What is the stretch reflex in squatting? (Part 2)

How is it that I cannot utilize the SSC (stretch-shortening cycle) every time I squat?

A few factors may contribute to an alteration of the length-tension relationship of the hamstrings and adductors in the squat. Let me first clarify what ‘length-tension relationship’ is.

An elastic tissue like a muscle has a maximal stretching capacity and a maximal contraction (shortening) capacity. These 2 properties exist in an inverse relationship called the length-tension relationship. Muscles discussed in this concept refer to its initial resting length before contraction and may not apply fully to an isotonic movement like the squat. 

Regardless, there is a sweet spot to the length of muscle that enables greatest strength generation. Beyond that at either ends of the chart, strength is almost non-existent. This applies for both full contraction (shortening) and full lengthening. The difference is that at the end of full lengthening under load, there exists the stretch reflex mechanism that can help to shorten the muscle closer to its optimal length for it to generate greater strength.

Hence, when someone loses a neutral spine position (and there are a multitude of reasons why that may happen) and goes into either a posterior tilt or excessive anterior tilt of the pelvis, this causes the hamstrings to shorten from its proximal origins and the stretch reflex is lost at the point of reversal.

“But I can still bounce out of the hole and I definitely still feel some stretch reflex going on somewhere..”

Yes, that is because all soft tissue structures have the ability to utilize at least one or more of the SSC phases. These soft tissue structures also include your ligaments and joint capsule. By ‘dive-bombing’ into the ligaments of your knee/hip joints or other muscles not made for taking such great loads, you may not feel pain now but later as damage to these structures exceed the healing capacity of the body. 

Having just covered how the pelvis plays a role in the stretch reflex of the hamstrings, let’s talk about how the knees and ankles may be a culprit.

While many of us struggle to improve and maintain our ankle mobility into dorsiflexion (angle of shin tilt over feet), some people do struggle with the opposite. Having a huge amount of ankle dorsiflexion range of motion, the amount of knee flexion allowed past parallel is greater and this means that this individual may only be able to finally lengthen the hamstring sufficiently at a much greater squat depth utilizing more hip flexion. That being said, this is perfectly ideal in Olympic weightlifters who really only want to bounce out of a hole as low as the platform beneath their heels. 

With a greater depth required due to reasons stemming from hyper mobility, one’s ability to maintain a neutral spine at greater ranges of hip flexion is paramount. This also means that greater hip mobility must be maintained along with strong gluteal and core muscles. Any loss of hip mobility may be detrimental to these individuals wherein the strain to remain in neutral pelvic tilt (relative anterior pelvic tilt) is borne by the lower back extensor muscles in greater magnitude. 

Dangers of engaging your hamstring stretch reflex too early in your squat descent

So you have been hyper-diligent about preventing the butt wink when you squat and have been tensing your lower back muscles before every repetition but the butt wink still appears - if not you just cannot hit depth!

Fortunately (or unfortunately for some), most things in life require balance.

We fall back to the length-tension relationship concept. During the squat descent, premature lengthening of the hamstrings with overly arching one’s back (resulting in excess anterior pelvic tilt) at any point before reaching close to parallel thighs causes the hamstrings to lose their ability to generate hip extension upon initiating ascent. During the descent, the lengthened hamstrings will further limit squat depth due to its acting like a tied-arch bridge and restricting any further hip flexion that is necessary to maintaining the position of the barbell over mid foot. This results in a pseudo depth limitation and would usually feel like a hamstring tightness instead of an unintended ‘bounce’ out of the bottom most position of the squat. Other accompanied sensations may include hip flexor tightness which will be explained next. Common errors resulting in these experiences include an over-emphasis on hamstring and hip flexor stretching which may further exacerbate this problem instead.

Additionally, with the hips in full flexion, the rectus femoris (one of the quadriceps muscles which control both hip flexion and knee extension through the patellofemoral joint) is shortened. This results in increased joint pressure in the knee cap and as greater knee flexion increases with greater depth, a grinding effect within the joint occurs predisposing the lifter to an earlier onset of knee osteoarthritis. 

Finally, overarching your back with load directly on it is undeniably dangerous and detrimental to spinal health (google ‘spinal stenosis’). Aiming to maintain a neutral lumbar spine is paramount to being able to engage the hamstring stretch reflex. 

So what can I do to learn how to utilize the SSC effectively?

Common squat accessories aimed at developing ‘explosiveness’ include the box squat and pause squats. Done correctly, both exercises are highly effective with their own merits.

Box squats for powerlifting emphasize the hip hinging mechanism. With the presence of a box and momentary rest for the quadriceps and gluteus, the lifter is able to practice rocking the body forward on the pelvis with direct tactile feedback on the ischial tuberosities (sit bones). This anterior pelvic tilt relative to the hips joint (done without arching the lower back into greater extension) puts the hamstrings on a quick stretch thus engaging the hamstring stretch reflex at a greater intensity. This also makes it a non-ideal exercise for someone training around hamstring tendinopathy but may be ideal for one with stand-alone quadricep/patella tendinopathy.

Pause squatting emphasizes hip extension force generation from a dead stop, making the lifter less reliant on the hamstring stretch reflex. This instead puts the tendons and muscles under greater static load, utilizing the other 2 phases of the SSC —  storage of elastic energy and active state. As a result, there is greater gluteal muscle/ quadriceps development in lifters who predominantly rely on the stretch reflex i.e. Olympic weightlifters, track and field athletes. This may be more suitable for someone trying to train around the squat with quadriceps and hamstring tendinopathy.

In conclusion, the stretch reflex is simply one of the various mechanisms in which humans and animals move with greater efficiency. The pre-requisite to any efficient movement at greater loads and speeds requires a solid foundation of core stability and sound biomechanics.

And if all else fails, just squat with control: 2 seconds down and 2 seconds up.

Lift safe, lift strong