The purpose of this study is to provide new mechanical insights and morphological parameters of in vivo muscle contractile properties by combining MRE and ultrasound techniques. The growing muscle process followed by the very beginning of the aging process was characterized. MRE and ultrasound tests were performed on the Vastus Medialis (VM) muscle of children (N=7, 8-12yrs) and adults (N=19, 24- 58yrs). For MRE tests, mechanical parameters such as shear modulus (
) and attenuation coefficient (
) were measured on the VM muscle at rest and at contracted positions (10% and 20% MVC) for each group. Longitudinal ultrasonic images (13 MHz) were performed in the same area as the shear wave’s propagation and fascicle angles were measured for each subject at 10% and 20 % of MVC. The shear modulus increases with the degree of contraction for young adults representing progressive muscle fiber recruitment. Middle-aged adults revealed a maximum
at 10 % of MVC due to the aging process. Shear wave propagation follows the path of the fascicles providing geometrical insights. At rest children had the lower
correlated to a non oriented fascicules pattern. This result is confirmed with the high attenuation coefficient reflecting a less compact media. However, at 10% MVC children revealed the highest
showing randomized muscle fiber recruitment, correlated with a high increase of fascicle angle. Subcutaneous tissue had a lower stiffness compared to VM muscle. MRE is a potential clinical tool to characterize the mechanical and morphological properties of the children and adult muscle and could be used for the follow-up of patients suffering from neuromuscular disorders and for the diagnosis of muscle disorders related to the aging process.