The aim of this study was to compare the reliability of mechanical and neuromuscular measures during common plyometric exercises and to ascertain if these measures can differentiate plyometric intensity. 7 recreationally active male participants performed 7 plyometric exercises: countermovement jump, rebound jump, 30cm drop jump, 40cm drop jump, hop, rebound hop, step hop. Reactive exercises were defined as those involving ground impact (eg drop jump) rather than beginning and ending with feet in contact with the ground eg countermovement jump.
Surface electromyography (SEMG) was measured at the vastus lateralis (VL) and biceps femoris (BF) with consideration to eccentric (ECC) vs concentric (CON) phases of movement. Mechanical measures of ground reaction forces included: peak force (PF), peak eccentric power (PEP), and impulse (IMP). Intraclass correlation coefficients indicated high reliability (r>0.82) for all SEMG results excluding ECC-VL during 40cm drop jump. Mechanical measures demonstrated high reliability (r>0.85).
Authors determined that CON muscle activity did not differ between reactive and non-reactive exercises when intent was maximal. However, ECC muscle activity was higher during reactive exercises. Additionally, all mechanical measures could distinguish between reactive and non-reactive movements with reactive exercises consistently demonstrating higher PF, PEP and IMP.
Concentric muscle activity was the same regardless of the type of contact/impact made during the movement (reactive vs non-reactive OR plyometric vs ballistic). This could indicate a benefit of using plyometric exercises with athletes participating in sports with mixed biomechanical demands. For example, a sprinter is thought to benefit from ballistic/non-reactive movements particularly to improve acceleration. This phase of a sprint involves longer contact times and consequently greater impulse generated to overcome the inertia of the athlete’s body mass. This is thought to be best developed using movements with similar contact times, emphasising an explosive concentric phase. During later phases of the sprint, contact times will decrease requiring greater musculotendinous stiffness to perform a quicker physical reaction to the ground and maintain speed.
The current findings indicate that reactive movements generate equal concentric forces when performed with maximal intent. Therefore, if an athlete is capable of performing reactive drills and is physically prepared to do so (i.e. they demonstrate good posture and mechanics during landing and have undergone a period of physical preparation) then they should spend a lot more time doing reactive vs non-reactive exercises. The coach should ensure the athlete is ready for the intensity experienced and regress the intensity/type of movement if they need time to build competent dynamic and explosive movements.
Rather than replicating the physical contact times of the sport, a coach should aim to physically prepare the athlete for focussed plyometric/reactive training.
Jarvis, M. M., Graham-Smith, P., & Comfort, P. (2014). A Methodological Approach to Quantifying Plyometric Intensity. Journal of strength and conditioning research/National Strength & Conditioning Association.