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Role of Knee Joint in Locomotion

- See:
     - Knee Joint Anatomy and Function:
     - Gait Menu:

- Diagram:

- Mid-Stance:
    - knee joint is in approx 15 deg of flexion & is moving toward extension;
    - primary flexors of ankle are beginning to demonstrate low grade activity;
           - this activity gradually increases until immediately after heel off position;
    - inasmuch as line of gravity passes in front of knee joint when body is erect, gravity tends to produce a locking of knee joint;
    - in ordinary standing, however, knee joint is not usually locked, but is maintained a bit short of full extension;
    - when it is in full passive extension, quadriceps is relaxed, patella can be passively moved from side to side, and the hamstring muscles and 
           gastrocnemius act to maintain joint in nearly extended position;

- Heel Off:
     - at heel off the resultant force passes in front of the knee and acts to extend the joint;
     - as resultant forces passes in front of metatarsal heads, knee joint flexes & mechanical forces act in direction of flexion once again;
     - knee has extended to approx 178 deg;
     - is probably being initiated by the biarticular gastrocnemius which is also helping to prevent hyperextension of the knee from occuring;
     - peak electrical activity of gastroc occurs shortly after heel off;
     - since mechanical forces are acting to extend knee at this time, and since hamstrings are inactive, it is reasonable to assume that 
           gastrocnemius is helping to prevent knee hyperextension;
     - at "heel off" maximum electrical activity of gastroc is recorded;
           - it is not until later that the remaining plantar flexors show peak activity (TA, Soleus, PB, PL and FHL);

- Push Off:
    - is probably being initiated by the biarticular gastrocnemius which is also helping to prevent hyperextension of the knee from occuring;
    - later in the push off period, the floor reaction indicatess that a flexion moment is generated at the knee, the magnitude of which continues 
           to increase until slightly before toe off;
    - it is interesting that to note that electromyographic tracings show little quadriceps activity during this time;
           - the vastus intermedius and rectus femoris become active just before toe off but not earlier;
    - it would seem likely, that the uniarticular plantar flexors - particlarly the soleus, peroneus longus, tibialis posterior, flexor digitorum longus,
            and FHL, play an important role in counteracting the knee flexion moment by extending the tibia on the fixed foot;
    - double support:
           - during the normal gait there is a period of double support when the two extermities are in contact with the ground simulaneously;
           - this occurs between push off and toe off on one side and between heel strike and foot flat on the contralateral side;
           - as the cadence decreases and the length of time spent in double support increases;
           - the absence of double support is used to distinguish walking from running;

- Between Push Off and Toe Off:
     - push off phase corresponds to the time interval between heel off and toe off positions of the same foot;
     - heel off occurs following mid stance, when the heel of the supporting extremity rises from the floor, the ball of foot & toes are still in 
           contact w/ ground;
     - knee joint moves from almost complete extension to about 40 deg of flexion;
     - as torso moves forward over the supporting extremity, resultant force also moves forward reducing magnitude of flexion moment;
     - between push off and toe off a flexion moment of considerable force develops at knee joint, but quadriceps remain relatively inactive;
         - it is possible that the ankle plantar flexors exclusive of the gasstrocnemius, are acting to retard the forward rotation of tibia and 
               thereby exert an extension force at the knee;

- Between Toe Off and Mid Swing:
     - maximum knee flexion angle during gait, which has a value of approx 65 deg, is reached;

- Toe Off:
     - contraction of the quadriceps, initiated before toe off, apprarently serves two purposes;
          - it prevents heel from rising too high in a posterior direction;
          - it helps to initiate the forward swing of the leg

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