I posted this elsewhere, looking for everyones general thoughts on training this element
Compliant training surface – effect on sprint performance
I was discussing with some of my athletes the other day regarding why i wanted them to stay in running flats for longer then we previously had throughout the winter, I started to explain things in regards to the system sprint etc and its cross over affects
Anyway in my own notes is the information below.
Does anyone have any good papers or discussion points on the cross over effect when training using a more compliant surface and then racing accordingly etc
Is system stiffness fixed or can it be changed?
System fitness is the sum of muscle, tendon joint and surface interactions. The stiffness can change depending on joint angles, muscle recruitment and surface stiffness. The stiffer the surface of the ground the less stiffer the leg or system stiffness. The less stiffer or more compliant a surface, the greater the system stiffness. This quality allows performance to be the same regardless of the surface. Human athletes maintain the same effective vertical stiffness on all surfaces when hopping, running or sprinting. They can maintain a constant vertical stiffness by increasing leg stiffness to offset reductions in surface stiffness. There is a tendancy for leg stiffness to increase when the surface is compliant. The size of the increase or decrease of system stiffness depends on compliance of stiffness. If the surface is stiff then system stiffness decreases. If the surface is compliant then system stiffness increases. The ratio of surface-to-leg stiffness will determine the change in system stiffness. When the surface is very compliant then the system stiffness increase is much larger. For example, hopping on a sprung surface such as a trampoline will increase the system stiffness significantly. Vertical displacement on all surfaces is kept the same. Stride frequency and ground contact can be kept the same as a result of system stiffness adjustment. Contact time and frequency will be affected if the surface is compliant but not elastic such as running on sand.
How important is system stiffness to running or sprinting?
Well, it is fundamental to the attainment and maintenance of high level performance. High leg-spring stiffness has been linked to sprint performance, McMahon & Cheng (1990). Cadence in sprinting is directly proportional to leg spring stiffness (K). High hopping frequency and stride frequency is proportional to vertical stiffness (Kvert) or the displacement of the center of gravity. In short; the higher the stiffness the higher the frequency in both sprinting and hopping. Running stiffness and hopping stiffness are the same. The implications for training are profound.
Training on compliant and elastic surfacesTraining on a compliant elastic surface will require the legs to increase stiffness. A negative relationship between surface compliance and oxygen consumption has been found. Compliant elastic surface should store and return energy with each step, reducing the mechanical work. Training on grass is a good alternative to many surfaces. Grass has a tendancy to dissipate energy yet it is compliant. The dampening effect may increase the work done by muscles because energy will not be returned fully or at the right time. As a general rule a surface that is fatiguing is likely to be less compliant. Performing hops or plyometrics on a compliant elastic surface such as a sprung gymnastics floor or trampoline should increase the leg or system stiffness. Soft compliant sport shoes should improve leg stiffness when sprinting on a stiff surface, Bishop et al (2006).