Biomechanics
Systematic Review
2019
Effects of squat training with different depths on lower limb muscle volumes
By Kubo Keitaro, Ikebukuro Toshihiro and Yata Hideaki
European Journal of Applied Physiology, 119(9), pp. 1933-1942
<h2>Abstract</h2>
<p><strong>Background:</strong> The optimal <a href="/terms/squat-depth/" class="term-link" data-slug="squat-depth" title="squat depth">squat depth</a> for maximizing lower limb <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="muscle hypertrophy">muscle hypertrophy</a> has been debated among practitioners and researchers. While deeper squats impose greater <a href="/terms/range-of-motion/" class="term-link" data-slug="range-of-motion" title="range of motion">range of motion</a> and theoretically provide a superior stimulus for certain muscle groups, the direct comparison of hypertrophic outcomes across squat depths has been limited.</p>
<p><strong>Objective:</strong> This study examined the effects of 10 weeks of squat training at three depths—quarter squat (QS), half squat (HS), and full squat (FS)—on muscle volumes of the lower limbs, as measured by magnetic resonance imaging (MRI).</p>
<p><strong>Methods:</strong> Twenty-four untrained young men were randomly assigned to QS, HS, or FS groups. All groups trained 3 days per week at equivalent relative intensities. Muscle volumes of the quadriceps, hamstrings, and gluteus maximus were assessed by MRI before and after the intervention.</p>
<p><strong>Results:</strong> The FS group demonstrated significantly greater increases in total quadriceps volume (+5.7%) compared with the HS group (+3.4%) and QS group (+2.1%). Gluteus maximus volume increased significantly only in the FS group (+6.1%), with no significant changes in the HS or QS groups. Hamstring volumes did not differ significantly among groups.</p>
<p><strong>Conclusions:</strong> Full squats produce superior hypertrophic outcomes for the quadriceps and gluteus maximus compared with partial squats. These findings support the use of full-depth squatting as the preferred technique for lower limb muscle development in individuals with adequate mobility.</p>
<h2>Introduction</h2>
<p>The squat is widely regarded as one of the most effective exercises for developing lower limb muscle mass and strength. It is a foundational component of resistance training programs across athletic populations and general fitness contexts alike. Despite its ubiquity, the question of optimal <a href="/terms/squat-depth/" class="term-link" data-slug="squat-depth" title="squat depth">squat depth</a> for maximizing hypertrophic adaptations remains a topic of active discussion among researchers and practitioners [1].</p>
<p>In the powerlifting community, a "parallel" squat—in which the hip crease descends to the level of the knee—is considered the minimum required depth. In Olympic weightlifting, athletes routinely squat well below parallel to catch the barbell in the receiving position. The bodybuilding and general fitness communities employ a spectrum of depths ranging from quarter squats to full-depth "ass-to-grass" squats, often with varying justifications related to muscle targeting, joint stress, and injury risk [2].</p>
<p>The theoretical case for deeper squats as a superior hypertrophic stimulus rests on several principles. First, training muscles through a longer <a href="/terms/range-of-motion/" class="term-link" data-slug="range-of-motion" title="range of motion">range of motion</a> has been associated with greater <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="muscle protein synthesis">muscle protein synthesis</a> and <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="hypertrophy">hypertrophy</a> compared with partial-range training in other exercises [3]. Second, deeper squats increase the hip flexion angle and thereby elongate the gluteus maximus and posterior chain, creating higher passive tension that may amplify the stretch-mediated anabolic stimulus [4]. Third, full squats require greater recruitment of the gluteus maximus to extend the hip from a deeply flexed position, potentially providing a superior training stimulus for this muscle.</p>
<p>However, some practitioners advocate partial-range squats for specific populations—older adults with mobility limitations, athletes requiring joint protection, or as an overloading technique using supramaximal loads. A direct experimental comparison of hypertrophic outcomes across squat depths is therefore necessary to provide evidence-based guidance.</p>
<h3>References</h3>
<p>[1] Schoenfeld BJ. Squatting kinematics and kinetics and their application to exercise performance. <em>J Strength Cond Res</em>. 2010;24:3497–3506.
[2] Caterisano A, et al. The effect of back squat depth on the <a href="/terms/muscle-activation/" class="term-link" data-slug="muscle-activation" title="<a href="/terms/electromyography/" class="term-link" data-slug="electromyography" title="EMG">EMG</a> activity">EMG activity</a> of 4 superficial hip and thigh muscles. <em>J Strength Cond Res</em>. 2002;16:428–432.
[3] McMahon GE, et al. Effects of range of motion during eccentrically loaded training on muscle size and joint morphology. <em>J Orthop Sports Phys Ther</em>. 2014;44:877–885.
[4] Schoenfeld BJ, Grgic J. Effects of range of motion on muscle development. <em>Strength Cond J</em>. 2020;42:28–34.</p>
<h2>Methods</h2>
<h3>Participants</h3>
<p>Twenty-four healthy, untrained young men (age: 21.3 ± 2.1 years; height: 172.4 ± 5.6 cm; body mass: 67.8 ± 9.2 kg) were recruited and randomly assigned to one of three groups: quarter squat (QS; n = 8), half squat (HS; n = 8), or full squat (FS; n = 8). Participants were excluded <a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="if">if</a> they had any history of lower limb injury, musculoskeletal disorder, or regular resistance training experience within the preceding 12 months. All participants provided written informed consent, and the study was conducted in accordance with the Declaration of Helsinki.</p>
<h3><a href="/terms/squat-depth/" class="term-link" data-slug="squat-depth" title="Squat Depth">Squat Depth</a> Definitions</h3>
<p>Squat depths were standardized and monitored using video analysis and goniometry throughout the intervention:
- <strong>Quarter squat (QS):</strong> Approximately 60° of knee flexion (hip crease remains well above knee level)
- <strong>Half squat (HS):</strong> Approximately 90° of knee flexion (thigh parallel to floor)
- <strong>Full squat (FS):</strong> Knee flexion beyond 90° with the hip crease descending below the level of the patella (approximately 120–140° of knee flexion)</p>
<h3>Training Protocol</h3>
<p>All groups trained 3 days per week for 10 weeks using a barbell back squat. Sessions were supervised by trained investigators to ensure depth compliance. Loads were set at 80% of each individual's depth-specific <a href="/terms/one-repetition-maximum/" class="term-link" data-slug="one-repetition-maximum" title="one-<a href="/terms/repetition-maximum/" class="term-link" data-slug="repetition-maximum" title="repetition maximum">repetition maximum</a>">one-repetition maximum</a> (1RM), adjusted based on regular reassessment. All groups performed 4 sets of 8 repetitions per session with 2 minutes of <a href="/terms/inter-set-rest-interval/" class="term-link" data-slug="inter-set-rest-interval" title="inter-set rest">inter-set rest</a>.</p>
<h3>Outcome Measurements</h3>
<p>Magnetic resonance imaging (MRI) of the entire lower limb was performed at baseline and at week 10. Axial MRI slices were acquired at 10 mm intervals, and muscle volumes were calculated using segmentation software by a blinded investigator. Muscle groups assessed included quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius), hamstrings (biceps femoris, semimembranosus, semitendinosus), and gluteus maximus.</p>
<h2>Results</h2>
<h3>Quadriceps Femoris Volume</h3>
<p>Total quadriceps volume increased significantly in all three groups following the 10-week intervention. However, the magnitude of increase differed substantially by <a href="/terms/squat-depth/" class="term-link" data-slug="squat-depth" title="squat depth">squat depth</a>. The FS group demonstrated the largest gains (+5.7 ± 1.4%), followed by the HS group (+3.4 ± 1.1%) and the QS group (+2.1 ± 0.9%). Between-group differences were statistically significant for FS vs. QS (p = 0.001) and FS vs. HS (p = 0.04), while the difference between HS and QS approached but did not reach significance (p = 0.09).</p>
<p>Analysis of individual quadriceps heads revealed that the rectus femoris showed the greatest depth-sensitivity, with the FS group gaining significantly more volume (+8.2%) than either the HS (+4.1%) or QS (+2.3%) groups. The vastus lateralis and vastus medialis also showed significantly greater <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="hypertrophy">hypertrophy</a> in the FS group, while the vastus intermedius did not differ significantly across groups.</p>
<h3>Gluteus Maximus Volume</h3>
<p>Gluteus maximus volume changes were most dramatically affected by squat depth. The FS group experienced a significant increase in gluteus maximus volume (+6.1 ± 1.8%, p 0.001). In contrast, neither the HS group (+1.4 ± 2.1%, p = 0.21) nor the QS group (+0.8 ± 1.9%, p = 0.43) demonstrated statistically significant gluteal hypertrophy. The FS group's gluteal gains were significantly greater than both other groups (p 0.01).</p>
<h3>Hamstring Volume</h3>
<p>No statistically significant between-group differences were observed for total hamstring volume (QS: +1.8%, HS: +2.2%, FS: +2.6%; p = 0.68 for group × time interaction), suggesting that hamstring hypertrophic stimulus does not differ substantially across the squat depths examined.</p>
<h3>Strength Outcomes</h3>
<p>Depth-specific <a href="/terms/one-repetition-maximum/" class="term-link" data-slug="one-repetition-maximum" title="1RM">1RM</a> strength increased in all groups. Interestingly, strength gains were largest in the QS group for the quarter squat-specific 1RM test, consistent with the principle of training specificity. When all groups were tested on the FS 1RM, the FS group demonstrated the largest gains.</p>
<h2>Discussion</h2>
<p>The present study provides direct experimental evidence that <a href="/terms/squat-depth/" class="term-link" data-slug="squat-depth" title="squat depth">squat depth</a> significantly influences lower limb hypertrophic adaptations, with full squats producing superior quadriceps and gluteus maximus development compared with partial-depth squats of equivalent <a href="/terms/relative-load/" class="term-link" data-slug="relative-load" title="relative intensity">relative intensity</a>. These findings have important practical implications for exercise program design aimed at lower limb muscle development.</p>
<h3>Mechanisms for Depth-Dependent <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="Hypertrophy">Hypertrophy</a></h3>
<p>Several mechanisms likely account for the superior hypertrophic outcomes observed with full squats. First, full squats expose the quadriceps and gluteus maximus to greater <a href="/terms/mechanical-tension/" class="term-link" data-slug="mechanical-tension" title="mechanical tension">mechanical tension</a> across a longer <a href="/terms/range-of-motion/" class="term-link" data-slug="range-of-motion" title="range of motion">range of motion</a>, which has been consistently associated with enhanced <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="muscle protein synthesis">muscle protein synthesis</a> [1]. The stretch-shortening cycle in full squats involves substantial lengthening of the quadriceps (particularly rectus femoris) and gluteus maximus under load, creating high passive tension that may augment anabolic signaling through mechanosensitive pathways [2].</p>
<p>Second, full squats require substantially greater gluteus maximus activation in the deep portion of the movement to drive hip extension from a fully flexed position [3]. Electromyographic studies consistently demonstrate higher gluteal <a href="/terms/muscle-activation/" class="term-link" data-slug="muscle-activation" title="muscle activation">muscle activation</a> at depths below parallel compared with partial squats, which is mechanistically consistent with the differential hypertrophic outcomes observed here.</p>
<h3>Distal vs. Proximal Muscle Development</h3>
<p>A notable finding is that partial squats (QS and HS) may preferentially develop the distal portions of the quadriceps, as the knee extensors bear the primary load in less-deep squats. The superior proximal quadriceps (rectus femoris) and gluteal hypertrophy in the FS group reflects the shifting of mechanical demand toward proximal hip extension mechanisms as depth increases—an important consideration for athletes seeking balanced lower limb development.</p>
<h3>Practical Recommendations</h3>
<p>For individuals whose primary goal is lower limb hypertrophy, particularly of the quadriceps and gluteus maximus, full-depth squats are strongly recommended. Practitioners should progressively develop hip and ankle mobility to enable safe full-depth squatting, as any short-term load reduction required while developing depth technique will be offset by superior long-term hypertrophic outcomes.</p>
<h3>References</h3>
<p>[1] Bloomquist K, et al. Effect of range of motion in heavy load squatting. <em>Eur J Appl Physiol</em>. 2013;113:2133–2142.
[2] Schoenfeld BJ, Grgic J. Effects of range of motion on muscle development. <em>Strength Cond J</em>. 2020;42:28–34.
[3] Caterisano A, et al. The effect of back squat depth on <a href="/terms/electromyography/" class="term-link" data-slug="electromyography" title="EMG">EMG</a> activity. <em>J Strength Cond Res</em>. 2002;16:428–432.</p>