Muscle imbalance due to reduced muscular endurance is a significant risk factor. Thus, for the lower limb, muscle imbalance is one of the most common causes of traumatic injury. A number of studies on isoinertial technology have demonstrated that it allows the development of forces similar to or greater than those generated in the same exercise but performed with traditional weights. Our research aimed to analyze the morpho-functional changes at the muscle level using ultrasound, and the evolution of muscle power output express of maximal muscle strength at the level of the knee extensors, specifically the rectus femoris muscle, following an isoinertial training program. The study included 11 female soccer-practicing sportswomen with average age (15.18 ± 1.08). The assessment included an ultrasound assessment of the rectus femoris muscle, before and after isoiniertial training (post acute moment); a muscle force assessment using dynamometry; and an evaluation of isoinertial parameters as concentric and excentric power, in terms of coefficients that represent report between the left and right sides. The isoinertial protocols training included three weekly sessions of isoinertial exercises. The results show an increase in the values of the ultrasound dimensions, approximately at the same level for both measured dimensions, which is significant for demonstrating the existence of an increase in muscle volume. An important progress is observed in the mean maximum strength and maximum force, but especially in the duration of maintenance of the maximum loading force. There is no statistically significant symmetry at the level of the rectus femoris muscle for concentric power and no statistically significant symmetry tendency for eccentric power, although there is a favorable evolution in terms of values.