Matt and Scott explore the reasoning and application of the "knees out" component of the Squat and Deadlift model in Starting Strength. While knees out is an important element in the teaching model, we need to understand exactly what we are trying to accomplish with its use. For those with knee slide and knee valgus issues, it's equally important to understand when to use the cue and when NOT to use it.
Shoving the knees out during the squat (and deadlift, for that matter) accomplishes two important things. First, it clears space for the torso to drop between the thighs, allowing the lifter to hit proper depth, which the model defines as the hip crease dropping below the top of the knee. If the knees were not shoved out -- so that the thighs track over the toes, which should be turned out about 30 degrees -- the thighs would jam into the torso and cause impingement, a trapping of soft tissues between the femur and ASIS. Secondly, knees out satisfies the criteria of using the most muscle mass by engaging the adductors and abductors in the movement. As the knees are shoved out, the abductors (the glute minimus and medius, as well as the smaller "PT muscles" like the piriformis, obturator, and TFL) act to external rotate the femur away from the hip. This causes the adductors to elongate, creating a strong stretch reflex at the bottom of the squat which aids in hip extension. Thus shoving the knees out accomplishes the objective of using more muscle mass in the movement, and thereby getting stronger.
Many trainees will at some point experience "knee slide," in which the knees move forward throughout the descent of the squat, instead of stopping their forward travel 1/3 to 1/2 of the way down as the model describes. Knee slide causes a number of problems. From a mechanical standpoint, knee slide increases the range of motion necessary to hit to depth, since the hip must go lower and lower to pass the top of the knee as the knee angle closes. From an anatomical perspective, the more closed knee angle means that the hamstrings cannot fully lengthen distally. The back angle tends to become more vertical at the bottom with knee slide, to preserve midfoot balance, causing moment force that should be placed on the hips (where the biggest muscle groups are) to transfer to the knee, which has only one muscle group contributing to extension, the quads.
For the knee-slider, cueing "knees out" early and reinforcing it at the bottom of the squat will often fix knee slide, as the lengthening effect of the hamstrings and hip external rotators described above helps pull the knee back during the descent. Sometimes the lifter must be reminded of the midfoot cue (the Master Cue) while shoving her knees out. When verbal cues fail to work, the TUBOW serves as a handy tactile cue. This is a block of wood or PVC pipe about 2ft tall that is placed in front of the toes. The objective is to touch the TUBOW with your knee, but not knock it over.
Knee valgus, or knee caving, in which the knees come inward during the squat, can be much tougher to correct. Matt observes that knee valgus presents a bigger problem for women, some of whom have wider hips and a naturally more "toed-in" femur, although many male lifters struggle as well. Scott likes to use the TUBOW with recalcitrant knee valgus, except he will place it on the side of the foot and instruct the lifter to hold their knee against the TUBOW throughout the squat. Matt prefers an elastic hip band that goes around the thighs and gives the lifter something to push against as they try to feel the knees out position throughout the squat. He also often programs tempo squats, to ensure that the knees remain out during the ascent, coming of the hole. For these lifters, knee valgus may likely be a lifelong issue, but with dedicated work to correct it, the weights at which valgus kicks in will continue to climb, facilitating productive strength training in the range where good form can be adhered to.
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