Although many studies have focused on stretching techniques for athletes, no comprehensive studies have investigated the use of the instrument-assisted soft tissue mobilization (IASTM) technique in young basketball players. The active properties of muscle and subjective reporting of functional ability were used to identify the effects of IASTM on the calf muscle. Active range of motion (AROM), functional fitness, and isokinetic lower strength in the knees and ankles were measured in 40 healthy young basketball players. They were divided into the IASTM group (n= 20) and a control group (CG, n= 20). Twenty asymptomatic young basketball players were treated with IASTM six times per week for 8 weeks. The remaining 20 participants did not receive a treatment intervention between tests and served as the control. Ankle AROM (two knee positions of 0∘ and 45∘ flexion), functional fitness at the knee and ankle (side-step and vertical jump), and isokinetic peak torque were determined during ankle dorsiflexion (DF) and plantar flexion (PF) and knee extension (EX) and flexion (FX). The data were analyzed using repeated-measures analysis of variance. Significant differences were observed between the two groups in IASTM and control AROM 0∘ knee flexion (right: DF; p< 0.001 and PF; p< 0.001; left: DF; p< 0.001 and PF; p= 0.011), AROM 45∘ knee flexion (right: DF; p< 0.001 and PF; p= 0.009; left: DF; p< 0.001 and PF; p= 0.001), functional fitness (side step; p= 0.001, sit and reach; p= 0.025, vertical jump; p= 0.001), ankle isokinetic strength 30∘/sec (right: DF; p= 0.001 and PF; p= 0.001; left: DF; p< 0.001 and PF; p= 0.002), ankle isokinetic strength 120∘/sec (right: DF; p= 0.049 and PF; p= 0.001; left: DF; p= 0.023 and PF; p< 0.001), knee isokinetic strength 60∘/sec (right: EX; p= 0.001, FX; p= 0.001 and hamstring and quadriceps ratio [H/Q]; p= 0.001, left: EX; p= 0.001, FX; p= 0.001 and H/Q; p= 0.001), and knee isokinetic strength 180∘/sec (right: EX; p= 0.001, FX; p= 0.001 and H/Q; p= 0.001; left: EX; p= 0.001, FX; p= 0.010 and H/Q; p= 0.001). These results suggest that IASTM improves functional fitness and lower body muscle strength in young basketball players.