Date Published: February 7, 2019
Publisher: Public Library of Science
Author(s): Francisco Piqueras-Sanchiz, Saúl Martín-Rodríguez, Luis Manuel Martínez-Aranda, Thiago Ribeiro Lopes, Javier Raya-González, Óscar García-García, Fábio Yuzo Nakamura, Massimo Sacchetti.
Flywheel iso-inertial training has been shown to positively affect muscular strength and sports performance (e.g. agility). However, implementing such eccentrically-biased training during a microcycle needs to be carefully planned due to its purported effects on the neuromuscular system that can last for hours/days post-exercise. This study aimed at using tensiomyography to verify the effects of different inertias during the hip extension exercise on the contractile function of biceps femoris and semitendinosus muscles of the dominant leg for up to 72 hours post-exercise. Thirty participants (24.4 ± 3.4 years) were divided into 0.075 or 0.1 kg·m2 inertia groups and a control group. Magnitude-based analysis was used for the comparisons. Several tensiomyography parameters were changed after both intensities of flywheel exercise (in most cases indicating a decrement in muscle stiffness), whereas most between-group differences suggested that in the semitendinosus muscle, the higher inertia (0.1 kg·m2) influenced the muscle stiffness parameters more (e.g. Dm = maximal radial displacement) while in the biceps femoris, the greater effect was caused by the lower inertia (0.075 kg·m2) (e.g. Tc = contraction time). Most changes in contractile properties of the investigated muscles occur within 24 hours post-exercise, but can persist for up to 72 hours. However, higher inertia (0.1 kg·m2) influenced the stiffness of the semitendinosus muscle more, while in the biceps femoris, the greater effect was caused by the lower inertia (0.075 kg·m2). These findings should be considered by practitioners when prescribing flywheel iso-inertial training.
Several investigations to date have revealed that eccentric (ECC) training has superior outcomes compared to concentric (CON) training in the development of both muscle hypertrophy and strength . Compared to CON actions, isolated ECC actions are characterized by producing higher torque  with lower muscle activation  and electromechanical delay . Additionally, ECC actions require lower metabolic cost compared with CON actions , as well as higher solicitation of Type IIx fibres . In addition, ECC actions can optimize the efficacy of training (i.e. better adaptation in less time) of elite sports athletes [1, 7]. On the other hand, although ECC-based training has many advantages over CON training, it has been known for 40 years that this type of training produces swelling and a decreased range of motion, being related to changes in passive and active muscle stiffness, as well as a pronounced loss of strength between 24 and 72 hours after exercising [8, 9].
Our hypothesis in this study was that passive muscle belly stiffness of the hamstrings would increase with both moderate and high FLY-RT loads. However, this seems not to be confirmed.
In summary, the data presented here indicate that the inertia 0.075 kg·m2 resulted in higher contraction velocities in both concentric and eccentric phases, while 0.1 kg·m2 resulted in higher rating of perceived exertion. A decrement in muscle stiffness was found after both intensities of flywheel exercise, but the higher inertia (0.1 kg·m2) influenced the muscle stiffness of ST more, while in the BF, the greater effect was caused by the lower inertia (0.075 kg·m2).