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Understanding The Reactive Strength Index (RSI)

Oppdatert: 11. jul. 2023

By Manuel Alfaro - Sport Performance Engineer

The Reactive Strength Index (RSI) is one of the metrics utilized to evaluate the performance of our body's locomotion system. An integral component of this system is the muscle-tendon unit. This unit illustrates that muscles are interconnected with bones via tendons rather the muscle being directly attached to them. The RSI specifically measures the muscle-tendon unit’s capability to produce a powerful contraction following a swift pre-contraction stretch, a process known as the Stretch-Shortening Cycle (SSC). In essence, the RSI provides insights into the functioning of the muscle-tendon unit and the SSC, particularly how our muscles and tendons work together to maximize power output and efficiency. Plyometric training is one type of training that leverage the SSC and consequently, helps in enhancing the RSI. Plyometric training involves quick, powerful movements that stretch and then immediately contract the muscle-tendon unit much like the action of jumping. However, to truly appreciate the significance of the RSI and its relationship with plyometric training, it is essential first to investigate the anatomy and physiology of the muscle-tendon unit and the SSC. Through this approach, we can fully understand the underlying mechanisms that impact the RSI and how it can be used to enhance athletic performance.

In the following article, we aim to clarify the interactions within our musculoskeletal system understanding the muscle-tendon unit and the SSC, and how these elements relate to the RSI and plyometric training. By gaining a comprehensive understanding of the RSI, we can provide strength and conditioning coaches with insights that enable them to customize plyometric exercises for specific individuals, helping them achieve their performance goals more effectively.

Energy flow within The Muscle-tendon Unit

The muscle-tendon unit is an integral part of our body's locomotion system, where muscles actively generate force and tendons, acting as springs, store and release this force to amplify the movement. This interplay between muscles and tendons regulates the flow of energy in our body, which can be understood through three primary patterns (Figure 1):

  1. Energy conservation (A): The body's energy can be saved and used later by our tendons. For example, when we run, the energy from our foot hitting the ground is saved in the tendons in our legs as they stretch. This stored energy is then used to help push our body forward for the next step.

  2. Power amplification (B): The energy generated by a muscle contraction is stored in a tendon and released when the muscle relaxes, thereby boosting the overall force of movement.

  3. Power attenuation (C): When landing from a jump, energy is temporarily stored in a tendon to reduce the impact. This process involves a muscle lengthening in an eccentric (stretching) motion to dissipate the force of the landing. This works like a shock-absorber, reducing the impact on muscles and joints.

Figure 1. Muscle-tendon unit energy flow: A graphic representation showing how the movement of energy within muscle-tendon systems guides mechanical operations. (Roberts, T. J., & Azizi, E. (2011)).

The Stretch-Shortening Cycle

The SSC is a crucial part of the neuromuscular process and the muscle-tendon complex that underlies powerful actions like jumps. The SSC involves a rapid sequence of muscle-tendon lengthening (stretching) followed by muscle-tendon contracting (shortening), which is particularly important in movements that involve an impact that induces stretch, like jumping or plyometric exercises. The ability of the muscle-tendon unit to leverage the SSC for powerful contraction after a stretch is called reactive strength. Reactive strength is often measured using the RSI. The body´s effectiveness of this ability is influenced by four key factors, including:

  • Tendon Properties and Energy Transfer: The potential energy to elastic energy transition within the tendons and the stiffness of the tendons both play crucial roles. The stiffer the tendons, the more efficiently they can store and release elastic energy during the SSC. This property is essential for the rapid transfer of forces during SSC activities.

  • Neural Activation and Muscle Pre-activation: The activation level of the nervous system significantly impacts reactive strength. A preceding eccentric contraction can increase this activation level, allowing the following concentric phase to begin from a higher level of activation increasing the power production.

  • Stretch Reflex Mechanism: The reflex pathways detects muscle lengthening and subsequently trigger a contraction. This plays a vital role in the SSC. This reflex mechanism, known as the stretch reflex, is a natural response that aids in the production of force during SSC movements.

  • Actin-Myosin Bridging: The ratio of actin and myosin bridges also influences the SSC. A previous eccentric contraction increases this ratio, creating more cross-bridges between these protein filaments. This setup provides a better mechanical position for the muscles to generate force in the subsequent concentric contraction.

Plyometrics and the SSC

Plyometric training is a type of exercise that focuses on enhancing athletic performance, improving physical fitness levels, and boosting overall strength and agility, making it a popular choice among athletes and fitness enthusiasts. The core idea of this training revolves around the concept of the SSC. Simply put, plyometric training involves a rapid sequence of movements that stretch and then immediately contract your muscle-tendon unit, similar to the motion of jumping. By doing this, we aim to optimize the muscle-tendon unit's performance, allowing the body to produce more force and power. Plyometric training movements can involve a fast SSC or slow SSC.

Fast SSC:

  • Fast SSC occurs when the ground contact time (GCT) during a movement is less than 250 milliseconds, such as a drop jump. In a drop jump, the body is subjected to a high load over a short period, inducing a quick stretch of the muscle-tendon unit.

  • Fast SSC requires a rapid transition from muscle-tendon stretching to muscle-tendon shortening to achieve powerful movement outputs, and the joints involved have a limited range of motion or their movement is fixed.

  • Fast SSC primarily trains an athlete's reactivity and ability to produce force rapidly, honing the nervous system, reflex pathways, and characteristics/stiffness of tendons due to reduced joint excursion and short GCT. In these fast SSC scenarios, muscles usually work isometrically or only concentrically because the impact forces are not as significant, requiring less extensive muscle eccentric work and lengthening, allowing the tendons and the nervous system play a more prominent role.

Slow SSC:

  • Slow SSC occurs when the GCT during a movement is more than 250 milliseconds, such as in the CMJ. In this movement, GCT is often more than 500 milliseconds.

  • The slow SSC involves a slower, more deliberate loading phase (muscle-tendon stretching), allowing for a greater build-up of elastic energy in the muscle-tendon unit before being released in the shortening phase.

  • The slow SSC is typically used in movements requiring a higher force output, as it provides a longer period for force generation during the loading phase.

  • It primarily trains an athlete's maximal muscular strength and power, contributing to overall improvements in performance in explosive strength-demanding activities.

The ground contact time during plyometric exercises is an important variable to consider for strength and conditioning coaches. By examining ground contact times, coaches can understand what type of SSC (fast or slow) is being used, which helps adjust the plyometric exercises to the specific demands of an athlete's sport or the task they wish to improve.

Plyometrics and The Reactive Strength Index (RSI)

The RSI is a tool used to quantify plyometric or SSC performance. It is derived from the height achieved in a jump and the time spent on the ground developing the forces needed for that jump. The RSI is calculated by dividing the jump height by the ground contact time before take-off (Figure 2). RSI reflects an individual's ability to change quickly from the stretch to the shortening phase and is considered a measure of "explosiveness". It's also been described as a tool to monitor stress on the muscle-tendon unit.

Figure 2. Formula for calculating the RSI: The Reactive Strength Index can be increased by improving jump height, minimizing ground contact time, or implementing a combination of both strategies (Flanagan, E. P., & Comyns, T. M. (2008)).

Understanding the SSC, which involves both fast and slow SSC, is vital in interpreting RSI values in different plyometric exercises.

Fast SSC movements, characterized by short GCT, are often observed in exercises such as low drop jumps. A high RSI in these movements indicates an efficient transfer of elastic energy stored in stiff tendons and a responsive nervous system that rapidly triggers muscular contractions. The muscle work in fast SSC exercises is primarily isometric or concentric, with the tendons taking the lead. However, if an athlete records a lower RSI in these movements, it could be a signal of some potential deficits. Firstly, it may hint at sub-optimal tendon stiffness, affecting the ability to efficiently store and release elastic energy. Secondly, it could indicate a reduction in muscular strength. In cases where the muscle strength is not sufficient to maintain an isometric contraction under the impact forces, the muscle will resort to an eccentric contraction, which prolongs the ground contact time, thus reducing the RSI.

Slow SSC movements are characterized by longer GCT as seen in the countermovement jump. These exercises offer an extended time for the muscle-tendon unit to generate and release force, leading to a higher force output. Although a higher RSI in slow SSC movements signifies improvements in muscle power generation, the roles of tendon stiffness and energy transfer shouldn't be disregarded. Hence, slow SSC exercises predominantly train an athlete's maximal muscular strength and power while simultaneously influencing the efficiency of the tendons.

In all plyometric exercises, it's important to remember that muscles and tendons function as an interconnected system. Each component's performance can impact the other, contributing to the overall efficiency of the SSC and, ultimately, the RSI. Therefore, a balance between fast and slow SSC exercises is crucial to maximizing improvements in both muscle power and tendon stiffness. This balanced approach will lead to a more comprehensive enhancement of the individual's reactive strength, as quantified by the RSI.

In conclusion, understanding the relationship between RSI and the SSC can guide coaches and athletes in making more informed decisions about the appropriate plyometric exercises to incorporate into their training programs. This will enable them to meet specific athletic demands and objectives, tailoring training to optimize the use of the muscle-tendon unit and improve overall performance.

Practical applications of the RSI

The AlphaPWR system utilizes the Reactive Strength Index (RSI) during plyometric exercises such as repeated jumps, which have a clear ground contact phase. Repeated jumps are a frequently used and researched plyometric activity. In this exercise, an athlete continuously jumps for a certain number of repetitions without a pause between jumps. Given that the RSI is a ratio of ground contact time to jump height, both these factors must be considered when evaluating the overall RSI score.

The duration of ground contact during plyometric exercises is a crucial factor for strength and conditioning trainers to monitor. By analyzing the ground contact time while a plyometric exercise is being performed, the supervising coach can accurately determine the type of SSC, fast or slow, being employed. The principle of specificity (the concept that training should be relevant and appropriate to the sport for which the individual is training), suggests that the requirements of an athlete's sport or the requirements of a task where an athlete wants to enhance performance will directly influence how the plyometric exercises should be executed. The jump height in vertical jumping actions reflects an athlete's power production capabilities. Monitoring both the jump height and the RSI during plyometric training can help ensure that athletes are performing with high effort and maximal power production. For example, athletes aiming to increase maximum jump height may benefit from longer ground contact times to generate maximum force and jump height, while those wishing to improve maximum velocity sprinting speed, which relies on fast SSC utilization, would need plyometric training with shorter contact times.

However, if a coach only considers GCT during plyometric training, athletes may adjust their jumping strategies to reduce GCT, but potentially at the cost of their power output. Conversely, if jump height is the only variable considered, athletes might produce high power outputs, but with longer GCT, which would go against the principle of specificity in training.

Utilizing RSI Feedback to Boost Performance

The AlphaPWR system revolutionizes plyometric training by providing instant feedback on jump height, ground contact time, and RSI. This capability is not just about collecting data; it serves a vital role in enhancing athletic performance.

Studies underline the significant impact that specific guidance can have on improving jumping performance. As an example, Arampatzis et al. discovered that when athletes are encouraged to “jump high and a bit quicker than your last jump”, it often results in significantly shorter GCT during depth jumps, compared to simply instructing them to “jump as high as possible”.

Feedback is a potent motivator in plyometric training. With the AlphaPWR system, athletes can immediately see their jump heights, GCT, and RSI after each attempt. This instant knowledge about their performance can push them to give near-maximum effort in their exercises, optimizing the benefits of the training. By offering real-time performance metrics, the AlphaPWR system bridges the gap between effort and understanding, enabling athletes to maximize their training outcomes. This immediacy of feedback is what makes the AlphaPWR system not only an innovative tool but a game-changer in the world of plyometric training.

Thank you for choosing Alphatek, and for allowing us to be part of your journey to enhancing athletic performance! We look forward to your feedback on our new RSI feature and the impacts it brings to your training routine.

Keep training, keep evolving, and stay Alpha!


Roberts, T. J., & Azizi, E. (2011). Flexible mechanisms: The diverse roles of biological springs in vertebrate movement. The Journal of Experimental Biology, 214, 353-361.

Flanagan, E. P., & Comyns, T. M. (2008). The use of contact time and the reactive strength index to optimize fast stretch-shortening cycle training. Strength and Conditioning Journal, 30(5), 32-38.

Arampatzis, A., Schade, F., Walsh, M., & Brüggemann, G. P. (2001). Influence of leg stiffness and its effect on myodynamic jumping performance. Journal of Electromyography and Kinesiology, 11(5), 355–364.

Ramirez-Campillo, R., Thapa, R. K., Afonso, J., Perez-Castilla, A., Bishop, C., Byrne, P. J., & Granacher, U. (2023). Effects of Plyometric Jump Training on the Reactive Strength Index in Healthy Individuals Across the Lifespan: A Systematic Review with Meta-analysis. Sports Medicine, 53, 1029–1053.

Ebben, W. P., & Petushek, E. J. (2010). Using the reactive strength index modified to evaluate plyometric performance. Journal of Strength and Conditioning Research, 24(8), 1983–1987.

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