Caring for and Counseling the Youth Runner

Growth Plate and Epiphysis

Child runners are uniquely different from adult runners because their bones are actively growing ( Fig. 21.1 ). The areas of the greatest growth are in the axial skeleton and the long bones such as the femur, tibia, humerus, and radius. Starting from the midpoint of a long bone and progressing out to the articular cartilage, long bones are defined by specific regions. The first region is the diaphysis or main bone shaft, second is the metaphysis or subgrowth plate, third is the primary physis or growth plate, and last is the epiphysis or articular region. The diaphysis and metaphysis are covered by the periosteum, which is a metabolically active layer that lays down new bone. Circumferential growth occurs at the periosteum whereas longitudinal growth occurs at the primary physis. Growth at the primary physis is dependent on multiple hormonal growth factors and can consume about 10% of the total energy production of young athletes. Structural changes and hormonal influences during puberty can weaken the growth plates, which can make these sites susceptible to injury, especially with increased loading forces during running.

At birth, the epiphysis is composed of three types of hyaline cartilage: physeal, epiphsyeal, and articular. Physeal cartilage is a highly cellular cartilage that is responsible for the conversion of cartilage to new bone called endochondral ossification. This occurs at the primary physis with the most immature cells on the side of the epiphysis and the most mature cartilage forming new bone toward the metaphysis. In the core of the epiphysis, or secondary ossification center, epiphyseal cartilage dominates and is composed of a collagen-rich matrix with numerous chondrocytes. As the secondary ossification centers grow and mature, they also undergo endochondral ossification in a process analogous to the physeal cartilage. Secondary ossification centers are also common locations for tendons to attach, placing them under additional stress. At the outermost region of the epiphysis, articular cartilage lines the articular surface. Where the physeal cartilage and epiphyseal cartilage are highly vascular, the articular cartilage is avascular and is almost completely acellular, composed of aggrecan, type II collagen, and water. As the skeletal system matures the physeal cartilage and epiphyseal cartilage are replaced by bone, only the articular cartilage persists in the adult.

Peak Height Velocity

Peak height velocity is the concept used by physiologists to describe the time at which an individual is growing the fastest in height. This timing is dependent on various genetic and environmental factors, but on average most girls achieve peak height velocity at the age of 12 years and most boys at the age of 14 years. In addition to peak height velocity varying by age, peak height velocity also varies by body region, with the lower limbs achieving the greatest rate of lengthening before the trunk. This time of great lengthening in the skeleton leads to specific physiologic consequences and implications in biomechanics and performance.

During peak height velocity, several areas become relatively weak. First, bones are weakest during peak height velocity because bone mineral density is lowest right before maximal growth rate. Second, as the joint surfaces of long bones increase in size, articular cartilage grows to keep pace and becomes relatively weak, weaker than when bones were growing at a slower rate and weaker than mature cartilage. Third, the muscle tendon complex does not lengthen at the same time as long bones, which results in greater tension being placed on this structure. These changes lead to greater strain being placed on growing bones, on their thinning growth plates, and on their apophyses, which explains why young runners are more vulnerable to injuries at these sites.

Interestingly, as peak height velocity results in relative physiologic weakness, there is evidence that running performance increases. Papaikovou et al. studied the running times of children competing in the 30-m sprint and found that running performance steadily increases and then plateaus when peak height velocity is reached. There are several explanations for why this occurs. Fast-twitch motor units become more prevalent with age resulting in greater force development; better coordination develops between agonist and antagonist muscle groups, leading to increased running efficiency; and stride length increases with lengthening of the long bones, leading to greater distance traveled per running gait cycle. Cadence generally does not increase during peak height velocity and is not a factor in increased running performance. Therefore, while the child runner is obtaining the greatest skeletal growth and developing anatomical sites of relative weakness, they are also reaching greater running performance. This is important to consider, as youth runners become more proficient and are increasing their training duration and intensity.