Abstract
This study aimed to examine longitudinal changes in isometric strength of the knee
extensors (ImKE) and knee flexors (ImKF) at 30° and 60°. The sample was composed of
67 players aged 11.0–13.9 years at baseline over five years. Stature, body mass, skinfolds,
and isometric strength (ImKE30°, ImKF30°, ImKE60° and ImKF60°) were measured. Fat
mass and fat-free mass (FFM) were derived from skinfolds. Skeletal age was obtained
using TW2 RUS. Multilevel random effects regression analyses extracted developmental
polynomial models. An annual increment on chronological age (CA) corresponded to 5.6 N
(ImKE30°), 2.7 N (ImKF30°), 4.6 N (ImKE60°) and 1.5 N (ImKF60°). An increment of 1 kg in FFM predicted isometric strength as
follows: 1.2 N (ImKE30°), 2.1 N (ImKF30°), 3.1 N (ImKE60°) and 2.0 N (ImKF60°). The
following equations were obtained: ImKE30°=5.759×CA+1.163×FFM; ImKF30°=−19.369+2.691×CA+0.693×CA2+2.108×FFM; ImKE60°=4.553×CA+3.134×FFM; and, ImKF60°=-19.669+1.544×CA+2.033×FFM. Although
skeletal maturity had a negligible effect on dependent variables, age and body size,
based on FFM, were relevant longitudinal predictors. During adolescence, systematic
assessment of knee extensors and knee flexors are strongly recommended to prevent
impairment of knee muscle groups.
Key words
multilevel modeling - growth - skeletal maturation - youth athletes