COMPARATIVE ANALYSIS OF ISOKINETIC, ISOMETRIC AND ISOTONIC
MUSCLE PERFORMANCE MEASURES
Jeffrey W. Ryder1, Roxanne E. Buxton2, Robert Ploutz-Snyder 1 and Lori Ploutz-Snyder1
1Universities
Space Research Association, Houston, TX,
Abstract
INTRODUCTION: Spaceflight is associated with a loss of
muscle mass, strength, power, and endurance. All current
U.S. International Space Station crew perform pre- and
postflight isokinetic knee extension strength testing as a
medical requirement. Although isokinetic testing has a
long standing history in spaceflight research, the
functional relevance of the results has been questioned
due to the nature of the strength testing being performed
at fixed angular velocities. The aim of this investigation
was to compare isokinetic strength measures to other
muscle performance modalities.
METHODS: Subjects (up to 76) performed isokinetic,
isometric and isotonic muscle testing. Knee extension
strength was determined at 0 or 60 °/s on a Biodex
isokinetic dynamometer. A 20 repetition isokinetic knee
extension – flexion total work test was determined at 180
°/s. Knee extension maximal isometric force testing was
performed using a knee extension device modified for
isometric strength testing and equipped with a load cell.
Leg press maximal isometric force, isotonic power (at
40% of maximal isometric force), and total isotonic work
was assessed using a customized and instrumented 35°
leg press machine. Pearson’s correlation (r) was used to
evaluate agreement among baseline strength tests. A
subset of subjects (n = 25) also performed the isokinetic
and leg press tests following unloading during bed rest or
spaceflight. Students’ t-test was used to evaluate and
compare pre-/post- changes as evaluated by the different
muscle performance indices.
RESULTS: Correlation values (r) for isokinetic knee
extension at 60 °/s were 0.71 (n = 54) with isometric knee
extension strength, 0.73 (n = 76) with isometric leg press
strength, and 0.67 (n = 76) with isotonic leg press power.
The r value for isometric knee extension strength between
2 different devices (Biodex versus Modified Nautilus) was
0.67 (n = 15). The isokinetic versus isotonic total work r
value was 0.55 (n = 47). The rank order for each of the
muscle performance tests in order of lowest to highest
sensitivity to change are: isokinetic work at 180 °/s (p =
0.38); isometric leg press strength (p = 0.06); isotonic leg
press isotonic power (p = 0.04); isokinetic strength at 60°
per second (p = 0.005); and leg press isotonic work (p =
0.005).
DISCUSSION: Positive correlations of 0.31 to 0.75
revealed moderate to strong associations between
isokinetic strength and each of the other muscle
performance metrics measured prior to, or without
unloading. However, these measures revealed differential
sensitivities to detecting change resulting from chronic
axial unloading. Assuming that each of these measured
parameters is affected similarly by unloading, we
conclude that the isokinetic strength at 60 °/s and isotonic
leg press work tests have superior sensitivities to change.
2University
of Houston, Houston, TX
Methods
Muscle Performance Equipment
Subjects:
Test Equipment. Left: Biodex isokinetic dynamometer. Middle: Instrumented Nautilus knee extension machine
fixed with knee angle. Right: Instrumented Nebula Fitness inclined leg press machine with magnetic braking
system that allows for ballistic concentric power testing.
Knee Extension on a Biodex
Isokinetic Dynamometer:
Results
-Peak torque at 60°/s.
-Peak torque at 0°/s (isometric), knee angle
of 90°.
-Total work during 20 repetitions at 180°/s.
Knee Extension Machine:
-Maximal Isometric force using a knee
angle of 75°.
Comparisons versus isokinetic knee extension strength for knee extension maximal isometric force (left r =
0.71), leg press maximal isometric force, (middle, r = 0.73) and leg press peak isotonic power (right, r = 0.67)
Current US International Space Station (ISS)
crew undergo pre- and post-flight isokinetic
medical requirements testing.
Inclined Leg Press:
-Maximal Isometric force. The knee angle
was 90°.
-Peak ballistic isotonic power using a load
equal to 40% of maximal isometric force.
Starting knee angle was 90°.
-Total work during a 21 repetition
power/endurance test (40% of maximal
isometric force load, 90° starting angle).
Statistics:
Introduction
Isokinetic dynamometry has a well established
history for use during research studies of
space flight and space flight analogs such as
bed rest.
Healthy adult subjects (76) performed all or
some of the following isokinetic, isometric
and isotonic muscle performance tests. A
sub-set of subjects (25) performed the tests
again following unloading due to bed rest
(14 days, n = 9; 70 days, n = 10) or space
flight (ISS missions of approximately 6
months, n = 6).
Isometric knee extension strength comparison
using Biodex versus Nautilus knee extension
machine (r = 0.67)
Total work comparison for isokinetic knee
extension/flexion versus isotonic leg press (r =
0.55)
Measure
n
t score
p value
Isokinetic work
25
0.90
0.38
The functional relevance of isokinetic
measurements can be questioned as few real
life movements occur at a fixed joint velocity.
Leg Press Strength
25
1.98
0.06
Leg Press Power
25
2.16
0.04
The purpose of this investigation was to
compare isokinetic muscle measures to other
modes of muscle performance testing, as well
as determining the sensitivities of the
measures to change in response to axial
unloading by bed rest or space flight.
Isokinetic Strength
25
3.08
0.005
Leg Press Work
25
3.10
0.005
Muscle performance tests in order of lowest to highest sensitivity to unloading-induced changes.
This work was funded by the NASA Human Research Program
-Pearson’s r correlations were used to
determine agreement between baseline
muscle performance tests.
-Students’ t-tests were used for pre- postunloading comparisons.
Conclusions
Moderate to strong correlations exist
between isokinetics and other muscle
performance metrics.
Assuming similar changes are induced by
unloading, isokinetic strength at 60°/s and
isotonic leg press work have the highest
sensitivities to change.