THE ULTIMATE GUIDE TO PLYOMETRICS
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Plyometrics are often the final stage of a physio’s rehab program as they involve teaching the patient how to apply their newly gained strength at rapid speeds that will see them excel on the field and have a reduced risk of injury. But when it comes to prescribing plyometrics for our patients, how do we know where to start and how to progress these exercises to ensure that we’re maximizing adaptations and aren’t just guessing.
To begin, let’s look at the fundamentals of plyometrics. Plyometric exercises are rapid and powerful movements that involve the application of force into the ground at a high velocity. Some examples of these types of movements are bounding, hurdle jumps and drop jumps. At the basis of all plyometric movements is the stretch-shortening cycle (SSC) of the musculotendinous unit (MTU). The SSC is made up of two phases, the eccentric phase where the MTU is lengthened prior to contraction and the concentric phase where the MTU shortens to produce a rapid movement. In the case of a box jump, the eccentric phase is where your hips, knees and ankles move into a flexed position as you squat down and the concentric phase is the jump which involves extension of the hips, knees and ankles. The SSC is allowing the body to produce movements in a more efficient manner that requires less energy than a purely concentric motion. For instance, you can jump a lot further if there’s an eccentric phase preceding the concentric phase as opposed to a concentric phase only.
The mechanisms responsible for the SSC are still not well understood, however, they are thought to be due to several factors:
1. Storage and release of tension
Mammalian tendons are composed almost exclusively of collagen fibrils that are highly structured and organized into collagen fibers which are the main structural component of tendons (7). Collagen is viscoelastic, meaning that it is capable of elongating and then returning to its original length. This can be observed during a countermovement where the tendon elongates and while doing so absorbs potential energy. This energy is then released during the concentric contraction which enhances power output while reducing the energy demands of the working muscles. Interestingly it has been shown that during the eccentric phase of the SSC the muscle fibers remain isometric or even shorten (6). This is important because it suggests that the tendon alone stores all of the potential energy during the SSC.
2. Residual force development
Residual force enhancement (RFE) refers to an increase in force output when an isometric or concentric concentration is immediately preceded by an eccentric contraction (1). For a long time, RFE was thought to be due to sarcomere length non-uniformity. It is well established that when a muscle lengthens, the sarcomeres within muscle fibers do not lengthen in a uniform manner. Some will lengthen more than others, creating non-uniformity in sarcomere length (1). The popular theory once held by many was that when a muscle is actively stretched along the ‘unstable’ descending limb of the force-length relationship a small number of weaker sarcomeres would be stretched beyond myofilament overlap. This would allow most of the remaining sarcomeres to undergo minimal stretch and remain closer to the optimal ‘plateau’ position of the length-tension relationship, thereby allowing the muscle to generate more force (3).
However, recent studies have shown that sarcomere length non-uniformity does not explain what happens during eccentric contractions and therefore does not explain RFE. Researchers (2) stretched active and passive myofibrils beyond myofilament overlap and found that the forces produced by actively stretched myofibrils were 2-4 times greater than those passively stretched. And this effect was abolished when the protein titin was eliminated. In addition to this, RFE was also seen in sarcomeres that demonstrated sarcomere length uniformity (4) and the forces produced following an eccentric contraction have been shown to vastly exceed those produced by sarcomeres at optimal lengths (2).
Therefore the cross-bridge model cannot explain the observed increased force production during eccentric contractions nor does is explain RFE. Based on the experimental observations and theoretical support it has been proposed that the sarcomeric protein titin is responsible for this phenomenon (9). Titin is attached to myosin filaments and during active muscle lengthening calcium binds to the protein which triggers titin to increase its stiffness. It also causes titin to bind to actin which reduces its free spring length and allows for greater force production during eccentric contractions and explains the observed residual force enhancement.
Although, research that shows no muscle lengthening occurs during the SSC would refute the importance of RFE as a potential mechanism.
3. The Time Available for Force Development
The eccentric portion of a plyometric movement provides time between the initiation of movement and the concentric phase where power is expressed. In fact, it takes between 300-500ms to achieve 90% of maximum voluntary force (10). This time delay is due to the time taken for the central nervous system to generate the signal, for the signal to travel through the central and peripheral motor neurons and then for the machinery within the muscle to generate force. If the concentric contraction begins as soon as the force starts to rise, the force produced will be sub-maximal. This is especially so in rapid plyometric movements that typically only last between 200-400ms. A prior counter movement allows time for force to increase so that when the concentric contraction begins, the amount of force generated will be greater than without the countermovement.
4. The Stretch Reflex
It has been thought that during the countermovement, muscle stretch is detected by muscle spindles which then triggers the muscle spindle reflex (8). This increases the activation of motor neurons and their associated muscle fibers, thus allowing a supra-maximal force to be generated during the concentric phase. However as previously mentioned, research suggests that there is no muscle fiber lengthening (6) and therefore no trigger for stretch reflexes. This is supported by research that shows no significant increase in muscle EMG when comparing plyometrics with and without a pre-stretch (9).
Credit picture: www.muscleandstrength.com
PLYOMETRICS IN PRACTICE
Now that we understand how the SSC works (or rather understand that we don’t fully understand), let’s look at it practically. The SSC can be classified as either slow or fast depending on ground contact time (GCT). Fast SSC have a GCT of less than 0.25s and involve a fast eccentric phase with a rapid transition time between the eccentric and concentric phase. Slow SSC have a GCT longer than 0.25 seconds and a longer eccentric phase with a slower transition.
Despite being assessed with the same jump tests by practitioners, slow and fast SSC are poorly correlated to each other and represent different physical capacities. Slow SSC are more closely correlated to maximal strength while fast SSC are more representative of reactive strength. In a study by Young et al. (2015) a plyometric program training the fast SSC improved this quality by 20% whilst having no effect on the slow SSC as measured by maximum jump height.
That being said, there is a positive correlation between maximal strength and reactive strength. In a study of 20 academy rugby players, those with higher levels of maximal strength had greater reactive strength levels (16). However, only 40% of the variance in reactive strength could be explained by maximal strength and so 60% of reactive strength is associated with other factors. This is a fairly common observation as there are many athletes who demonstrate high levels of power but are quite weak when it comes to the big lifts.
Reactive strength a simple ratio measured by how high you can jump and how quickly you can do so. It is calculated by dividing the height of the jump by ground contact time. For instance, an athlete jumping 40cm with a contact time of 0.2s would have a RSI of 2.0 units (0.4m / 0.2s). The RSI is also a measurement of stress on the calf and Achilles muscle/tendon units as well as being significantly correlated to 10m acceleration times and change of direction ability (11).
Countermovement jumps involve a relatively long eccentric phase and as such are a great way to assess an athlete’s ability to exert maximal force using a slow SSC. And to assess an athlete’s ability to exert force rapidly using a fast SSC, the RSI can be measured using a force plate with the following tests:
1. Drop/ depth jumps. The athlete performs a drop jump from a range of drop heights (for example 15, 30, 45, 60cm) and the practitioner records the jump height, GCT and RSI for each. This will allow you to identify the drop height for an optimal RSI as well as the height that the athlete’s RSI starts to break down. Interestingly, research by Byrne et al (2010) has shown that training at optimal drop jump height significantly improved reactive strength across all drop jump heights.
2. The rebound jump: The athlete performs a countermovement jump and upon landing immediately jumps again trying to achieve maximal height with minimal GCT. The rebound jump assesses reactive strength with less eccentric load than drop jumps and is also less time intense.
3, The 10/5 drill: The athlete again performs a countermovement jump but upon landing immediately performs 10 repeated bilateral hops. The average of the five jumps with the highest RSI is taken as measurement. This is a good way to measure RSI using repeat jump efforts which may be more specific for field-based running athletes.
The RSI also doubles as a very sensitive measure for monitoring adaptation to training loads and fatigue in athletes. Countermovement jump height is a popular test for this however its major drawback is that GCT is not considered and athletes can use a multitude of strategies to increase jump height. In a study by Cormack et al. (2008) the RSI via a drop jump was found to be a sensitive indicator of neuromuscular fatigue in AFL players compared to jump height alone (15).
By now you should be able to tell that not all plyometrics are created equal and it’s important to consider the demands of your athlete’s sport. For instance, sprinters require higher levels of reactive strength whereas field based athletes will also require a certain level of reactive strength but maintaining this for longer periods will be more important to them.
So how do we know what plyometrics to prescribe to our clients and how do we progress these. Well, first of all, we need to understand how much stress we are placing through the muscles, tendons and joint structures during plyometric movements. This will be determined by the eccentric load and the amount of time the load is applied in. Below are all of the factors that need to be considered when looking at the intensity of plyometrics and how to progress them.
1. Single leg plyometrics are more intense than bilateral plyometrics. Double leg plyometrics with a spatial difference between the feet (split jumps) or a time difference between the feet (step up jumps) are more intense than double leg but less intense than single leg variations.
2. The height that athletes jump up to or down from is a strong indicator of stress applied to the body. Jumping from a 60cm height places more stress through the legs than from a 30cm height and jumping onto a box places virtually no load through the legs.
3. Plyometrics with shorter ground contact times (fast SSC) will place more stress through the legs than those performed with longer contact times (slow SSC).
4. Performing jumps in a repeated fashion will place more stress through the legs than individual jumps with a break between each. So 5 continuous hurdle hops are more intense than 5 countermovement jumps.
Another factor to consider when programming plyometrics is the direction that they generate force in. This can be vertical, horizontal, lateral or a combination.
If you can remember these factors, you will always be able to gradually increase the intensity of plyometrics and therefore enhance neuromuscular adaptations and performance.
A good starting point is making sure your athlete is able to stick a landing, either from a countermovement or from a height. And it’s critical that your athlete is able to do so while remaining stiff through the ankle, knee and hip joints. Regulating this stiffness will ensure that the SSC is utilized for better power application into the ground and also offers protection from joint injuries. Teaching athletes to land “softly” by flexing at the hips, knees and ankles (as many of us are taught at uni) is not only inefficient and slow but it reduces trunk stiffness and increases the risk of injury. In the early phase, slow SSC exercises such as countermovements and box jumps should be introduced to work on jumping technique. Lateral shuffling over hurdles is a low-intensity way to introduce lateral and multi-directional movements important for performance and injury prevention.
The athlete should then be introduced to fast SSC exercises that focus on minimizing GCT. Pogo hops, split jerks and bounding are all great exercises for this purpose and emphasize keeping a strong upright posture. Multi-directional pogo hops can be introduced as a progression to lateral work.
Once the athlete is proficient at minimizing ground contact time, the athlete can then be progressed to power endurance progressions such as continuous pogo hops in multiple directions. The next logical progression is to work up to maximal effort jumps such as drop jumps and multiple single leg hops. These plyometrics are the most intense as they involve the greatest eccentric loads with the shortest GCT. Drop jumps can be progressed by increasing jump height, using a single leg and performing multiple jumps after landing. Lateral plyometrics can also be progressed to single leg lateral hops and these can be performed in combination with other multidirectional hops to challenge the athlete. This is where you can get creative to simulate the playing environment and maximize adaptations.
When training athletes to improve reactive strength it’s generally recommended to perform two plyometric sessions per week. When focussing on reactive strength endurance the athlete should perform between 100-120 jumps and between 50-60 jumps if training with maximal effort. However, for a majority of athletes, training and performing for their sport will provide much of the stimulus required to improve but dedicated plyometric exercises can be included within this training to maximize adaptations.
This blog was written by Mitchell Casey. Check out his Facebook page HERE
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