# Effects of Successive Annual Training on Young Swimmers' Strength Asymmetries and Performance
## 1 Introduction  
Swimmers use both their arms and legs optimally to minimize resistive drag and increase propulsion during swimming [1]. Most of the time, swimmers may use one of their limbs (arm or leg) more efficiently compared to the other due to asymmetries in strength [2]. Thus, asymmetries may affect swimmers' technique as well as performance after a period of training [3]. It is known that comparable force application from both the right and left sides of the body may positively affect swimming performance by lowering intracycle velocity variations [1]. Moreover, more than 96% of the human population presents a noticeable asymmetry level [4], especially in cycling and continuous sports activities [2, 5].  
The evaluation of strength asymmetries for upper limbs may be assessed with specific dryland tests (i.e., shoulder isometric strength and hand grip isometric strength) [6, 7] or with a specific in-water evaluation such as the 30 s tethered swimming test [8]. Recently, a moderate to large direct relationship was observed between dryland strength and swimming force production [9] and performance during a training set (i.e., 4 \(\times\) 50 m) [10]. However, no relationship was found between strength asymmetries calculated by a 30 s tethered swimming test and a shoulder isokinetic test [9]. Additionally, it was found that bilateral asymmetries and peak force production during a maximum isometric voluntary test were similar between tethered swimming and isometric strength tests [11]. Recently, it was highlighted that evaluating strength asymmetries in different angular velocities (60\({}^{\circ}\)/s or 180\({}^{\circ}\)/s) during an isokinetic test may result in different levels of asymmetries [12], andthis may affect swimmers' coordination during propulsion in swimming [12]. In addition, strength asymmetries may be associated with swimmers' technical characteristics, such as stroke rate (SR) or stroke length (SL), or with muscular imbalances during a tethered swimming test [9].  
By emphasizing muscular imbalances, it was observed that shoulder rotator muscles became stronger than antagonists [13]. This may increase muscle imbalance and the risk of injury during propulsion after a 16-week macrocycle training period [13]. Furthermore, biological maturity has been proposed as an important factor that may affect swimmers' biomechanical characteristics and be associated with their efficiency in the water [14]. It is well documented that higher-level swimmers have better efficiency during propulsion due to their better swimming technique [14]. Additionally, swimmers' biological maturation will likely affect their dryland and in-water strength gains or the likely induced asymmetries after a training period. Recently, it was reported that bone asymmetries in tennis players may increase substantially, which is connected with maturity status in both males and females [15].  
To our knowledge, however, there is no study in swimming testing strength asymmetries in dryland and in-water strength changes during successive annual training plans. Therefore, the purpose of the current study was to examine the influence of swimming training on performance, strength asymmetries, dryland, and in-water strength changes as well as biomechanical characteristics after twelve weeks including similar training content applied in two successive annual cycles of training. We hypothesized that swimmers' strength asymmetries will differ between the successive annual training plans. Furthermore, the changes in swimmers' strength asymmetries will be associated with changes in their biological maturation.