Quadriceps Leverage and Force Production in Deep Squats (Beyond 90° Knee Flexion)

Quadriceps Moment Arm at Varying Knee Angles

During a squat, the patella acts as a fulcrum that increases the quadriceps’ internal moment arm (distance from knee joint axis) and thus mechanical leverage. However, this leverage is angle-dependent. Research shows that the knee extensor moment arm is greatest in mid-range knee flexion (approximately 20–45°) and then progressively decreases as the knee flexes deeper past 90°ijspt.scholasticahq.comijspt.scholasticahq.com. In other words, at deep knee angles the patella sits in a position that shortens the effective lever arm of the quadriceps. By ~100° of flexion the quadriceps’ moment arm has diminished from its peak, reducing the mechanical advantage for force production ijspt.scholasticahq.comijspt.scholasticahq.com. Im et al. (2015, PMID: 26520912) confirmed via MRI that the quadriceps tendon moment arm peaks near ~20–30° knee flexion and then declines with further flexion ijspt.scholasticahq.comijspt.scholasticahq.com. Similarly, a recent modeling study (Wheatley et al., 2021, PMID: 34404228) noted that increased knee flexion shifts the knee’s axis and patellar position, yielding smaller extensor moment arms in deep flexion pubmed.ncbi.nlm.nih.gov. Practically, this means squatting well below 90° requires the quadriceps to produce higher muscle force to achieve the same joint moment, due to the loss of leverage at deep angles.

Active Insufficiency of the Rectus Femoris in Deep Squats

“Active insufficiency” occurs when a multi-joint muscle shortens over both joints simultaneously, limiting its force output. The rectus femoris, the only bi-articular quadriceps muscle (crossing hip and knee), can encounter active insufficiency in a deep squat. At the bottom of a squat (e.g. > 90° knee flexion with hips deeply flexed), the rectus femoris is shortened at the hip while also attempting to shorten (contract) at the knee, which can impair its force production. Fitness literature often claims the rectus femoris “shuts off” in deep squats due to this effect. Recent analyses support that active insufficiency is primarily a concern for rectus femoris, not the other quad muscles ijspt.scholasticahq.com. Potvin et al. (2022) noted that active insufficiency considerations “may only apply to the rectus femoris,” as the other vasti are single-joint knee extensors and thus less affected ijspt.scholasticahq.com.

Importantly, the rectus plays a dual role: knee extensor (agonist at the knee) and hip flexor (antagonist to the hip extensors). In the squat’s ascent, the hips must extend – an action directly opposed by rectus femoris activation. Biomechanics studies have observed that the rectus femoris contributes less in deep squats to avoid counteracting hip extension. For example, de Souza et al. (2023) found that in the eccentric (lowering) phase of a squat, rectus femoris EMG activity was significantly higher at 90° than at 140°, suggesting the rectus was less active in the deeper squat range mdpi.com. The authors attributed this to the rectus’s length-tension disadvantage and its antagonistic role at the hip: with a more forward-leaning trunk and flexed hip in a deep squat, the rectus is shortened at the hip, reducing its effective length and neural drive mdpi.com. Indeed, they reported that a greater trunk flexion (as in a deep squat) “would reduce RF length and, consequently, its activation” mdpi.com. In short, at deep flexion the rectus femoris is biomechanically less able to produce force – it is near active insufficiency – so the body relies more on the other quadriceps muscles and hip extensors.

Muscle Length–Tension in the Quads at Deep Knee Angles

Muscle force output is highly dependent on muscle fiber length (the length–tension relationship). Peer-reviewed studies mapping quadriceps fiber lengths indicate that the rectus femoris and the three vasti operate on different portions of the length–tension curve during squats. The rectus femoris, due to its two-joint nature, tends to operate on the ascending limb of its length–tension curve in many knee positions ijspt.scholasticahq.com. This means the rectus is often at a shorter-than-optimal length (especially when the hip is flexed), so increasing its length would initially increase force – indicating potential active insufficiency when overly shortened. In contrast, the monoarticular vasti (vastus lateralis, medialis, intermedius) operate closer to the plateau or descending limb of their length–tension curve at typical squat depths ijspt.scholasticahq.com. Even at 90° of knee flexion and beyond, studies have found the vasti’s sarcomeres are at or beyond optimal length. For example, Chen et al. (2016, PMID: 27481293) used microendoscopy to directly measure sarcomere lengths in vivo: in the vastus lateralis they found sarcomeres lengthened from ~2.84 µm at 50° knee flexion to ~3.17 µm at 110°, indicating that at 110° the fibers were longer than the optimal length for force productionpubmed.ncbi.nlm.nih.gov. Similarly, Son et al. (2018, PMID: 29258751) reported that at 90° knee flexion the vastus lateralis and medialis fibers were on the long (descending) side of their tension curve (e.g. vastus medialis average sarcomere ~4.1 µm at 90° vs ~3.6 µm at 45°) pubmed.ncbi.nlm.nih.govpubmed.ncbi.nlm.nih.gov. These findings mean that in deep knee bends, the vasti are somewhat overstretched relative to their optimal length, which can reduce their maximal force output despite still being active. Notably, because the rectus femoris is shortened by hip flexion, its fibers remain on the shorter, ascending limb even in deep knee flexion – further evidence that only rectus experiences a true active insufficiency in the squat position ijspt.scholasticahq.com. In summary, at squat depths beyond 90° the vasti are stretched (potentially past optimal), and the rectus femoris is shortened – both factors can limit force. The net result is that the quadriceps may produce less force per unit activation in the deep range compared to mid-range angles.

Force Production and EMG in Deep vs. Partial Squats

Because of the above factors (reduced moment arm and suboptimal muscle lengths), the effective knee extension force output diminishes at extreme flexion angles. Isometric strength testing confirms this: peak knee extension torque occurs around mid-range angles (~60–100°), and torque drops off at very shallow or very deep angles. In one study of maximal voluntary contractions at various knee angles, extension torque increased as the knee moved from 130° up to ~110° flexion, then plateaued or slightly decreased moving to 120°–130°mdpi.com. By ~120–130° of flexion, the quadriceps produced less torque than at 90–100°, despite maximal effort. Crucially, this reduction in torque at deep angles was not due to lack of muscle activation – EMG showed the vastus lateralis and medialis were fully activated even at 120–130°mdpi.com. Kukić et al. (2022) observed no significant drop in quadriceps EMG amplitude from 80° out to 130° knee positions, indicating that the nervous system can still maximally drive the quads in deep flexion, but the output is lower due to mechanical factors (i.e. shorter lever arm and less favorable fiber length) mdpi.commdpi.com. This aligns with the length–tension evidence above: the quads are trying as hard as ever, but their force translating to joint torque is reduced past 110° flexion.

Dynamic squat experiments echo these findings. In deep squats (e.g. 140° knee flexion depth), the quadriceps must overcome both a large external moment and their internal disadvantages. EMG studies show shifts in muscle contribution with depth. de Souza et al. (2023) found that during the concentric ascent, vastus lateralis activation was actually higher in a full-depth squat (0–140°) than in a half-squat (0–90°) mdpi.com, presumably because the vasti had to compensate more in the deep range as rectus femoris contribution was limited. In contrast, rectus femoris activity tended to be lower in the deepest squat, especially during the lowering phase mdpi.com. The authors noted this is consistent with rectus femoris’s reduced efficacy at long hip flexion – the muscle cannot contribute as much when it’s near its shortened limit mdpi.com. Thus, in a full squat the body relies on the monoarticular vasti (and the gluteals for hip extension) to a greater extent, whereas in a partial squat the rectus femoris can play a more active role.

Summary of Key Findings (2010–2025 Literature)

Peer-reviewed research from the past decade supports that squatting below 90° of knee flexion places the quadriceps at a mechanical disadvantage, but not an insurmountable one: The quadriceps’ internal moment arm decreases in deep flexion, meaning less leverage for knee extension ijspt.scholasticahq.comijspt.scholasticahq.com. The vasti muscles are stretched beyond optimal length at deep knee angles, and the rectus femoris may suffer active insufficiency due to concurrent hip flexion ijspt.scholasticahq.compubmed.ncbi.nlm.nih.gov. These factors lead to reduced force output at the knee joint in deep squats, even though neural activation of the quads remains high mdpi.com. In practical terms, the quads are still highly engaged in a deep squat, but each fiber’s force translates to a slightly lower torque due to geometry and length-tension limitations. This is evidenced by both in vivo measurements of muscle length (PMID: 27481293, 29258751) and strength/EMG tests at varied angles (PMID: 26520912). Overall, while the quadriceps can certainly be trained and strengthened in deep squats, the research confirms a modest loss of mechanical advantage and potential force output in the quad muscles once the knee is flexed beyond ~90°, largely attributable to patellofemoral leverage changes and muscle length dynamics ijspt.scholasticahq.compubmed.ncbi.nlm.nih.gov.