MUSCLE FUNCTION
Knee Extensor Muscle Function
The quadriceps femoris muscle group is the only muscle crossing
anterior to the axis of the knee and is the prime mover for knee extension.
Other muscles that can act to extend the knee require the foot to be fixated,
creating a closed chain. In this situation, the hamstrings and the soleus
muscles can cause or control knee extension by pulling the tibia posteriorly.
Closed-chain function
During
standing and the stance phase of gait, the knee is an intermediate joint in a
closed chain. The quadriceps muscle controls the amount of flexion at the knee
and also causes knee extension through reverse muscle pull on the femur. In the
erect posture, when the knee is locked, the quadriceps need not function when
the gravity line falls anterior to the axis of motion. In this case, tension in
the hamstring and gastrocnemius tendons supports the posterior capsule.
Patella
The
patella improves the moment arm of the extensor force by increasing the
distance of the quadriceps tendon from the knee joint axis. Its greatest effect
on the leverage of the quadriceps is during extension of the knee from 60_ to
30_ and rapidly diminishes from 15_ to 0_ of extension.
Torque
The peak torque of the quadriceps muscle occurs between 70_ and
50_. The physiological advantage of the quadriceps rapidly decreases during the
last 15_ of knee extension because of its shortened length. This, combined with
its decreased moment arm in the last 15_, requires the muscle to significantly
increase its contractile force when large demands are placed on the muscle
during terminal extension. During standing, assistance for extension comes from
the hamstring and soleus muscles as well as from the mechanical locking
mechanism of the knee. In addition, the anterior cruciate ligament and the pull
of the hamstring muscle group counter the anterior translation force of the
quadriceps muscle. During open-chain knee extension exercises in the sitting or
supine position, when the resistive force is maximum in terminal extension
because of the moment arm of the resistance a relatively strong contraction of
the quadriceps muscle is required to overcome the physiological and mechanical
disadvantages of the muscle to complete the final 15_ of motion.83 However, it
is worth mention- ing that the compressive loads on the patella also decrease
in terminal extension because of its superior location with respect to the
trochlear groove and the resultant force of the line of pull of the quadriceps
and patellar tendon. The therapist needs to be aware of the effect of the resistance
and where in the range of motion the muscle is being challenged. During
open-chain exercises with fixed resistance, when the resistance torque
challenges the quadriceps in terminal extension there is little challenge mid-range
where the muscle is capable of generating greater tension.
Knee Flexor Muscle Function
The hamstring muscles are the primary knee flexors and also
influence rotation of the tibia on the femur. Because they are two-joint
muscles, they contract more efficiently when they are simultaneously lengthened
over the hip (during hip flexion) as they flex the knee. During closedchain activities,
the hamstring muscles can assist with knee extension by pulling on the tibia. The
gastrocnemius muscle can also function as a knee flexor, but its prime function
at the knee during weight bearing is to support the posterior capsule against
hyperextension forces. The popliteus muscle supports the posterior capsule and acts
to unlock the knee. The pes anserinus muscle group (sartorius, gracilis, semitendinosus)
provides medial stability to the knee and affects rotation of the tibia in a
closed chain.
Dynamic Stability of the Knee
Because of the incongruity of the femoral condyles and tibial
plateaus, there is little stability from the bony architecture. The cruciate
and collateral ligaments provide significant passive stability in the various
ranges of joint motion. Dynamic stability involves motor control of the neuromuscular
system to coordinate muscle activity around the joint. The complex feedforward
and feedback responses mediated by the central nervous system modulate muscle stiffness
and are important for providing dynamic knee stability under varying loads and
stresses imposed on the joint structures. As summarized in a clinical
commentary by Williams,254 clinical and scientific evidence is accumulating to
substantiate exercise programs that have the purpose of training dynamic knee
stability; that is, to improve control of the knee via neuromuscular responses in
order to reduce knee ligament stress and injury during high-intensity
activities.
0 Comments