Table 1.

Participants, intervention, comparators, outcomes, and study design (PICOS) framework for study inclusion and exclusion criteria.

Category	Inclusion criteria	Exclusion criteria
Participant	Healthy males and females of any age	Individuals who had sustained a recent injury
Intervention	A plyometric jump training programme that conformed to the following definition: “Lower body unilateral and bilateral bounds, jumps, and hops that utilise a pre-stretch or countermovement that incites usage of the stretch-shortening cycle”31,68	Interventions that were carried out in conjunction with alternative training methods such as strength or balance training Interventions that were carried out in water or that used additional manipulative techniques such as electrostimulation Interventions <3 weeks
Comparator	Studies must have included an experimental group that undertook plyometric training and a control group to which it could be compared. The control group could not have been engaged in any plyometric training	Studies that did not have a control group
Outcome	Each study must have included a measure of direct or indirect lower body stiffness, taken both prior to and after the intervention period. Lower-limb stiffness can be assessed using either laboratory measures such ultrasonography to quantify muscle and tendon stiffness directly29,31 or field-related measures such as vertical hopping stiffness.11 In our meta-analysis, when the included studies used methods to quantify muscle and/or tendon stiffness, the resultant term tissue stiffness is used. In contrast, when included studies used field-based measures, the term quasistiffness is used.3,64 The classification of stiffness, as we judged it, must have conformed to one of the following, as described by Latash and Zatsiorsky3: Stiffness: The elastic properties of tendons and passive muscles Apparent stiffness: The response of active muscles to stretch forces Quasistiffness: Proxies of the above qualities as measured with tests such as rebound hopping	Studies with no measure of stiffness, apparent stiffness or quasistiffness
Study design	Controlled training intervention studies containing 2 independent groups for comparison	The second treatment sequence of a crossover study, cross-sectional studies, or studies that evaluated acute performance variables only (i.e., postactivation potentiation)