C(passive-EMG)) was significantly greater in OW but independent of the activation capacities. These results indicate that a weight-related additional loading of the MT structures in OW children caused the MT system to response accordingly to the functional demand, i.e., higher stiffness of the MT structures due to a concomitant increase in the stiffness of the SEC passive and active fraction. This study also reveals that possible differences in the activation capacities influence the determination of MT stiffness of the SEC active fraction.