Speed and acceleration are integral athletic qualities in rugby union, impacting line breaks, activity rate, metres advanced and tries scored. While there are many methods of training speed, plyometric training utilises similar force-generating mechanisms as sprinting during limited ground contact times, making it ideal to transfer gym-based improvements to the sports field. However, at present a lack of information exists regarding the optimal dose and manipulation of plyometric programme parameters, thereby limiting our understanding of best practice in the context of physical preparation. Thus, the primary purpose of this thesis was to investigate functional and mechanistic characteristics of plyometric training to better inform speed and acceleration training in professional and semi-professional rugby players. Accordingly, the main questions this thesis sought to answer were: 1) What is the lowest necessary plyometric dose for improving sprint performance? 2) What is the best training manipulation of key variables for improving sprint performance? 3) What are the resulting force-velocity sprint profile adaptations from plyometric training and how best do we implement them for position-specific demands? To better understand these questions, extensive literature searches were conducted surrounding plyometric loading mechanisms and adaptations relating to rugby union (narrative literature review) and plyometric variable manipulation (systematic literature review). The former provided evidence for the use of plyometric training for all positions, despite unique positional demands, while the latter critically analysed plyometric variable manipulation and volume loads for improving sprint performance. From an acute perspective, the second section of this thesis provided a more comprehensive understanding of these ideas and their relevance to current practice. Results from a cross-sectional investigation into competition-level and position-specific speed demands reiterated the importance for all players to train speed and acceleration (Chapter 4). Specifically, international, and professional rugby players across all positions (n = 152) showed faster split times (-0.01 to -0.06 s), greater maximal velocity (Vmax: +0.10 to +0.26 m.s-1) and a more force-dominant force-velocity sprint profile (F0: +92.2 to +233.2 N) than academy players (p < 0.01; ES: 0.22 – 1.42). Irrespective of competition level, split times (30 m: 4.402 – 4.046 s) and maximal velocity characteristics (Vmax: 8.02 – 8.97 m.s-1) demonstrated a linear trend across positions, wherein outside backs were the fastest and tight-5 players were the slowest (ES: 0.38 – 2.22; Chapter 4). Notably, loose forwards shared similar force attributes to tight forwards (F0: 955.7 N vs. 918.6 N), but similar maximal velocity characteristics (Vmax: 8.51 m.s-1 vs. 8.64 m.s-1) to inside backs, providing insight to individualised programme needs. For Chapter 5, an international survey of 61 elite strength and conditioning practitioners’ current plyometric practice revealed a large gap between published recommendations and reported practical applications. In particular, 68.4% of international practitioners reported frequently using very low plyometric volumes (≤20 ground contacts (GC)) during off-season periods compared to professional practitioners (30.8%) and semi-professional practitioners (16.7%). Collectively, these real-world findings contrast the current high-volume literary recommendations (120 – 400 GC) (Chapter 5). Additionally, competition-level and sport analysis revealed differences in several plyometric variables including weeks of plyometric training during competition (Chi-Square = 50.65; p < 0.03), sessional volume loads across competition phases (Q = 15.74 – 36.66; p < 0.05), and exercise choice (Chi-Square = 8.83 – 12.62; p < 0.02) (Chapter 5). Results herein provided numerous considerations for practically relevant interventions, attempting to bridge the gap between practice and theory. A unique characteristic of plyometric training, unlike many other forms of training, is using the athlete’s own bodyweight as a primary stimulus, rather than an external load. However, absence of an external load has previously created difficulties in monitoring plyometric quality and volume loads, resulting in scarce information surrounding dose response. This thesis has provided novel insight to the effects of progressively increased horizontal vs. vertical plyometric session volumes on direction-specific kinetic performance (Chapter 6). While rugby players were typically able to maintain or improve performance during plyometric training induced fatigued states, several kinetic characteristics were altered during high-volume sessions (Chapter 6). For example, both training directions resulted in increased eccentric impulse following 40 horizontal GC (+12.9%) and 80 vertical GC (+4.9%) which was generally maintained for the remainder of the session. Moreover, these kinetic fluctuations corresponded with a shift in vertical force producing strategies in the horizontal condition only (p < 0.05). Results therein provided evidence for the use of minimally effective dosing strategies. Accordingly, short-term low-volume plyometric training was found to improve (∆30 m time: -0.020 s; ES = -0.23) or maintain sprint performance (∆30 m time: + 0.049 s; ES = 0.17) better than rugby specific training and resistance training only (∆30 m time: +0.071 s; ES = 0.36), and that the magnitude of adaptation may be in part related to fitness levels (R = 0.434 to -0.568). Moreover, vertical plyometric training was found to improve secondary acceleration (Vertical ∆10 – 20 m split: -0.01 s; ES = -0.28 vs. Horizontal ∆10-20 m split: 0.00 s; ES = -0.10) and force-centric variables (Vertical ES = 0.43 – 0.78 vs. Horizontal ES = 0.13 – 0.35) more so than horizontally applied training (Chapter 7). Further investigation into horizontal dose response showed low-volumes (40 – 60 GC), but not ultra-low volumes (50% reduction) of single-leg horizontal drop jump were sufficient for improving 10-, 20- and 30- m time (-0.03 s to -0.05 s; ES = 0.32 – 0.54) in semi-professional players (Chapter 8). However, both protocols were similarly effective at improving vertical jump performance (19.2 – 22.6%; ES = 1.34 – 1.42). Overall, this thesis adds to the existing literature on plyometric manipulation and minimally effective dose strategies for improving sprint performance, particularly relevant for trained rugby players. Specifically, this thesis provides substantial support for the use of low volume (40 – 60 GC), but not ultra-low volume (<35 GC), moderate-high intensity plyometric training strategies. In particular, practitioners may want to implement exercises facilitating a sizeable and fast eccentric component for adaptations pertinent to maximal speed. Whereas, longer duration exercises enabling greater concentric impulse may be more suited to improving accelerative performance. Additionally, rugby athletes may benefit from greater use of horizontal plyometrics in their programming. However, when introducing a new stimulus practitioners are advised to consider an athlete’s individual characteristics, positional demands, and adaptation periods for optimal programming. Importantly, while acute performance can be maintained in trained individuals during high-volume (i.e., 200 GC) sessions, even moderate volumes may impose kinetic decrements in movement speed and power. These decrements raise questions about the purpose of additional plyometric volumes, and whether high-volume sessions are beneficial or just added stress in trained rugby players.