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Presented at NACOB 98:
North American
Congress
on Biomechanics
Canadian Society for Biomechanics -
American Society of Biomechanics
University of Waterloo
Waterloo, Ontario, Canada
August 14-18, 1998
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PREACTIVATION OF LUMBAR SPINAL MUSCULATURE
DECREASES TRUNK
FLEXIBILITY AND MUSCULAR
RESPONSE TO PERTURBATION
Ian A.F. Stokes, Mack Gardner-Morse and
David Norton
Department of Orthopaedics and Rehabilitation,
University of Vermont
Burlington, VT 05405.
INTRODUCTION
The ligamentous lumbar spine is inherently
unstable, and must therefore be stabilized by
the stiffness of the muscles to prevent
buckling and consequent self-injury'.
Analytically, muscle stiffness (which increases
with intensity of activation) has the ability
to prevent lumbar spine buckling which would
occur otherwise {1-5}. The way in which lumbar
muscles are recruited must provide both
postural control and sufficient stiffness to
ensure stability.
REVIEW AND THEORY
In the spine, as well as in other joints,
stability can be increased by increasing the
coactivation of antagonistic muscles; although,
this coactivation of antagonists produces
increases in the spinal loads {6}. Stability
here is taken to mean the ability of a system
to return to its equilibrium position after a
perturbation.
The purpose of these experiments was two-fold:
(1) to investigate our overall hypothesis that
the stiffness of pre-activated muscles
stabilizes the spine, obviating the need for a
reflex or voluntary change in muscle activation
in response to a small perturbation; (2) to
obtain measurements of the trunk stiffness as a
function of the steady-state load.
PROCEDURES
Subjects stood in an apparatus with the pelvis
effectively immobilized by a support structure.
A harness around the subjects' thorax was
connected to a system for applying a
predetermined load, together with a
superimposed perturbation of variable (and
controlled) magnitude and duration. The
connection between the thoracic harness and the
loading apparatus was made by a cable passing
through a pulley
(Figure 1). The pulley was attached to one of
five anchorage points on a wall track
surrounding the subject at angles of 0°, 45°,
90°, 135° and 180° to the forward direction. A
load cell recorded the force applied, and
displacement transducer recorded the co-linear
motion of the subject's thorax.
The subjects first equilibrated with a
steady-state load of 20% or 40% of their
maximum extension effort, (using an analog
voltmeter as a visual feedback). Then a single
full sine-wave force perturbation pulse
(amplitude 7.5% or 15% of maximum effort,
duration 80 ms or 300 ms) was applied at a
random time by the investigator. There were
three repeated trials of each test condition.
The perturbation amplitudes averaged 26 N when
set to 7.5% of maximum effort, and 55 N when
set to 15% of maximum effort.
Muscle responses
Twelve EMG electrodes recorded signals from
six right and left pairs of muscles (rectus
abdominus, internal and external obliques,
longissimus, iliocostalis and multifidus). For
multifidus, fine wire electrodes were inserted
intra-muscularly; other electrodes were
conventionally placed surface electrodes. All
EMG signals were pre-amplified, recorded at
2048 Hz, processed through a 10 to 500 Hz
elliptical no-lag filter, and rectified by an
RMS moving average.
Trunk stiffness
The trunk dynamics were modeled using a 2nd
order differential equation. The trunk
stiffness was evaluated by a two-stage
mathematical curve fitting process. First, the
applied force perturbation was fit to a sine
wave to determine the amplitude and the period.
Second, the driving-point stiffness and the
inertia were found using a nonlinear least
squares curvefit.
RESULTS
Muscular responses
It was considered that there was a change of
muscle activity when a rectified and filtered
EMG signal differed from the steady-state
values by more than 3 standard deviations from
its prior level for a period of at least 50 ms.
Twelve muscles were monitored in six subjects,
with five loading directions, two steady state
load magnitudes, two perturbation magnitudes
and two perturbation durations, with three
repetitions of each test condition, but with 12
missed because of technical difficulties. The
overall number of muscle responses was 956 or
13%. There were 12/203 trials (3% of the
total) in which no response was detected in any
of the 12 monitored muscles. The muscles which
did respond were most commonly those which were
agonists for the steady state loading, except
during large slow perturbations when
antagonists responded more frequently with
greater latency.
As expected, high magnitude of load
perturbation was associated with more muscle
responses (p< 0.01). Also, as hypothesized, a
greater preactivation of muscles (steady state
effort) was associated with fewer muscle
responses, especially among the agonist muscles
for the steady state effort. This effect is
demonstrated in Figure 2 for rectus abdominus,
in which this behavior occurred at all angles.
Trunk stiffness
The trunk stiffness varied with loading
direction and with steady state effort (Figure
3) (ANOVA: p<0.001) but was independent of
perturbation magnitude. The most significant
effect was an average 43% increase in trunk
stiffness with increased muscle activation.
The effective mass averaged 11.3 kg. This is
plausible, based on the fact that not all of
the subjects' trunk mass would be directly
coupled to the harness and the perturbing
force.
DISCUSSION
This study confirms that trunk stiffness
increases with increased levels of muscle
activation. Also, it was found that a change
in muscular activation does not always occur
when the pre-loaded trunk is perturbed.
This is consistent with the hypothesis that the
spine is stabilized by the stiffness of
activated muscles, and the likelihood of a
muscular response was less for agonistic
muscles (those opposing the steady state
loading) when the steady state load was higher
and these muscles were more strongly activated.
Increased magnitude of perturbation increased
the number of responses, suggesting that there
may be a threshold below which a response is
not required. The short latency of agonist
muscle response is indicative of a monosynaptic
(probably stretch reflex) or 'medium' latency
reflex. When antagonistic muscles were
recruited, these responses occurred with
generally longer latencies.
REFERENCES
1. Bergmark A: Stability of the lumbar spine.
A study in mechanical engineering.
Acta-Orthop-Scand-Suppl. 1989; 230: 1-54.
2. Cholewicki J, Panjabi MM, Khachatryan A:
Stabilizing function of trunk flexor-extensor
muscles around: a neutral spine posture.
Spine 1997, 22(19): 2207-2212.
3. Crisco JJ, Panjabi MM: Euler stability of
the human ligamentous lumbar spine. Part 1:
Theory. Clin. Biomech 7: 19-26, 1992
4. Gardner-Morse M, Stokes IA and Laible JP:
Role of muscles in lumbar spine stability in
maximum extension efforts. J Orthop Res, 1995,
13(5):802-8
5. Gardner-Morse MG, Stokes IAF: The effects
of abdominal muscle coactivation on lumbar
spine stability. Spine 23(1), 86-92,
1998.,30,32}
6. Thelen DG, Schultz AB, Ashton-Miller JA:
Co-contraction of lumbar muscles during the
development of time-varying triaxial moments.
J Orthop Res, 1995, 13(3):390-8
ACKNOWLEDGMENTS
Supported by NIH R01 AR 44119
