## 1 Introduction  
Reactive strength is the ability to perform fast muscle contractions by taking advantage of the stretch-shortening cycle (SSC), which consists of quick eccentric-concentric muscular contractions [1]. This muscular coupling produces a more powerful contraction than the simple concentric action due to the storage and re-utilization of elastic energy [2; 3]. The transition between eccentric and concentric contraction should be as short as possible to optimally use the stored energy (i.e., \(<\)250 ms) [4]. In track and field sprinting, the ground contact time is typically below 0.250 ms during each phase of the race, i.e., the acceleration phase, the maximum velocity phase and the deceleration phase [5]. Track and field sprinting requires a fast SSC, and consequently, the efficiency of the mechanisms of reactive strength represents a crucial component of the performance, especially during the maximum velocity phase [6; 7]. The reactive strength index (RSI) is the most common method for assessing the reactive strength of lower limbs [8]. The RSI is usually assessed as the ratio between jump height and contact time in specific jumps [9], such as drop jumps [10; 11], rebound jumps [12] and hopping jumps (i.e., ankle jumps) [13; 14]; thus, the RSI represents a commonly accepted measurement of the efficiency of the SSC [8]. Despite several studies investigating the relationship between RSIs and sprint performance in athletes of different disciplines [15], the association between RSIs and sprinting remains questionable [16]. Non-univocal results may be due to the different strategies adopted to assess the RSI (bilateral or unilateral jump, box height of the drop jump, etc.) in regard to the performance level of athletes involved in the studies and to the different sprint distances (from 20 to 60 m). Some authors reported moderate negative correlations between the RSI and sprint acceleration time (r= \(-\)0.426) and between the RSI and top speed (r = \(-\)0.326) [15]. Other studies [17; 18] reported large negative correlations between the RSI and 100 m time (r = \(-\)0.566 and r = \(-\)0.75, respectively); however, some authors suggest that an RSI calculated by a horizontal jump test (bi- and unilateral) may be a better predictor of sprint performance (5-50 m) compared to an RSI calculated in vertical jumps [19; 20; 21; 22]. Experimental studies investigating RSIs in horizontal jumps, however, present some methodological limitations in the calculation of the RSI, such as the use of non-standardized jumps. Stride length is considered a fundamental parameter for sprint performance [5] that does not require specific coordination skills (such as jumps or a bounding run); thus, an RSI based on this parameter may be more specific for sprinting compared to previously used indices [10; 11; 12; 13]. Recently, muscle ultrasounds have been used to investigate the relationships between the muscle architecture of the lower body muscles and sprint performance in different athletic populations, such as track and field sprinters [23] and field hockey players [24]. Significant correlations were detected between the pennation angle, fascicle length and echo intensity of the vastus lateralis and sprint performance. The latter was recently indicated as the best predictor of running speed in physically active young individuals sprinting on a motorless treadmill [25]. Echo intensity is influenced by the characteristics of the contractile components of the muscle in addition to other elements, such as fat tissue and connective and fibrotic tissue within the muscle. A low level of echo intensity has been associated with low levels of intramuscular fat and connective tissue, indicating a good muscle \"quality\" [26].