Marilia Coutinho, Ph.D.*
Introduction: studying equipment carry-over
The study of equipment carry-over in strength sports or power/strength movements poses serious methodological problems. There is practically nothing in the scientific literature on biomechanics or exercise physiology concerning the subject.
However, it is a main concern for lifters and practitioners alike. Support equipment for strength sports and strength exercises is designed for two purposes: historically, the first was to provide protection against injury. As the strength sports evolved into more competitive and commercially relevant activities, a second purpose became more prominent: the improved performance provided by the equipment in the form of an additional weight the athlete was able to lift. This is known as the equipment's "carry-over".
Measuring carry-over is not simple, since it is an additional weight to a "raw" (unequipped) lift and the equipment alters the movement itself in a substantial manner. Ideally, the equipment's carry-over should be the 1RM load executed with the equipment, minus the 1RM raw load. However, a number of variables may alter one or both values for the same lifter, at the same period: if the lift is done at a competition or at laboratory conditions (or gym); how long the lifter has been familiar and developed technique for the use of the equipment; or how long the lifter has used the particular equipment under test (a new shirt or a new wrap respond differently to the same maximum effort).
Indirect attempts to measure equipment effect have been done before. Zink et al (2001) have studied the effects of the usual belt on trunk and leg muscle activity and joint kinematics during the squat. They observed that the employment of a belt did not change the myoelectric activity during the movement, but it did alter its speed and the path of the bar. Consequently, myoelectric activity is a poor indicator of equipment effect on lifting, since the belt obviously provided a substantial advantage ("significant" alterations in movement speed and bar path both indicate the movement was facilitated and intuitively, an unknown carry-over provided).
We are left, therefore, with anecdotal information from lifters, each adopting their own subjective measure. This article is based on a few considerations of critical issues related to the squat movement, some technical information concerning the elastic material sling (the "knee wrap") itself, my own measurements with the wraps I have used and the comments I received from other lifters.
The squat is either a competitive lift in the sport of Powerlifting or a traditional weight training exercise. As a "lift", the squat has small variations according to the powerlifting federations' rulebooks. In general terms, it consists of a combined flexion of knee and hip joint with a loaded bar over the lifter's trapezius. The bar-downward flexion goes until either a position where hip and knee joint are parallel, or hip joint has "broken" the parallel line, in a knee flexion smaller than 90o. The upward movement then begins, with knee and hip extension until the lifter has resumed a locked-knees straight position. As an exercise, the squat has a myriad variations, using linear guided motion apparatus such as the Smith machine and other devices that basically change the center of gravity of the "bar-lifter" system (traditionally, vertical over the lifter's feet). These variations allow for playing with the joint forces operating in the flexion-extension movements in a way that the "lift-squat" does not allow (Abelbeck 2002).
The lift-squat generates important forces on the knee and hip joints that may be soothed or modified with the exercise devices.
All in all, the squat is considered one of the most complete and important exercises because the recruitment during the movement involves a great number of muscle groups. There has been, however, for many years, some concern over the great forces imposed on the knee and hip joints during the barbell(lift) squat.
The main forces implied in the squat movement are the posterior cruciate ligament (PCL) tensile forces, anterior cruciate ligament (ACL) tensile forces, Tibiofemoral (TF) compressive forces and patellofemoral (PF) compressive forces. All such forces are increased with increased knee flexion (Escamilla et all 2001), and further augmented under external resistance (Wallace et al 2002). Particularly, TF and PF tend to assume greater values with increased knee flexion (Escamilla 2001).
Squat stance also alters the forces operating on the lifter's body. A wider stance tends to increase the movement on both hip and knee joint, thus aggravating the PF/TF augmentation (Escamilla et al 2001b).
One of the most interesting studies concerning this article's focus is Senter and Hame's biomechanical analysis of tibial torque and knee flexion angle (Senter & Hame 2006). According to the authors, there are many knee injuries in sports in general associated with hyper-flexion of the knee joint. At such angles, PCL and meniscus are usually compromised. PCL deficient knees exhibit greater external tibial rotation, which is also detrimental to joint integrity. The authors conclude by claiming that the reduction of forces over PCL, ACL and meniscus should be sought through appropriate protective equipment, proper training and safe surfaces.
The knee wrap
The knee wrap is one such protective equipment. It consists of a long (2m or more) elastic material sling that is wrapped in a tight manner around the lifter's knee, a few inches bellow and above the patella. When the knee is flexed under an external resistance (the loaded bar), the elastic material is further stretched, hopefully reaching its highest expansion during the bottom-most position of the squat. The potential energy thus accumulated on the stretched material is then transferred back to the lifter as kinetic energy, adding to the concentric movement strength during knee extension.
Since most of the stress of the deep squat is on the knee joint, and not the hip joint, the knee wrap allows the lifter to overcome the limiting factor of the lift and employ glutes and quadriceps power to execute the full motion.
It obviously protects the knee joint as well. However, the increased resistance to aggravating forces present at the critical knee hyper-flexion are the main source of knee wrap carry-over.
Measuring knee wrap carry-over
A raw squat will go unaltered in form until a certain point where the forces operating on the knee joint restrict the angle of knee flexion. It would not be appropriate to compare the load at this point with a 1RM competition lift, where, in most federations, the 90o angle of knee flexion must be broken. The comparison must be made, then, with the maximum load when the lift is executed in perfect form in raw conditions ("deep squat"). As far as I know, no one published one such comparison and I had no access to quantitatively accurate case-study measurements. Therefore, this is only my own "self-experiment". The "perfect-deep-form-raw-lift" point of my measurement corresponded to a 209lb load (several repetitions were done beyond 220lb, but they "paralleled", which may be an instinctive protective reaction). The "equipped" 1RM "under laboratory conditions" (the gym) is 303.6lb, which gives a carry-over of 45.3%. The predicted equipped 1RM "under competition conditions" is 319lb, which gives a carry-over of 52.6%. This was all performed with the Blue Power APT ProWristStraps knee wrap. The carry-over obtained with other brands of knee wrap, even with an APT knee wrap other than Blue Power, differed from these values. Anecdotal evidence suggests these are extremely individual preferences.
Although there is no controlled experimentation performed on knee wrap carry-over and protective effect on the knee joint, lifters' experience shows that this is one of the highest performance-supportive and protective lifting devices. Indirect biomechanical information about the squat may indicate why this is so.
Abelbeck K.G. (2002). "Biomechanical Model and Evaluation of a Linear Motion Squat Type Exercise". Journal of Strength and Conditioning Research, 16(4), 516-524.
Escamilla R.F., Fleisig G.S., Lowry T.M., Barrentine S.W., Andrews J.R. (2001b). "A three-dimensional biomechanical analysis of the squat during varying stance widths". Medicine & Science in Sports & Exercise, 33(6):984-98.
Escamilla R.F., Fleisig G.S., Zheng N., Lander J.E., Barrentine S.W., Andrews J.R., Bergemann B.W., Moorman C.T. (2001). "Effects of technique variations on knee biomechanics during the squat and leg press". Medicine & Science in Sports & Exercise, 33(9):1552-66.
Escamilla RF (2001). "Knee biomechanics of the dynamic squat exercise". Medicine & Science in Sports & Exercise, 33(1):127-41.
Senter C., Hame S.L. (2006). "Biomechanical analysis of tibial torque and knee flexion angle: implications for understanding knee injury". Sports Medicine, 36(8):635-41.
Wallace D.A., Salem G.J., Salinas R., Powers C.M. (2002). "Patellofemoral joint kinetics while squatting with and without an external load". Journal of Orthopaedic & Sports Physical Therapy, 32(4):141-8.
Zink A.J., Whiting W.C., Vincent W.J., and Mclaine A.J. (2001). "The Effects of a Weight Belt on Trunk and Leg Muscle Activity and Joint Kinematics During the Squat Exercise". Journal of Strength and Conditioning Research, 5(2), 235-240.
* Marilia Coutinho is a 121lbs APTProWristStraps sponsored powerlifter.