(04) THE EFFECTS OF RESISTANCE EXERCISE TRAINING ON ISOMETRIC AND TENSILE STRENGTH AND MUSCLE CROSS-SECTIONAL AREA IN PREPUBESCENT YOUTH

Purpose: To evaluate changes in isometric muscular strength, cross-sectional area (CSA), and tensile strength (TS) of the right leg extensors in prepubescent youth ages 8 to 10 years pre-and post-resistance exercise training program (RET).

Methods: Twenty-nine healthy, prepubescent subjects (15 males, 14 females, age=9.3±0.9 yrs, hheight=137.6±8.5 cm; body mass=34.8±9.6 kg) completed an 8-week (3 days/week) linear periodization RET or served as controls (CON). Muscle CSA was analyzed at pre-and post-testing via ultrasound images of the vastus medialis, rectus femoris, and vastus lateralis. Total CSA of the three muscles (cm²) was calculated for each subject. An isometric dynamometer was used to record isometric maximal voluntary contractions (MVCs) of the right leg extensors. The highest 0.25 sec epoch of force (N) during 2-3 MVCs was determined as peak. Tensile strength (TS) was calculated by dividing peak MVC by total CSA. RET consisted of whole-body training sessions featuring 4 lower body exercises (step-ups, squats, seated leg extension, etc.) and 2 upper body exercises (bench press, shoulder press, lat pulldown, etc.). Volume decreased over time as load increased to implement progressive overloading of the muscles (e.g., week 1: 3 sets of 8-12 repetitions; week 8: 5 sets of 3-5 repetitions). Separate two-way mixed factorial ANOVAs (group [RET vs. CON] x time [pre vs. post]) were used to examine potential changes in peak MVC, total CSA, and TS. Partial eta square (η_p²) was calculated for significant interactions with Cohen’s d calculated for t-tests.

Results: Table 1 contains means and standard deviations. For peak MVC, there was a significant two-way interaction (p=0.002, η_p²=0.304). Peak MVC increased (p< 0.001, d=1.445) pre- to post-RET, but not for the CON (p=0.497) with no differences between groups at pre- (p=0.132) or post-intervention (p=0.990). For total CSA, there was a significant two-way interaction (p=0.018, η_p²=0.189). However, there were no significant increases pre- to post-intervention for the RET (p=0.163) and CON (p=0.067) or differences between groups at pre- (p=0.150) or post-intervention (p=0.470). For TS, there was no two-way interaction (p=0.233) or main effect for group (p=0.917), but there was a main effect for time (p< 0.001). Collapsed across groups, TS increased (p< 0.001,d=0.845) from pre- (3.22±0.68 N/cm²) to post-intervention (3.74±0.88 N/cm²).

Conclusions: Isometric strength increased by 22.4% in prepubescent youth with no changes in total CSA. Interestingly, TS did not significantly increase for the RET more than CON, however, Cohen’s d would suggest there was a large magnitude of increase for the RET (d=0.993) with a moderate increase for CON (d=0.529). PRACTICAL APPLICATIONS: RET is effective for increasing maximal strength in youth despite no increases in CSA. These results support efforts to increase youths participating in RET as a method of improving strength, thus improving physical performance.


 

Acknowledgements: This study was funded by the National Strength and Conditioning Association Foundation's Senior Investigator Grant awarded to Dr. Trent J. Herda.