Why do we use plyometric and ballistic exercises?
There are two basic concepts that are useful to understand when using plyometrics. The first idea is that of power production and the other is rate of force development.
The ability to carry out any athletic movements successfully (powerfully) requires both muscular force production and the speed at which this is produced, Power = Force x Velocity.
Plyometric and ballistic exercises have repeatedly shown to improve both force and velocity.
Rate of force development (RFD) is simply the time it takes to generate maximal force during a movement. The rapid movements carried out by elite athletes require maximal force to be developed by the muscles within 50-250ms. However it can take 400ms or more to reach peak force. This is why it is highly desirable for athletes to build both power and speed.
The usual resistance training programmes will help athletes to become stronger and increase their maximal force, though these kinds of lifts are significantly slower than explosive exercises so this alone cannot develop velocity and therefore power. Luckily for beginner athletes and gym goers RFD will develop easily through many different methods of training, but for more highly trained individuals they must seek to carry out exercises specifically to improve this – by carrying out plyometric and ballistic exercise alongside their strength programmes. Olympic lifting is often used by coaches as a way of training explosive power in their athletes. These lifts require the athlete to transition from stationary to maximal power very rapidly. Training RFD is also extremely important for those whose take part in Olympic Weightlifting as its own sport because these explosive exercises improve the power, speed and elasticity required for lifting technique. Plyometrics training also has shown to lead to physical adaptations such as increased muscle size and increased tendon stiffness, as well as reducing the risk of injury when taking part in a sport.
How does it all work?
There are two models that can explain this, the mechanical model and the neurophysiological model.
The Mechanical Model: This involves harvesting the elastic energy in the muscular and tendinous parts of the moving limb (series elastic component) to produce force.
The Neurophysiological Model: Using a phenomenon called the stretch reflex. This is the body’s involuntary response to a stimulus that stretches the muscle, leading to a corrective movement. A common example that can be observed is the seated patellar reflex, if the patella tendon is tapped the quadriceps get stretched and this starts the reflex response of the quadriceps contracting and the leg raising upwards.
How do these concepts work together? This process is called the Stretch Shortening Cycle, during which the energy storage involved in the mechanical model and the stimulation of the stretch reflex work together. There are three phases: eccentric, amortization and concentric. The eccentric phase is the stretch portion of the cycle, imagine a spring being stretched, storing elastic energy. A real-life example would be when carrying out a countermovement (fast partial squat) to initiate a jump. The partial squat is a rapid eccentric movement that produces an elastic energy store ready to be used in the jump. Amortization is the time gap between the stretch and shortening movements. The aim of plyometric exercises is to keep the duration of this as short as possible, otherwise the stored energy is lost as heat instead of being used during the concentric phase. Think of the concentric phase as the spring pinging back to its usual length, this is the shortening of the muscle, where the stored energy can be used for force production. In terms of the real life example this is the jump phase – if the time between the partial squat and jump remains very short the athlete is able to produce more power which in turn makes them jump higher.
How to programme it
Incorporating plyometric exercises into programs must be done gradually and in a carefully planned format to ensure correct learning and minimised injury risk. Intensity must be increased over time as the athlete becomes more conditioned to plyometric exercises. Several factors are considered in what affects intensity, these are: points of contact, speed, jump height, body weight. It is important to start with learning landing mechanics and very low intensity movements to reinforce correct form. This first phase will last a few weeks until the athlete is prepared enough to move on. After this, low intensity fast plyometric exercises are introduced, focusing on short ground contact time (amortization phase). Exercises in this phase will include things like ankle jumps and skipping. After this the intensity can be increased first to higher repeated jumps e.g. squat jumps, still focusing on short ground contact time, and then to depth jumps which require the athlete to step or jump off a box. The flow chart below shows a progression of exercises from low to high intensity:
Before doing any plyometric drills it is important to warm up correctly. This is slightly different to a general warm up, as it involves low intensity dynamic movements. The format depends on what plyometric exercises you will be using in the workout, as the warm up movements must be specific to those carried out later. In general it could include marching, jogging, skipping, alternating lunges and other bodyweight exercises.
Plyometrics is about quality over quantity – you should be focusing on power output and not endurance, so ensure rest time allows adequate recovery between sets so that your power output is consistent. Plyometric exercises can be deceptively taxing and easily overdone, due to the stress placed on joints and surrounding tissues. This is why you must ensure that you train from low intensity up without moving on to the next stage before you are ready.
Here are some exercises that we have recently been doing. They apply a vertical emphasis due to the application to weightlifting. If you participate in a sport that includes running and horizontal/lateral movement there are also exercises which are more specific to this, for example hurdle jumps and repeated bounds or lateral bounds. You must select exercises that are relevant to the movement in your sport.
Barbell Squat Jump The aim of the barbell squat jump is to increase the explosive power produced by the legs and in turn assist your ability to get out of the bottom of squats quickly, increase your vertical jump height and improve the power of your triple extension, and has been linked to increasing sprinting speed. Anyone who can jump comfortably unweighted can give these a try. The bar is unracked the same way as if you were performing ordinary back squats, it must be held tight against your shoulders to keep it in place. Squat down and then explode up out of the squat to jump vertically from the ground, landing softly. Carry out 4 sets of 3-5 reps. Start with an empty bar to familiarise yourself with the exercise, continue up to a comfortable weight within 20-30% of your back squat maximum. The aim is to produce maximum power for all reps so it’s best to keep it light. Jump squats can also be performed holding dumbbells at your side, or with a sandbag on your shoulders.
Vertical Jump Vertical jump height tests can be used periodically to monitor increases in leg power output acquired from Olympic lifting and plyometric exercises. The lift-off phase requires simultaneous and explosive extension of the hip, knee and ankle (triple extension). Olympic lifting involves replicating this movement under resistance and can be used to improve jump height in sports such as basketball and rugby. For example it could be tested once a month
to gauge improvement.
The picture illustrates the correct landing position and starting position for
this exercise. Once the athlete has landed they must immediately repeat the jump. Notice that the hips are still above the knee and the shoulders are in alignment with the knees. This keeps the centre of gravity over the body's base of support.
Single leg push off
The starting position for this exercise involves the athlete standing with one foot on the box and one foot on the ground, making sure that the heel is close to the edge of the box and shin is kept vertical. The athlete then jumps, using double arm swing to help generate upward force, pushing up through the leg that is on the box and must land with the same foot back on the box before the other reaches the floor.
Counter-movement box jump
This exercise is relatively low in intensity. Start standing quite close to the box, perform a quick counter-movement partial squat and jump onto the box using arm swing for assistance. Land with two feet on the box in a half squat position and then step off the box before repeating the exercise.
High intensity. The athlete will step off a box and land on the balls of the feet with legs slightly flexed. This is then immediately followed by a vertical take-off. The aim is to increase speed and acceleration of take-off before changing the drop height. Depth jumps above 110cm are thought ineffective as the change from eccentric to concentric is too slow. If the heels are forced onto the floor when landing from the drop the box is also too high and there is an increased chance of injury. There is some evidence that box height does not significantly affect adaptations, studies have shown that depth jumps from 40 cm or 110 cm have yielded similar results.
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Science For Sport, Rate of Force Development
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