Klopt bij 90 graden treedt er een piek belasting op. Het lijkt dat ook onzinnig om juist op dat punt af te remmen, te stoppen en weer te versnellen.
Squat depth:
Safety and Importance
The full squat is the preferred lower body exercise for safety as well as athletic strength. Yet physical therapists, orthopedic surgeons, chiropractors, nurse practitioners, and inexperienced athletic trainers regularly condemn the time-honored full range of motion squat as being harmful to the knees and apparently, as a result, the soul. Tha squat, when performed correctly, is not only the safest leg exercise for the knees, it produces a more stable knee than any other leg exercise. The important part of the last statement is the "when performed correctly" qualifier. Correctly is deep, with hips dropping below level with the top patella. Correctly is a full range motion.
Any squat that is not deep is a partial squat, and partial squats stress the knee and the quadriceps without stressing the glutes and the hamstrings. The hamstrings and glutes perform their function in the squat when the hip is stretched to the point where they get tight, at full hip flexion - the deep squat position. The hamstring muscles, attached to the tibia and to the ischial tuberosity of the pelvis, reach a full stretch at the very bottom of the squat, where the pelvis tilts forward with the torso, stretching the two ends of the muscle apart. At this stretched position they provide a slight rebound out of the bottom, wich will look like a "bounce", and this in fact is a useful thing to coach into the movement if done carefully and with good explanation to the trainee. The tension of the stretch pulls the tibia backwards, the posterior direction, balancing the forward-pulling force produced by the quadriceps, wich pull from the front. The hamstrings finish their work, with help from the glutes, by straightening out, "extending", the hip.
In a partial squat, wich fails to provide a full stretch for the hamstrings, most of the force against the tibia is forward, from the quadriceps and their attachement to the front of the tibia below the knee. This produces an anterior shear, a forward-directed sliding force, on the knee, with the tibia being pulled forwardfrom the patellar tendon and without a balancing pull from the opposing hamstrings. This shearing force - and the resulting strain on the prepatellar area - may be the biggest problem with partial squats. Many spectacular doses of tendinitis have been produced this way, with "squats" getting the blame.
The hamstrings benefit from their involvement in the full squat by getting strong in direct proportion to rheir anatomically proper share of the work in the movement, as determined by the mechanics of the movement itself. This fact is often overlooked when considering aterior cruciate tears and their relationsship to the conditioning program. The ACL stabilizes the knee; it prevents the tibia from translating forward relative the the femur. As we have already seen, so does the hamstring group of muscles. Underdeveloped, weak hamstrings thus play a role in ACL injuries, and full squats work the hamstrings while partial squats do not. In the same way the hamstrings protect the knee during a full squat, hamstrings that are stronger due to full squats can protecht the ACL during the sport we are squatting to condition for. In fact, athletes who are missing an ACL can safely squat heavy weights, because the ACL is under no stress in a correctly performed full squat.
Another problem with partial squats is the fact that very heavy loads may be moved, due to the shorter range of motion and the greater mechanical efficiency of the quarter squat position. This predisposes the trainee to back injuries as a result of the extreme spinal loading that results from putting a weight on his back that is possibly in excess of three times the weight that can be safely handled in a correct deep squat. A lot of football coaches are fond of partial squats; it allows them to say that their 17 year old linemen are all squatting 600 lbs. Don't let this happen to you or your program - you don't need the problems that come with trainees handling unsafe loads. If it's too heavy to squat below parallel, it's too heavy to have on the back. Olympic weightlifters provide a perfect illustration of the safety and benefits of the full squat. Currently 167 of the 12 countries in the world compete in Olympic Weightlifting. More than 10,000 individuals compete in International Weightlifting Federation events alone, and the number of participants in total from the 167 countries would be staggering, likely on the order of 2 to 5 million (China alone boasts over 1 million lifters - per Ma Jian Ping). All over the world, weightlifters squat way below parallel safely, most often using some form of the exercise, either back squats or front squats, every day. That is correct: they squat way below parallel every training day, and most programs call for six days per week. Isn't it fascinating that they are both strong and not under the care of an orthopedic surgeon?
There is simply no other exercise, and certainly no machine, the produces the level of central nervous system activity, improved balance and coordination, skeletal loading and bone density, muscular stimulation and growth, connective tissue stress and strength, psychological demand ad toughness, and overall systemic conitioning as the correctly performed full squat. In the absence of an injury that prevents their being performed at all, everyone that lifts weights should learn to do them correctly.
