Strength training: squats and leg extensions

by Alessandro Stranieri
Published on n. 30 of Volleyball HiTech - September 2004

In sport the forms in which the capacity for strength is manifested are many and different. Jumping, throwing, running, are all terms in common use among athletes of different disciplines, but few of these would think of the aforementioned gestures as a particular expression of strength.

Definition of force

Motor power capacity is usually referred to as a person's ability to oppose or overcome resistance by means of a muscle contraction.
In physics, this is expressed with the formula F = M a, that is Mass x acceleration. Applying this concept to sports movements, it is deduced that, with the same force used, the greater the mass and the lower the acceleration imparted to the body. This means that, between two people with a hypothetical lower limb musculature of equal development, the the higher the lighter. This assumption is one of the reasons that explain why volleyball players have an excellent muscle tone for jumping and not hypertrophic and heavy like a bodybuilder.

The capacity for strength, therefore, is generally always present, expressing itself each time in different ways and in conjunction with the other conditional capacities (speed, endurance). Each sport has its particular need for strength, (maximum strength, resistant strength, fast strength) on the basis of which the ideal training program for the right muscles of the athlete will have to be determined.
Team sports, in particular, require a specialization of the neuro-muscular system in order to improve the ability to develop high forces in a very short time, which will be followed by a refinement of the coordination of movements during high and rapid strength commitments.

Basically, the programming of strength training generally pursues some main purposes, including:

Improvement and maintenance of the state of muscular form for the purpose of performance;

Muscle-tendon and bone strengthening for the evolution of the stability of the musculoskeletal system, according to the high specific loads;

Compensation of the less used muscle groups in the specific technical gesture of each sport.

Training methods

Strength-related muscle strengthening is generally given with isotonic (dynamic) or isometric (static) methods. In the first case, the affected muscle shortens during the active phase of the movement (concentric phase), releasing gradually, later, during the passive phase (eccentric phase), to return to its original length.
Static methods include exercises in which the muscle contracts while maintaining, or almost, its length and the distance between the joint heads. However, of the two models, isotonic is the most widely used and easily applicable.

Exercises and means

Strength training for the lower limbs is divided into 2 different types of exercises:

closed kinetic chain (CCC) like the Squat and the thrusts to the Leg Press, in which the feet are constrained to the thrust plane;

open kinetic chain (CCA) like the Leg Extension, which does not provide for a constraint of the ends or the movement of the other segments (thigh, pelvis, trunk).

Squat

This represents the most complete exercise for the acquisition of the strength quality of the lower limbs. The attitude of the body during the execution of the exercise in free form with the barbell is very similar to the pushing movements made by the athlete during the jumps.

We can in effect consider it a multiarticular exercise, by virtue of the action it exerts on all the extensor muscles of the lower limbs and on the stabilizing muscles of the exercise such as the gastrocnemius and hamstrings for the knee, the abdominal and lumbar muscles for trunk.
The only point against this exercise (provided it is correctly performed) is represented by the weight of the barbell which presses longitudinally on the spine. In this regard, the exercises al Leg press to partially eliminate the load on the spine during execution.
Often, due to a reduced excursion of the ankle joint, due to retraction of the soleus or due to difficulty in anteroversion of the pelvis, some athletes are unable to perform an exact bend of the legs without lifting the heels and adopt a thickness under the heel to facilitate the correct alignment of all body segments. Personally, I do not agree with this practice, both because it moves the body weight too far forward, overloading the structure of the knee, and because it does not encourage an athlete to seek improvement of the joint range. Also in this case the execution of the push exercise Leg Press it can help the problem, mainly because, apart from the differences in the involvement of the upper part of the body, in the two exercises the involvement of the muscles of the lower limbs is substantially the same.

Indications for the correct execution of the exercise

Begin the descent by moving the pelvis back, as in the movement of sitting on a chair. In this way the knees will not move forward but will remain perpendicular to the foot.

The toes are slightly turned outwards and have a hint of a "V." Do not seek further intra or external rotations of the feet believing that you are emphasizing certain muscles: it is harmful and above all useless.

Keep your back well aligned and taut throughout descent and ascent, looking forward-upward.

During strengthening programs, you should try to use overloads without placing excessive strain on the knee joint and ligaments.
In the drawing it is possible to observe the different load to which the knee is subjected during two different types of exercise for the lower limbs: Leg extension (CCA-open kinetic chain), Leg press / Squat (CCC-closed kinetic chain).

During the extension of the knee to the leg extension a component of shear force develops. The figure shows how the vector of the force exerted by the limb (a) crosses the vector of the direction of resistance opposed by the ligaments (b) at a point ( C) external to the action system of the leg, which is subjected to the resistance (R) represented by the bearing of the machine. In this situation there is a forward translation of the tibia and a backward sliding of the femur (anterior drawer), prevented by the resistance offered by the ligaments (86% of the ACL).

In the Leg Press, on the other hand, the intersection point (C) is located within the "system" creating a much shorter lever arm (BC) and a lower shear force; moreover, in this case, the hamstring muscles are activated and reduce the anterior drawer phenomenon by reducing the stress on the ACL.

In the squat as in the leg press, it is good practice to respect a correct alignment between the foot and the knee as shown by the green arrow.

Leg Extension

It is another excellent exercise for the muscles of the leg extensors. In addition to not aggravating the already heavy load of the intervertebral discs, it allows a more selective training of the muscles that make up the quadriceps femoris, especially the medial and lateral vastus (both in the Squat and in the Leg Extension, the rectus femoris is the least involved as it is partially "detended" due to the inclination of the trunk forward and the consequent flexion of the hip.).
Here too there is the possibility of a structural overload, this time however, on the anterior cruciate ligament (ACL), particularly stressed in this exercise especially during the last 40 ° of knee extension. (see box). For this reason it is recommended to use the Leg Extension with not excessively high loads, such as general muscle work with a high number of repetitions.

Indications for the correct execution of the exercise

Do not start the extension movement from a starting position with the knee excessively bent;

Do not hyper-extend the knee in the final phase; some machines have the ability to adjust the working angles.

Do not detach the buttocks and loins from the seat during the outward and return phases of the movement;

In conclusion

Although the rather recent studies by Moore and Beynnon disagree on the lower danger to the ACL, in general we can say that, if well performed, closed kinetic chain exercises, especially the squat with free barbell, are less harmful to the body. knee compared to those in an open kinetic chain, also bringing a series of further positive sides such as:

Best simulation of the athletic jumping gesture;

Greater protection of the ACL by simultaneous intervention of quadriceps and hamstrings;

Lower stress on the patellofemoral joint;

Greater stability due to the compression given by the forces that act;

A greater hormonal response that has a positive effect on the strengthening of the muscle-tendon and bone systems.

(For further information on the subject you can consult the works of authors such as: Henning, Nisell, Yasuda, Renstrom, Grabiner, Markolf, Lutz, More, Pope, Escamilla, Wilk, Bosco).