By: Wyatt Christiansen
Hamstring anatomy and function
The hamstrings are a group of muscles, 3 to be specific, on the posterior side of your thigh. They are the biceps femoris (long and short head), semimembranosus, and the semitendinosus. All three (except the short head of the biceps femoris) originate at the ischial tuberosity of the pelvis and insert on the head of the fibula (biceps femoris), the medial condyle of the tibia (semimembranosus), and the proximal end of the tibia (semitendinosus). In other words, the hamstrings originate on the pelvis and insert onto the bones of the lower leg just below the knee.
This means the hamstrings are a biarticular muscle group, they cross over both the hip and knee joint. And because of this, when the hamstrings contract 2 movements can occur, knee flexion and/or hip extension. In fact a mathematical model by Vigotsky and colleagues (2016) showed that the adductor magnus was the primary hip extensor, the hamstrings acted as a secondary extensor in the bottom half of the squat while the glutes kicked in in the upper half of the squat. Although, it should be noted that you likely will see little to no hamstring hypertrophy from only squatting (Kubo et al, 2019). This is likely because other musculature (the quads, adductors, and glutes) take on most of the force and lengthening, while the hamstrings may only experience a smaller proportion of that load and even less range of motion.
Biomechanics During the Squat
During the squat both the knees and hips flex simultaneously until the hip joint goes below the top of the knee cap, powerlifting standard, at which point a lifter begins to ascend creating both hip and knee extension. A multitude of factors will contribute to how this movement is achieved. They
include bar placement, load,, stance width, anatomical proportions, and technique used. Each of these will affect how much hip and knee flexion occur which will determine how much the hamstrings are utilized. If we think of the hamstrings in its neutral, or resting, length, let's denote this as “0”, at lockout, then we would need the hamstring to lengthen during the eccentric (a positive number) in order for it to actively contract during the concentric to aid in hip extension during the squat. If the hamstring muscle length remains unchanged or shortens (a negative number) then we could say that the hamstrings do not contribute.
Thus, given that hip flexion will increase hamstring length while knee flexion decreases hamstring length, in theory comparing the 2 joints should yield whether or not the hamstring contributes to hip extension.
In powerlifting the bar is placed on your back, most commonly in the low bar position, although in training high bar, front squats and/or safety bar squats are common accessories. Generally, as the bar is higher up on the body, such as in the high bar position or the elevation of a safety squat bar, or more forward, such as in a front squat, we see the following changes compared to a low bar squat with the same stance:
A more vertical torso
Less hip flexion
Greater knee flexion
Greater ankle dorsiflexion
Oppositely with the bar lower a lifter will usually experience the opposite:
A more inclined or horizontal torso
Greater hip flexion
Decreased knee flexion
Decreased ankle dorsiflexion
Do note that these are generalized joint angles; it is possible to alter technique and have a higher degree of knee and ankle flexion with a low bar position, if the lifter deliberately uses that technique.
Now if we compare the amount of hip and knee flexion using data from Glassbrook et al. (2017), powerlifters using a low bar squat averaged 121° ±10° of hip flexion and 117° ±12° of knee flexion. If we subtract knee flexion which shortens the hamstrings from hip flexion which lengthens them, we are left with 4 degrees or “units”. Although small this does indicate some lengthening of the hamstrings.
Comparatively, Olympic lifters in the same study using a high bar technique averaged 109° ± 9° of hip flexion and 124° ± 7° of knee flexion. This results in -15 degrees, or “units” meaning the hamstrings shortened during the eccentric and cannot contribute to hip extension during the concentric phase.
It should be noted that the olympic lifting test subjects in this study were all male, had an average 1RM of 169.5 ± 26.5 kg (373.7 ± 58.4 lbs), and an average experience of 3.75 ± 2.72 years. The powerlifting subjects had an average 1RM of 181.2 ± 21.8 kg (399.5 ± 48 lbs) and an average experience of 5.05 ± 4.56 years. We could theorize that in more experienced powerlifters, or on an individual level, other factors such as stance, footwear, and technique could result in a greater hamstring lengthening.
When squatting, the goal is to reach depth and stand back up while maintaining balance of the barbell lifter system. This is often taught as keeping the bar or center of mass over the midfoot. And we have a multitude of ways to achieve this without changing any factors other than technical execution. There are generally 2 main techniques, although they truly exist on a spectrum. A “knees forward” technique involves more ankle dorsiflexion, greater knee flexion, more stress on the quads, and a more upright torso. Oppositely, a “sit back” technique reduces knee flexion, often trying to achieve near vertical shins with a knee angle of 90° which in turn results in much higher amounts of hip flexion.
As with all other variables in the squat, if a technique results in more hip flexion than knee flexion, the lifter should then experience eccentric lengthening of the hamstring and consequently contribution to concentric hip extension from the hamstrings.
In addition to bar placement, stance will also affect biomechanics. If we view the squat directly from the side we will notice 4 things that occur with a wider stance:
Decreased ankle dorsiflexion
Decreased knee flexion
Increased external rotation of the hip (this brings the hips more forward, and artificially shortens the femur)
Decreased hip flexion (a more upright torso)
The opposite is generally true when a lifter utilizes a narrower stance.
Again the utilization of the hamstrings will depend on the ratio of hip to knee flexion. In this case a wider stance decreases both. However, anecdotally, a wider stance is often used in conjunction with a “sit back” technique. As previously stated, to maintain balance over the mid foot while using a “sit back” technique, a lifter will utilize less knee flexion and less ankle dorsiflexion to create a more upright shin. To compensate for this, the lifter will also increase hip flexion.
Of course not every wide stance lifter will squat this way. Thus we can say that stance width may indirectly influence the potential contributions of the hamstrings for squat hip extension.
Every human is different not only in absolute size, but our bodily proportions. This of course means that to achieve a squat to powerlifting standards, different strategies will be used.
Let’s visualize 2 lifters who are each 6’ tall. Lifter A has 3’ 6” long legs and a 2’ 6” long torso while Lifter B has 2’ 6” long legs and a 3’ 6” long torso. Despite being the same height, using the same bar position, the strategies for completing a squat will be quite different. Lifter A will generally be more bent over while lifter B will remain more upright. These are compensations each lifter will have to make in order to maintain balance over the midfoot. If Lifter A attempted to be more upright with knees forward, they would likely put the bar behind their midfoot. Oppositely, if Lifter B attempted to be more bent over and sit back, they would put the bar over their toes.
Now neither strategy is “wrong/right”, it is simply what works best for said individual’s anatomy.
We can look at lifter anatomy like the other factors to determine if the hamstrings experience any eccentric lengthening. Lifter A is more likely to experience increased hip flexion and decreased knee flexion, making them more likely to utilize the hamstrings in aiding hip extension. Lifter B will experience less hip flexion and more knee flexion, resulting in the hamstrings lengthening less or not at all.
Putting It All Together
So do your hamstrings contribute to squatting? It depends. If you are a squatter who has longer legs and a shorter torso, uses a “sit back” technique, has the bar in a low bar position, and limits knee flexion with a wider stance, it is more likely that you are lengthening the hamstring as you approach the hole and gain. However, if you are a lifter who has shorter legs but a longer torso, uses a “knees forward” technique, and a generally narrower stance, even with a low bar position it appears unlikely you significantly stretch your hamstrings to have them aid in hip extension. And if you find yourself somewhere in the middle they may provide some contribution.
Now, while the amount of hamstring contribution will be individualized, they are still important for squatting. Even in those that do not lengthen their hamstrings, they are actively contracting to varying degrees during the squat (Jensen 2000). Of the musculature that contributes to maximal squat strength, the hamstrings may not be the most important… However, they are not something that should go untrained either. If you are a powerlifter, you probably already perform some hamstring training with your deadlift work. But, doing more hamstring curls or finding a slot for good mornings could still work to aid both your squat and your deadlift.
Train your hamstrings. It’s unlikely an extra hamstring exercise or two will be too much to recover from, and even if you don’t experience hamstring lengthening during a squat, they may act to stabilize the hip and knee during the movement. That said, they aren’t a magic pill if you already have decent squat training and do not need to be prioritized over other more bang for your buck exercises.