*Communication*

### **Optimum Angle of Force Production Temporarily Changes Due to Growth in Male Adolescence**

### **Junya Saeki 1,2,\*, Satoshi Iizuka 1, Hiroaki Sekino 3, Ayahiro Suzuki 3, Toshihiro Maemichi 3 and Suguru Torii 1**


Received: 1 December 2020; Accepted: 1 January 2021; Published: 3 January 2021

**Abstract:** The peak increase in lean mass in adolescents is delayed from peak height velocity (PHV), and muscle flexibility temporarily decreases as bones grow. If the decrease in muscle flexibility is caused by muscle elongation, the relationship between the exerted torque and the joint angle could change in adolescents. The purpose of this study was to investigate the change in the optimum angle of force production due to growth. Eighty-eight healthy boys were recruited for this study. Isokinetic knee extension muscle strength of the dominant leg was recorded. The outcome variable was the knee flexion angle when maximal knee extension torque was produced (optimum angle). The age at which PHV occurred was estimated from subjects' height history. We calculated the difference between the age at measurement and the expected age of PHV (growth age). A regression analysis was performed with the optimal angle of force exertion as the dependent variable and the growth age as the independent variable. Then, a polynomial formula with the lowest *p*-value was obtained. A significant cubic regression was obtained between optimum angle and growth age. The results sugges<sup>t</sup> that the optimum angle of force production temporarily changes in male adolescence.

**Keywords:** force–angle relationship; isokinetic muscle strength; muscle–tendon unit; maximal voluntary contraction; growth spurt; children
