Table 2.

Study characteristics.

Study	Age (year)a	Height (cm)a	Weight (kg)a	Training status	Weeks	Mean frequency (per week)	Total sessions	Mean weekly jumps	Jump type	Test	Type of stiffness
Chaouachi et al. (2014)68	13.7 ± 0.8	161.5 ± 7.7	45.9 ± 9.7	Inactive (physical education only)	8	3.0	24	292.5	Countermovement jump, line jump (standing distance jump), drop jump + 1 step, front to back cone hops, lateral box jump push off, 1-leg distance jump + 1 step, single-leg cone jumps front to back, single-leg cone jumps side to side, single-leg box push off	Leg stiffness (submaximal hopping test)	Direct
Cornu et al. (1997)5	22.3 ± 2.4	—	—	Athletes (basketball and volleyball)	7	2.0	14	1200.0	Squat jumps, drop jumps, hopping, jumps from high (70 cm) and low (40 cm) platforms, jumps over hedges using 1 or both feet	Mechanical impedance	Indirect
Fouré et al. (2009)27	18.8 ± 0.9	179.2 ± 6.1	68.5 ± 7.1	Athletes (basketball, volleyball, and handball)	8	2.0	16	400.0	Squat jumps, counter-movement jumps, drop jumps from either low (40 cm), medium (60 cm), or high (80 cm) platforms, jumps over hedges using 1 or both feet	Maximal musculo-articular stiffness with dynamometer	Direct
Fouré et al. (2010)26	18.8 ± 0.9	177.3 ± 6.2	68.4 ± 6.5	Active (10.5 h per week)	14	2.4	34	485.7	Squat jumps, counter-movement jumps, drop jumps from either low (40 cm), medium (60 cm), or high (80 cm) platforms, jumps over hedges using 1 or both feet	Maximal musculo-articular stiffness with dynamometer (stiffness index)	Direct
García-Pinillos et al. (2020)64	27.2 ± 8.6	172.0 ± 10.0	66.0 ± 10.4	Recreationally trained (3‒5 weekly running sessions)	10	3.2	32	1000.0	Bilateral and unilateral—alternating jump rope	Arch stiffness	Indirect
Hirayama et al. (2017)69	22.0 ± 3.0	172.0 ± 5.8	66.9 ± 10.5	Recreationally active with no plyometric experience	12	3.0	36	300.0	Unilateral depth jumps	Achilles’ tendon stiffness (ultrasound)	Direct
Houghton et al. (2013)70	21.0 ± 4.0	174.6 ± 3.1	73.7 ± 10.3	Athletes (no prior plyometric experience)	8	1.9	15	223.1	Land off box, standing long jump in squat jump position, standing long jump, forward jump over hurdle, vertical countermovement jump, lateral jump over hurdle, reactive jumps, forward jump (50 cm apart), bounding forward hurdles, forward jumps to box, box jumps, bounding forward repeated hurdles, forward jumps, lateral jumps to box, drop jumps, drop jump and jump over hurdle	Achilles’ tendon stiffness at 90% MVC (dynamometer)	Direct
Jeffreys et al. (2019)58 (HPG)	20.3 ± 1.6	182.0 ± 5.0	91.6 ± 10.4	Trained (1‒2 years plyometrics)	6	2.0	12	320.0	Standing vertical jumps (tuck jumps), multiple 2-foot hurdle jumps, repeated 2-foot jumps (horizontal), alternate leg bounds, lateral 2-foot jumps, multiple 2-foot hurdle jumps, single-foot hops, drop jumps, lateral 1-foot jumps, single-foot drop jumps	Leg stiffness (force plate)	Indirect
Jeffreys et al. (2019)58 (LPG)	20.3 ± 1.6	183.0 ± 5.0	91.6 ± 10.4	Trained (1‒2 years plyometrics)	6	2.0	12	80.0	Standing vertical jumps (tuck jumps), multiple 2-foot hurdle jumps, repeated 2-foot jumps (horizontal), alternate leg bounds, lateral 2-foot jumps, multiple 2-foot hurdle jumps, single-foot hops, drop jumps, lateral 1-foot jumps, single-foot drop jumps	Leg stiffness (force plate)	Indirect
Laurent et al. (2020)65 (KE)	22.5b	180.5 ± 5.8	68.7 ± 14.0	Active but untrained	10	2.0	20	298.0	Stationary hopping, drop jump on the spot, drop jump with displacement	Achilles’ tendon stiffness (ultrasound)	Direct
Laurent et al. (2020)65 (KF)	22.5b	180.9 ± 10.5	69.7 ± 10.8	Active but untrained	10	2.0	20	298.0	Stationary hopping, drop jump on the spot, drop jump with displacement	Achilles’ tendon stiffness (ultrasound)	Direct
Lloyd et al. (2012)71 (G12)	12.3 ± 0.3	151.8 ± 7.9	44.8 ± 9.4	Inactive (physical education only)	4	2.0	8	92.5	Squat jump, countermovement jump, pogo hopping, standing long jump, lateral hops, hop scotch, bilateral “power” hops, ankle jumps, “power” skipping, unilateral pogo hops, max rebound hops, drop jumps, hurdle “power” hops, total foot contacts	Absolute leg stiffness (submaximal hopping)	Indirect
Lloyd et al. (2012)71 (G15)	15.3 ± 0.3	174.4 ± 6.6	65.0 ± 8.9	Inactive (physical education only)	4	2.0	8	92.5	Squat jump, countermovement jump, pogo hopping, standing long jump, lateral hops, hop scotch, bilateral “power” hops, ankle jumps, “power” skipping, unilateral pogo hops, max rebound hops, drop jumps, hurdle “power” hops, total foot contacts	Absolute leg stiffness (submaximal hopping)	Indirect
Lloyd et al. (2012)71 (G9)	9.4 ± 0.5	133.2 ± 8.7	32.6 ± 7.0	Inactive (physical education only)	4	2.0	8	92.5	Squat jump, countermovement jump, pogo hopping, standing long jump, lateral hops, hopscotch, bilateral “power” hops, ankle jumps, “power” skipping, unilateral pogo hops, max rebound hops, drop jumps, hurdle “power” hops, total foot contacts	Absolute leg stiffness (submaximal hopping)	Indirect
Spurrs et al. (2003)15	25.0 ± 4.0	178.0 ± 4.0	72.4 ± 5.5	Trained athletes (10 years)	6	2.5	15	131.1	Squat jump, split scissor jump, double-leg bound, alternate leg bound, single-leg forward hop, depth jump, double-leg hurdle jump, single-leg hurdle hop	Musculotendinous stiffness (seated calf raise)	Direct
Wu et al. (2010)25	22.1 ± 1.6	174.4 ± 7.6	65.8 ± 8.6	Untrained students	8	2.0	16	180.0	Squat jump, split squat jump, cycled split squat jump, pike jump, double-leg tuck jump, double-leg zigzag hop, double-leg hop, in-depth jump, box jump	Stiffness (Chemomechanical potentiation)	Direct

Abbreviations: G9 = age 9 experimental group; G12 = age 12 experimental group; G15 = age 15 experimental group; HPG = high volume plyometric group; KE = knees extended; KF = knees flexed; LPG = low volume plyometric group; MVC =  maximum voluntary contraction.

^aThe data are presented as mean ± SD.

^bThese data are represented by a median.