The Effect of Long Term Isometric Training on Core/Torso Stiffness

Abstract

While core stiffness enhances athletic performance traits controversy exists regarding the effectiveness of isometric vs dynamic core training methods. This study aimed to determine if long term changes in stiffness can be trained, and if so, what is the most effective method. Twenty four healthy male subjects (23 ± 3 years, 1.8 ± 0.06 m, 77.5 ± 10.8 kg) were recruited for passive and active stiffness measurements before and after a six week core training intervention. Twelve subjects (22 ± 2 years, 1.8 ± 0.08 m, 78.3 ± 12.3 kg) were considered naïve to physical and core exercise. The other twelve subjects (24 ± 3 years, 1.8 ± 0.05 m, 76.8 ± 9.7 kg) were Muay Thai athletes (savvy). A repeated measures design compared core training methods (Isometric vs. Dynamic, with a Control group) and subject training experience (naïve vs. savvy) before and after a six week training period. Passive stiffness was assessed on a 'frictionless' bending apparatus and active stiffness assessed via a quick release mechanism. Passive stiffness increased following the isometric training protocol. Dynamic training produced a smaller effect and as expected there was no change in the Control group. Active stiffness did not change in any group. Comparisons between subject and training groups did not reveal any interactions. Thus, an isometric training approach was superior in terms of enhancing core stiffness. This is important since increased core stiffness enhances load bearing ability, arrests painful vertebral micromovements and enhances ballistic distal limb movement. This may explain the efficacy reported for back and knee injury reduction.

Full text

EFFECT OF LONG-TERM ISOMETRIC TRAINING
ON CORE/TORSO STIFFNESS
BENJAMIN C. Y. LEE AND STUART M. MCGILL
Spine Biomechanics Laboratory, Department of Kinesiology, Faculty of Kinesiology, University of Waterloo, Waterloo,
Ontario, Canada
ABSTRACT
Lee, BCY and McGill, SM. Effect of long-term isometric
training on core/torso stiffness. J Strength Cond Res 29(6):
1515–1526, 2015—Although core stiffness enhances athletic
performance traits, controversy exists regarding the effective-
ness of isometric vs. dynamic core training methods. This
study aimed to determine whether long-term changes in stiff-
ness can be trained, and if so, what is the most effective
method. Twenty-four healthy male subjects (23 63years;
1.8 60.06 m; 77.5 610.8 kg) were recruited for passive
and active stiffness measurements before and after a 6-week
core training intervention. Twelve subjects (22 62years;
1.8 60.08 m; 78.3 612.3 kg) were considered naive to
physical and core exercise. The other 12 subjects (24 63
years; 1.8 60.05 m; 76.8 69.7 kg) were Muay Thai athletes
(savvy). A repeated-measures design compared core training
methods (isometric vs. dynamic, with a control group) and sub-
ject training experience (naive vs. savvy) before and after
a 6-week training period. Passive stiffness was assessed on
a “frictionless” bending apparatus and active stiffness assessed
through a quick release mechanism. Passive stiffness
increased after the isometric training protocol. Dynamic train-
ing produced a smaller effect, and as expected, there was no
change in the control group. Active stiffness did not change
in any group. Comparisons between subject and training
groups did not reveal any interactions. Thus, an isometric
training approach was superior in terms of enhancing core
stiffness. This is important since increased core stiffness
enhances load bearing ability, arrests painful vertebral micro-
movements, and enhances ballistic distal limb movement.
This may explain the efficacy reported for back and knee
injury reduction.
KEY WORDS spine, performance, athleticism, rehabilitation
INTRODUCTION
Core exercises are a staple among athletically
trained individuals and clinical populations in
an effort to strengthen musculature (35,46),
improve muscular endurance (31), reduce low
back pain (15,21,36,43), and improve sport performance
(19,44). Greater torso stiffness enhances performance
through 3 mechanisms. As explained by McGill (29): (a)
briefly stiffening the torso proximal to the shoulders and hips
transfers the full force and movement of muscles to the distal
side of these ball and socket joints resulting in greater limb
strength and speed; (b) muscularly stiffening the spinal col-
umn enhances its load bearing capacity preventing buckling;
and (c) the muscular turgor associated with stiffness creates
an armor over vital structures enhancing resilience during
contact sports. McGill’s explanation builds on the stiffness-
stability relationship of the spine described by Bergmark (3)
in which muscular stiffness stabilizes the spine against per-
turbation from external load and movement. This has been
demonstrated in athletic tasks, such as strongman events
(33), martial arts striking (30), and single leg exercises (47).
Although not directly affecting performance, stiffness also
arrests micromovements of the spinal joints reducing pain
in those with instability.
Typical athletic training programs involve the program-
ming of core exercises to induce long-term strength, speed,
and endurance adaptations (14,24,38,45) but little is known if
the effect is long lasting. This is the seminal question
explored here. Many traditional core training programs
involve the use of movement-based torso exercises due to
the high level of challenge placed on the core musculature
(2,13). Although challenge and subsequent strength adapta-
tions to the core musculature is thought to be great, many of
these exercises violate mechanisms found to cause injury to
the spine and subject the spine to high shear and compres-
sive loads (9,11,27,41). In contrast, isometric core exercises,
based on challenging the core musculature through static
braced postures, have also been investigated and when com-
pared with their dynamic counterparts shown to create
moderate levels of core activity while minimizing imposed
spine loads (1,10,12,32). Given the breadth of data suggest-
ing enhanced core stiffness enhances athleticism, it would be
beneficial for athletes to participate in core training regimens
Address correspondence to Stuart McGill, mcgill@uwaterloo.ca.
29(6)/1515–1526
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as part of their strength and conditioning programs, but
what is the best method of doing so?
Seminal work from Burgess et al. (8) and Kubo et al. (25)
suggested that stiffness can be altered in the lower limbs
through isometric and plyometric training, but whether
residual torso stiffness is created through core training is
not known. To the authors’ knowledge, so such studies
exist examining core stiffness adaptations from training,
but examining this effect may prove very useful for athletes
to determine the best methods to induce enhanced athlet-
icism associated with core stiffness. From this, specific
questions addressed in this study were as follows: can
passive torso stiffness be increased, and if so, is it better
to use a dynamic exercise program or an isometric one; is
there a difference between athletically naive or savvy
populations; and in addition, can active stiffness be altered
with these 2 approaches to training? It was hypothesized
that long-term isometric training would enhance passive
and active stiffness to a greater degree than dynamic
training or control, and naive subjects would see greater
stiffness increases than savvy subjects.
METHODS
Experimental Approach to the Problem
A repeated-measures test/retest protocol was used to exam-
ine changes in active and passive stiffness after a 6-week core
training protocol consisting of isometric bracing or dynamic
movement exercises in 24 male subjects. All subjects were
collected and trained between March 2013 and June 2013
during day time hours. As physiological markers of health and
performance were not within the scope of the study, controls
for nutrition and hydration were not used. Subjects had
passive and active stiffness measured before and after a 6-week
training or waiting period. After the initial data collection,
subjects were divided into an isometric training group,
dynamic training group, or control group. Isometric and
dynamic training groups performed a training program
progressing in intensity based on static bracing exercises
Figure 1. Frictionless bending apparatus used for passive flexion and extension (top left), lateral bend trials (top right), and frictionless twisting apparatus used
for passive axial twist trials (bottom center).
Isometric Training on Core Stiffness
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and movement/speed-based exercises, respectively. The
training/control groups were also evenly divided into “naive”
and “savvy” groups (see Subjects for more detail) to determine
whether athletically savvy individuals responded differently
to training than their naive counterparts.
Subjects
Twenty-four young healthy university-aged males (22.9 6
2.7 years; range 18-29 years; 1.79 60.06 m; 77.5 6
10.8 kg) were selected for this study. Of these, 12 subjects
(21.7 61.9 years; range 18-26 years; 1.80 60.08 m; 78.3 6
12.3 kg) were selected with limited experience in physical
training and little to no experience in performing core exer-
cises. These are referred to as the exercise “naive” group. The
remaining subgroup of 12 subjects (24.2 62.9 years; range
21-29 years; 1.8 60.05 m; 76.8 69.7 kg) was selected from
a population of athletes with experience in core training.
Inclusion criteria for this subgroup consisted of individuals
highly experienced in core training methods, having regu-
larly performed direct core exercises for at least 1 year. This
special population consisted of club Muay Thai fighters,
a martial art native to Thailand involving standing striking
with the fists, elbows, knees, and shins. These are referred to
as the “savvy” group. Exclusion criteria for both subgroups
consisted of any individuals who have experienced low back
pain or injury currently or within the past year. The majority
of naive subjects were active in recreational/intramural
sports, but had no background in core training and limited
experience in physical or weight training. Savvy subjects
were trained in Muay Thai boxing for at least 1 year (ranging
from 1.5 to 6 years of consistent training) with the majority
(10) having competitive amateur records and 2 subjects
being provincial and international amateur champions in
their respective weight classes.
All subject recruitment and data collection procedures
were performed in accordance with University Office of
Research Ethics guidelines. The participants were informed
of the purpose and method of the study to ensure that they
understood completely, and each provided written informed
consent to participate. Participants were also informed that
at any time during the data collection or training protocol
they were free to withdraw from the study. Written informed
consent was gained in agreement with MSSE and ACSM
guidelines.
Procedures
Two different methods were used to measure passive and
active stiffness. Passive stiffness was assessed through
a “frictionless” bending apparatus in 3 planes of motion (sag-
ittal, frontal, and transverse) after Brown and McGill (5,6),
whereas active stiffness was measured through a “quick
release” mechanism after Brown et al. (7). Active and passive
torso stiffness values were collected in 2 data collections, one
before and another after a 6-week core training intervention.
The training intervention consisted of 3 groups: 1 group
performed isometric core exercises, 1 group performed
dynamic core exercises, and the control group performed
no special exercises during this period. Eight subjects were
placed into each group, 4 from the naive subject group and 4
from the savvy subject group.
Passive Stiffness Measurement. Sagittal and frontal plane
passive bending trials were performed in which participants
were secured at the hips, knees, and ankles on a solid lower-
body platform. Each participant’s upper body was secured to
a cradle with a glass bottom surface, about their upper arms,
torso, and shoulders (Figure 1). The upper-body cradle was
free to glide overtop of a similar glass surface with precision
nylon ball bearings between the 2 structures. This created
a frictionless float influenced by gravity and allowed trunk
movement about either the flexion-extension or lateral bend
axis. Participants laid on their right side for the flexion-
extension trials and on their back for the lateral bend trials.
Their torsos were supported in each position to ensure that
participants adopted and maintained a nondeviated spine
posture throughout the testing.
Passive axial twisting trials were performed in a separate
apparatus consisting of a rotating wheel platform mounted
to a fixed base through ball bearings with a frictionless
contact (Figure 1). The participant stood upright on the
platform maintaining upright spine posture with their
upper body fixed through a harness strap to a vertical
post (approximately at the level of T9). Lumbar spine
Figure 2. Quick release experimental setup consisting of a chair used
to foster a neutral hip and spine posture, harness, electromagnet, and
cable stack. The cable pull is shown with the arrow.
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TABLE 1. Six-week isometric core training program.
Exercise
Week 1 Week 2 Week 3 Week 4
Sets 3
repetitions Frequency*
Sets 3
repetitions Frequency*
Sets 3
repetitions Frequency* Sets 3repetitions Frequency*
Plank 5 35, 4, 3, 2, 1 4 5 35, 4, 3, 2, 1 7
Bird dog 5 35, 4, 3, 2, 1 4 5 35, 4, 3, 2, 1 7
Side bridge 5 35, 4, 3, 2, 1 4 5 35, 4, 3, 2, 1 7
Torsional buttress 5 35, 4, 3, 2, 1 7
Anterior pallof press 5 310 s 4 Same volume, increase
load
4
Posterior pallof press 5 310 s 4 Same volume, increase
load
4
Suitcase hold 5 310 s per
side
4 Same volume, increase
load
4
Antirotation pallof Press 5 310 s per
side
4 Same volume, increase
load
4
Stir the pot
Inverted row
Kettlebell unilateral rack
walk
Half kneeling woodchop
Exercise
Week 5 Week 6
CommentsSets 3repetitions Frequency* Sets 3repetitions Frequency*
Plank Focus on quality of core contraction and postural cues.
Descending pyramid sets (start at 5 repetitions at 10 s
each, next set decrease 1 repetition, continue to
decease 1 repetition per set)
Bird dog
Side bridge
Torsional buttress Focus on quality of core contraction and postural cues.
Use a hold time before shaking begins, maximum 10 s
Anterior pallof press Focus on quality of core contraction and postural cues
Posterior pallof press
Suitcase hold
Antirotation pallof Press
Stir the pot 5 310 s per
direction
45310 s per
direction
4 Begin on knees, progress to toes. If 10 s is not feasible,
train below and progress through the phase
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motion was measured with an electromagnetic transducer
described below.
During each trial, participants were instructed to “feel
completely relaxed, like you are going to sleep.” Muscular
activation was monitored by multichannel electromyogra-
phy (EMG) to ensure that a truly passive response was ob-
tained. Subjects easily learned to relax with this feedback.
Three trials of each bending direction were performed in
a randomized order.
Quick Release Trials. During quick release trials, participants
were placed in a semiseated position in a restraint jig that
restricted hip and lower limb motion while leaving the
trunk free to move in all directions (Figure 2). This has
been shown to foster a neutral spine posture and elastic
equilibrium for the hips and spine (39). Participants were
statically preloaded anteriorly with a 16-kg mass, applied
through a cable at the level of T7, which was randomly
released without the participant’s prior knowledge
through an electromagnet (Job Master Magnets, Oakville,
Canada). Participants were instructed to use core bracing
techniques to prevent displacement after release. This trial
was repeated 3 times.
Instrumentation
Electromyography. Electromyography signals were collected on
unilateral core musculature using pregelled, disposable, monop-
olar Ag-Cl disc-shaped surface electrodes (30 mm diameter,
Medi-traceTM 100 Series Foam Electrodes, Covidien, MA,
USA) placed on the skin over each muscle of interest (rectus
abdominis, external oblique, internal oblique, latissimus dorsi,
upper erector spinae, lower erector spinae). The purpose of
EMG collection was to verify that core muscular activation was
below 5% to ensure a passive response. In fact, postprocessing
of data revealed that all trials turned out to be below 3%
maximal voluntary contraction (MVC). Electromyography data
were not used for any other purpose. Briefly, normalized signals
were obtained as follows. Signals were amplified (62.5 V;
AMT-8, Bortec, Calgary, Canada; bandwidth 10–1,000 Hz,
common mode rejection ratio = 115 db at 60 Hz, input imped-
ance = 10 GX) and sampled at 2,048 Hz, low-pass filtered with
a 500-Hz rectified and low-pass filtered at 2.5 Hz (single pass
second order) to mimic the frequency response of torso muscle
afterBreretonandMcGill(4);andnormalizedtothemaximum
voltage produced during MVC trials to produce a linear enve-
lope mimicking the muscle force output; a technique used
many times before (6). Maximal voluntary contractions were
obtained using 3 postures: (a) a modified sit-up position in
which participants isometrically attempted to produce trunk
flexion, side bend, and twist motions against resistance; (b)
isometric trunk extension while cantilevered in a prone position
over the edge of a table (Biering-Sorensen position) against
external resistance; and (c) isometric wide grip pull-up posture
in which the subject attempted to isometrically pull against
a horizontal bar while being resisted with instructions of
Inverted row Up to 5 310 4 5 310 4 If 10 repetitions are not feasible, perform as many
repetitions as possible and maintain static posture.
Focus on keeping torso straight (avoid hip hiking/
sagging)
Kettlebell unilateral rack
walk
3330 m walk per
side
4 Same volume,
increase load
4 Focus on core contraction and upright posture (avoid
lateral lean)
Half kneeling woodchop Up to 5 310 per
side
45310 4 If 10 repetitions are not possible, perform as many
repetitions as possible and progress through the phase
*Frequency denoted as number of training sessions per week.
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TABLE 2. Six-week dynamic core training program.
Exercise
Week 1 Week 2 Week 3 Week 4
Sets 3repetitions Frequency*
Sets 3
repetitions Frequency* Sets 3repetitions Frequency*
Sets 3
repetitions Frequency*
Curl up Up to 5 310 4 5 310 7
Superman Up to 5 310 4 5 310 7
Side curl up Up to 5 310 per
side
45310 per side 7
Twisting curl up Up to 5 310 per
side
45310 per side 7
Advanced curl up (limbs
extended)
Up to 5 310 4 5 310 4
Back extension Up to 5 310 4 5 310 4
Russian barbell twist Up to 5 310 per
side
4535–10 per
side
4
Curl up twitch
Superman twitch
Lateral medicine ball throw
Rotational medicine ball throw
Exercise
Week 5 Week 6
CommentsSets 3repetitions Frequency*
Sets 3
repetitions Frequency*
Curl up Focus on quality of muscular contraction; visualize muscular activation
throughout motion
Superman
Side curl up
Twisting curl up
Advanced curl up (limbs
extended)
Begin with 5 35 and progress repetitions to 10. If 10 repetitions per
side is too easy add weight
Back extension
Russian barbell twist
Curl up twitch Up to 5 310 4 5 310 4 Begin unweighted and focus on twitch speed and rate of activation/
relaxation
Superman twitch Up to 5 310 4 5 310 4
Lateral medicine ball
throw
Up to 5 310 per
side
45
310 per side 4 Ball velocity comes from torso movement, not arms
Rotational medicine ball
throw
Up to 5 310 per
side
45310 per side 4 Ball velocity comes from torso movement, not arms
*Frequency denoted as number of training sessions per week.
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maintaining a maximally tight grip and attempting to “bend the
bar” while pulling vertically.
Torso Kinematics. Three-dimensional lumbar spine motion
was recorded using an electromagnetic tracking system
(Isotrak, Polhemus, Colchester,
VT, USA) with the source
secured over the sacrum and
the sensor over T12 (6). The
trunk motion data were sam-
pled digitally at 60 Hz and
dual-pass filtered (effective
fourth order 3 Hz low-pass,
zero lag, Butterworth) (4).
Applied Moment. To ob t ai n
passive stiffness, the mo-
mentsappliedtothetorso
were calculated as the prod-
uct of the force applied per-
pendicular to the distal end of
the upper-body cradle, and
the distance between the
point of force application
and the L4/L5 disc for the
bending trials; or the radius of
the rotating platform for
twisting trials. Active stiffness
measurements were obtained as the product of the
momentarmofthecableappliedatthelevelofT7,to
the level of L4/L5, and the cable force. Force was
recorded with an inline force transducer (Transducer
Techniques Inc., Temecula, CA, USA) and digitally
Figure 3. Curve fit moment/deflection data of pre- and post-training. Data were taken at 50, 65, 80, 90, 95, and
100% of peak pretraining moment.
Figure 4. Summary stiffness curves for passive flexion trials with applied moment (N$m) plotted on the y-axis and deflection (degrees) on the x-axis. Top left:
naive isometric group. Middle left: savvy isometric group. Bottom left: Overall isometric. Top middle: Naive dynamic group. Center: savvy dynamic group. Bottom
middle: Overall dynamic. Top right: Naive control group. Middle right: savvy control group. Bottom right: Overall control.
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sampled at 2,160 Hz. Force signals were dual-pass
filtered (effective fourth order 3 Hz low-pass Butter-
worth). Both the linear enveloped EMG and force
signals were downsampled to 60 Hz to match the trunk
motion data.
Core Training Protocols. Subjects were trained for 6 weeks
using either isometric or dynamic core exercises (the control
group did not train). All subjects were asked to refrain from
performing any core exercises outside of those assigned by
researchers during the study. The Isometric training group
performed static exercises designed to challenge the core
musculature through bracing cues. The dynamic training
group performed exercises based on torso movement. Both
training programs were periodized to increase challenge
every 2 weeks, dividing each program into 3 phases (Tables 1
and 2 for a description of the progressive programs).
Moment Angle Relationship and Measurement of Stiffness
Passive Trials. The applied moment and corresponding trunk
angle were normalized in time to ensure equal trial length
across all trials and participants. Trunk angles were normal-
ized as a percentage of the maximum range of motion that
participants were able to obtain in the pretraining bending
trials. Exponential curve fits of the following form were
performed for each trial type:
M¼lefq
;
where Mis the applied moment (N$m), land fare curve
fitting constants, and qis the angular displacement of the
Figure 5. Summary of preisometric/postisometric stiffness curves for passive extension, left lateral bend, right lateral bend, left axial twist, and right axial twist
(top to bottom); applied moment (N$m) is denoted on the y-axis and deflection (degrees) on the x-axis. The curves plotted represent training response for all
subjects.
Isometric Training on Core Stiffness
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torso. The calculated moment was normalized as a percentage
of the maximum applied moment of the pretraining trials and
calculated at 50, 65, 80, 90, 95, and 100% of pretraining peak
moment for pre- and post-training conditions (Figure 3).
Quick Release Trials for Active Stiffness. On magnet and force
release, an event was detected from the load-cell signal by
visually identifying when the force-time slope changed. Over
the next 250 milliseconds, the force at release and the peak
angular displacement of the lumbar spine were obtained to
calculate a gross stiffness measure, after Sutarno and McGill
(39). A gross lumbar measure of stiffness (N$m/degree) was
then obtained from the following equation:
k¼
M
q;
where krepresents the stiffness calculated from the slope of
the moment (M) and absolute angular deflection (q) curve.
Statistical Analyses
Statistical tests were performed using IBM SPSS Statistical
software (version 19, IBM, Corp., Somers, NY, USA). 3 3
232 repeated-measures analysis of variance (3 training
group levels, 2 subject group levels, and before and after
training) was conducted for comparing range of motion
values at each specific instance of applied moment before
and after the training protocol (50, 65, 80, 90, 95, and 100%
of pretraining applied moment). Where applicable, post
hoc analyses were performed using the Tukey HSD test
when a significant effect was detected with statistical sig-
nificance set at p#0.05. To the researchers’ knowledge, no
studies currently exist examining stiffness adaptations with
Figure 6. Summary of predynamic/postdynamic stiffness curves for passive extension, left lateral bend, right lateral bend, left axial twist, and right axial twist
(top to bottom); applied moment (N$m) is denoted on the y-axis and deflection (degrees) on the x-axis. The curves plotted represent training response for all
subjects.
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core training, thus it is difficult to establish intra-class cor-
relations and statistical power.
RESULTS
An example from Figure 4 illustrates the overall effects for
the passive flexion test. The biggest changes were observed
in the isometric training group, regardless of subject group.
Isometric Training
Significant stiffness increases were measured in both savvy
and naive populations after 6-week isometric training for the
majority of bending tests and at multiple levels of applied
moment (p#0.05). In naive subjects, only extension and
right lateral bending stiffness did not significantly increase
(although right lateral bending stiffness at 80% of applied
moment was significantly different from pretraining condi-
tions) (Figure 5). The majority of trials showed stiffness in-
creases near end range of motion; flexion stiffness increased
significantly at 95% of applied moment and beyond while
left lateral bending and right axial twisting stiffness showed
significant increases at 80% of applied moment and beyond.
Only left axial twist stiffness increased at all levels of applied
moment. Savvy subjects experience similar results to their
athletically naive counterparts after 6-week isometric train-
ing. Extension and right lateral bending stiffness were shown
to not significantly change but significant differences were
experienced for all other directions. Interestingly, savvy sub-
jects experienced greater magnitude of changes in flexion
and left axial twist stiffness near end range of motion than
the naive subjects (p,0.001 at 95% of applied moment and
beyond for flexion, and p,0.01 at 90% of applied moment
and beyond for right axial twist). Active stiffness did not
significantly change in either subject group after long-term
isometric training.
Dynamic Training
The results of 6-week dynamic training yielded far fewer
stiffness changes in both subject groups (Figure 6). Only
right lateral bend at all moment levels except at 80% for
naive subjects, left axial twist at 90% of applied moment
and beyond for savvy populations, and a single significant
difference at 90% of applied moment in extension showed
differences after dynamic training (p#0.05). No significant
differences in active extension stiffness were experienced
after long-term dynamic training.
Control groups did not experience any significant changes
in response after the 6-week period, within each subject
group for all directions and between subject groups.
DISCUSSION
The results suggest that the isometric exercise approach was
superior in enhancing torso stiffness over the dynamic
approach in a 6-week trial. Both the naive and savvy groups
responded similarly to training. These findings generally
agree with the results of previous work investigating iso-
metric training on tendon stiffness in the lower limbs
performed by Burgess et al. (8) and Kubo et al. (25) respec-
tively. However, the Burgess work differed in that they re-
ported increases in tendon stiffness with dynamic lower limb
training, whereas this study’s dynamic protocol did not show
such changes.
It was possible that the 6-week protocol could have
caused physical adaptations of hypertrophy and strength
gain. However, if this were true, one would have expected
similar stiffness gains with the dynamic training approach.
Some evidence exists that some components of the core
musculature experienced thickness increases after selected
trunk strengthening exercises, including the bird dog and
side bridge isometric exercises (42). Perhaps this is linked
to the muscular hypertrophy/time under tension relation-
ship—greater time under tension has been shown to
increase skeletal muscular hypertrophy (23,32,37). Obvi-
ously, time spent under muscular contraction was much
higher when performing isometric exercises. For example,
a 10-second plank requires continual peak activation of
anterior core musculature for the full 10-second period,
whereas a 10-repetition curl up incorporates a duty cycle
resulting in far less time under tension. In addition, sharp
increases in passive stiffness near-maximum applied mo-
ments were measured in isometrically trained subjects.
Burgess and Kubo both proposed that after training, col-
lagen structures remodeled similar to that with muscular
hypertrophy, and demonstrated by Michna (34) and
Zamora and Marini (48). An alternate explanation is that
the 6-week program stimulated neural changes and a resid-
ual stiffness. Although active stiffness was not significantly
affected, training journals kept by subjects revealed a per-
ception of training effects. Comments of being better able
to control activation of specific core and hip musculature
were common in both isometric and dynamic groups.
These comments may be related to increased voluntary
muscular activation levels after resistance exercise. Garfin-
kel and Cafarelli showed increases in MVC after 8 weeks of
isometric limb training (16); not only did muscular cross-
sectional area increase but increased EMG amplitude dur-
ing MVC trials was also observed. Other groups have re-
ported similar findings where multiweek periods of
resistance training resulted in increased EMG amplitude
during maximal exertions (17,22,24).
The results suggest that isometric core training is
superior to dynamic training for enhancing torso stiffness.
Enhanced core stiffness allows the spine the bear greater
loads (10) and express greater distal limb athleticism (26).
The next step would be to examine specific changes in
athletic performance.
To the author’s knowledge, no research investigating
training-related changes in core stiffness have been per-
formed. Thus, without existing values to estimate statistical
power, it was not possible to establish a suitable sample
population. However, the work performed provided statisti-
cally supported evidence of isometric training effects on core
Isometric Training on Core Stiffness
1524
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stiffness laying the groundwork for more investigations into
adaptations from specific training styles.
PRACTICAL APPLICATIONS
This is the first study that quantified the effects of multiweek
core training programs to enhance torso stiffness. Isometric
training was superior to dynamic training to enhance core
stiffness. Given the enhancement of limb athleticism, this
study gives some foundation to the practice of including
isometric core exercises into athletes’ training regimens and
pregame warm-up. In fact, isometric core exercise programs
are part of successful injury prevention programs (40).
Some coaches argue that performing compound load
bearing exercises, such as squats and deadlifts, are sufficient
for core activation but authors contend that an isometric
core regimen is superior for creating 3-dimensional spinal
stability. Although these compound exercises require sub-
stantial activation of the core musculature (18), the stability
challenge lies mainly in the sagittal plane. Many athletic
tasks involve stability about the frontal and transverse planes;
consider a football player who sprints 5 yards forward and
powerfully cuts left. If lateral core stiffness is insufficient,
energy leaks causing buckling at the torso compromises
speed and increases known injury mechanisms of spine
bending under load (28) and knee valgus buckling (20). In
essence, when sufficient core stiffness is lacking, athlete
movement becomes inefficient and manifested by perfor-
mance decrements and increased injury risk. In addition,
coaches must carefully monitor loads experienced by the
athlete during sport and strength and conditioning training.
Exceeding tissue tolerance by loading the athlete too fre-
quently or too greatly increases the odds of tissue injury
(26). This is where the beauty of the isometric core exercises
come into play; athletes may develop core stiffness attributes
while minimizing imposed loads to the spine (1,10,23), free-
ing up capacity to put toward sport practice. With the low
spinal loads experienced during these exercises, athletes can
perform these almost daily as done during the study training
period. The researchers believe that a 15- to 45-minute iso-
metric training regimen when used in conjunction with
a strength and conditioning program creates the core stabil-
ity necessary to allow the athlete to fully express their
athleticism.
ACKNOWLEDGMENTS
The authors thank the Natural Science and Engineering
Research Council of Canada (NSERC) for their financial
support.
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