The interaction of resistance training and maturation on drop jump kinetics in young boys
Existing paediatric literature has highlighted the various growth- and maturation-related neuromuscular changes that children experience as they move through adolescence into adulthood. These changes have been linked to increased force output, sprint velocities and jump height (Radnor et al., 2021; Radnor et al., 2018; Tumkur Anil Kumar et al., 2021). The performance of these movements is underpinned by the stretch shortening cycle (SSC) muscle action, which involves a braking phase, rapidly followed by a propulsive phase (Komi, 2000). Drop jumps (DJ) are rebound based tasks where the sequence of muscle actions involved in the execution replicate those observed in locomotive tasks, such as sprinting, making it an appropriate tool for the examination of SSC function. When using the DJ as an assessment tool, the measures reported by previous studies have primarily been simple field-based metrics such as jump height, ground contact time and reactive strength index (RSI). However, these metrics fail to provide an insight into the phases of the jump and differences in jump strategy, and it therefore becomes necessary to look beyond these measures and examine the kinetics during the braking and propulsive phase of the DJ. Study 1 identified that jump height has a relatively greater influence than ground contact time on RSI during a DJ, and subsequently revealed that jump height, ground contact time and RSI might best represent relative net impulse, relative propulsive force and relative power, respectively. The study also identified the various kinetic qualities that these performance measures failed to reflect, thereby highlighting the need for the examination of DJ kinetics. Study 2 established moderate to excellent relative reliability and acceptable absolute reliability for most DJ kinetic variables across all maturity groups (ICC = 0.59 to 0.91; CV = 3.9% to 9.9%), with the acceptable threshold for random variation being exceeded in some cases, such as braking peak force and braking rate of force development (CV = 10.4% to 34.3%). The findings of the study revealed similar magnitude of differences in the kinetic variables between successive maturity groups and highlighted that the more mature participants exhibited better SSC performance. Study 3 highlighted that post-PHV boys responded better to a short-term combined strength and plyometric training intervention compared to their pre-PHV counterparts. The post-PHV boys exhibited large improvements in mean and peak forces, impulse, power, work done and centre of mass velocity, underpinning small to moderate improvements in jump height and RSI. While the pre-PHV boys exhibited moderate increases for several kinetics variables, they showed no significant changes for majority of the kinetics resulting in small increases in jump height and leaving RSI unaffected. Study 4 revealed that the combination of neuromuscular and traditional resistance training elicited improvements in all absolute impulse, as well as braking and propulsive work done, underpinning small increases in jump height but leaving RSI unchanged. While findings from the study suggested that the combination of neuromuscular and traditional resistance training might help to enhance training-related adaptations, they also highlighted that the adaptations following resistance training are specific to the nature of the stimulus.