## 1 Introduction
Late-phase acceleration is characterised by shorter ground contact times (GCTs) and a more vertical application of ground reaction forces (GRFs) compared to the early phase [22]. During early acceleration, a pronounced forward lean and longer GCTs facilitate the production of large anterior-posterior (horizontal) forces necessary to overcome inertia and initiate forward motion [3; 13]. As acceleration progresses, athletes adopt a more upright posture and shorter GCTs, resulting in a shift toward a more vertically oriented force application. Consequently, the anterior-posterior component of the GRF decreases in magnitude, though it remains essential for continued propulsion. This necessitates a more rapid and efficient application of horizontal force within a reduced time frame [23; 24]. Such findings have been replicated in the literature across varying athletic populations and varying sprint distances. [PERSON] [25] reported RSI to be significantly related to 30 m (r = \(-\)0.79) and 100 m (r = \(-\)0.75) sprint performance in female competitive sprinters. Reactive strength was assessed via 30 cm DJs with reported contact times of 0.185 s \(\pm\) 0.018. [PERSON] et al. [26] also reported a significant negative relationship between reactive strength (assessed via 30 cm DJ) and acceleration over 8 m (r = \(-\)0.55) among field and court sport athletes. Contact times in the reactive strength assessment were not reported. However, there have been some contrasting results in the literature with respect to the relationship between reactive strength and acceleration or sprinting performance. [PERSON] et al. [27] found no relationship between RSI and sprint performance in male- and female-trained sprinters. Reactive strength was assessed across two testing protocols: 30 cm DJs with a 0.25 s contact constraint and a hopping task in which the hopping frequency was constrained to a frequency of 2.2 Hz. The constrained frequency of hopping produced GCTs of 0.157 \(\pm\) 0.019 and 0.158 \(\pm\) 0.015 for the male and female groups, respectively. The 30 cm DJ protocol produced contact times of 0.170 s \(\pm\) 0.028 and 0.183 s \(\pm\) 0.028 for the male and female groups, respectively. Similar to [PERSON] [28], [PERSON] et al. [27] suggested that the sprint athletes studied may have struggled to tolerate the eccentric loads during the DJ test. [PERSON] reported a wide range of DJ contact times (men: 0.137-0.249 s; women: 0.144-0.227 s), indicating varied reactive strength capacities. [PERSON] further argued that DJ RSI may be a poor indicator of fast SSC performance in heterogeneous groups. DJ contact times were notably longer than those observed during sprint acceleration, leading the authors to question the suitability of RSI for sprint-based assessments. The repeat hopping protocol was also considered inappropriate, as athletes appeared to limit jump height to maintain rhythm, resulting in submaximal efforts that did not reflect true impulse generation. Similarly, [PERSON] et al. [29] found no relationship between RSI and either early acceleration or maximum speed in track and field athletes, though DJ contact times were not reported. [PERSON] and [PERSON] [30] also observed no association between RSI from an incremental DJ protocol (drop heights: 12-84 cm) and sprint performance over 10 m, 30 m, and 60 m in female rugby players; DJ contact times ranged from 0.30 to 0.39 s, exceeding the range typically associated with fast SSC. The extant literature reveals that there is considerable ambiguity in the data regarding the association of reactive strength qualities with acceleration ability. Firstly, previous research utilised reactive strength assessments with contact times that were not indicative of fast SSC performance [30]. Secondly, some research included reactive strength assessments with a level of eccentric loading that was too great for the athletes' abilities [26]. Finally, studies used acceleration split distances of too great a distance, resulting in the aggregation of multiple acceleration phases [28].