A COMPARISON OF MAXIMAL KNEE MOMENTS GENERATED DURING SINGLE JOINT
KNEE EXTENSION AND LEG PRESS TASKS
1
Mark T. Gordon, 2Brian W. Schulz, and 1James A. Ashton-Miller
1
University of Michigan, Ann Arbor, MI, USA
2
VA HSR&D/RR&D Center of Excellence, Maximizing Rehabilitation Outcomes,
James A. Haley VA Hospital, Tampa, FL, USA
email: [email protected], web: http://me.engin.umich.edu/brl
INTRODUCTION
The peak knee extensor moment developed during
an isolated knee extension exercise and a leg press
exercise are often recorded under isotonic and/or
quasi-isometric conditions [1,2]. From a training
perspective it is important to know how those
moments compare during the shortening and
lengthening phases of exercises on such machines.
So the present study compares the peak knee
moment generated using a Biodex (‘single joint’)
dynamometer with that on servo-controlled (‘multijoint’) leg press machine (BLAST!, BioLogic
Engineering, Inc., Dexter, MI) during the shortening
and lengthening phases of quadriceps contraction.
METHODS
Seven male and seven female healthy younger
adults (ages 20-35 years) were recruited to be part
of the study. The average body mass was 74 (14) kg
and the average height was 173 (11) cm. The data
from one male subject was excluded from the study
due to the subject stopping the Biodex machine
during the trials. The data from the 90°/s and
120°/s trials shortening contraction of five
additional subjects (2 male and 3 female) were
excluded due to data recording errors.
started with the subject’s knee at flexed at ~90°.
The subject first exerted maximal knee moment
using a shortening quadriceps contraction to extend
the knee, then exerted a maximal knee extensor
moment while the dynamometer flexed the knee
back to ~90° with a lengthening quadriceps
contraction.
The leg press task was performed at three different
knee angle velocities (15°/s, 25°/s, and 35°/s). The
subject again started the trial with the knee at ~90°
flexion. The subject then pushed on the foot plate
with maximal force while the plate was driven
distally at a prescribed rate, resulting in a
quadriceps shortening contraction. Foot force was
recorded normal to the foot plate. After 1-5 sec.
rest, the subject pushed maximally against the foot
plate as it returned towards them using a
lengthening quadriceps contraction. The trials were
randomized in the same manner as the knee
extension trials. Single joint testing was completed
one or more days prior to the leg press testing.
Kinematic data were collected at 120 Hz using a
Vicon camera system and 36 markers and were low
pass filtered at 6 Hz. Force data were collected at 80
Hz from the BLAST! machine.
A subject-specific 3-D biomechanical model was
generated in OpenSim. Positional data were used to
scale a torso and leg musculoskeletal model and
generate the kinematics of the leg press exercise.
The force data was then combined with the
kinematic data and inverse dynamics was used to
solve for the knee joint moment. Knee joint moment
data were normalized by dividing the joint moment
by the subject’s weight (N) and height (m) for both
the knee extension and the leg press tasks.
The knee extension exercise was performed on a
‘single joint’ BIODEX dynamometer. Trials were
performed at three different velocities (60°/s, 90°/s,
and 120°/s). The order of the velocities was
randomized and the subjects completed the order
three times for a total of nine trials. Each trial
Since there was a difference in knee joint rotational
velocities on the BIODEX and leg press machines,
the data could not be directly compared across
machines at the same velocity. Instead the values
were averaged to provide a peak knee extension
moment for each apparatus during the quadriceps
On both machines, the quadriceps shortening phase
peak knee moment could be used to predict the peak
moment developed during the quadriceps
lengthening phase (Figure 3).
Two limitations of this study include the modest
sample size (n=13) and the leg press machine only
recording the normal foot plate force. The latter is a
relatively minor source of error based on
unpublished data gathered on an earlier version of
the machine equipped with a 6-axis force plate.
CONCLUSION
The peak knee joint extensor moment generated in a
leg press task was significantly greater than the
maximal knee extensor moment developed in the
‘single joint’ task. This has implications for the
training and rehabilitation of the knee extensor
muscles.
REFERENCES
Leg Press
Peak Normalized Knee Moment
The peak ‘single joint’ knee extensor moment was
not a reliable predictor of the peak ‘leg press’ knee
extensor moment in quadriceps shortening (Figure
1) or lengthening (Figure 2) contraction phases.
This was surprising since each task involved a
maximal exertion involving the knee joint. The
bilateral deficit for the peak force during a leg press
task is known to be approximately 80% [4] (dotted
line in Figures 1 and 2). The data from the present
study fall above that line and the peak knee extensor
moment generated during the ‘leg press’ exercise
proved significantly greater than during the ‘single
joint’ knee extension exercise (p≤0.05).
This study was supported by a VA RR&D Career
Development Award (E2964F) and Research
Award Enhancement Program.
0.30
0.25
y = 0.9497x + 0.045
R² = 0.6012
0.20
0.15
0.10
Line of Unity
0.05
Assumed Bilateral Deficit
0.00
0.00
0.05
0.10
0.15
0.20
0.25
Single Joint
Peak Normalized Knee Moment
Figure 1: Comparison of peak knee moment data
from both the ‘single joint’ and leg press machines
during the quadriceps shortening contraction phase.
Leg Press
Peak Normalized Knee Moment
RESULTS AND DISCUSSION
ACKNOWLEDGEMENTS
0.40
0.35
y = 0.5219x + 0.1143
R² = 0.3249
0.30
0.25
0.20
0.15
Series2
0.10
0.05
Series3
0.00
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
Single Joint
Peak Normalized Knee Moment
Figure 2: Comparison of peak knee moment data
from both the ‘single joint’ and leg press machines
during the quadriceps lengthening contraction
phase.
0.40
Quadriceps Lengthening Phase
Peak Normalized Knee Moment
shortening and lengthening phases. The peak ‘leg
press’ knee moment was averaged across both
knees to compare with the ‘single joint’ knee
moment value after a 13% adjustment for the
difference in knee joint velocities [3].
0.35
0.30
0.25
y = 1.0987x + 0.0484
R² = 0.8248
y = 1.3266x + 0.0343
R² = 0.6721
0.20
Single Joint
0.15
Leg Press
0.10
0.05
0.00
1. Escamilla RF, et al. Med&Sci in Sports Exercise
39:4, 556-569, 1998.
2. Wilk KE, et al. Am J Sport Med 24, 518-527,
1996.
3.Tihanyi J, et al. European J Appl Phys 48, 331343, 1982.
4. Vandervoort AA, et al. Phys Soc 46-51, 1983.
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Quadriceps Shortening Phase
Peak Normalized Knee Moment
Figure 3: Peak knee extension moment during the
quadriceps shortening and lengthening contraction
phases for the knee extension and leg press tasks.