# The Effects of Flywheel Training with a Portable Device on Physical Performance in Soccer Players
## 1 Introduction  
Soccer is a team sport in which players expend many high-intensity efforts through predominantly unilateral actions [1] and in different planes of movement [2]. One of the most repeated actions during a match is change of direction (COD), with more than 800 changes of direction (CODs) per match [3]. There are multiple factors on which the COD ability depends--neuromuscular coordination, strength, and power of the lower limbs [1, 4, 5, 6, 7, 8]. Among these changes of direction, we can differentiate those involving a deceleration phase followed by acceleration in a different direction [9], typically characterized by directional changes at angles of less than 90\({}^{\circ}\)[10], and, by contrast, the curve sprint (CS), which is defined by a gradual and continuous change of direction throughout the entire route [10, 11]. This sprint can be performed to the right, with the right leg remaining on the inside of the curve, or to the left, with the left leg remaining on the inside of the curve. [PERSON] et al. [12] point out the existence of a strong side and weak sides in the CS, finding a similar performance between the CS towards the strong side and the linear sprint. The CS is a neuromuscular action determining performance in soccer [11]. Around 85% of the sprints performed in a soccer match are executed in a curved trajectory; all outfield players complete CSs during the match [11]. It is a resource used by players to avoid offside and toevade, follow, or lure an opponent [13]; therefore, it is highly important in both offensive and defensive situations. Eccentric strength is particularly important to CODs [9]. The scientific literature has shown that the COD correlates with the eccentric strength of the lower limb [14; 15]. To target the eccentric phase of the movement, training devices known as rotary inertial devices (RIDs) have been designed [16; 17]. RIDs provide a source of linear resistance from a tether wrapped around a horizontal axis in the shape of a cylinder or a vertical axis in the shape of a cone [18]. Concentric action unwinds the tether belt by rotating the flywheel in one direction, and eccentric action occurs during rewinding by breaking the generated motion [19]. The literature shows that for the same working load, RIDs generate more resistance to eccentric phase motion than traditional means [19]. Eccentric overload training using RIDs has been shown to enhance muscle activation in soccer players during various tasks involving CODs [14]. Following the implementation of a training protocol with RIDs, significant improvements in COD performance were observed in soccer players [6; 20]. Repeated actions under eccentric overload may result in improvements in braking capacity, a critical factor in COD performance [18]. The use of RIDs to overload multidirectional movements yields greater performance enhancements compared to conventional soccer training [21]. Additionally, isoinertial resistance training demonstrated superior results compared to cable resistance training methodologies [22]. However, one of the drawbacks of the use of RIDs in soccer training is the difficulty of transporting them to different places where players train and their high cost, which makes their use almost unattainable for modestly funded soccer teams [23]. Recently, new, easily portable, and low-cost RID technology has been developed [24]. The device features compact dimensions (12 \(\times\) 11 \(\times\) 11 cm) and a lightweight design (<1 kg), making it easily transportable in a backpack [25]. RIDs can operate with either a conical or a cylindrical pulley system. In this case, the portable RID uses a conical pulley, operating similarly to traditional conical RIDs while offering a lower cost and enhanced portability. The conical shape of the RID enables the execution of specific multi-directional and multi-joint movements with optimal power [21].