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Here you will find an article on one exercise
(new one every month). With a detailed analysis of the
muscles used during the exercise and a informative description
of how to both perform and assist in the actual performance
of the exercise.
A full library of all previous exercises
can be found at members online under the sub menu
Exercise:
Leg Extension
Client Instruction:
Starting Position
1. Take a seat in a leg extension machine
2. Position yourself so that your knees are aligned with
the axis of rotation of the machine. The back of your knees
should be close to the edge of the seat
3. If possible adjust the back support so that you can
lean against it while maintaining a relatively straight
back
4. Your feet should be curled beneath the roller pads and
you should hold the hand rails located on the side of the
machine
5. Tighten your abdominals with your chest slightly elevated
Up Phase
6. Begin the exercise by straightening your legs at the
knee joint in a slow and controlled manner
Down Phase
7. Pause at the top of the movement
and then slowly lower the weight down slowly to the original
starting position
Muscle Analysis
Chart - Leg Extension
| Up Phase |
| Joint |
Action |
Contraction |
Muscle Group |
Specific Muscles |
| Knee Joint |
Extension |
Concentric |
Knee Joint Extensors |
Rectus Femoris
Vastus Intermedius
Vastus Lateralis
Vastus Medialis
|
| Down Phase* |
| Knee Joint |
Flexion |
Eccentric |
Knee Joint Extensors |
Rectus Femoris
Vastus Intermedius
Vastus Lateralis
Vastus Medialis
|
Down Phase*
In the down phase of the exercise the weight is lowered slowly
with gravity. The muscles that concentrically contracted to
lift the weight are the same muscles that are eccentrically
contracting to lower the weight.
General
Kinesiological Analysis
In leg extension exercises, the weight is lifted
by knee joint extension caused by the concentric contraction
of the vastus medialis, vastus lateralis, vastus intermedius
and the rectus femoris.
Advanced Kinesiological
Analysis
The prime movers in knee joint extension are the vastus medialis,
vastus lateralis, vastus inter-medius and the rectus femoris.
The rectus femoris is the only quadriceps muscle that crosses
the hip joint and is classified as a two joint muscle. It
has an origin on the pelvic girdle, whereas the origin of
the vasti muscles is on the femur. Because of its different
position on the pelvic girdle, it is possible to place the
rectus femoris on additional stretch prior to it contracting
in knee joint extension, by tilting the pelvic girdle backwards.
This can be facilitated in a leg extension machine by slightly
adjusting the back support to allow the client’s trunk
to lean back. This tilts the pelvis backwards and stretches
the rectus femoris before it contracts. The rectus femoris
most effectively contributes to single joint movements. When
used in combined movements of knee joint extension and hip
joint flexion, the extension becomes more dominant.
The range of motion possible in knee joint extension is affected
by the degree of flexibility of the hamstrings. If the hamstrings
are too tight they act as a brake to prevent the knee joint
from extending. This only happens from this seated position
as the hamstrings are already placed on stretch due to the
hip in flexion. It has been suggested that the position of
the feet prior to the knee extension exercise can affect the
contribution of the vasti muscles. When the leg extension
exercise is performed with the feet in a neutral position,
the vasti muscles and the rectus femoris all contribute maximally.
When the exercise is performed with the feet rotated in towards
the body (hip joint medial rotation) the vastus lateralis
and the vastus intermedius are the major contributors, while
the vastus medialis and the rectus femoris are used very little.
When the leg extension is performed with the feet turned out
(hip joint lateral rotation) the vastus medialis, rectus femoris
and the vastus intermedius are contributing the most, with
the vastus lateralis contributing the least.
In some clients it is possible that the sartorius will be
used in the leg extension exercise. This does not occur in
all cases due to the change in position of its insertion in
some people. When the insertion is anterior to the axis of
rotation of the knee it will contribute. However, when it
is located behind the axis of rotation of the knee joint it
will not. The vastus medialis is often cited as a potential
cause of retro patella pain caused by chrondro-malacia patella.
This condition is a wearing away of the cartilage located
in the patella groove that lies on the distal anterior femur.
When the patella does not sit snugly within this groove it
wears away the cartilage. The patella is acted upon by the
vastus medialis and the vastus lateralis that are located
on opposite side of the bone. During knee joint extension
they pull on the patella in different directions. The vastus
medialis appears to be the weaker than the vastus lateralis.
Clinicians have suggested that to develop the strength of
the vastus medialis it is important to contract the muscle
over the last 15-20 degrees of knee joint extension. It is
difficult for the vastus medialis to compete with the vastus
lateralis, as the latter is involved in almost all knee extension
exercises. There have been reports that allowing the leg to
move into full extension, particularly the last 5-10 degrees
can increase the shear forces in the knee joint.
Disclaimer: No responsibility is accepted for any loss
or damage suffered as a result of the use of the above
information
or any reliance on it. Users should satisfy themselves
as to their own or clients medical and physical condition
before adopting/using
the information or recommendations made. No responsibility
or liability is accepted for any loss or damage suffered
by any person as a result of adopting the above information
or recommendations.
Further information
1300 136 632 Phone +61 02 9212 7185 or Fax +61 02 9211
0002
Suite 505/410 Elizabeth Street Surry Hills 2010 Sydney
Australia
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