《Force: The biomechanics of training (English Edition) 作者:Dan Cleather》
14 DISTRACTED BY TRIPLE EXTENSION
What is triple extension?
So, as we have just seen, many athletic movements involve us extending out leg to exert force into the ground. These movements generally require the coordinated extension of the ankle, knee and hip-hence triple extension An example that is often given of a triple extension pattern is the ‘clean pull’ in Olympic weightlifting Figure 18).
Triple extension certainly occurs in a range of movements, so why do I say people get distracted by it? In order to understand this we need to talk about observability bias.
83
FORCE
Figure 18. Triple extension at the end of the second pull in Olympic-style weightlifting
Observability bias
One of Steve Magness’ Rules of Everything is that we overemphasise the importance of what we can measure and what we already know”. In coaching this manifests itself in what I call observability bias’. That is, we can only coach what we can see, and this will lead us to focus on the aspects of the skill that are most visually evident to us. However, the problem here is that the most obvious things may not be the most important.
A good example of observability bias is triple extension. How do we assess whether an effective triple extension has occurred? In high speed extension movements it is often difficult for the inexperienced eye to pick up fine technical details during the course of the movement. Instead, the easiest position to spot is the fully extended end position (for instance, the end of the second pull that is depicted in Figure 18). Natucally, if a coach is particularly interested in improving an athlete’s triple extension this can lead them to cue the athlete to ensure that they hit a fully extended end position. Unfortunately, this can often be counter-productive.
DISTRACTED BY TRIPLE EXTENSION
Full extension is follow through…
The fully extended end position seen in a range of triple extension movements (eg, jumping and weightlifting is actually a consequence of things that happen earlier in the movement. For instance, Figue 19 shows a Series of stills of an athlete jumping side-by-side with the force-time graph from the movement. As the top image shows, the end of peak force expression occurs when the lower limb is still relatively flexed. From this point onwards, force production drops rapidly, such that in the fully extended position, force production is zero. Essentially, it is the momentum that the athlete accred in the earlier part of the jump that carries them through to the fully extended position. The instant at which full extension occurs is of minor importance to the execution of the movement as little force is expressed at this time.
The problem with cueing an athlete to hit a fully extended position is that you are emphasising a less important position. This isn’t always necessarily a bad thing – such a cue might be effective if the athlete complies with it by exerting more force at the appropriate time and then has a greater ‘follow through’. However, there will be negative consequences if the che results in the athlete delaying their peak effort until later in the movement.
A misguided focus on the end position is frequently a problem in Olympic weightlifting. This is because, after exerting the maximum possible impulse on the bar, the athlete then needs to reverse their body’s direction of motion and descend to catch the bar. Over-cueing the athlete to hit an extended position can badly compromise an athlete’s technique by making them slow to get under the bar. This then compounds the problem of the athlete reserving their effort until too late in the movement.
FORCE
Figue 19. Force production during vertical jumping
DISTRACTED BY TRIPLE EXTENSION
The triple extension myth
Clearly, winning also involves a triple extension pattern of the lower limb (Figure 20). The latter part of the extension takes place when the foot is behind the athlete. What is important to realise is that for many elite sprinters the leg will not be fully extended at the point the foot leaves the ground. However, to the untrained eye it can appear like full extension is reached. The legs move quickly when we sprint, it is hard to pick out the shapes that athletes make, and it seems logical that full extension should осс.
Figure 20. Kinogram of an elite sprinter personal best 100m performance 9.948) running at 11.1ms!
Similar to jumping or weightlifting, coaching an athlete to focus on fully extending at push-off can have a disastrous effect. Again, force production should be miniscule at this point, and instead the athlete should be bringing the leg forward to prepare for its nest contact with the ground. Asking an athlete to emphasis push-off will cause them to spend more time on the ground which will in then make them slower.