the lump from the outside looks like a pit
the lump sticks into the joint
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The extensor muscle has a maximum force of about 14 N (1.4 kg). The flexor muscle can only produce a force of about 0.7 N (70 gms). So how can this weak flexor muscle hold the tibia flexed against the mighty force of the extensor? Why doesn't the leg start to extend as soon as the extensor muscle produces more force than the flexor?
The answer lies in a particular structure inside the joint known as the lump.
In the "elbow" of the hind leg there is a structure which looks like a small black pit. This pit is in fact a lump that sticks into the cavity of the femur.
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the lump from the outside looks like a pit |
the lump sticks into the joint |
(If we wanted to be highly technical, we could call the lump a "sclerotised cuticular invagination of the femoral-tibial joint", but, for once, even the scientific literature has kept things simple, and it is generally known as the lump.)
The lump is absolutely crucial for the jump, because it enables the weak flexor muscle to hold the tibia flexed against the strong extensor muscle during the energy build-up. Two features account for this:
The lump changes the angle with which the flexor tendon pulls on the tibia. When the tibia is fully flexed, the flexor muscle has a very direct line of pull on the tibia, while the extensor has a very indirect line of pull. The flexor thus has a large mechanical advantage over the extensor muscle. As the tibia starts to extend, this very rapidly drops.
An additional feature comes into play in the fully flexed position. There is a small pocket in the middle of the flexor tendon, close to where it joins onto the tibia. As the tibia comes into the fully flexed position, this pocket arrives over the lump, and slides down onto it. This further increases the ability of the flexor muscle to hold the tibia flexed against the strong extensor muscle.
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Changing lever ratios
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Putting these two things together explains why full tibial flexion is an essential pre-requisite for the jump. If the tibia doesn't flex fully, then the tendon pocket cannot engage over the lump, and the flexor does not get its full mechanical advantage over the extensor. This means that the extensor muscle "wins" against the flexor muscle and the tibia starts to extend before sufficient energy has been stored in the springs. The resulting behaviour is a weak hop rather than a vigorous jump.
The story so far is (I hope) quite convincing, but the only real evidence comes from the video of the joint. To prove the story we need to know what really happens to the nerves and muscles during the behaviour. This will be described next.
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