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ORIGINAL ARTICLE
Effect of range of motion in heavy load squatting on muscle
and tendon adaptations
K. Bloomquist •H. Langberg •S. Karlsen •
S. Madsgaard •M. Boesen •T. Raastad
Received: 16 November 2012 / Accepted: 5 April 2013 / Published online: 20 April 2013
ÓSpringer-Verlag Berlin Heidelberg 2013
Abstract Manipulating joint range of motion during
squat training may have differential effects on adaptations
to strength training with implications for sports and reha-
bilitation. Consequently, the purpose of this study was to
compare the effects of squat training with a short vs. a long
range of motion. Male students (n=17) were randomly
assigned to 12 weeks of progressive squat training (repe-
tition matched, repetition maximum sets) performed as
either a) deep squat (0–120°of knee flexion); n=8 (DS)
or (b) shallow squat (0–60 of knee flexion); n=9 (SS).
Strength (1 RM and isometric strength), jump performance,
muscle architecture and cross-sectional area (CSA) of the
thigh muscles, as well as CSA and collagen synthesis in
the patellar tendon, were assessed before and after the
intervention. The DS group increased 1 RM in both the SS
and DS with *20 ±3 %, while the SS group achieved a
36 ±4 % increase in the SS, and 9 ±2 % in the DS
(P\0.05). However, the main finding was that DS train-
ing resulted in superior increases in front thigh muscle
CSA (4–7 %) compared to SS training, whereas no dif-
ferences were observed in patellar tendon CSA. In parallel
with the larger increase in front thigh muscle CSA, a
superior increase in isometric knee extension strength at
75°(6 ±2 %) and 105°(8 ±1 %) knee flexion, and
squat-jump performance (15 ±3 %) were observed in the
DS group compared to the SS group. Training deep squats
elicited favourable adaptations on knee extensor muscle
size and function compared to training shallow squats.
Keywords Resistance training Hypertrophy Patellar
tendon Jumping performance
Abbreviations
CV Coefficient of variation
CJ Counter movement jump
CSA Cross-sectional area
DS Deep squat
DEXA Dual energy X-ray absorption
LBM Lean body mass
MRI Magnetic resonance imaging
r Pearson correlation coefficient
PINP Procollagen type 1 N-propeptide
RM Repetition maximum
SEC Series elastic component
SS Shallow squat
SJ Squat jump
SD Standard deviation
SE Standard error
SSC Stretch shortening cycle
Communicated by William J. Kraemer.
K. Bloomquist
The University Hospitals Centre for Health Research,
Copenhagen University Hospital, Copenhagen, Denmark
H. Langberg
CopenRehab, Institute of Social Medicine,
Department of Public Health and Centre for Healthy Ageing,
Faculty of Health Sciences, University of Copenhagen,
Copenhagen, Denmark
S. Karlsen S. Madsgaard T. Raastad (&)
Norwegian School of Sport Sciences,
P.O. Box 4014, U.S., 0806 Oslo, Norway
e-mail: truls.raastad@nih.no
M. Boesen
Sportsmedicine and surgery, Parkens private hospital,
Copenhagen, Denmark
123
Eur J Appl Physiol (2013) 113:2133–2142
DOI 10.1007/s00421-013-2642-7
Introduction
Strength training is associated with improvements in muscle
strength through adaptations in neural control (Aagaard
et al. 2002; Del Balso and Cafarelli 2007), muscle cross-
sectional area (CSA) (Wickiewicz et al. 1984; Kawakami
et al. 1995; Aagaard et al. 2001), muscle architecture
(Blazevich et al. 2003; Aagaard et al. 2001; Alegre et al.
2006), fibre-type transformation (Andersen and Aagaard
2000) and alterations in the length–force characteristics
(Abe et al. 2000). Furthermore, muscular adaptations appear
to be dependent on loading parameters, volume of exercise,
velocity of exercise, and movement intent of the exercises
used in training (Hakkinen et al. 1985; Thepaut-Mathieu
et al. 1988; Weir et al. 1994; Rimmer 2000; Blazevich et al.
2003; Lockie et al. 2003; Markovic et al. 2007).
Improved muscle strength increases the forces distrib-
uted from the muscles through the tendons (Kannus 2000)
and increases the stress on the connective tissue within the
muscle, as well as on the tendons in series with the mus-
cles. It is thus likely that the biomechanical properties of
the connective tissue are influenced by the force-generating
capacity of the muscles, although little research has been
conducted to address this (Kongsgaard et al. 2007; Couppe
et al. 2008).
Exercise has been shown to increase the turnover of
tissue within tendons both acutely and as a result of a
prolonged training intervention (Langberg et al. 1999,
2000; Miller et al. 2005; Kongsgaard et al. 2007; Langberg
et al. 2007). Animal studies show that exercise improves
the physical properties of tendons, e.g. maximal tensile
strength (Elliott 1965; Woo et al. 1981), and newly pub-
lished data indicate that this also is the case in humans
(Haraldsson et al. 2005; Kongsgaard et al. 2007; Couppe
et al. 2008).
Strength training utilizing the squat exercise can be
performed in various ways, among those being a full range
deep squat (DS) or a limited range shallow squat (SS). To
our knowledge, only one study has explored the effect of
squat training at different joint angles (Weiss 2000).
Hypothetically adaptations to squat training performed
using full- or limited range of motion could lead to dif-
ferential adaptations, with implications for, e.g. power
sport performance or during preventative rehabilitation
programs against certain musculotendinous injuries.
Theoretically, in maximal lifts the forces on the knee
extensors and patellar tendon are the same in both the DS
and SS even though the SS can be performed with sub-
stantially more load (Fig. 1). This is because the external
moment arm is approximately twice as long when the
femur is parallel to the ground (DS) compared to a limited
range of motion of 60°of knee flexion (SS). Thus,
assuming that the force on the muscle–tendon system is the
same in both ranges of motion, the only difference is the
length at which the working muscles contract.
However, changes in the patellar tendon moment arm
with increasing knee angles also need to be considered, as
the moment arm of the knee extensor muscles is described
by the moment arm of the patellar tendon. Peak values for
the patellar tendon moment arm are estimated to be near
45°of knee flexion (Krevolin et al. 2004). Interestingly, at
90°of knee flexion the patellar tendon moment arm seems
to decrease by approximately 50 %, and most likely con-
tinues to decrease with increasing knee angles (Krevolin
et al. 2004). Assuming this to be true, the DS would thus
elicit higher tendon and muscles forces compared to the
SS, and hypothetically be a catalyst for patellar tendon
hypertrophy and collagen synthesis. This hypothesis is
supported by findings reported by Tsaopoulos et al. (2006).
The purpose of this randomised study was therefore to
explore whether the DS and SS exercise had a differential
effect on specific adaptations in the front thigh muscles and
patellar tendon, as well as on jump performance. It was
hypothesized that SS training would be superior in eliciting
increased strength in the SS. In contrast, DS training would
Fig. 1 Illustration of the deepest position in the SS (left) and DS
exercise (right). The external moment arms indicated are estimated
from an average subject with regard to lifting technique and height
(180 cm). The ground reaction forces represent a body mass of 80 kg
and an external load of 200 kg in the SS, and 100 kg in the DS
2134 Eur J Appl Physiol (2013) 113:2133–2142
123
be superior in increasing strength in the DS and front thigh
muscle CSA as well as increasing patellar tendon CSA and
collagen synthesis. Furthermore, it was hypothesized that
these superior muscle and tendon adaptations with DS
training would translate into a more positive effect on
jumping performance compared to the SS.
Materials and methods
Subjects
It was calculated that ten subjects in each training group
would give a statistical power of 90 %. Normally, the drop-
out rate in training interventions is around 10–20 %.
Therefore, twenty-four males were recruited for the study
(Table 1). All subjects were sports science students. If they
had been squat training more than once weekly during the
preceding 6 months, or if they were engaged in strength or
power sports, they were excluded from the study. During
the intervention, subjects were requested not to participate
in endurance sports more than three times per week, or to
engage in strength training of the lower extremities. After a
1-week familiarization period, subjects were tested and
paired according to their initial DS strength. From each pair
one subject was drawn, by envelope, into either the DS or
SS group with the other member of the pair allotted to the
opposite group. Four subjects withdrew preceding training.
This left 20 subjects, where an additional two subjects
withdrew due to illness and injury. Training attendance was
set at C80 %, and one additional subject was excluded due
to a lack of attendance. This left 17 subjects with nine in the
SS group, and eight in the DS group.
Training
Both groups engaged in strength training, three times per
week for 12 weeks. Each session started with a 10-min
general warm-up, followed by a specialized warm-up
consisting of 1–3 submaximal squats (shallow or deep
according to training group). Both groups performed bar-
bell squat free weight exercises. The SS group performed
the squat from complete knee extension (0°)to60°of knee
flexion, and back to extended knee, while the DS group
performed a full range of motion squat, with the femur
parallel to the floor in the lowest position (120°of knee
flexion) (Fig. 1). Both squat variations were executed with
an eccentric phase lasting 2–4 s followed by a maximal
effort in the concentric phase with the subjects’ feet staying
on the ground. The training program was periodized, and
loads progressively increased during the 12 weeks
(Table 2). All training sessions were supervised to ensure
correct range of motion and safety. The study complied
with the Declaration of Helsinki and was approved by the
Regional Ethics Committee of Southern Norway.
Testing procedures
Microdialysis and ultrasonography were carried out during
the familiarization week, while the remainder of the pre-
tests were carried out the following week. All tests were
carried out at pre-intervention and after 12 weeks. Testers
were blinded in regard to training group.
1 RM strength
All subjects were tested using 1 RM for both the DS and SS
after a general and specialized warm-up consisting of a
series of 10–6–3–1 repetitions, without subjects reaching
fatigue. Based on the last sub-maximal series of 1 repeti-
tion, a plausible load was chosen. Hereafter, loads were
increased with a minimum of 5 or 10 kg and a maximum of
15 or 30 kg, for DS and SS, respectively, until the subjects
failed to lift the load with correct technique.
Isometric strength
Isometric strength of the knee extensors on the right leg
was measured in a dynamometer (Technogym REV 9000,
Gambettola, Italy) at knee angles of 40°,75°and 105°(full
knee extension at 0°). After a specific warm-up with four
isokinetic knee extensions with increasing intensity, two
maximal contractions of 5 s were performed at each knee
angle with a 30 s rest between attempts. Peak torque at
each knee angle was used for analysis (coefficient of var-
iation (CV) \5 %).
Cross-sectional area (CSA)
The CSA of the front thigh muscles [m. sartorius and
quadriceps (and adductors in the most proximal sections)],
Table 1 Pretest characteristics of subjects in the SS group and in the
DS group (mean ±SD)
Shallow squat
group (n=9)
Deep squat
group (n=8)
Age (years) 23 ±325±6
Weight (kg) 80 ±15 79 ±6
Height (cm) 178 ±6 181 ±5
Peak torque (Nm)
(isometric at 105°)
241 ±66 242 ±29
Jump height (cm)
(squat jump)
33.9 ±3.6 32.8 ±3.3
Muscle CSA (cm
2
) (front thigh) 95.6 ±14.1 95.2 ±7.3
Tendon CSA (mm
2
) (middle part) 162 ±9 166 ±12
Eur J Appl Physiol (2013) 113:2133–2142 2135
123
back thigh muscles (hamstrings) and the patellar tendon
were obtained using magnetic resonance imaging [(MRI),
GE Signa 1.5 Tesla EchoSpeed, GE Medical Systems,
Milwaukee, WI]. A total of nine slices were analysed for
muscle CSA from both legs. The first slice was placed
10 cm proximal to the lateral femoral epicondyle and was
defined as the most distal slice. The remaining eight slices
were then placed proximally to this reference point with
10 mm between each slice. To measure the CSA of the
patellar tendon seven slices per leg were taken. The first
slice was placed 5 mm distal to the tibial plateau (reference
point). The second slice was placed on the tibial plateau,
and the remaining slices were taken proximal to the tibial
plateau with 5 mm between each slice.
A line was manually drawn along the perimeter of the
muscle bellies and the tendon on each slice, and the CSA
was automatically generated in the software (OsiriX 3.9.3,
Pixmeo, Bernex, Switzerland).
Lean body mass (LBM)
LBM of the legs and body composition were measured
using Dual energy X-ray absorption [(DEXA), Lunar
Prodigy densitometer, GE Medical Systems, Madison, WI].
Subjects were requested not to eat or drink during the 2 h
preceding the scanning and to eat identical meals at iden-
tical times at both pre and post test.
Muscle architecture
Using ultrasound imaging (Toshiba Sonolayer Just Vision
400) pennation angle and muscle thickness of the right m.
vastus lateralis were measured. Ultrasound was performed
with subjects relaxed and lying in supine with the knees fully
extended. Using a point midway between the greater tro-
chanter and the lateral condyle, isolated muscle thickness and
pennation angle were measured in vivo using the ultrasono-
graph, and pictures were stored and blinded. Muscle thick-
ness was determined by measuring the distance between the
superficialand deep aponeurosis whereas the pennation angle
was defined as the angle between the fascicle and the deep
aponeurosis (Alegre et al. 2006). Five measurements were
taken. The highest and lowest values were withdrawn, and a
mean value was determined from the three remaining mea-
surements. Reliability measurements were not conducted in
the present trial; however, CV of 3 % for muscle thickness
and 5 % for pennation angle has hence been performed uti-
lizing same analysis procedures and ultrasonograph techni-
cians. The posttest was performed 1 day after a submaximal
training session during the last training week.
Collagen synthesis in the patellar tendon
Pretest microdialysis was performed, with no exercise or
testing done during the preceding 2 days (Langberg et al.
1999). The post sampling was executed the day after the last
training session (16–28 h after exercise). On each leg, one
microdialysis catheter, custom-made in the laboratory, was
placed in front of the patellar tendon before and after the
intervention. The active part of the membrane was 30-mm
long covering the width of the patellar tendon. The sterilized
(ETO) fibres were all high molecular mass cut-off fibres
(3,000 kDa, membrane length 30 mm, catheter outer diam-
eter 0.05 mm). In vivo recovery was determined using
labelled glucose (Glucose D-[3-
3
H]), 250 lCi in 2.5 ml
ethanol/water (9:1), as no radioactively labelled procollagen
type 1 N-propetide (PINP) was commercially available. The
catheters were perfused (CMA 100) at a rate of 2 ll/min.
After the microdialysis catheters had been positioned, the
subjects rested for at least 90 min before starting the sampling
(4 h), to ensure that the insertion trauma was minimized
(Langberg et al. 1999). The samples were immediately frozen
at -80 °C until analyses were done. Collagen synthesis was
analysed as the peritendinous concentration of PINP in the
microdialysis samples by a sandwich ELISA (Christensen
et al. 2008). The dialysate samples were diluted: 1:9, 1:10 or
1:20 before the analysis, based on previous analysis of the
sample. The detection level was 41 pg/ml and the intra-assay
variation (CV) of 4.9 % at 4.2 ng/ml. All the samples from
the same subject were analysed on the same ELISA plate.
Jump performance
Squat jumps (SJ) and counter movement jumps (CMJ)
were performed on a forceplate (SG–9, Advanced
Table 2 Periodization and progression of strength training
Week Monday Wednesday Friday
1 Familiarization Familiarization Familiarization
2 Pretesting Pretesting Pretesting
33910 RM 3 98 (submax)
a
495RM
43910 RM 3 910 (submax) 4 95RM
53910 RM 3 98 (submax) 4 95RM
63910 RM 3 910 (submax) 4 95RM
73910 RM 3 98 (submax) 4 95RM
83910 RM 3 910 (submax) 4 95RM
9396RM 398 (submax) 5 93RM
10 3 96RM 3910 (submax) 5 93RM
11 3 96RM 398 (submax) 5 93RM
12 3 96RM 3910 (submax) 5 93RM
13 3 96RM 398 (submax) 5 93RM
14 3 96RM 3910 (submax) 5 93RM
15 Posttesting Posttesting Posttesting
a
Eight reps with a 12–13 RM load
2136 Eur J Appl Physiol (2013) 113:2133–2142
123
Mechanical Technologies, Newton, MA, USA) and low-
pass filtered at 1,050 Hz. SJ were performed with no
counter-movement from a knee angle of 90°with hands
fixed at the hip. The CMJ started from a standing position
with the hands fixed at the hip. Jump height was calculated
from the impulse during takeoff. At each test, the subjects
did three to six jumps, and the best result was used for
analysis (CV \5 %).
Statistical methods
A per-protocol analysis was applied, thus all results are
based on the 17 subjects who completed the training
intervention. Means, standard deviations (SD) and standard
errors (SE) were calculated, and all values are presented as
means and standard errors unless otherwise noted. The
paired ttest was performed to assess changes over time
within each training group, whereas the un-paired ttest was
performed to assess the statistical significance of between-
group differences. Pearson’s correlation coefficient (r) was
used to calculate SS and DS collapsed variables. Signifi-
cance was set at 5 % (PB0.05).
Results
The pretest characteristics of the subjects are presented in
Table 1with no between-group differences.
Strength
Both training groups increased 1 RM when tested in the SS
and DS (Fig. 2). For the DS group, an increase of
*20 ±3 % was observed in both squat ranges (P\0.05),
while the SS group achieved a 36 ±4 % increase in the SS
and 9 ±2 % in the DS (P\0.05). The SS group increased
1 RM in the SS more than the DS group, and the DS group
increased 1 RM in the DS more than the SS group
(P\0.05). Maximal isometric knee extensor torque
increased in the DS group at knee angles of 75°(6 ±2%)
and 105°(8 ±1%) (P\0.05). At 105°, the increased
torque in the DS group was larger than in the SS group
(8 ±1 vs. 0 ±5 %, respectively) (P\0.05) (Fig. 3).
Thigh muscle CSA
The three most distal slices were a mix of muscle and
tendon tissue (especially in the tallest subjects) and were
therefore not analysed. The muscle CSA of the front thigh
was increased at all measured sites in the DS group
(4–7 %), while increases at the two most proximal sites
were observed in the SS group (P\0.05) (Fig. 4, upper
panel). The change in muscle CSA at all measured sites
was greater in the DS group (P\0.05). The muscle CSA
of the back thigh was increased at the second most proxi-
mal site in the DS group (P\0.05) (Fig. 4, lower panel).
Lean body mass
LBM of the legs increased by 2.0 ±0.8 % (P\0.05) in
the DS group while no increase was observed in the SS
group (1.5 ±0.9 %) (Fig. 5). The DS group achieved
increases in body mass 1.7 ±0.6 kg (2.2 ±0.6 %)
(P\0.05) and total LBM 1.2 ±0.4 kg (1.8 ±0.8 %)
(P\0.05). No changes in body mass were detected in the
SS group. There were no differences between training
groups in body composition, and no changes in fat percent
for either group.
Muscle architecture
No changes in muscle thickness were observed in either
training group. However, increases in pennation angle were
observed for both the SS and DS group (23 ±5 and
22 ±6%) (P\0.05), respectively. There were no dif-
ferences between groups for changes in muscle thickness or
pennation angle (Table 3).
Tendon CSA and collagen synthesis
We found no changes in the CSA of the patellar tendon at
any of the measured sites in any group. Collagen synthesis,
indicated by the PINP content measured with microdialy-
sis, did not increase after training in either group.
Jump height
CMJ height increased by 7 ±4 % in the SS group and
13 ±2 % in the DS group (P\0.05) (Fig. 6). SJ height
0
50
100
150
200
250
300
350
Shallow group Deep group Shallow group Deep group
Deep squat Shallow squat
1 RM (kg)
Pre
Post
**
*
*
#
#
Fig. 2 One repetition maximum (1 RM) in the DS and SS exercise
measured pre and post intervention. Asterisk significant change from
pretest, hash significant difference between groups from pre to
posttest
Eur J Appl Physiol (2013) 113:2133–2142 2137
123
increased in the DS group (15 ±3%)(P\0.05) and was
greater than in the SS group (P\0.05).
Correlations
Front thigh muscle CSA/LBM legs and strength
Correlations between muscle CSA of the front thigh and 1
RM DS strength were detected at both pretest and posttest,
whereas correlations between muscle CSA of the front
thigh and 1 RM SS strength only were observed at pretest
(Table 4). No correlations were detected between isometric
knee extension strength and front thigh muscle CSA.
However, correlations between isometric knee extension
strength at both 75°and 105°and LBM of the legs were
found at pretest.
Muscle strength and architecture
Correlations between isometric strength at 105°and pen-
nation angle were found at pretest (Table 4).
Front thigh muscle CSA and patellar tendon CSA
No correlations between front thigh muscle CSA and
patellar tendon CSA were observed. However, at pretest, a
correlation was found between the mean patellar tendon
CSA and LBM of the legs. Furthermore, there was a cor-
relation between mid-patellar tendon CSA and maximal
isometric knee extensor strength at 105°(Table 4).
Jump height and muscle architecture/strength
A correlation was found between SJ height and isometric
knee extensor strength at 105°both at pretest and posttest
(Table 4). No correlations between jump performance and
muscle architecture were observed. However, a negative
-6 %
-4 %
-2 %
0 %
2 %
4 %
6 %
8 %
10 %
12 %
Shallow
g
roup Deep
g
roup
% change in knee extension torque
40°
75°
105°
*
*
#
Fig. 3 Change in isometric knee-extension peak torque measured at
knee angles of 40°,75°and 105°(0°is full extension). Asterisk
significant change from pretest, hash significant difference between
groups from pre to posttest
-4.0 %
-2.0 %
0.0 %
2.0 %
4.0 %
6.0 %
8.0 %
10.0 %
987654
Cross sectional area
(% change)
Shallow group
Deep group
Front of thigh
*
*
*
*****
###
#
#
-4.0 %
-3.0 %
-2.0 %
-1.0 %
0.0 %
1.0 %
2.0 %
3.0 %
4.0 %
5.0 %
987654
Cross sectional area
(% change)
Shallow group
Deep group
Back of thigh
*
Fig. 4 Change in front thigh muscle CSA (upper panel) and back
thigh (lower panel). Asterisk significant change from pretest (Sec-
tion 9 was the most proximal), hash significant difference between
groups from pre to posttest
21
21
22
22
23
23
24
Shallow
g
roup Deep
g
roup
LBM legs (kg)
Pre
Post *
Fig. 5 Leg lean body mass (LBM) pre and post intervention. Asterisk
significant change from pretest
2138 Eur J Appl Physiol (2013) 113:2133–2142
123
correlation was found between pennation angle and the
difference between the CMJ and SJ height.
Discussion
We found that 12 weeks of progressive heavy load squat
training, regardless of range of motion, resulted in increa-
ses in 1 RM strength and pennation angle, as well as
increases in CMJ height. However, only the DS group
increased SJ height, LBM of the legs, isometric strength at
75°and 105°, and front thigh muscle CSA at all measured
points. The SS group elicited front thigh muscle CSA
increases only at the two most proximal sights.
Maximal muscle strength
In accordance with our hypothesis, the SS exercise elicited
larger strength gains in the SS, while the DS exercise
resulted in larger strength gains in the DS. It is, however,
worth noting that DS training elicited similar results in both
the DS and SS. In contrast, isometric knee extension
strength measurements revealed no increases of strength in
the SS group despite higher loads of training, whereas the
DS group achieved increases at both 75°and 105°knee
flexion. These results are similar to those reported by Weiss
et al. (2000) who concluded that the DS was superior to the
SS in regard to strength and squat performance.
In the SS group, the CSA of the front thigh muscles was
increased only at the two most proximal sites. In con-
junction, no isometric strength gains were found in the SS
group, as well as no increases in leg LBM or in back thigh
muscle CSA. This indicates that the increases in 1 RM
strength observed in the SS group likely were due to neural
adaptations and/or to muscular adaptations in muscles not
analysed. The two most proximal sites included adductor
muscles. Substantially higher loads were lifted by the SS
group, and it seems reasonable to assume that this poten-
tially resulted in an increased load on the adductor muscles,
with hypertrophy to follow as well as favourable adapta-
tions to other muscles working over the hip. Indeed, it is a
Table 3 Muscle architecture before and after training in the SS and DS group
Shallow squat group Deep squat group
Pre Post Pre Post
Muscle thickness (cm) 2.47 ±0.37 2.54 ±0.33 2.51 ±0.26 2.60 ±0.32
Fasicle angle (°) 18.5 ±3.0 22.6 ±3.7* 18.6 ±2.8 21.7 ±2.0*
Pre and post values given as mean ±SD
*P\0.05
0%
4%
8%
12%
16%
20%
Squat J CMJ
Change in jump height (%) .
Shallow group
Deep group
**
#
*
Fig. 6 Change in jump height in SJ and CMJ. Asterisk significant
change from pretest, hash significant difference between groups from
pre to posttest
Table 4 Correlation of selected outcomes at pre and posttest
Outcome Outcome Pretest
(r)
Posttest D
(r)
Isometric strength at
105°Leg LBM 0.79* 0.29
Isometric strength at
75°Leg LBM 0.75* -0.03
1 RM deep squat Front thigh muscle
CSA
0.66* 0.53*
1 RM shallow squat Front thigh muscle
CSA
0.77* -0.22
Mean patellar tendon
CSA
Leg LBM 0.56* 0.11
Mid patellar tendon
CSA
Isometric strength at
105°0.60* 0.29
SJ height Isometric strength at
105°0.54* 0.55*
SJ height–CMJ
height
Pennation angle 0.66*
Isometric strength at
105°Pennation angle 0.53* 0.06
*P\0.05
Eur J Appl Physiol (2013) 113:2133–2142 2139
123
study limitation that other muscles (e.g. hip extensors)
were not measured, as these may have been affected dif-
ferentially by the two squat training modes.
The DS group increased front thigh muscle CSA at all
measured sites in accordance with our hypothesis. These
findings are supported by the increases of leg LBM, and
increases in both isometric and 1 RM strength in the DS
group as well as pre and posttest correlations between 1
RM strength and front thigh CSA. Estimations of the
external moment arms in the DS and SS showed that in the
deepest position the external moment arm was approxi-
mately twice as long in the DS as in the SS (Fig. 1). This
difference in external moment arm corresponded with the
observed doubling of external load that could be lifted in
the SS compared to the DS exercise. However, the patellar
tendon moment arm is reduced when knee flexion is above
60°(deepest position in the SS) (Krevolin et al. 2004).
Consequently, although the knee joint torque was similar in
the deepest positions, the muscle force (and tendon force)
working in the DS was probably 50–100 % higher than in
the SS training due to the 25–50 % shorter patellar tendon
moment arm in this position (Krevolin et al. 2004). Fur-
thermore, when body mass is taken into consideration, the
difference in muscle and tendon force between the two
conditions may have been even larger. The greater hyper-
trophy observed in the extensor muscles in the DS group
may therefore partly be explained by the larger muscle
force developed in the DS training.
Muscle architecture
Both training groups achieved increases in muscle thick-
ness and pennation angle of the m. vastus lateralis. There
were no differences between groups, indicating no differ-
ential muscle architecture adaptations in response to the
two different ranges of motion. As stated above, front thigh
muscle CSA increases were larger in the DS than in the SS
group, indicating that other muscle bellies could have been
more affected by DS training than the m. vastus lateralis.
In the present study, a correlation between pennation
angle and isometric strength at 105°was observed at pre-
test, and though no posttest correlation was found, it is
reasonable to assume that the increases in pennation angle
in both training groups may have had a positive influence
on the observed strength development.
Patellar tendon properties
We had expected to find increases of patellar tendon CSA
and collagen synthesis as a result of the squat training.
However, neither group elicited gains in patellar tendon
CSA or collagen synthesis. At pretest, correlations were
found between the patellar tendon CSA and leg LBM and
strength consistent with previous studies that have shown a
relationship between muscle strength/size and patellar CSA
(Elliott 1965; Kongsgaard et al. 2007). However, no
change in patellar tendon CSA was observed in either
group despite the increase in strength and CSA of the
adjacent muscle, nor was any correlations found. Though
not expected, these results are in accordance with several
resistance training studies that have shown that increases in
strength were not accompanied by increases in tendon CSA
(Reeves et al. 2003; Kubo et al. 2007; Seynnes et al. 2009).
Rather, a markedly altered elastic modulus was found in
these studies, implying a change in the composition of the
tendon structure instead of the size. However, in the
present study no changes in collagen synthesis were
observed which may indicate that 12 weeks of single-mode
squat training is not sufficient time to generate a detectable
change in patellar tendon CSA, nor collagen synthesis, in
well trained subjects. Future studies are warranted to better
understand the coordinated response of muscle and tendon
as this knowledge could be of significant value in pre-
venting overuse injuries of the tendon.
Jump height
CMJ height increased in both groups, however, only the DS
group achieved gains in the SJ. As summarized by Earp
et al. (2010), CMJ is a movement that involves a stretch
shortening cycle (SSC) that allows the body to store and
redirect energy through an eccentric movement quickly
followed by a concentric movement. Due to the SSC, more
force and power can be generated during the concentric
phase of the jump, than if no eccentric phase was involved.
This is seen when comparing the CMJ height to the SJ
height.
We found no correlations between front thigh muscle
CSA or strength gains and CMJ jump height in the present
study. However, improvements in jumping performance
could be caused by changes in muscle CSA only in variable
individual combinations with changes in pennation angle,
as these two variables are governing factors for the train-
ing-induced change in physiological CSA, and hence
muscle force. As the SJ lacks the eccentric phase, and
therefore the benefits of the SSC, the SJ is more dependent
on maximal muscle strength. In the present study, we
observed no between-group differences in CMJ height. In
contrast, the DS group was superior to the SS group in the
SJ. In addition, increases in isometric strength were largest
in the DS group and a correlation both at pre and posttest
were found between SJ height and isometric strength at
105°. Thus, the increases in strength achieved by the DS
group, could account for the superior SJ performance. This
positive relationship between increases in SJ height and
strength underlines that increases in isolated muscle
2140 Eur J Appl Physiol (2013) 113:2133–2142
123
strength are beneficial for jump performance despite the
increase in LBM and body mass due to muscle
hypertrophy.
Conclusion
In conclusion, we found that 12 weeks of heavy load DS
training was superior to the SS in regard to increases in
front thigh muscle CSA and leg LBM, with no changes in
patellar tendon CSA. Parallel with these adaptations,
superior changes in isometric knee extension strength and
SJ performance were observed with DS training. In both
groups, increases in 1 RM squat strength and CMJ height
were observed. We suggest that larger muscle–tendon
forces over the knee joint, more internal (patellar tendon)
work produced, and longer muscle length of the knee
extensors during the DS compared to the SS exercise were
the main explanations for the superior adaptations observed
with DS training in this study. Adaptations in muscles
involved in hip extension and stabilization were not mea-
sured in this study, and possible favourable adaptations
with SS training in these areas can therefore not be ruled
out.
Acknowledgments The authors express their thanks to the subjects
for their time and effort, and a special thanks goes to Oliver Faul and
Tron Krosshaug for the construction of the DS and the SS figures.
Conflict of interest No conflicts of interest, financial or otherwise
are declared by the authors(s).
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