Should I perform my plyometrics training with added load?

Plyometrics are accepted in the realm of strength and conditioning as an effective and reliable mean to improve performance in power-based movements such as jumping and sprinting.

The topic is heavily researched as there are still uncertainty on the most effective way to train plyometrics to yield maximal gains. Whether it be the type of jumps that should be performed and if any additional external load should be added.
If proven useful, you would also look at the most effective way to add external load the an athlete performing plyometric training.

In the world of traditional weightlifting, it is a common assumption that increased load (almost) always leads to improvements, but what about plyometrics training?

Research papers on the subject compare the effect of loaded and unloaded training protocols by measuring performance in movements such as: counter-movement jumps (CMJ), squats jumps (SJ), drop jumps (DJ) and sprinting (5, 10, 20 and/or 50 meters).

Loaded jump training is performed with external load applied through weight vests (WV), dumbbells (DB) or plates, and resistance bands.

Overall, the research seems to converge towards an agreement that performing plyometrics training with added load leads to increased performance when compared to unloaded training.
However, not all methods of loading yield the same results. 

Loaded jump training (LJT) performed with weight vest have shown better results than LJT performed with dumbbells with equal load (between 8-11% body mass) [1,2,5,8]. Moreover, Harrison et al. [8]  claim that jumps performed with dumbbells as external load can disrupt the relative timing of the jump action. We can presume that hand-held loading may interfere with the natural arm sequence occurring during a CMJ, leading to a decrease in performance. 

One research article by Sheppard et al. [9] did find improvements with accentuated eccentric load CMJ using hand-held loading. However, the loading was much greater than similar research (45 lbs. plate in each hand compared to 25 lbs. dumbbell/weight vest) and the arm swing motion was modified for the bodyweight group to match this of the loaded group. 

They suggested the improvements were correlated to the unique and novel training stimulus offered by accentuated eccentric loaded jumps, promoting superior results in a population already performing high volumes of body weight jumping as part of they regular training.  

Multiple research papers also assessed the effectiveness of plyometrics as a mean to improve sprinting performance. 

Loaded plyometrics have seen greater improvements than their unloaded variations [5] but gains remain minimal when compared to training regimens featuring heavy squat protocols (40-60% 1RM) [4].

It is recommended to combine traditional weight lifting and plyometrics to maximise improvements in power and acceleration-based movements [3].

Regarding the safety of performing loaded power-based movements, Jansen et al. [6] examined the kinetics and kinematics variables of landings following unloaded and loaded CMJs and found no noticeable differences. This suggests that performing loaded jump does not correlated to an increases in injury risk. 

Bottom line, plyometrics performed with moderate loads between 8-12% BW, preferably using a weight vest, stand as an effective training method to improve jumping performance but fails to generate satisfying results when it comes to sprinting.

References

1. Kobal, R., Pereira, L., Zanetti, V., Ramirez-Campillo, R. and Loturco, I. Effects of Unloaded vs. Loaded Plyometrics on Speed and Power Performance of Elite Young Soccer Players. Frontiers in Physiology (2017).

2. Khlifa, R, Aouadi, R, Hermassi, S, Chelly, M, Jlid, M, Hbacha, H, and Castagna, C. Effects of a Plyometric Training Program With and Without Added Load on Jumping Ability in Basketball Players. J Strength Cond Res 24(11), 2010.

3. Rodriguez-Rossell, D, Torress-Torrelo, J, Franco-Márquez, F, González-Suarez, J, González-Badillo, J. Effects of light-load maximal lifting velocity weight training vs. combined weight training and plyometrics on sprint, vertical jump and strength performance in adult soccer players. J of Science and Medicine in Sport 20(7), 695-699, 2017.

4. de Hoyo, M, Gonzalo-Skok, O, Sañudo, B, Carrascal, C, Plaza-Armas, JR, Camacho-Candil, F, and Otero-Esquina, C. Comparative effects of in-season full-back squat, resisted sprint training, and plyometric training on explosive performance in U-19 elite soccer players. J Strength Cond Res 30(2): 368–377, 2016.

5. Negra, Y, Chaabene, H, Sammoud, S, Prieske, O, Moran, J, Ramirez-Campillo, R, Nejmaoui, A, and Granacher, U. The Increased Effectiveness of Loaded Versus Unloaded Plyometric Jump Training in Improving Muscle Power, Speed, Change of Direction, and Kicking-Distance Performance in Prepubertal Male Soccer Players. Int J Sports Phys and Performance 15(2), 189-185, 2020.

6. Janssen, I, Sheppard, J, Dingley, A, Chapman, D, and Spratford, W. Lower extremity kinematics and kinetics when landing from unloaded and loaded jumps. (2012). Faculty of Science, Medicine and Health – Papers: part A. 122, 2012.

7. Psycharakis, S, Eagle, S, Moir, G, Rawcliffe, A, Mckenzie, C, Graham, S, Lamont S, and Connaboy C. Effects of Additional Load on the Occurrence of Bilateral Deficit in Counter-Movement and Squat Jumps, Research Quarterly for Exercise and Sport, 2019.

8. Harrison, A, Byrne, P, and Sundar, S. The effects of added mass on the biomechanics and performance of countermovement jumps. J Sport Sciences, 37(14), 1591-1599, 2019.

9. Sheppard, J, Hobson, S, Barker, M, Taylor, K, Capman, D, McGuigan, M, and Newton, R. The Effect of Training with Accentuated Eccentric Load Counter-Movement Jumps on Strength and Power Characteristics of High-Performance Volleyball Players. Int J Sport Science & Coaching 3(3), 355-363, 2008.