Great post Jeremy. Thank you for commenting on the kinematics as we in this thread are at a point where we have a good commen understanding of the kinetics of sprinting.
Isometrics excercises: What intensity are you refering to?
Tendon will not be contributing to force return? Could you explain more in dept and do you have a reference?
When you speak of changing speeds from 10 m/s to 12 m/s and risk of injury are your refering to the increased load on the tendons when dealing with breaking forces. Is this comparable to overspeed running going from 10 – 12.. I would think the load is different but i cant put my mind on why? ideers?
For the tendon to contribute significantly it needs to be stretched; F=kx where x is the distance of stretch and k is the stiffness. You might be able to increase “k” by 64% from isometric training but is there any point (see below). The human body doubles (exaggerated) the effectiveness in a deep stretch by reflexively activating more muscle to counter the stretch; i.e. the stretch reflex. The stretch reflex applies only at the block start according to literature; however it is likely that it would be usable to a lesser extent in the first step and second. (In addition, and as far as I’m aware, no-one has measured/examined the stretch reflex of the forward swinging leg – but it works a treat)
Of course the tendon will transfer the forces produced within the muscle; a stiffer tendon may transfer this force with less delay because of less stretch. However Kubo (Acta Physiol Scand 2000;168,327-335) found that the elasticity of tendon structures of the VL and MG at high force production levels, which might be associated with the storage and subsequent release of energy, are similar in both sprinters and untrained persons; suggesting a lack of stimulus in sprinting to produce an adaptation. Kubo (2000) did find a strong correlation (r=-0.757) between VL compliance and 100-m time but no relationship for the MG (r=0.228). Kuittunen (Med Sci Spor Exer 34;1:166-173, 2002) found no correlation between ankle or knee joint stiffness (as opposed to tendon compliance Kubo 2000) and running speed; ankle or knee joint stiffness may not be a limiting factor in increasing running speed.