In the squat, how do the hamstrings contract to provide hip extension whilst lengthen to allow knee extension? I am thinking that the motor units and therefore fibres are seperated for movements at either the hip or the knee. ie. individual fibres do not cross both the hip and knee. The same would go for other double joint muscles. Is this correct or wrong…what is the answer?
Cheers Charlie, theone.
Good grief Tomo, nice to see you here at last. I’ve not been injured, I’ve just had to swing things round a bit to go down the 400 route - be back next week though.
This is explained by the co-contraction phenomenon. There were some threads on the Supertraining site about it, I’ll try find them.
Sorry I couldn’t find any threads, there might be some in cyberspace somewhere though, I’ll keep looking.
This is an interesting article that is somwhat relevant. http://www.wsu.edu/~strength/hilobar.htm
Also http://www.ecybex.com/education/perf_training/muscular.html
Thanks Jimbo, much appreciated.
The co-contraction phenomenon states that the hams and other double joint muscles contract at both joints. At one joint they are a prime mover and at the other a co-contractor (acting against the direction of that joint) resulting in greater stability of that joint. This seems surprising since when the hip is extending powerfully, the knee can still extend very easily. Anyway, any more information would be appreciated.
good grief richard. r you from liverpool at all?? 
Please look for the thread on Lombard’s Paradox on the old forum. This was also covered on the Supertraining site.
Here is the original post I submitted on the old site-
The concept of Lombard’s Paradox has recently been a point of discussion on the Supertraining List. I thought the group would find this interesting.
This is the post I submitted 6-17-2002.
Here is an excerpt from:
Relative activity of hip and knee extensors in sprinting – Implications for training
Klaus Wiemann and Gunter Tidow
New Studies in Athletics
10(1): 29-49, 1995
from page 32-33:
2.3 The hamstrings as knee extensors
The hamstrings (HS; m. semitendinosus, m. semimembranosus, m. biceps femoris caput longum) also act as extensors at the hip joint. The reason for the hesitant consideration of these muscles as prime movers during the sprint seem to be the fact that the biarticular HS are generally considered not only as hip extensors but also as knee flexors, whereas a knee extension is demanded, during the support phase of the sprint. However, as early as in 1903, LOMBARD drew attention to the “paradoxical” function of biarticular muscles. Fischer (1927), Molbech (On the paradoxical effect of some two-joint muscles. Acta Morphologica Neerlando-Scandinavica. 6: 171-178, 1965) and Andrews (A general method for determining the functional role of a muscle. J Biomech Eng. 107(4): 348-353) described this paradoxical function in more detail, and Carlsoo & Molbech (The functions of certain two-joint muscles in a closed muscular chain. Acta Morphologica Neerlando-Scandinavica. 6: 377-386, 1966), Gregor et al. (1985) and Andrews (The functional roles of the hamstrings and quadriceps during cycling: Lombard’s Paradox revisited. J Biomech. 20(6): 565-575, 1987) applied this paradox to the function of the hamstrings in cycling. According to this principle and provided that the free end of the two-link kinematic chain of the leg is guided (inertia, support reaction), the HS have not only a hip extending function but, paradoxically, also a knee extending function. Apart from a short note made by Donskoi (1961), the so-called LOMBARD paradox has been applied only recently to the function of the HS during sprinting (Wiemann 1989, 1990, and 1991). By means of vector splitting and model formation, it can be shown that according to the LOMBARD paradox and unless the knee angle is smaller than 145 degrees, the HS during the support phase of the sprint bring about both a hip extension and a knee extension. To this extent, the HS organize exactly that movement – namely a synchronous hip and knee extension – which is required in the support phase.
The action of the HS becomes especially clear, if one observes the pelvis from below during a leg position corresponding to the front support in the sprint. One can see that the HS extend, like reins, from the ischial bone to the lower leg, exactly in the direction of the pull of the leg under the pelvis during the support phase.
However, in previous cinematographic and electromyographic studies of sprinting, the HS were either given little attention (Simonsen et al. Activity of mono- and biarticular leg muscles during sprint running. Eur J Appl Physiol. 54: 524-532, 1985; Mero & Komi. Electromyographic activity in sprinting at speeds ranging from sub- maximal to supra-maximal. Med Sci Sports Exerc. 19(3): 266-274, 1987) or they were still treated as knee flexors (Bober et al. The mechanics of the leg swing in running. Techniques in Athletics, Cologne, 7-9 June 1990 Conference proceedings, Vol. 2, pp. 507-510; McClay et al. Muscle activity in running. Biomechanics of Distance Running. Chapter 6: 165-186, 1990). Wood (Optimal performance criteria and limiting factors in sprint running. New Studies in Athletics. 2: 55-63, 1986) who at least regards the contractility of the HS as the limiting factor in the sprint, also identifies the HS as knee flexors. Even Lemaire & Robertson (Power in Sprinting. Track and Field Journal. 35: 13-17, 1989) do not make a clear statement about the contentious function of the HS in the knee joint, although they recommend that more attention should be paid, during strength training for sprinting to the hip flexors and extensors than to the muscles affecting the knee joint. Only Wiemann, on the basis of electromyographic pilot study (Wiemann. Die Muskelaktivitat beim Laufen. Leistungssport. 4: 27-31, 1986) and the results of vector analysis (Wiemann 1989 and 1991) postulated the extensor function of the HS at the knee in the support phase of the sprint. Jollenbeck et al. (1990), in experiments, revealed a relationship between the length and force of the HS and sprinting speed.
From the above we can assume that, in sprinting, the movement of the support leg, from the moment the thigh begins to move down from the high knee lift position to the completion of the push-off, is caused by two muscle ‘reins’, namely
(a) by the HS, which form a long biarticular rein from the ischial bone to the lower leg, the m. semitendinosus and the m. semimembranosus forming the inner rein and the m. biceps femoris caput longum forming the outer rein, and
(b) by a short, uniarticular rein running from the pelvis to the thigh, consisting of the GM as the outer traction rope and the AM as the inner traction rope.
During the support phase these muscle loops produce a force which is directed horizontally backward, the reaction to which propels the body forward. However, the backward rotating torque, in the form of the “sprinter’s forward lean”. It can be assumed that, in the course of the sprint cycle, the activity of the synergistic partners within both reins must be adjusted to one another, in order consistently to direct both the free leg, in the swinging phase, and the knee of the support leg, through the sagittal movement plane.