Morphological and Mechanical Properties of the Quadriceps Femoris Muscle-Tendon Unit From Adolescence to Adulthood: Effects of Age and Athletic Training
To investigate the development of the quadriceps femoris muscle-tendon unit from early adolescence (EA), late adolescence (LA) to young adulthood (YA), and examine how it is influenced by athletic training in a cross-sectional design.
Forty-one male athletes and forty male non-athletes from three different age groups (EA: 12–14 years,n= 29; LA: 16–18 years,n= 27; and YA: 20–35 years,n= 25)
Sports and Training Routine in 3 groups:
Athletes: American football, volleyball, handball, basketball, judo, kick-boxing, fencing, gymnastics, dancing, hockey, vaulting, track and field, acrobatics, decathlon, and trained at least three times per week for at least 75 min per session.
Untrained adolescent: School sports and a maximum of one session of recreational sports per week
Adults: One session of recreational sports per week
Measures: The measurements of muscle strength (i.e., knee extension moments), vastus lateralis (VL) architecture and patellar tendon mechanical properties were carried out on the dominant leg (i.e., leg used for kicking a ball) following a standardized warm-up consisting of 2–3 min ergometer cycling, ten submaximal isometric contractions, and three maximum voluntary isometric contractions (MVC)
Tendons adapt to increased mechanical loading and enhance their stiffness already in EA. In EA participants, the average enhancement in patellar tendon stiffness due to training was ∼25%, which can be interpreted as clear and functionally relevant adaptation.
The main alteration in muscle strength and tendon stiffness due to maturation seems to occur between EA and LA. In this stage also the main changes in the femur length, body height and body mass occur, which indicates an analogous development of the functional and mechanical muscle-tendon properties with the skeletal system.
Aside from higher levels of muscle strength, muscle thickness and tendon stiffness in athletes, the development of the properties of the knee extensor muscle-tendon unit from early-adolescence to adulthood is similar in athletes and non-athlete controls, with the major alterations occurring between early and LA. The frequency of imbalances in the quadriceps femoris muscle-tendon unit seem to increase with both age and athletic training during the adolescence-to-adulthood development and result in an increased mechanical demand for the patellar tendon.
The integration of a specific training that increases tendon stiffness and facilitates a balanced adaptation between muscle and tendon might be an important approach for the athletic practice.
An effective training stimulus for tendon adaptation is a combination of high loading magnitude, an appropriate loading duration in every repetition (i.e., 3 s) and repetitive loading. In children, the development of resistance training competency should precede the application of high loads, yet it has already been shown that specific tendon training in accordance to the exercise recommendations above can be successfully applied in children to increase their tendon stiffness.
ORIGINAL RESEARCH ARTICLE
Front. Physiol., 27 August 2019 | https://doi.org/10.3389/fphys.2019.01082