Effect of Long-Term Isometric Training On Core Torso Stiffness
Effect of Long-Term Isometric Training On Core Torso Stiffness
Effect of Long-Term Isometric Training On Core Torso Stiffness
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 INTRODUCTION
C
Lee, BCY and McGill, SM. Effect of long-term isometric ore exercises are a staple among athletically
training on core/torso stiffness. J Strength Cond Res 29(6): trained individuals and clinical populations in
1515–1526, 2015—Although core stiffness enhances athletic an effort to strengthen musculature (35,46),
performance traits, controversy exists regarding the effective- improve muscular endurance (31), reduce low
ness of isometric vs. dynamic core training methods. This
back pain (15,21,36,43), and improve sport performance
(19,44). Greater torso stiffness enhances performance
study aimed to determine whether long-term changes in stiff-
through 3 mechanisms. As explained by McGill (29): (a)
ness can be trained, and if so, what is the most effective
briefly stiffening the torso proximal to the shoulders and hips
method. Twenty-four healthy male subjects (23 6 3 years;
transfers the full force and movement of muscles to the distal
1.8 6 0.06 m; 77.5 6 10.8 kg) were recruited for passive side of these ball and socket joints resulting in greater limb
and active stiffness measurements before and after a 6-week strength and speed; (b) muscularly stiffening the spinal col-
core training intervention. Twelve subjects (22 6 2 years; umn enhances its load bearing capacity preventing buckling;
1.8 6 0.08 m; 78.3 6 12.3 kg) were considered naive to and (c) the muscular turgor associated with stiffness creates
physical and core exercise. The other 12 subjects (24 6 3 an armor over vital structures enhancing resilience during
years; 1.8 6 0.05 m; 76.8 6 9.7 kg) were Muay Thai athletes contact sports. McGill’s explanation builds on the stiffness-
(savvy). A repeated-measures design compared core training stability relationship of the spine described by Bergmark (3)
methods (isometric vs. dynamic, with a control group) and sub- in which muscular stiffness stabilizes the spine against per-
ject training experience (naive vs. savvy) before and after turbation from external load and movement. This has been
a 6-week training period. Passive stiffness was assessed on demonstrated in athletic tasks, such as strongman events
(33), martial arts striking (30), and single leg exercises (47).
a “frictionless” bending apparatus and active stiffness assessed
Although not directly affecting performance, stiffness also
through a quick release mechanism. Passive stiffness
arrests micromovements of the spinal joints reducing pain
increased after the isometric training protocol. Dynamic train-
in those with instability.
ing produced a smaller effect, and as expected, there was no
Typical athletic training programs involve the program-
change in the control group. Active stiffness did not change ming of core exercises to induce long-term strength, speed,
in any group. Comparisons between subject and training and endurance adaptations (14,24,38,45) but little is known if
groups did not reveal any interactions. Thus, an isometric the effect is long lasting. This is the seminal question
training approach was superior in terms of enhancing core explored here. Many traditional core training programs
stiffness. This is important since increased core stiffness involve the use of movement-based torso exercises due to
enhances load bearing ability, arrests painful vertebral micro- the high level of challenge placed on the core musculature
movements, and enhances ballistic distal limb movement. (2,13). Although challenge and subsequent strength adapta-
This may explain the efficacy reported for back and knee tions to the core musculature is thought to be great, many of
injury reduction. these exercises violate mechanisms found to cause injury to
the spine and subject the spine to high shear and compres-
KEY WORDS spine, performance, athleticism, rehabilitation 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
Address correspondence to Stuart McGill, mcgill@uwaterloo.ca. moderate levels of core activity while minimizing imposed
29(6)/1515–1526 spine loads (1,10,12,32). Given the breadth of data suggest-
Journal of Strength and Conditioning Research ing enhanced core stiffness enhances athleticism, it would be
Ó 2015 National Strength and Conditioning Association beneficial for athletes to participate in core training regimens
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
Isometric Training on Core Stiffness
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).
as part of their strength and conditioning programs, but training or control, and naive subjects would see greater
what is the best method of doing so? stiffness increases than savvy subjects.
Seminal work from Burgess et al. (8) and Kubo et al. (25)
suggested that stiffness can be altered in the lower limbs METHODS
through isometric and plyometric training, but whether Experimental Approach to the Problem
residual torso stiffness is created through core training is A repeated-measures test/retest protocol was used to exam-
not known. To the authors’ knowledge, so such studies ine changes in active and passive stiffness after a 6-week core
exist examining core stiffness adaptations from training, training protocol consisting of isometric bracing or dynamic
but examining this effect may prove very useful for athletes movement exercises in 24 male subjects. All subjects were
to determine the best methods to induce enhanced athlet- collected and trained between March 2013 and June 2013
icism associated with core stiffness. From this, specific during day time hours. As physiological markers of health and
questions addressed in this study were as follows: can performance were not within the scope of the study, controls
passive torso stiffness be increased, and if so, is it better for nutrition and hydration were not used. Subjects had
to use a dynamic exercise program or an isometric one; is passive and active stiffness measured before and after a 6-week
there a difference between athletically naive or savvy training or waiting period. After the initial data collection,
populations; and in addition, can active stiffness be altered subjects were divided into an isometric training group,
with these 2 approaches to training? It was hypothesized dynamic training group, or control group. Isometric and
that long-term isometric training would enhance passive dynamic training groups performed a training program
and active stiffness to a greater degree than dynamic progressing in intensity based on static bracing exercises
the TM
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the TM
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
Figure 2. Quick release experimental setup consisting of a chair used release” mechanism after Brown et al. (7). Active and passive
to foster a neutral hip and spine posture, harness, electromagnet, and
torso stiffness values were collected in 2 data collections, one
cable stack. The cable pull is shown with the arrow.
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
and movement/speed-based exercises, respectively. The dynamic core exercises, and the control group performed
training/control groups were also evenly divided into “naive” no special exercises during this period. Eight subjects were
and “savvy” groups (see Subjects for more detail) to determine placed into each group, 4 from the naive subject group and 4
whether athletically savvy individuals responded differently from the savvy subject group.
to training than their naive counterparts.
Subjects Passive Stiffness Measurement. Sagittal and frontal plane
Twenty-four young healthy university-aged males (22.9 6 passive bending trials were performed in which participants
2.7 years; range 18-29 years; 1.79 6 0.06 m; 77.5 6 were secured at the hips, knees, and ankles on a solid lower-
10.8 kg) were selected for this study. Of these, 12 subjects body platform. Each participant’s upper body was secured to
(21.7 6 1.9 years; range 18-26 years; 1.80 6 0.08 m; 78.3 6 a cradle with a glass bottom surface, about their upper arms,
12.3 kg) were selected with limited experience in physical torso, and shoulders (Figure 1). The upper-body cradle was
training and little to no experience in performing core exer- free to glide overtop of a similar glass surface with precision
cises. These are referred to as the exercise “naive” group. The nylon ball bearings between the 2 structures. This created
remaining subgroup of 12 subjects (24.2 6 2.9 years; range a frictionless float influenced by gravity and allowed trunk
21-29 years; 1.8 6 0.05 m; 76.8 6 9.7 kg) was selected from movement about either the flexion-extension or lateral bend
a population of athletes with experience in core training. axis. Participants laid on their right side for the flexion-
Inclusion criteria for this subgroup consisted of individuals extension trials and on their back for the lateral bend trials.
highly experienced in core training methods, having regu- Their torsos were supported in each position to ensure that
larly performed direct core exercises for at least 1 year. This participants adopted and maintained a nondeviated spine
special population consisted of club Muay Thai fighters, posture throughout the testing.
a martial art native to Thailand involving standing striking Passive axial twisting trials were performed in a separate
with the fists, elbows, knees, and shins. These are referred to apparatus consisting of a rotating wheel platform mounted
as the “savvy” group. Exclusion criteria for both subgroups to a fixed base through ball bearings with a frictionless
consisted of any individuals who have experienced low back contact (Figure 1). The participant stood upright on the
pain or injury currently or within the past year. The majority platform maintaining upright spine posture with their
of naive subjects were active in recreational/intramural upper body fixed through a harness strap to a vertical
sports, but had no background in core training and limited post (approximately at the level of T9). Lumbar spine
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1518
Plank 5 3 5, 4, 3, 2, 1 4 5 3 5, 4, 3, 2, 1 7
Bird dog 5 3 5, 4, 3, 2, 1 4 5 3 5, 4, 3, 2, 1 7
Side bridge 5 3 5, 4, 3, 2, 1 4 5 3 5, 4, 3, 2, 1 7
Torsional buttress 5 3 5, 4, 3, 2, 1 7
Anterior pallof press 5 3 10 s 4 Same volume, increase 4
load
Posterior pallof press 5 3 10 s 4 Same volume, increase 4
load
Suitcase hold 5 3 10 s per 4 Same volume, increase 4
side load
Antirotation pallof Press 5 3 10 s per 4 Same volume, increase 4
side load
Stir the pot
TM
Inverted row
Kettlebell unilateral rack
walk
Half kneeling woodchop
Week 5 Week 6
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Instrumentation
Electromyography. Electromyography signals were collected on
unilateral core musculature using pregelled, disposable, monop-
olar Ag-Cl disc-shaped surface electrodes (30 mm diameter,
increase load
Same volume,
5 3 10
Up to 5 3 10 per
side
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1520
Sets 3 Sets 3
Exercise Sets 3 repetitions Frequency* repetitions Frequency* Sets 3 repetitions Frequency* repetitions Frequency*
Curl up Up to 5 3 10 4 5 3 10 7
Superman Up to 5 3 10 4 5 3 10 7
Side curl up Up to 5 3 10 per 4 5 3 10 per side 7
side
Twisting curl up Up to 5 3 10 per 4 5 3 10 per side 7
side
Advanced curl up (limbs Up to 5 3 10 4 5 3 10 4
extended)
Back extension Up to 5 3 10 4 5 3 10 4
Russian barbell twist Up to 5 3 10 per 4 5 3 5–10 per 4
side side
Curl up twitch
Superman twitch
TM
Week 5 Week 6
Sets 3
Exercise Sets 3 repetitions Frequency* repetitions Frequency* Comments
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the TM
Figure 4. Summary stiffness curves for passive flexion trials with applied moment (N$m) plotted on the y-axis and deflection (degrees) on the x-axis. Top left:
naive isometric group. Middle left: savvy isometric group. Bottom left: Overall isometric. Top middle: Naive dynamic group. Center: savvy dynamic group. Bottom
middle: Overall dynamic. Top right: Naive control group. Middle right: savvy control group. Bottom right: Overall control.
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Isometric Training on Core Stiffness
Figure 5. Summary of preisometric/postisometric stiffness curves for passive extension, left lateral bend, right lateral bend, left axial twist, and right axial twist
(top to bottom); applied moment (N$m) is denoted on the y-axis and deflection (degrees) on the x-axis. The curves plotted represent training response for all
subjects.
sampled at 2,160 Hz. Force signals were dual-pass every 2 weeks, dividing each program into 3 phases (Tables 1
filtered (effective fourth order 3 Hz low-pass Butter- and 2 for a description of the progressive programs).
worth). Both the linear enveloped EMG and force
Moment Angle Relationship and Measurement of Stiffness
signals were downsampled to 60 Hz to match the trunk
Passive Trials. The applied moment and corresponding trunk
motion data.
angle were normalized in time to ensure equal trial length
across all trials and participants. Trunk angles were normal-
Core Training Protocols. Subjects were trained for 6 weeks ized as a percentage of the maximum range of motion that
using either isometric or dynamic core exercises (the control participants were able to obtain in the pretraining bending
group did not train). All subjects were asked to refrain from trials. Exponential curve fits of the following form were
performing any core exercises outside of those assigned by performed for each trial type:
researchers during the study. The Isometric training group
performed static exercises designed to challenge the core M ¼ le fq ;
musculature through bracing cues. The dynamic training
group performed exercises based on torso movement. Both where M is the applied moment (N$m), l and f are curve
training programs were periodized to increase challenge fitting constants, and q is the angular displacement of the
the TM
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the TM
Figure 6. Summary of predynamic/postdynamic stiffness curves for passive extension, left lateral bend, right lateral bend, left axial twist, and right axial twist
(top to bottom); applied moment (N$m) is denoted on the y-axis and deflection (degrees) on the x-axis. The curves plotted represent training response for all
subjects.
torso. The calculated moment was normalized as a percentage where k represents the stiffness calculated from the slope of
of the maximum applied moment of the pretraining trials and the moment (M) and absolute angular deflection (q) curve.
calculated at 50, 65, 80, 90, 95, and 100% of pretraining peak
moment for pre- and post-training conditions (Figure 3). Statistical Analyses
Statistical tests were performed using IBM SPSS Statistical
Quick Release Trials for Active Stiffness. On magnet and force software (version 19, IBM, Corp., Somers, NY, USA). 3 3
release, an event was detected from the load-cell signal by 2 3 2 repeated-measures analysis of variance (3 training
visually identifying when the force-time slope changed. Over group levels, 2 subject group levels, and before and after
the next 250 milliseconds, the force at release and the peak training) was conducted for comparing range of motion
angular displacement of the lumbar spine were obtained to values at each specific instance of applied moment before
calculate a gross stiffness measure, after Sutarno and McGill and after the training protocol (50, 65, 80, 90, 95, and 100%
(39). A gross lumbar measure of stiffness (N$m/degree) was of pretraining applied moment). Where applicable, post
then obtained from the following equation: hoc analyses were performed using the Tukey HSD test
when a significant effect was detected with statistical sig-
M nificance set at p # 0.05. To the researchers’ knowledge, no
k¼ ;
q studies currently exist examining stiffness adaptations with
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Isometric Training on Core Stiffness
core training, thus it is difficult to establish intra-class cor- performed by Burgess et al. (8) and Kubo et al. (25) respec-
relations and statistical power. tively. However, the Burgess work differed in that they re-
ported increases in tendon stiffness with dynamic lower limb
RESULTS training, whereas this study’s dynamic protocol did not show
An example from Figure 4 illustrates the overall effects for such changes.
the passive flexion test. The biggest changes were observed It was possible that the 6-week protocol could have
in the isometric training group, regardless of subject group. caused physical adaptations of hypertrophy and strength
gain. However, if this were true, one would have expected
Isometric Training
similar stiffness gains with the dynamic training approach.
Significant stiffness increases were measured in both savvy
Some evidence exists that some components of the core
and naive populations after 6-week isometric training for the
musculature experienced thickness increases after selected
majority of bending tests and at multiple levels of applied
trunk strengthening exercises, including the bird dog and
moment (p # 0.05). In naive subjects, only extension and
side bridge isometric exercises (42). Perhaps this is linked
right lateral bending stiffness did not significantly increase
to the muscular hypertrophy/time under tension relation-
(although right lateral bending stiffness at 80% of applied
ship—greater time under tension has been shown to
moment was significantly different from pretraining condi-
increase skeletal muscular hypertrophy (23,32,37). Obvi-
tions) (Figure 5). The majority of trials showed stiffness in-
ously, time spent under muscular contraction was much
creases near end range of motion; flexion stiffness increased
higher when performing isometric exercises. For example,
significantly at 95% of applied moment and beyond while
a 10-second plank requires continual peak activation of
left lateral bending and right axial twisting stiffness showed
anterior core musculature for the full 10-second period,
significant increases at 80% of applied moment and beyond.
whereas a 10-repetition curl up incorporates a duty cycle
Only left axial twist stiffness increased at all levels of applied
resulting in far less time under tension. In addition, sharp
moment. Savvy subjects experience similar results to their
increases in passive stiffness near-maximum applied mo-
athletically naive counterparts after 6-week isometric train-
ments were measured in isometrically trained subjects.
ing. Extension and right lateral bending stiffness were shown
Burgess and Kubo both proposed that after training, col-
to not significantly change but significant differences were
lagen structures remodeled similar to that with muscular
experienced for all other directions. Interestingly, savvy sub-
hypertrophy, and demonstrated by Michna (34) and
jects experienced greater magnitude of changes in flexion
Zamora and Marini (48). An alternate explanation is that
and left axial twist stiffness near end range of motion than
the 6-week program stimulated neural changes and a resid-
the naive subjects (p , 0.001 at 95% of applied moment and
ual stiffness. Although active stiffness was not significantly
beyond for flexion, and p , 0.01 at 90% of applied moment
affected, training journals kept by subjects revealed a per-
and beyond for right axial twist). Active stiffness did not
ception of training effects. Comments of being better able
significantly change in either subject group after long-term
to control activation of specific core and hip musculature
isometric training.
were common in both isometric and dynamic groups.
Dynamic Training These comments may be related to increased voluntary
The results of 6-week dynamic training yielded far fewer muscular activation levels after resistance exercise. Garfin-
stiffness changes in both subject groups (Figure 6). Only kel and Cafarelli showed increases in MVC after 8 weeks of
right lateral bend at all moment levels except at 80% for isometric limb training (16); not only did muscular cross-
naive subjects, left axial twist at 90% of applied moment sectional area increase but increased EMG amplitude dur-
and beyond for savvy populations, and a single significant ing MVC trials was also observed. Other groups have re-
difference at 90% of applied moment in extension showed ported similar findings where multiweek periods of
differences after dynamic training (p # 0.05). No significant resistance training resulted in increased EMG amplitude
differences in active extension stiffness were experienced during maximal exertions (17,22,24).
after long-term dynamic training. The results suggest that isometric core training is
Control groups did not experience any significant changes superior to dynamic training for enhancing torso stiffness.
in response after the 6-week period, within each subject Enhanced core stiffness allows the spine the bear greater
group for all directions and between subject groups. loads (10) and express greater distal limb athleticism (26).
The next step would be to examine specific changes in
DISCUSSION athletic performance.
The results suggest that the isometric exercise approach was To the author’s knowledge, no research investigating
superior in enhancing torso stiffness over the dynamic training-related changes in core stiffness have been per-
approach in a 6-week trial. Both the naive and savvy groups formed. Thus, without existing values to estimate statistical
responded similarly to training. These findings generally power, it was not possible to establish a suitable sample
agree with the results of previous work investigating iso- population. However, the work performed provided statisti-
metric training on tendon stiffness in the lower limbs cally supported evidence of isometric training effects on core
the TM
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the TM
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