Monday, October 17, 2011

Cycling & Lombard's Paradox

The motion of cycling can be split into 2 phase: the 'power phase' and the 'recovery phase'. The power phase begins when the leg is in a flexed position (top dead center - TDC) and continues through to when it is fully extended (bottom dead center - BDC). '

A: Top Dead Center (start of power phase)

B: Bottom Dead Center (end of power phase and start of recovery phase)

With regard to moving the leg through the phases, two muscles groups are primarily concerned with motions about the hip and knee joint; The Quadriceps and Hamstrings. Both of these muscle groups are 'bi-articular', meaning they span across both the hip and knee joint. The action of the Quadriceps group involves flexion of the upper leg about the hip joint and extension of the lower leg about the knee joint. In comparison, the Hamstring muscle group extends the upper leg about the hip joint and flexes the lower leg about the knee joint. When comparing the actions of these muscle at their respective joints, it can be seen that they are antagonists of each other (their actions oppose each other).

During the 'power phase' of cycling, both the upper and lower leg extend at the hip and knee joint, respectively. To initiate this movement, both the hamstrings (hip extension) and quadriceps (knee extension) must be active. This situation is referred to as 'Lombard's Paradox' as both muscle groups are active at the same time with each respective muscle apposing the desired motion at one joint. The situation is opposite during the 'recovery phase' - the quadriceps are needed for hip flexion and the hamstrings are needed for knee flexion. Once again, 'Lombard's Paradox' arises.

The reason this phenomenon can exist without compromising smooth and efficient motion is because each muscle group is able to selectively control how much force it produces across each joint. The locations at which the muscles insert into bone also assists in allowing this paradox to occur.


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