The effect of short-term in-season Isotonic vs. Isoinertial strength training on speed and power in elite young football players

The aim of this study was to compare the effect of isotonic and isoinertial strength training on speed and power over a short-term period during the in-season in elite young football players. Using a controlled study design, 19 players were randomly split up into 2 groups: ISOTONIC (n=10, age: 15 ± 0.36 years, height: 1.68 ± 0.53 m, body mass: 58.7 ± 4.5 kgs) and ISOINERTIAL (n=9, age: 15 ± 0.51 years, height: 1.69 ± 0.35 m, body mass: 59.2 ± 5.60 kgs). Both groups took part in a 6-week (12 sessions) intervention program consisting of either the barbell back squat (ISOTONIC) or the flywheel squat (ISOINERTIAL). Pre and post-tests were carried out on 1RM squat, thigh CSA (circumference surface area), CMJ (countermovement jump) and 15 m sprint. There was a significant interaction between pre/post tests and CSA (p = 0.007), with post hoc analysis revealing significant higher improvement in the ISOINERTIAL group (from 25.80 ± 6.99 cm² to 31.22 ± 6.83 cm²). There were no interactions from pre to post intervention (p > 0.05) in the 1RM squat, CMJ and 15m sprint. There was also no correlation (p > 0.05) between the performance (CMJ/15 m sprint) and morphological (CSA) or neurological (1RM squat) changes. The results of this study support results from other studies confirming that isoinertial training has a better impact on 15 m sprint and CMJ performance. However, this was only statistically significant in the CSA, and therefore we cannot conclude that one type of training is preferred over the other to improve speed and power over a short-term period.