4.1. Comparison of the Calculated Compressive Forces in the Half and Quarter Squat with Experimentally-Determined Strength Values of Vertebral Bodies and Segments
To the best of the authors’ knowledge, there are no calculations about the compressive forces of the lumbar spine between different squatting depths with maximal loads. These estimates are therefore extrapolated from the existing literature. Cappozzo et al.
] calculated compressive forces on the L3/L4 segment for four subjects, who performed squats with turning points at a 112° down to an 87° knee angle. Weights of between 0.8- and 1.6-fold bodyweight resulted in compressive forces of 6–10-fold bodyweight at the turning point of the squat (3100–7340 N). With increasing loads, these authors calculated a raise in compressive forces acting on the vertebral bodies [19
] that led to increases of intradiscal pressure [20
]. For soccer players performing the half back squat for a period of eight weeks, Helgerud et al.
] have presented significant increases in 1-RM half squat (49.2%, p
≤ 0.001) with improvements from 1.5-fold bodyweight (115.7 ± 23.1 kg) to 2.2-fold bodyweight (176.4 ± 18.2 kg). Cormie et al.
] have reported that untrained male subjects were able to perform 1-RM in the half squat with 1.4-fold bodyweight without familiarization sessions. Rønnestad [26
] had the ability to perform a 1-RM in the half squat with 2.2-fold bodyweight (150.0 ± 15.3 kg) as inclusion criteria for his study. However, his participants (recreational sportsmen) showed significant gains of 24.2% (p
≤ 0.01) to 186.4 ± 21.9 kg in five weeks of training, which is an increase to 2.7-fold bodyweight. Physical education students exercising the quarter squat to a 120° knee angle for a period of 10 weeks increased their performance on average from 2.87- (220.00 ± 42.16 kg) to 3.89-fold bodyweight (297.89 ± 41.58 kg) (37.5%, p
≤ 0.05) [17
]. Thus, the L3/L4 vertebral segment would have to tolerate compressive forces of more than 17-fold bodyweight in the half [26
] and more than 20-fold in the quarter squat [17
]. The calculated compressive forces in the half squat after five weeks of training [26
] and 10 weeks of training in the quarter squat [17
] sum up to 12,359 N and 14,648 N, respectively, which clearly exceeds the maximally-tolerated values presented in Table 2
. For the deadlift (212–335 kg, relative strength 3.5 to 4.4× bodyweight), Granhed et al.
] have reported even higher axial compressive forces on the L3 vertebral body of 18,800–36,400 N in six elite powerlifters (bodyweight: 59–93 kg). Calculations had been performed via data extracted from movement analysis.
For the loading peaks in the squat, which were analyzed by Cappozzo et al.
] through ground-reaction forces, they have been far beyond 1 s. For the parallel squat in the study of Lander et al.
], only relative time scales were used without information about their duration. However, Jäger and Luttmann [49
] argued that (calculated) dynamic compressive forces could not be compared to experimentally-determined strength values, since loading peaks of less than a one-second duration may be tolerated by intervertebral discs (in vivo
) because of their viscoelastic properties. Therefore, a transfer to in vivo
situations should be done restrainedly. Nevertheless, those comparisons raise the question to what extent a performance-oriented strength training including the half and/or quarter squat, which demands comparatively supramaximal loads (with the deep squat), increases the risk of injury for the spine, especially in female athletes. For example, female sport students (n
= 23), who were mostly inexperienced with strength training, were already able to perform the quarter squat with 2.49-fold bodyweight in the pretest [17
]. Due to the significantly smaller female corpus vertebrae [50
], their vertebral bodies tolerate significantly less maximal compressive forces than men’s [51
]. Consequently, if equivalent compressive forces are applied, the axial stress is higher for female vertebral bodies [50
]. This should especially be accounted for in the training regimen for young athletes. Furthermore, Zatsiorsky and Kraemer [15
] emphasized the importance of stabilization by the trunk muscles (especially erector spinae) when performing squats to counteract potentially damaging anterior shear forces on intervertebral discs resulting from ventral flexion [23
]. However, the utilization of comparatively supramaximal loads in the half and quarter squat increases the risk of not being able to stabilize the thoracic and lumbar spinal region. Therefore, the probability of ventral flexion under these circumstances rises, which increases the risk of a disc prolapse [42
4.2. Comparison of the Calculated Compressive Forces in the Parallel and Deep Squat with Experimentally-Determined Strength Values of Vertebral Bodies and Segments
After 10 weeks of training, two groups of physical education students increased their performance in the deep front squat on average from 1.01-fold (73.00 ± 26.48 kg) to 1.26-fold bodyweight (90.88 ± 25.61 kg) and in the deep back squat from 1.12-fold (80.25 ± 30.28 kg) to 1.41-fold bodyweight (101.50 ± 30.00 kg) (29%–30%, p
≤ 0.05) [17
]. The calculated compressive forces [19
] of 3100–7324 N in the lumbar region (L3/L4 vertebral segment) are, according to Table 2
, within the range of tolerance for a 39-year-old male subject.
According to Lander et al.
], the calculated axial compressive forces on the L5/S1 vertebral segment were on average 9983 N at the turning point in the parallel squat with 70%–90% 1-RM (intensity range of hypertrophy-oriented training [7
]). Indeed, this calculated value exceeds the average values of 6000–8000 N presented in Table 1
, but it has to be taken into account that those experiments had been performed on very heterogeneous cadavers [40
]. The maximally-tolerated compressive force is dependent on sex, age, size of corpus vertebrae [40
], bone mineral density (BMD) and bone mineral content (BMC) [52
]. For example, the maximally-tolerated compressive force in the L5/S1 vertebral segment of a 22-year-old male (ex
) was 12,740 N and therefore within the calculated range by Lander et al.
]. Furthermore, the participants (n
= 6) in the study by Lander et al.
] were able to perform 1-RM in the parallel squat with 2.38-fold bodyweight, which can only be achieved after several years of intensive strength training.
In comparison, using the half squat as training exercise, similar values have been achieved by soccer players after only eight weeks [25
]. It can be expected that compressive stress on the spine applied for several years, as for example in weightlifters [53
] and powerlifters [48
], leads to structural adaptations of vertebral bodies (L2–L4). Those include increased BMD [53
] and BMC [48
], which result in higher maximally-tolerated compressive forces, as there is a linear, positive relationship between maximal compressive strength and BMD (r
= 0.91; p
≤ 0.00001) and BMC (r
= 0.84; p
≤ 0.00001) (L3) [52
]. Further, a cross-sectional study with 25 elite young weightlifters (17.4 ± 1.4 years) with more than 2.5 years of strength training experience (on average) revealed significantly increased values in BMD of L2–L4 (33%; p
≤ 0.05) compared to an age-matched control group (n
= 11) without group differences for body height and weight. Moreover, their BMD was 13% (p
≤ 0.05) higher compared to reference values of 400 males aged between 20 and 39 years [53
]. A follow-up investigation showed that another year of weightlifting training increased those values even further [54
] (p. 119). Lang et al.
] and Loehr et al.
] detected statistically-significant increases in BMD of L1–L2 of between 7% and 12.3% after four months of periodized strength training in parallel squats and deadlifts (6–10 RM, [55
]; 70%–80% 1-RM [56
]). Compared to young women, young men gained statistically significantly more in BMD of the lumbar spine (2.7%–7.7% vs.
−0.8%–1.5%) after six months of periodized strength training using the same training exercises (67%–95% 1-RM). Intragroup changes were not analyzed statistically [57
]. The remodelling processes of bones range from 4–6 months [58
]. Based on these facts, Chilibeck et al.
] emphasized the importance of strength training periods that last two to three times longer than this adaptation period to bring about substantial improvements in BMD. Several other studies with male [60
] and female [63
] long time weightlifters have reported significantly increased BMD values in comparison with controls [60
] and athletes of other sports, as for example cycling, cross-country skiing and orienteering [63
Concerns about an increased risk of injury performing the deep squat have been disproven by plenty of cross-sectional studies with weightlifters. Neither the extent nor the prevalence of spinal abnormalities was increased compared to other athletes [29
] or controls [30
]. Compared to controls (non and recreational sportsmen) that were age and gender matched (n
= 1347), Dalichau and Scheele [31
] determined a tendency toward an increasing kyphosis and inclination in the sagittal projection for 29 active weightlifters (C7–S2). These athletes were aged 27.5 years and had a sporting exposure of 10.3 years. Dalichau and Scheele [31
] (p. 119) stated: “After examining sports-specific mechanic demands and taking account of the epidemiological investigations of incidence and prevalence of spinal injuries in weight lifting the results evaluated are to interpret as a functional adaptation of the spinal curvature to the weight lifting-specific straining (sic) profile”.
Granhed et al.
] reported that L3 bone mineral content correlated (r
= 0.90) with the yearly load (300–5000 tons) of eight elite powerlifters, which indicates a positive influence of the volume of strength training on the development of BMC. Sabo et al.
] have also presented data of 28 male weightlifters, whose weekly load was 68 tons, with significantly higher BMD values of their lumbar vertebral bodies (L1–L4) (24% compared to age-matched controls). Tszuzuku et al.
] have even found a significant correlation (r
= 0.74; p
≤ 0.05) between 1-RM in the parallel squat and BMD of L2–L4 in ten powerlifters (mean age: 20.7 years).
It can also be expected that intervertebral discs adapt to years of strength training. Many animal studies lasting 3–15 weeks with dogs [65
] and rats [67
] confirm the positive effects of training in different parcours (running, jumping, crawling) [65
] and of treadmill training [66
] on nutrition status [65
] and on the structure of intervertebral discs [66
]. For example, in vivo
experiments with rats have shown that caudal nucleus and annulus pulposus positively adapted after two weeks of dynamic compression with 1 MPa, which is about three-fold bodyweight [69
]. However, most mammalian species possess notochordal cells in the nucleus pulposus at birth, but unlike humans, maintain them throughout much of their adulthood. Notochordal cells have been suggested to be progenitor and/or organizer cells [70
]. “They may directly synthesize matrix proteins and eventually differentiate into the mature chondrocytic nucleus cells or they may help to recruit and co-ordinate other mesenchymal cells to synthesize the extracellular matrix” [70
] (p. 3). Therefore, findings gathered from animals that maintain notochordal cells well into adult life should be interpreted cautiously [70
]. However, Ishihara et al.
] have reported an increase in proteoglycan metabolism in human vertebral discs (explants) stressed with 2.5 MPa, while it decreased if pressure was increased to 7.5 MPa. For a comparison, a male (70 kg) lifting a beer case (20 kg) from a squatting position already exhibits values of 1.7 MPa (L4/L5) [72
]. This is an apparent contradiction to reports of catabolic metabolic reactions of cellular cultures of nucleus pulposus at a pressure of 2.5 MPa [73
]. However, several papers have shown that the missing extracellular matrix in single-layered cellular cultures causes them to react differently to mechanic stimuli [70
], which is why explants [71
] are physiologically of higher relevance. In comparison with untrained controls, Tittel [75
] diagnosed larger lengthwise and cross-sectional diameters of vertebral bodies of lower thoracic and all lumbar vertebral bodies in strength training athletes with several years of experience. Those economic enlargements had still been detectable even 10 years after the athletic career ended. Moreover, Neumann et al.
] have reported a positive and linear correlation between BMD of vertebral bodies (ex vivo
) and tensile strength (r
= 0.84, p
≤ 0.05), as well as the stiffness of ligamentum longitudinal anterior (r
= 0.78, p
≤ 0.05). In vivo
, this would represent an increased passive stability of vertebral segments. In combination with the gain in BMD, enlargement in vertebral body surface area and well-trained trunk musculature, one can state that strength training on a regular basis shows a protective effect.
4.3. Incidence and Risk of Injuries of the Vertebral Column in Deep Squats
In weightlifting, athletes perform deep front and back squats, as well as pull and assistance exercises. Both demand high accelerations and loads. International elite weightlifters exercise regularly 10-times per week with 400 and more reps and 70–90 tons of weight in high volume training periods [14
]. X-ray images of weightlifters (n
= 25, average age: 31.5 years) do not reveal a higher extent of degenerative changes of the spine compared to track and field athletes (n
= 25; average age: 27.0 years) [29
]. By means of magnetic resonance imaging (MRI) scans of T6/7 to L5/S1, Baranto et al.
] have reported weightlifters (n
= 21) and ice hockey players (n
= 19) to have the highest prevalence of degenerative disc abnormalities compared to other professional sportsmen (e.g., wrestlers (n
= 13) and orienteering (n
= 18)). However, the differences have not been statistically significant even in comparison with untrained, age-matched controls (n
= 21). A 15-year follow-up diagnosis revealed a deterioration of existing abnormalities in all four groups of athletes that mainly included disc degeneration, which was diagnosed in more than 90% of the athletes. These abnormalities eased to some degree in 88% of the athletes, with the highest probabilities for weightlifters and ice-hockey players, but most of them had been ice hockey players [30
]. In a recent review about possible adaptations and degenerative changes of intervertebral discs depending on a particular sport, the authors have come to the conclusion that “sports including, swimming, baseball, weightlifting, rowing and equestrian riding are more likely to lead to disc degeneration” [77
]. However, their statement about an increased incidence of degenerative effects of weightlifting had only been based on two studies [78
]. One study, with monozygotic twins, determined higher disc degeneration in the T6 to T12 region for the twins who performed weightlifting compared to the twins who executed endurance sports. There were no degenerative changes present in the lumbar spine of the weightlifters [79
]. However, these weightlifters had already been training in a time when the standing press or “military press” was still an Olympic exercise. Due to missing guidelines [80
] and the high injury risk of this exercise [81
], it was abandoned in 1972 [80
]. The same problem exists for the majority of weightlifters in the study by Baranto et al.
] and Granhed and Morelli [78
]. Unfortunately, in their review Belavy et al.
] did not refer to the strong positive indications of other research groups in this field [31
In contrast, four epidemiological studies, lasting 2.5–6 years, have not revealed any major injuries of the spine in competitive weightlifters [82
]. Furthermore, MRI scans of lumbar spines of former elite athletes have not shown any statistically-significant differences for vertebral body height (L1/2 to L5/S1) or lumbar flexibility between weightlifters (n
= 29, 59.4 years), long-distance runners (n
= 27, 59.6 years), shooters (n
= 28, 61.1 years) and soccer players (n
= 30, 56.6 years) [32
]. According to Baranto et al.
], the prevalence of disc abnormalities (e.g., reduced intervertebral disc height) of professional weightlifters (n
= 10, 42.0 years) is similar to untrained age-matched controls (n
= 10). In addition, the incidence of back pain has been reported to be less (23% vs.
31%) in former weightlifters (n
= 13) compared to the general population (n
= 716) [78
]. According to this, it may be concluded that intervertebral discs in humans are able to adapt to exercise-induced compressive forces in weightlifting training. In the long run, those adaptations lead to an increased stress tolerance.
Walsh et al.
] reported a statistically significantly (p
≤ 0.01) increased extension in the lumbar spinal region with growing loads (40%–80% 1-RM) in 48 strength-trained participants performing the half squat. Therefore, Walsh et al.
] postulate a performance-oriented strength training in the half squat to increase the risk of injury in the lumbar spine. This assumption is further based on ex vivo
measurements in the L4/L5 segments [86
]: during axial compression of 2000 N, an extension by 2° compared to 0° statistically significantly increased intradiscal pressure within the posterior annulus. The concerns by Walsh et al.
] are unfounded, as the angle of the hip also changes. This is why in the deep squat at the turning point, the risk of delordosing of the lumbar spine is raised [35
]. Video analyses have shown males exhibiting a smaller anterior pelvic tilt and a larger ventral flexion of the lumbar spine during the lowering phase of the parallel squat compared to females (p
≤ 0.001) [35
]. The gender differences could result from larger range of motion in the hip in females [87
]. However, participants performed squats with submaximal loads of 0.5-fold bodyweight. Higher loads should also increase the risk of delordosing in females. Lander et al.
] analyzed the parallel squat of five strength-training experienced males with 75%–80% 1-RM. Induced fatigue led to increased upper body tilt. Trafimov et al.
] confirmed that a fatigued quadriceps femoris affected lifting technique with regard to performing a back lift rather than a squat lift. However, an increased upper body tilt raises the risk of ventral flexion of the thoracic and the lumbar spine [33
To minimize delordosing, it is necessary to start extension before the turning point. This can be achieved with increased activity of the erector spinae, which keeps intervertebral distances constant due to the closing of the apophyseal joints and therefore reduces anterior shear forces on discs [23
]. Weightlifters also have to remain in lordosis at movement onset in snatch and clean. Calhoon and Fry [83
] reported an incidence of 3.3 injuries per 1000 h of weightlifting training in 27 Olympic weightlifters in a period of six years. Training interruption of one day or less accounted for 87.3% of injuries concerning the lower back, while the remaining cases interrupted training for less than one week. Based on a questionnaire for 13–16-year-old weightlifters (n
= 1634), Hamill [84
] reported a prevalence for injury of 0.0017 per 100 h. Therefore, the rate of injury is less than in basketball (0.03), track and field (0.57), American football (0.1) and gymnastics (0.044) [84
Byrd et al.
] could not document any kind of injury or training interruption in 11 adolescent weightlifters (three females: 13.7 ± 1.2 years; eight males: 12.5 ± 1.6 years) in an observational period of 29 months in which they had performed 534 trials in competitions. In a retrospective study (four years) with 1109 weightlifters aged 12–20 years, who had participated in national and international competitions, no injuries that induced a chirurgic treatment or a hospital stay were reported [85
4.4. Risk of Injuries of the Knee Joint and Vertebral Column Due to the Restriction of the Anterior Knee Displacement in the Squat
If the parallel or the deep squat are performed with the spine remaining as vertical as possible, the anterior movement of the knee exceeds the toes (unrestricted movement). This has been regarded with concern by practitioners, as retropatellar pressure and tibiofemoral shear forces have been expected to increase and cause cartilage damage. Based on a literature review, we have been able to show that this is without any reason [90
]. For the restricted movement, it is recommended that the tibia should move anteriorly only to the point where the knee joints and the toes form a vertical line [91
]. Fry et al.
] calculated statistically-significant increases in peak torque of the knee joint at the turning point in seven male subjects performing the back squat (with 1.0-fold bodyweight) without this movement restriction. The average knee angles at the turning point were 66.1° compared to the restricted condition that was performed to 73.4°. However, based on Figure 1
, the restricted condition in the high-bar squat bears the risk of ventral flexion of the spine and cannot be considered as a correct execution of the squat. Lorenzetti et al.
] confirmed these results for the unrestricted condition and reported statistically significantly higher relative knee joint torque of 25% (p
< 0.05) with 0.5-fold bodyweight. In this case, however, the turning point in the range of motion restricted variation for their 20 subjects had been on average 91° (= half squat). This was, again, confirmed by that workgroup with more participants (n
= 30) [33
If the dependence of squatting depth and load and their influence on the enlargement of the retropatellar articular surface are taken into account, usage of comparably supramaximal weight loads in the half squat lead to higher torques and therefore higher pressure in the tibio- and patello-femoral joints. The greatest retropatellar compressive forces (in N) [92
] and highest compressive stresses (N/mm2
, in MPa) [93
] are observed at 90°. With increasing flexion of the knee joint in the deep squat, a cranial displacement of facet contact areas with continuous enlargement of the retropatellar articulating surface occurs [93
]. The additional contact between the quadriceps tendon and the intercondylar notch as the tendofemoral support surface (“wrapping effect” [92
]) contributes to an improved load distribution and enhanced force transfer [92
]. Both factors (increasing facet contact areas, wrapping effect) reduce retropatellar compressive forces [92
] and stresses [93
]. Furthermore, the soft tissue contact between the back of the thigh and the calf plays an important role in reducing the tibiofemoral and patellofemoral joint forces beyond 60°–40° of knee extension in the deep squat [99
], depending on the cross-sectional area of the hamstrings and the calf muscles [101
]. Besides lower weights, deep squats benefit from these relief effects that compensate for increases in knee-joint torques due to unrestricted forward movement of the knee, provided that the heels stay in contact with the ground. These factors cannot arise when an evasive movement of the trunk is performed owing to the restriction of the forward movement of the knees. The accomplishment of the half squat with supramaximal loads will endanger the knee joint by degenerative changes in the long term [90
]. In addition, Lorenzetti et al.
] reported statistically significantly higher relative torques in the hip joint at the turning point (14.6%; p
< 0.05), when the half squat was performed under restricted anterior knee movement. Fry et al.
] even reported 10-fold higher peak torques in the hip joint under this condition. These higher torque values in the hip joint bring additional increases in compressive forces at the sacroiliac joint and, therefore, the lumbar spinal region when performing the half squat with supramaximal loads. The restriction of the forward knee displacement will result in greater forward leaning [91
] with the risk of ventral flexion of the thoracic and lumbar spine [33
]. This evasive movement induces greater anterior shear forces on intervertebral discs [23
] and elicits tensile forces on intervertebral ligaments [23
]. The load combination of high-axial compressive and shear forces in ventral flexion increases the risk of a spinal discus prolapse [42
The instruction about a restriction of the forward knee displacement is a misinterpretation of the existing literature (for a discussion, see [90
]). It is strongly emphasized that this instruction should not be further recommended.