Leg Press

1. ANALYSIS OF MUSCLE ACTIVATION DURING
DIFFERENT LEG PRESS EXERCISES AT
SUBMAXIMUM EFFORT LEVELS
EDUARDO MARCZWSKI DA SILVA, MICHEL ARIAS BRENTANO, EDUARDO LUSA CADORE,
ANA PAULA VIOLA DE ALMEIDA, AND LUIZ FERNANDO MARTINS KRUEL
Grupo de Pesquisa em Atividades Aqua´ticas e Terrestres, Laboratory of Exercise Research,
Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil
ABSTRACT
Da Silva, EM, Brentano, MA, Cadore, EL, De Almeida, APV, and
Kruel, LFM. Analysis of muscle activation during different leg
press exercises at submaximum effort levels. J Strength Cond
Res 22: 1059–1065, 2008—Many studies have analyzed
muscle activity during different strength exercises. Although the
leg press (LP) is one of the most common exercises performed,
there is little evidence of lower limb muscle activity patterns
during this exercise and its variations. Thus, this study aimed to
verify how mechanical changes and loads affect lower limb
muscle activity during the performance of different LP
exercises. Fourteen women performed 3 LP exercises: 45°
LP (LP45), LP high (LPH), and LP low (LPL) at 40% and 80% of
the 1 repetition maximum. The electromyographic activity of the
rectus femoris, vastus lateralis, biceps femoris, gastrocnemius,
and gluteus maximus was recorded. Results suggested that
mechanical changes affect lower limb muscle activity and that it
is related to the load used. At moderate effort levels, the rectus
femoris and gastrocnemius were more active during the LP45
and LPL than during the LPH. At a high effort level, the rectus
femoris and vastus lateralis (quadriceps) were more active
during the LPL than the LPH. Again, the rectus femoris and
gastrocnemius were more active during the LP45 and LPL than
the LPH. On the other hand, gluteus maximus activity was
greater during the LPH than the LPL. This study found that
coordination patterns of muscle activity are different when
performing LP variations at high or moderate effort levels
because of mechanical changes and different loads lifted
during the different LP exercises. These results suggest that if
the goal is to induce greater rectus femoris and vastus lateralis
(quadriceps) activation, the LPL should be performed. On the
other hand, if the goal is to induce gluteus maximus activity, the
LPH should be performed.
KEY WORDS electromyography, lower limb muscles, mechan-
ical changes, loads lifted
INTRODUCTION
R
ecently, many studies have analyzed muscle acti-
vity during different strength exercises (14,25,28,
29,37). The superficial electromyographic (EMG)
technique is often used to identify the participa-
tion of a muscle or muscle group in different performance
techniques of many exercises (2–4,6,17,27,33). Exercises com-
monly used in a strength training program seem to be more in-
teresting to analyze during those analyses (20,23,24,35,36,39).
The leg press (LP) is a multijoint (hip, knee, and ankle)
exercise, its variations (low foot placement [LPL], high foot
placement [LPH], and 45° [LP45]) are some of the most
common exercises performed by athletes to enhance per-
formance in sports (11,12). The hip and knee extension
observed during concentric phase on LP is a very important
motion for these individuals because it involves the activation
of large muscle groups of the lower body. The conditioning
of those muscles are directly related to improvement in run-
ning, jumping, and lifting for football, track and field, power
lifting, and Olympic weightlifting athletes (10–12). Identify-
ing how mechanical changes and different loads affect the
activation pattern in hip and knee extensor muscles may
improve physical performance in athletic and nonathletic
populations (10–18).
Caterisano et al. (5) evaluated hip and knee extensor muscle
activity performing squats at 3 ranges of motion. They found
that gluteus maximus and vastus medialis activity was
influenced by different mechanical changes in this exercise
(partial, parallel, and full depth). Escamilla et al. (12) quantified
the hip and knee extensors muscle activity during LPH and
LPL exercises in different stance widths and foot positions.
They found that the peak of EMG activity for the gastro-
cnemius muscle was greater during the LPL than during the
Address correspondence to Eduardo Marczwski da Silva,
eduardomarczwski@yahoo.com.br.
22(4)/1059–1065
Journal of Strength and Conditioning Research
Ó 2008 National Strength and Conditioning Association
VOLUME 22 | NUMBER 4 | JULY 2008 | 1059

2. LPH, indicating that mechanical changes could modify
muscle activity pattern during the performance of LP
exercises. However, only a single voluntary effort level was
used, gluteus maximus muscle activity was not measured, and
the LP45 was not performed during these analyses.
Although the mechanical changes during strength exer-
cises variations can modify muscle activity pattern, studies
have not quantified how mechanical changes affect the hip
and knee extensor muscle activity pattern during LP exercises
at different submaximum loads lifted (5,11,12). Furthermore,
these studies have been done only with men (5,6,12,35–37).
Thus, the specific purpose of this study was to analyze how
mechanical changes and the loads lifted could modify the hip
and knee extensor muscle activity in women during different
LP exercises (LPL, LPH, and
LP45). Based on the findings of
Escamilla et al. (12), Caterisano
et al. (5), Anders et al. (1),
Lawrence and De Luca (26),
and Woods and Bigland-Ritchie
(38), we propose the hypothesis
that muscle activity could differ
during performance of the 3 LP
exercises and that these differ-
ences would depend on the
load lifted.
METHODS
Experimental Approach to the
Problem
Articles in academic journals
and fitness periodicals have
never examined how mechani-
cal changes and loads lifted
affect muscle activity during
LP exercises. Thus, we used 3
of the most common LP varia-
tions to examine which posi-
tions adopted during LP
exercises could elicit the highest
level of electrical activity in 5
lower limb muscles. Electro-
myographic signals were col-
lected from each muscle during
performance of the different LPs
using different submaximum
(40% and 80%) effort levels.
Acquisition of all EMG signals
was performed on the same day
for each subject. According to
convention, the root mean
square of the EMG signal
(rmsEMG) was used to quantify
the average level of electrical
activity produced during each condition. The signals were
normalized by the signal collected during the maximum
repetition of the LP45 to reduce the effect of variations in
signal amplitude among muscles and subjects (37). Compar-
isons were made among exercises at 2 submaximum effort
levels. These procedures were designed to address the
effectiveness of each exercise targeting specific muscles;
however, some controversy regarding their relative efficacy
and safety still exists.
Subjects
Fourteen healthy young women (physical education stu-
dents) from the Federal University of Rio Grande do Sul
(UFRGS) were selected for this study. The participants’ mean
age, height, percentage of lean body mass and fat mass (6SD)
Figure 1. (A) LPL starting concentric position. (B) LPL final concentric position.
1060 Journal of Strength and Conditioning Research
the TM
Muscle Activity During Strength Exercises

3. were 21.5 6 1.6 years, 1.63 6 0.06 m, 74.23 6 3.63%, and
26.04 6 3.79%, respectively. Subjects reported an average of
1 hour twice weekly for at least 6 months of strength training.
All subjects had been performing LP exercises for at least
4 months. None of the subjects presented knee or hip injury
or had undergone any knee or hip surgery before the study.
The Ethics Committee of UFRGS approved the study.
Before their participation, all subjects signed a university-
approved informed consent form.
Procedures
In the first week, anthropometric measurements of the
subjects were taken, and they also performed a 1 repetition
maximum (1RM) voluntary test of the LP45, LPH, and
LPL (Figures 1, 2, and 3, respectively) exercises. The
anthropometric protocol con-
sisted of height and weight
measurements as well as body
density assessment using the
skinfold method suggested by
Jackson et al. (21) to later body
fat measurements according to
Heyward and Stolarczyk (19).
In the second step, subjects
performed the 1RM test ran-
domly with the 3 LP exercises,
using a protocol similar to that
previously proposed by Glass
and Armstrong (18), and it was
limited to a maximum of 5 trials
to reach 1RM. The rest period
among trials was 2 minutes and
5 minutes among exercises in
order to avoid problems related
to muscle fatigue (32). Exercise
cadence and range of motion
were controlled by a Quartz
metronome with 1-bÁmin21
res-
olution and a Biometrics elec-
tronic goniometer (model TM
180) (12,36). Subsequently, the
1RM values were used to cal-
culate the submaximum effort
levels (40% and 80% of the
1RM). The starting concentric
position to perform the exer-
cises was set by a manual
goniometer (CARCI), as 90°
of knee flexion during the 3
LP exercises and 90°, 105°, and
125° of hip flexion during the
LPL, LP45, and LPH, respec-
tively (Figures 1A, 2A, and 3A).
The foot position used was that
considered the most comfort-
able for each subject. The final concentric position in all
exercises was set as the full knee extension (Figures 1B, 2B
and 3B) (12). An LP machine with high and low pedals
(Taurus, Porto Alegre, Brazil) and an LP45 machine (Topline,
Porto Alegre, Brazil), both of invariable resistance, were used
to perform the different LP exercises.
Data Collection
Subjects returned for data collection 1 week after the initial
measurements. During this time, they were encouraged to
keep their exercise routine. Myoelectric activity was obtained
by bipolar (20-mm interelectrode distance) surface electrodes
(Noraxon 272) placed longitudinally to the direction of the
muscle fiber on the rectus femoris, gluteus maximus, vastus
lateralis, biceps femoris (long head), and gastrocnemius
Figure 2. (A) LP45 starting concentric position. (B) LP45 final concentric position.
VOLUME 22 | NUMBER 4 | JULY 2008 | 1061
Journal of Strength and Conditioning Research
the TM
| www.nsca-jscr.org

4. (lateral head), following recommendations by Pincivero et al.
(31) and Rainoldi et al. (34). The reference electrode was
placed over the medial shaft of the tibia ~ 6–8 cm below the
inferior pole of the patella. Before electrode placement, the
skin area was shaved, abraded to reduce skin impedance, and
cleaned with isopropyl alcohol. The EMG signal was
obtained using an 8-channel electromyograph (model
AMT-8 channel; Bortec, Calgary, Alberta, Canada) with
a sample rate of 2000 Hz coupled to a Pentium desktop (200
MHz, 32 Mb RAM) fitted to a digital-analog conversion
plate. The common mode rejection of the current system is
115 dB at 60 Hz with an input impedance of 10 gV. The
electronic goniometer was positioned on the lateral epi-
condyle of the right knee in each subject to set the 90° knee
flexion angle in the starting position of each exercise, as well
as to distinguish the concen-
tric (90°–180° full extension)
and eccentric (180°–90°)
phases during the signal in-
terpretation (12).
The submaximum exercise
protocol was performed ran-
domly as well. First, the sub-
jects performed the 1RM of
the LP45. Subsequently, they
performed 5 repetitions at
40% and 5 repetitions at 80%
of 1RM in each LP exercise
(LPL, LP45, and LPH). All
exercise protocols were per-
formed on the same day.
During the exercise protocol,
both concentric and eccentric
phases were set at about 2
seconds each to reduce the
acceleration effects on the re-
sistance offered by the weight
lifted. The rest period between
exercises was the same allowed
during the 1RM test. Data
acquisition was started at the
beginning of the first repetition
and finished at the end of the
fifth repetition. Between repe-
titions, subjects were instructed
to stop for 1 second at the end
ofconcentricphaseto promote
a clear separation between
them. Finally, the subjects per-
formed the 1RM in LP45
to verify the effect of fatigue
on the EMG signal ampli-
tude. The intraclass correla-
tion coefficient between 1RM
tests was at least 0.92 for
all muscles, indicating no fatigue induced by the exercise
protocol.
Data Analysis
The signal registered during 1RM of the LP45, and only the
signal of 3 central repetitions obtained at submaximum
intensities were analyzed. This procedure was adopted to
avoid problems with signal discrepancies regarding the inertia
at the beginning of exercises, as well as the possibility of
fatigue in the last repetition (12). The EMG signal collected
was analyzed on SAD32 (32 bits, 2.61.05 mp version)
software, developed at the Mechanic Measurements Labo-
ratory of UFRGS. For segmentation and quantification of the
EMG signal, the goniometer’s curves were used to identify
and to separate the concentric and eccentric phases.
Figure 3. (A) LPH starting concentric position. (B) LPH final concentric postion.
1062 Journal of Strength and Conditioning Research
the TM
Muscle Activity During Strength Exercises

5. Raw EMG signals were band-pass filtered (Butterworth 5th
order) at 20–500 Hz following the recommendations of
DeLuca (7). To examine the EMG signals in the time
domain, the raw signals were processed through an rms
calculation. The rmsEMG average was obtained from the
3 central submaximum effort repetitions of the LPL, LP45,
and LPH, and the rmsEMG was obtained from maximum
repetition (1RM) of the LP45 for each muscle during the
concentric phase. This treatment was similar to that proposed by
Escamilla et al. (12) and Pincivero et al (31). The rmsEMG mean
collected at 40% and 80% was normalized using the value
collected from different muscles during the 1RM of the LP45
(100%). This procedure was adopted due to limitations on
normalization by isometric actions (7). Muscle activity was
compared in the concentric phase between exercises at 40% and
80% of the 1RM.
Statistical Analyses
The Shapiro-Wilks statistical test was used to determine the
data normality. According to the result, repeated-measures
analyses of variance comparing the exercises (LP45, LPH, and
LPL) for each intensity (40% and 80% of 1RM) were applied
to the 5 muscle activity values to verify the differences in
muscle activity. Subsequently, to determine the source of the
significance, Bonferroni’s post hoc test was used. All statistical
procedures were adopted by using the SPSS 11.0 package for
Windows. Significance was set at p # 0.05.
RESULTS
Figure 4 shows the rmsEMG normalized mean values of
muscle activity among the 3 exercises at 40% and 80%. At
moderate effort levels, rectus femoris and gastrocnemius activity
was greater (p , 0.05) during the LP45 and LPL than during
the LPH (Figure 4A). At high effort level, the rectus femoris and
vastus lateralis (quadriceps) were more active (p , 0.05) in
during the LPL than the LPH. The rectus femoris and
gastrocnemius were more active (p , 0.05) during the LP45 and
LPL than during the LPH. However, during the LPH exercise,
gluteus maximus activity was greater (p , 0.05) than during the
LPL (Figure 4B). No statistical difference was observed in
biceps femoris activity among the 3 exercises (Figure 4).
DISCUSSION
In the present study, we examined young women performing
3 different LP exercises at 2 submaximum effort levels.
The EMG data were analyzed in order to compare muscle
activity among exercises. The principal differences found in
muscle activation patterns are related to the mechanical
changes and effort levels required (40% and 80% of 1RM)
during these exercises.
At the moderate effort level (40%), we found that rectus
femoris and gastrocnemius activity during the performance of
the LP45 and LPL was greater than during the LPH exercise.
However, the same result was found at the high effort level
(80%). It means that the activity patterns of rectus femoris and
gastrocnemius were different among these exercises and did
not depend on the effort level required. This was probably
due to mechanical changes during the performance of these
exercises.
For the rectus femoris, the fact that it is a biarticular (hip and
knee) muscle can explain these differences (10–12). Escamilla
(10) suggested that the greater rectus femoris activity found
during monoarticular exercises (knee extension) compared to
biarticular exercises (LP and
squat) for the lower limbs can
be explained by its biarticular
function. When comparing dif-
ferent types of LP, it can be
verified that during the LPH at
the starting position, the high
foot placement increases the
hip flexion angle (the biceps
femoris and gluteus maximus
are stretched and the rectus
femoris is shortened). This
could impair the rectus femoris
mechanism that shortens this
muscle. Thus, it would result in
a strength deficit because, in
that position, the rectus femoris
would not be at a favorable
length to increase force pro-
duction (8,9,30). On the other
hand, in the LP45 and LPL
exercises, the rectus femoris
would not be as shortened,
Figure 4. Root mean square electromyographic values of the muscle activity of rectus femoris (RF), vastus lateralis
(VL), biceps femoris (BF), gastrocnemius (GAS), and gluteus maximus (GM) muscles during different types of
leg press exercises at a 40% effort level (a
difference from LPH, p , 0.05) (A) and during different types of leg
press exercises at an 80% effort level (b
difference from LPL, p , 0.05) (B).
VOLUME 22 | NUMBER 4 | JULY 2008 | 1063
Journal of Strength and Conditioning Research
the TM
| www.nsca-jscr.org

6. thus increasing its force production capacity. Our result was
different from that of Escamilla et al. (12). Probably the
different hip ankle positions used during LPH and LPL
performance can explain these differences. For the gastroc-
nemius muscle, similar results were previously reported by
Escamilla et al. (12), although only the LPL and LPH were
compared in their study. These results were explained by the
subjects’ different ankle joint positions adopted during the 3
exercises. During the LPH, the subject’s ankle is positioned at
a greater degree of plantar flexion compared to its position
during the LP45 and LPL. This causes a shortening of the
gastrocnemius muscle (17), which can impair the mechanics
of the gastrocnemius during this exercise, suggested again by
the relationship between force production and muscle length
(force-length curves) (8,9,30).
On the other hand, at the high effort level (80%), we found
that vastus lateralis activity was greater during the LPL than
during the LPH and that gluteus maximus activity was greater
during the LPH than during the LPL. It means that activity
patterns of the vastus lateralis and gluteus maximus were dif-
ferent between these exercises, depending on the effort level
required. It probably was a result of mechanical changes in
the arrangement of the loads lifted while performing these
exercises.
A specific requirement of the vastus lateralis can occur
during the LPL, but it happened only at the high effort level,
confirming the different result obtained from that found by
Escamilla et al. (12). Woods and Bigland-Ritchie (38) found
nonlinear force–EMG relationships in muscles of mixed fiber
composition. They suggested that at the low to moderate
effort level, low threshold units would be selectively
recruited, while at the high effort level, high threshold units
would be responsible for increasing the EMG signal.
According to Johnson et al. (22), the vastus lateralis muscle
consisted of approximately 45% of type I fibers and 55% of
type II. Thus, selective recruitment of type II fibers at
increasing force levels (80%) in the vastus lateralis may still be
responsible for increasing the EMG signal at the high effort
level. It suggests that coordination patterns are different from
the high to moderate effort levels (1,25). This finding
combined with the rectus femoris results indicates that the
quadriceps muscle group (rectus femoris and vastus lateralis)
appears to require more mechanical changes during the LPL
than the LPH, indicating a specific activity of these muscles
mainly at the high effort level (80%).
For the gluteus maximus, this finding supports the
suggestions of Caterisano et al. (5) that a greater angle of
hip motion could increase gluteus maximus activity during
LPH performance. Compared to the other exercises, in the
starting position, the greater hip flexion angle observed could
be favorable to gluteus maximus force production (8,9,30).
We suggest that the high gluteus maximus activity during the
LPH helps the deficit caused by the rectus femoris and
gastrocnemius muscles. However, this result was found only
at the high effort level. According to Johnson et al. (22), the
gluteus maximus muscle consisted of approximately 40%–
70% of type I fibers and 30%–60% of type II fibers. Thus,
selective recruitment of type II fibers at increasing force levels
in the gluteus maximus may still be responsible for the
increased EMG signal at the high effort level. Again, it sug-
gests that coordination patterns are different from the high to
moderate effort levels (1,26). No difference was observed
between exercises in biceps femoris activity, in agreement
with the results found by Escamilla et al. (12).
In conclusion, the results presented suggest that the
mechanical changes in LPL, LP45 and LPH performance
affect coordination activity patterns in women’s lower limb
muscles. The differences can be related to the load lifted
(effort level) during these exercises.
PRACTICAL APPLICATIONS
LP exercises (LPL, LPH, and LP45) are commonly
performed in strength training programs. Due to the fact
that the primary purpose of the LP exercises is the devel-
opment of increased strength during knee and hip extension
simultaneously, identifying the participation of the different
muscles involved in these exercises at different loads is very
important to coaches, athletes, and general people. The
results of our study indicate that when a load at 40% of 1RM is
selected, LPL and LP45 are recommended to strengthen the
rectus femoris and gastrocnemius muscles. When a load at
80% of 1RM is selected, the exercises recommended to
strengthen the rectus femoris and gastrocnemius are the
same. To strengthen the quadriceps (rectus femoris and vastus
lateralis) muscle, we recommend performing the LPL
exercise. On the other hand, to strengthen the gluteus
maximus muscle, the LPH exercise should be performed.
REFERENCES
1. Anders, C, Bretschneider, S, Bernsdorf, A, and Schneider, W.
Activation characteristics of shoulder muscles during maximal
and submaximal efforts. Eur J Appl Physiol 93: 540–546, 2005.
2. Blackard, DO, Jensen, RL, and Ebben, WP. Use of EMG analysis in
challenging kinetic chain terminology. Med Sci Sports Exerc 31:
443–448, 1999.
3. Bu¨ll, M, Vitti M, Freitas, V, and Rosa, G. Electromyographic
validation of the trapezius and serratus anterior muscles in military
press exercises with open grip. Electromyogr Clin Neurophysiol 41,
179–184, 2001.
4. Bu¨ll, M, Vitti, M, Freitas, V, and Rosa, G. Electromyographic
validation of the trapezius and serratus anterior muscles in the
rowing and frontal-lateral cross, dumbbells exercises. Electromyogr
Clin Neurophysiol 42, 79–84, 2002.
5. Caterisano, A, Moss, RE, Pellinger, TK, Woodruff, K, Lewis, VC,
Booth, W, and Khadra, T. The effect of back squat depth on the
EMG activity of 4 superficial hip and thigh muscles. J Strength Cond
Res 16: 428–432, 2002.
6. Cogley, RM, Archambaut, TA, Fibeger, JF, Koverman, MM,
Youdas, JW, and Hollman, JH. Comparison of muscle activation
using various hand positions during the push-up exercise. J Strength
Cond Res 19: 628–633, 2005.
7. De Luca, CJ. The use of surface electromyography in biomechanics.
J Appl Biomech 13: 135–163, 1997.
1064 Journal of Strength and Conditioning Research
the TM
Muscle Activity During Strength Exercises

7. 8. Edman, KA, Elzinga, G, and Noble, MI. Enhancement of
mechanical performance by stretch during tetanic contractions of
vertebrate skeletal muscle fibres. J Physiol Lond 281: 139–155,
1978.
9. Edman, KA, Elzinga, G, and Noble, MI. Residual force enhancement
after stretch of contracting frog single muscle fibers. J Gen Physiol 80:
769–784, 1982.
10. Escamilla, RF. Knee biomechanics of the dynamic squat exercise.
Med Sci Sports Exerc 33: 127–141, 2001.
11. Escamilla, RF, Fleisig, GS, Zheng, N, Barrentine, SW, Wilk, KE, and
Andrews, JR. Biomechanics of the knee during closed kinetic chain
and open kinetic chain exercises. Med Sci Sports Exerc 30: 556–569,
1998.
12. Escamilla, RF, Fleisig, GS, Zheng, N, Lander, JE, Barrentine, SW,
Rews, JR, Bergemann, BW, and Moorman, CT. Effects of technique
variations on knee biomechanics during squat and leg press. Med Sci
Sports Exerc 3: 1552–1566, 2001.
13. Escamilla, RF, Fleisig, GS, Zheng, N, Lander, JE, Barrentine, SW,
Andrews, JR, Bergemann, BW, and Moorman, CT 3rd. The effects
of technique variations on knee biomechanics during the squat and
leg press. Med Sci Sports Exerc 29: S156, 1997.
14. Ferreira, M, Bu¨ll, M, and Vitti, M. The comparison of the response in
the deltoid muscle (anterior portion) and the pectoralis major
muscle (clavicular portion) determined by the frontal-lateral cross,
dumbbells and rowing exercises. Electromyogr Clin Neurophysiol 43:
75–79, 2003.
15. Ferreira, M, Bu¨ll, M, and Vitti, M. Participation of the deltoid
(anterior portion) and the pectoralis major (clavicular portion)
muscles in different modalities of supine and frontal elevation
exercises with different grips. Electromyogr Clin Neurophysiol 43:
131–140, 2003.
16. Fiebert, IM, Correia, EP, Roach, KE, Carte, MB, Cespedes, J, and
Hemstreet, K. A comparison of EMG activity between the medial
and lateral heads of the gastrocnemius muscle during isometric
plantar flexion contractions at various knee angles. Isokinetics Exerc 6:
71–77, 1996.
17. Flanagan, S, Salem, GJ, Wang, M, Sanker, S, and Greendale, G.
Squatting exercises in older adults: kinematic and kinetic
comparisons. Med Sci Sports Exerc 35: 635–643, 2003.
18. Glass, SC and Armstrong, T. Electromyographical activity of the
pectoralis muscle during incline and decline bench presses. J Strength
Cond Res 11: 163–167, 1997.
19. Heyward, VH and Stolarczyk, LM. Avaliacxa˜o da composicxa˜o corporal
aplicada. Sa˜o Paulo: Manole, 2001.
20. Isear, JA, Erickson, JC, and Worrell, TW. EMG analysis of lower
extremity muscle recruitment patterns during unloaded squat. Med
Sci Sports Exerc 29: 532–539, 1997.
21. Jackson, AS, Pollock, ML, and Ward, A. Generalized equations for
predicting body density of women. Med Sci Sports Exerc 12: 175–182,
1980.
22. Johnson, MA, Polgar, J, Weightman, D, and Appleton, D. Data on
the distribution of fibre types in thirty-six human muscle—an autopsy
study. J Neurol Sci 18: 111–129, 1973.
23. Karst, GM and Willett, GM. Effects of specific exercise instructions
on abdominal muscle activity during trunk curl exercises. J Orthop
Sports Phys Ther 34: 548–552, 2004.
24. Khazei, D. Push-up power: five variations on this classic exercise.
Mens Fitness 10: 56 –57, 1994.
25. Kouzaki, M, Shinohara, M, Masani, K, Kanehisa, H, and Fukunaga, T.
Alternate muscle activity observed between knee extensor synergists
during low-level sustained contractions. J Appl Physiol 93: 675–684,
2002.
26. Lawrence, JH and Deluca, CJ. Myoelectric signal versus force
relationship in different human muscles. J Appl Physiol 54:
1653–1659, 1983.
27. Matheson, JW, Kernozek, TW, Feter, DCW, and Davies,
GJ. Electromyographic activity and applied load during seated
quadriceps exercises. Med Sci Sports Exerc 33: 1713–1725, 2001.
28. McCaw, S and Melrose, D. Stance width and bar load effects on leg
muscle activity during the parallel squat. Med Sci Sports Exerc 31:
428–436, 1999.
29. Ninos, JC, Irrgang, JJ, Berdett, R, and Weiss, JR. Electromyographic
analysis of the squat performed in self-selected lower extremity
neural rotation and 30 degrees of lower extremity turn-out from
self-selected neutral position. J Orthop Sports Phys Ther 25: 307–315,
1997.
30. Peterson, DR, Rassier, DE, and Herzog, W. Force enhancement
in single skeletal muscle fibres on the ascending limb of
the force–length relationship. J Exp Biol 207: 2787–2791,
2004.
31. Pincivero, DM, Campy, RM, Salfetnikov, Y, Bright, A, and
Coelho, AJ. Influence of contraction intensity, muscle, and gender on
median frequency of the quadriceps femoris. J Appl Physiol 90: 804–
810, 2001.
32. Ploutz-Snyder, LL and Giamis, EL. Orientation and familiarization
to 1RM strength testing in old and young women. J Strength Cond
Res 15: 519–523, 2001.
33. Rabita, G, Pe´rot, C, and Lensel-Corbeil, G. Differential effect of knee
extension isometric training on the different muscles of the
quadriceps femoris in humans. Eur J Appl Physiol 83: 531–538,
2000.
34. Rainoldi, A, Melchiorri, G, and Caruso, I. A method for positioning
electrodes during surface EMG recordings in lower limb muscles.
J Neurosci Methods 134: 37–43, 2004.
35. Signorile, JF, Weber, B, Roll, B, Caruso, JF, Lowenstein, I, and
Perry, AC. An electromyographical comparison of the squat and
knee extension exercises. J Strength Cond Res 8: 178–183,
1994.
36. Signorille, JE, Applegate, B, Duque, M, Cole, N, and Zink, A.
Selective recruitment of the triceps surae muscles with changes in
knee angle. J Strength Cond Res 16: 433–439, 2004.
37. Signorile, JE, Zink, A, and Szwed, S. A comparative electromyo-
graphical investigation of muscle utilization patterns using various
hand positions during the lat pull-down. J Strength Cond Res 16:
539–546, 2002.
38. Woods, JJ and Bigland-Ritchie, B. Linear and non-linear surface
EMG/force relationships in human muscles. Am J Phys Med 62:
287–299, 1982.
39. Wright, GA, Delong, TH, and Gehlsen, G. Electromyographic
activity of the hamstrings during performance of the leg curl, stiff-leg
deadlift, and back squat movements. J Strength Cond Res 13: 168–174,
1999.
VOLUME 22 | NUMBER 4 | JULY 2008 | 1065
Journal of Strength and Conditioning Research
the TM
| www.nsca-jscr.org