Isometric training and long-term adaptations: Effects of muscle length, intensity, and intent: A systematic review

Abstract

Isometric training is used in the rehabilitation and physical preparation of athletes, special populations and the general public. However, little consensus exists regarding training guidelines for a variety of desired outcomes. Understanding the adaptive response to specific loading parameters would be of benefit to practitioners. The objective of this systematic review, therefore, was to detail the medium to long‐term adaptations of different types of isometric training on morphological, neurological and performance variables. Exploration of the relevant subject matter was performed through MEDLINE, PubMed, SPORTDiscus and CINAHL databases. English, full‐text, peer‐reviewed journal articles and unpublished doctoral dissertations investigating medium to long‐term (≥3 weeks) adaptations to isometric training in humans were identified. These studies were evaluated further for methodological quality. Twenty‐six research outputs were reviewed. Isometric training at longer muscle lengths (0.86‐1.69%/week, ES = 0.03‐0.09/week) produced greater muscular hypertrophy when compared to equal volumes of shorter muscle length training (0.08‐0.83%/week, ES = ‐0.003‐0.07/week). Ballistic intent resulted in greater neuromuscular activation (1.04‐10.5%/week, ES = 0.02‐0.31/week vs. 1.64‐5.53%/week, ES = 0.03‐0.20/week) and rapid force production (1.2‐13.4%/week, ES = 0.05‐0.61/week vs. 1.01‐8.13%/week, ES = 0.06‐0.22/week). Substantial improvements in muscular hypertrophy and maximal force production were reported regardless of training intensity. High‐intensity (≥ 70%) contractions are required for improving tendon structure and function. Additionally, long muscle length training results in greater transference to dynamic performance. Despite relatively few studies meeting the inclusion criteria, this review provides practitioners with insight into which isometric training variables (e.g. joint angle, intensity, intent) to manipulate to achieve desired morphological and neuromuscular adaptations.

Full text

Scand J Med Sci Sports. 2019;1–20. wileyonlinelibrary.com/journal/sms
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© 2018 John Wiley & Sons A/S.
Published by John Wiley & Sons Ltd
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INTRODUCTION
Resistance training is widely utilized as a component of phys-
ical preparation for populations ranging from elite strength
and power athletes to injured members of the general pub-
lic.1 Commonly documented resistance training adaptations
include increased muscle mass,2 tendon quality,3-5 strength,
power, and range of motion,6 delaying muscular fatigue,7,8
and improving voluntary activation.9 Dynamic movements
incorporating the stretch‐shortening cycle (SSC) comprise
the overwhelming majority of resistance training programs.10
However, isolated concentric, eccentric, and isometric
Received: 1 August 2018
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Accepted: 17 December 2018
DOI: 10.1111/sms.13375
REVIEW ARTICLE
Isometric training and long‐term adaptations: Effects of muscle
length, intensity, and intent: A systematic review
Dustin J. Oranchuk1
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Adam G. Storey1
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André R. Nelson2
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John B. Cronin1,3
1Sports Performance Research Institute
New Zealand,Auckland University of
Technology, Auckland, New Zealand
2Institute for Health and Sport,Victoria
University, Melbourne, Victoria, Australia
3School of Health and Medical
Science,Edith Cowan University, Perth,
Western Australia, Australia
Correspondence
Dustin J. Oranchuk, Sports Performance
Research Institute New Zealand, Auckland
University of Technology, Auckland, New
Zealand.
Email: dustinoranchuk@gmail.com
Isometric training is used in the rehabilitation and physical preparation of athletes,
special populations, and the general public. However, little consensus exists regard-
ing training guidelines for a variety of desired outcomes. Understanding the adaptive
response to specific loading parameters would be of benefit to practitioners. The
objective of this systematic review, therefore, was to detail the medium‐ to long‐term
adaptations of different types of isometric training on morphological, neurological,
and performance variables. Exploration of the relevant subject matter was performed
through MEDLINE, PubMed, SPORTDiscus, and CINAHL databases. English, full‐
text, peer‐reviewed journal articles and unpublished doctoral dissertations investigat-
ing medium‐ to long‐term (≥3 weeks) adaptations to isometric training in humans
were identified. These studies were evaluated further for methodological quality.
Twenty‐six research outputs were reviewed. Isometric training at longer muscle
lengths (0.86%‐1.69%/week, ES = 0.03‐0.09/week) produced greater muscular hy-
pertrophy when compared to equal volumes of shorter muscle length training
(0.08%‐0.83%/week, ES = −0.003 to 0.07/week). Ballistic intent resulted in greater
neuromuscular activation (1.04%‐10.5%/week, ES = 0.02‐0.31/week vs
1.64%‐5.53%/week, ES = 0.03‐0.20/week) and rapid force production (1.2%‐13.4%/
week, ES = 0.05‐0.61/week vs 1.01%‐8.13%/week, ES = 0.06‐0.22/week).
Substantial improvements in muscular hypertrophy and maximal force production
were reported regardless of training intensity. High‐intensity (≥70%) contractions
are required for improving tendon structure and function. Additionally, long muscle
length training results in greater transference to dynamic performance. Despite rela-
tively few studies meeting the inclusion criteria, this review provides practitioners
with insight into which isometric training variables (eg, joint angle, intensity, intent)
to manipulate to achieve desired morphological and neuromuscular adaptations.
KEYWORDS
eccentric, fascicle, force, mechanical loading, muscle, stiffness, strength, tendon

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ORANCHUK et Al.
contractions have specific advantages when improving mus-
culo‐skeletal properties and neuromuscular function11-13 and
are increasing in popularity.14 Isometric contractions (where
the muscle‐tendon unit remains at a constant length) and their
role as a training option provide the focus of this paper.
Training with isometric contractions has been purported
to have several advantages. First, isometric training allows for
a tightly controlled application of force within pain‐free joint
angles in rehabilitative settings.15,16 Second, isometric train-
ing provides a means to induce force overload as maximal iso-
metric force is greater than that of concentric contractions.17
Third, a practitioner who understands the physical demands
of a sport may be able to utilize isometric training to focus on
specific weak points in a range of motion that can positively
transfer to performance18 and injury prevention.19 Isometric
contractions can also be used to provide an acute analgesic
effect and allow for pain‐free dynamic loading20,21 by alter-
ing excitatory and inhibitory functions in the corticomotor
pathways.22 Additionally, isometric contractions are a highly
reliable means of assessing and tracking changes in force pro-
duction.23-25 However, the ability of isometric assessments to
predict dynamic performance is questionable,23-25 despite
multi‐joint appraisals showing promise.26-29
Isometric training can elicit changes in physiological
qualities including muscle architecture,30 tendon stiffness
and health,21,31 joint angle‐specific torque,31-33 and metabolic
functions.34 As with any mode of resistance training, several
variables can be manipulated to alter the stimulus. The most
common isometric training variations include altering joint
angles30-33,35-40 and contraction intensity or duration.34,39,41-47
Less frequently researched variations include contraction in-
tent (eg, ramp vs ballistic)43,47,48 and incorporating special
methods such as blood flow restriction,49,50 vibration,51,52
and electrical stimulation.53 Additionally, emerging research
has demonstrated unique neuromuscular characteristics
between “pushing” (ie, exerting force against an immov-
able object) and “holding” (ie, maintaining a joint position
while resisting an external force) isometric contractions.54-60
Understanding the loading parameters that achieve a desired
adaptive response in muscle and tendon would be of benefit
to practitioners. Therefore, the purpose of this review was to
systematically evaluate research directly comparing the out-
comes of isometric training variations and to provide training
guidelines for a variety of desired outcomes.
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METHODS
The systematic review conformed to the “Preferred
Reporting Items for Systematic Reviews and Meta‐Analyses”
(PRISMA) guidelines.61 Therefore, no Institutional Review
Board approval was necessary.
2.1
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Literature search methodology
An electronic search was conducted utilizing MEDLINE,
SPORTDiscus, PubMed, and CINAHL databases from incep-
tion to March 2018. Key terms were searched for within the
article title, abstract, and keywords using conjunctions “OR”
and “AND” with truncation “*.” Combinations of the following
Boolean phrases comprised the search terms: (Isometric train*)
AND (strength* OR stiff*); (Isometric train*) AND (muscle*
OR tendon*); (Isometric train*) AND (session* OR week*).
2.2
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Inclusion and exclusion criteria
Studies were included in the review based on the following cri-
teria: (a) full text available in English; (b) peer‐reviewed journal
publications or doctoral dissertations; and (c) the study com-
pared two or more variations of isometric training. Studies were
excluded if (a) they were conference papers/posters/presenta-
tions; (b) they focused on small joints or muscles such as fingers
or toes; (c) primary dependent variables were related to cardio-
vascular health; (d) they included non‐human subjects; (e) they
were in vitro; (f) the intervention period was less than three
weeks in duration; and (g) they included variables such as blood
restriction, vibration, or electrical stimulation. Search strategy
and inclusion/exclusion results are summarized in Figure 1.
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Quality assessment
Studies that met the inclusion criteria were assessed to de-
termine their quality based on established scales utilized in
the fields of sport and exercise science, kinesiology, health
care, and rehabilitation. Adapted from a systematic review
by Brughelli et al,62 the scale developed for the current re-
view is illustrated in Data S1. Ten items were scored as 0
(clearly no), 1 (maybe), or 2 (clearly yes) based on this scor-
ing rubric.62 Therefore, each study received a quality score
ranging from 0 to 20. Two researchers completed the quality
assessments of each paper with a third researcher settling any
discrepancies in scoring.
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Statistical analysis
Percent change and Cohen’s d effect sizes (ES) were cal-
culated wherever possible to indicate the magnitude of the
practical effect. Effect sizes were averaged across the length
of an intervention where applicable. As recommended by
Rhea,63 effect sizes were interpreted as follows: trivial <0.35,
small = 0.35‐0.80, medium = 0.80‐1.50, and large > 1.5 for
recreationally active participants.63 Where possible, data
were pooled and average ES change and % change (pre‐post)
per week were calculated. All reported ES and percentage
changes are pre‐post within‐group, unless otherwise stated.

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ORANCHUK et Al.
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RESULTS
A total of 26 studies with a mean quality score of 14.3/20
(range = 10‐18) met the inclusion criteria for the review (Data
S2). A total of 713 participants (463 male, 250 female) were
recruited with an average sample size of 27.4 ± 28.1 (4‐120).
Of the accepted investigations, the mean age of the reported
participants was 24.3 ± 3.3 years (19.3‐31.8); seven studies
failed to report participant mean age. Most studies (16/26)
recruited untrained participants, while the remainder (11/26)
utilized “active” or “recreationally trained” participants.
None of the accepted studies examined competitive athletes
or well‐trained participants. All 26 accepted investigations
clearly stated independent and dependent variables, and 10
included a non‐exercise control group. The mean length of
intervention was 8.4 ± 3.6 (range = 3‐14) weeks, with an
average of 3.5 ± 0.96 (range = 2‐7) sessions per week for
an average of 28.6 ± 13.2 (range = 15‐56) total training ses-
sions. Interventions were volume‐equated in 17/26 studies,
while 10/26 studies included a non‐exercise control group.
Closed‐chain movements were only utilized in two studies,
whereas 23/26 utilized single‐joint contractions.
Nine published journal articles and one unpublished doc-
toral dissertation examining the chronic (5‐12 weeks) effects
of isometric training at varying joint angles fulfilled the in-
clusion criteria (Table 1).30-33,35-38,40,64 Of the ten included
studies, eight centered on the knee extensors,30-33,35,38,40,64
with two utilizing the elbow flexors.36,37 Six published ar-
ticles examining the effect of contraction intensity (Table 2)
fulfilled the inclusion criteria.41,42,44-46,65 Of these studies,
three examined plantar flexors41,42,65 and one examined knee
extensors,46 while single studies examined the elbow flex-
ors45 and extensors, respectively.44 Training variations out-
side of joint position or contraction intensity were also
included. These variations include the following: (a) intent of
contraction which included “progressive” vs “rapid”48,66 and
“explosive” vs “sustained”43,47,67 contractions (Table 3); (b)
total volume39; (c) contraction duration13,34; (d) rest period
duration68; and (e) periodization schemes69 (Table 4).
When synthesizing statistically significant findings, mea-
sures of muscular size increased in nine studies (5%‐19.7%,
ES = 0.19‐1.23) by 0.84%/week and 0.043 ES/week.13,30-
32,34,43,44,67,69 Maximal isometric force significantly increased
in 14 studies (8%‐60.3%, ES = 0.34‐3.26) by 4.34%/week
and 0.20 ES/week.32,35,37,38,40,43,44,46-48,64-67 The comparison
between joint angle and hypertrophic adaptation (n = 3 stud-
ies) revealed that training with joint angles ≤ 70º (46 ± 6.9º)
improved muscle size by an average of 0.47 ± 0.48%/week
and 0.032 ± 0.037 ES/week, compared to 1.16 ± 0.46%/
week and 0.067 ± 0.032 ES/week when training at >70º of
flexion (Figure 2).30-32 When comparing the nine studies
that reported training joint angle and hypertrophic adapta-
tions, training with joint angles ≤ 70º (59.8 ± 11.1º) im-
proved muscle size by an average of 0.61 ± 0.42%/week and
0.045 ± 0.034 ES/week, compared to 0.88 ± 0.8%/week and
0.046 ± 0.027 ES/week when training at >70º (88.6 ± 6º)
of flexion (Data S3).13,30-32,34,43,44,67,69 The comparative ef-
fects of training intensity on muscular hypertrophy were that
FIGURE 1 Search strategy

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ORANCHUK et Al.
TABLE 1 Joint angle
Study, year
(quality) Subjects Intervention
Mechanical and neural
adaptations
(P < 0.05, ES ≥ 0.50)
Performance effect
(P < 0.05, ES ≥ 0.50)
Alegre, Ferri‐Morales,
Rodriguez‐Casares, & Aguado
(2014)30
(18/20)
Healthy, untrained
university
students
M = 22
F = 7
19.3 years
Isometric knee
extension
SML = 50°
LML = 90°
~74% of MVIC
8 wk, 2‐3/wk
SML:
↑VL thickness at 25% and
50% muscle length
(5.2%‐6.1%, ES = 0.23‐0.24)
↑isokinetic EMG at 60‐70°
(ES = 1.0) and 50‐60°
(P = 0.21, ES = 0.77)
LML:
↑VL thickness at 25%, 50%,
and 75% muscle length
(9%‐13.5%, ES = 0.31‐0.65)
↑VL pennation angle (11.7%,
ES = 0.45)
SML:
↓Optimum angle (7.3%, ES = 0.91)
LML:
↑Concentric torque at 60° s−1 (22.6%,
ES = 1.1)
↑Optimum angle (14.6%, ES = 1.38)
Bandy & Hanten (1993)38
(18/20)
Healthy, untrained,
university
students
F = 107
23.9 y
Isometric knee
extension
SML = 30°
MML = 60°
LML = 90°
100% of MVIC
8 wk, 4/wk
SML:
↑EMG at 15, 30, 45 and 60°
vs ↑EMG in control
(ES = 0.87‐1.65)
MML:
↑EMG at 15, 30, 45, 60 and
70° vs ↑EMG control
(ES = 0.36‐2.26)
LML:
↑EMG at 30, 45, 60, 75, 90,
and 105° vs ↑EMG in control
(ES = 0.74‐2.28)
SML:
↑MVIC at 15, 30, 45 and 60
(ES = 0.88‐1.94)
MML:
↑MVIC at 15, 30, 45, 60 and 75°
(ES = 1.01‐2.25)
LML:
↑MVIC at 15, 30, 45, 60, 75, 90, and 105°
(ES = 0.94‐3.26)
Bogdanis et al (2018)64
(15/20)
Healthy, active
university
students
M = 15
21.5 ± 2.1 y
Isometric leg press
(+countermove-
ment jumps)
SML = 35° of knee
flexion
LML = 95° of knee
flexion
100% of MVIC
6 wk, 3/wk
SML:
↓Optimum angle (9.7%, ES = 1.77)
↑MVIC at 18° (22%, ES = 0.88) and 34°
(57.4%, ES = 2.41)
↓RFD 0‐200 ms and 0‐300 ms at 80°
(11.8%‐13.8%, ES = 0.51‐0.60)
↑RFD 0‐200 ms and 0‐300 ms at 18°
(40.7%‐45.4%, ES = 1.2‐1.52) and 34°
(17.9%‐20.9%, ES = 0.62‐0.77)
↑1RM squat (9.6%, ES = 0.61)
↑CMJ height (7.2%, ES = 0.66)
LML:
↑MVIC (main time effect: P = 0.028) at all
joint angles (18‐98°) (~12.3%)
*↑RFD 0‐300 ms at 34° (14.4%, ES = 0.52)
↑1RM squat (11.9%, ES = 0.64)
↑CMJ height (8.4%, ES = 0.51)
Kubo et al (2006)31
(11/20)
Healthy university
students
M = 9
24 ± 1 y
Isometric knee
extension
SML = 50°
LML = 100°
70% of MVIC
12 wk, 4/wk
SML:
↑Quadriceps muscle volume
(10%, ES = 0.82)
↑EMG at all joint angles
(3.1%‐7.5%, ES = 0.25‐0.44)
LML:
↑Quadriceps muscle volume
(11%, ES = 1.06)
↑Tendon stiffness (50.86%,
ES = 1.22)
↓Tendon elongation
(−14.01%, ES = 0.62)
↑EMG at all joint angles
(7%‐8.84%, ES = 0.45‐0.72)
SML:
↑MVIC at 40, 50, 60, 70, and 80°
LML:
↑MVIC at 40, 50, 60, 70, 80, 90, 100, and
110°
(Continues)

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ORANCHUK et Al.
intensities ≤70% (68.9 ± 3.3%) of MVIC improved muscle
size by 0.77 ± 0.26%/week and 0.13 ± 0.12 ES/week, com-
pared to 0.70 ± 0.55%/week and 0.13 ± 0.21 ES/week when
training at >70% (85.3 ± 12%) of MVIC (Figure 3).13,30-
32,34,43,44,67,69 The comparisons of training intensity and
improvements in isometric force (n = 3 studies) found that
training at ≤70% (41.3 ± 16.5%) of MVIC improved muscle
size by 6.8 ± 3%/week and 0.32 ± 0.13 ES/week, compared
to 8.9 ± 5.5%/week and 0.36 ± 0.11 ES/week when training
at >70% (100 ± 0%) of MVIC (Figure 4).44,46,65 The joint
angle‐isometric force comparison (n = 7) showed that train-
ing at ≤70º (42.8 ± 16.4º) resulted in MVIC improvements
Study, year
(quality) Subjects Intervention
Mechanical and neural
adaptations
(P < 0.05, ES ≥ 0.50)
Performance effect
(P < 0.05, ES ≥ 0.50)
Lindh (1979)40
(13/20)
Healthy
F = 10
26.5 y
Isometric knee
extension
SML = 15°
LML = 60°
100% of MVIC
5 wk, 3/wk
SML:
↑MVIC in SML at 15° (32%)
↑MVIC at 60° (14%)
↑Con torque at 30° s−1
LML:
↑MVIC at 15° (11%)
↑MVIC at 60° (31%)
↑Con torque at 30° s−1
Noorkoiv, Nosaka, & Blazevich
(2014)32
(17/20)
Healthy, untrained
M = 16
23.7 ± 4.0 y
Isometric knee
extension
SML = 38.1 ± 3.7°
LML = 87.5 ± 6.0°
100% of MVIC
6 wk, 3/wk
SML:
↑Mid‐VL fascicle length
(5.6%, ES = 0.63)
LML:
↑Voluntary activation at 50°
(ES = 0.53) and 60°
(ES = 1.02)
↑Total quadriceps muscle
volume (5.2%, ES = 0.19)
↑Distal VL fascicle length
(5.8%, ES = 0.33)
SML:
↑MVIC at 40 and 50° (8.0%‐14.2%,
ES = 0.34‐0.54)
Noorkoiv, Nosaka, & Blazevich
(2015)33
(17/20)
Healthy, untrained
M = 16
23.7 ± 4.0 y
Isometric knee
extension
SML = 38.1 ± 3.7°
LML = 87.5 ± 6.0°
100% of MVIC
6 wk, 3/wk
LML:
↑Concentric torque at 30, *60, *90, and
120° s−1 (10.1%‐13%, ES = 0.55‐0.70)
Rasch & Pierson (1964)36
(13/20)
Healthy, untrained
university
students
M = 29
Isometric elbow
flexion
Single‐angle = 3
sets at 90°
Multi‐angle = 1 set
at 60, 90 and 120°
100% of MVIC
5 wk, 5/wk
Sterling (1969)35
(18/20)
University physical
education students
M = 120
Isometric “hip press”
SML = 25°
MML = 55°
LML = 85°
100% MVIC
7 wk, 3/wk
SML:
↑MVIC at 25 and 55° (21%‐37.2%)
MML:
↑MVIC at 25 and 55° (15.4%‐51.4%)
LML:
↑MVIC at 85° (3.1%)
Thepaut‐Mathieu, Van Hoecke,
& Maton (1988)37
11/20
Untrained
M = 24
31.8 y
Isometric elbow
flexion
SML = 60°
MML = 100°
LML = 155°
80% MVIC
5 wk, 3/wk
SML, MML, and LML:
↑EMG at all angles
SML:
↑MVIC at 60 and 80° (10%‐25%)
MML:
↑MVIC at 60‐155° (22%‐30%)
LML:
↑MVIC at 80‐155° (24%‐54%)
1RM, 1 repetition maximum; CMJ, countermovement jump; Con, concentric; ES, effect size (Cohen’s d); LML, long muscle length; MML, medium muscle length;
MVIC, maximal voluntary isometric contraction; RF, rectus femoris; SML, short muscle length; VL, vastus lateralis; VM, vastus medialis.
*Denotes P > 0.05.
TABLE 1 (Continued)

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ORANCHUK et Al.
TABLE 2 Contraction intensity
Study, quality Subjects Intervention
Morphological and neural adaptations
(P < 0.05, ES ≥ 0.50)
Performance effect
(P < 0.05, ES ≥ 0.50)
Adamantios Arampatzis,
Karamanidis, & Albracht
(2007)41
14/20
Healthy, untrained university students
M = 7
F = 14
28 y
Isometric plantar flexion
LI = 55% MVIC (24 contractions)
HI = 90% MVIC (16 contractions)
14 wk, 4/wk
LI:
↑Tendon elongation (16.2%, ES = 0.56)
↑Tendon strain (17.4%, ES = 0.57)
↑Calculated maximum tendon force
(28.4%, ES = 1.76)
HI:
↑Tendon stiffness (36%, ES = 1.57)
↑Tendon CSA at 60% and 70% of tendon
length
↑Calculated maximum tendon force
(43.6%, ES = 2.04)
Adamantois Arampatzis,
Peper, Bierbaum, & Albracht
(2010)42
14/20
Healthy, untrained university students
M = 11
23.9 y
Isometric plantar flexion
LI = 55% MVIC (20 contractions)
HI = 90% MVIC (12 contractions)
14 wk, 4/wk
LI:
↑Tendon elongation (14%, ES = 0.84)
↑Tendon strain (13.7%, ES = 0.67)
↑Calculated maximum tendon force
(11.7%, ES = 0.89)
HI:
↑Tendon stiffness (17.1%, ES = 0.82)
↑Calculated maximum tendon force
(11.9%, ES = 0.81)
Kanehisa et al (2002)44
16/20
Healthy, untrained
M = 12
27.5 y
Isometric elbow extension
LI = 60% MVIC (4 × 30 s)
HI = 100% MVIC (12 × 6 s)
10 wk, 3/wk
LI:
↑Muscle volume (5.3%, ES = 0.26)
HI:
↑Muscle volume (12.4%, ES = 0.28)
LI:
↑MVIC (61%, ES = 1.91)
HI:
↑MVIC (60.3%, ES = 2.71)
Khouw & Herbert (1998)45
11/20
51 untrained university students
M = 18
F = 33
Isometric elbow flexion
Each subject assigned to an
individual intensity between 0%
and 100% in 2% increments
6 weeks, 3/week
Greater ↑MVIC (slope = 0.19,
5.3%, P = 0.006) when training
closer to 100%
Szeto, Strauss, De Domenico,
& Sun Lai (1989)46
11/20
University students
M = 6
F = 12
Isometric knee extension
LI = 25% MVIC
MI = 50% MVIC
HI = 100% MVIC
3 wk, 5/wk
LI:
*↑MVIC (22.3%, ES = 0.61)
MI:
↑MVIC (31.3%, ES = 1.14)
HI:
↑MVIC (45.7%, ES = 1.44)
(Continues)

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ORANCHUK et Al.
of 4 ± 2.1%/week and 0.15 ± 0.1 ES/week, compared to
3.4 ± 4.2%/week and 0.15 ± 0.17 ES/week when training at
>70º (101.8 ± 24.2º) of flexion (Data S4).31,32,35,37,38,40,64
4
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DISCUSSION
4.1
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Morphological adaptations
Adaptations to the physical structure of tissues can be caused
by several factors, including mechanical, metabolic, and hor-
monal factors, and often result in altered function. The mor-
phology of the musculo‐skeletal system is of relevance to this
review and provides the focus for subsequent discussion.
4.1.1
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Muscle volume
While most methods of progressive resistance training can
result in increased muscular size, it is important to under-
stand how to optimally alter variables including intensity,
frequency, and duration of each training method for maximal
efficiency. Isometric resistance training has been demon-
strated to induce significant hypertrophy.13,30-32,34,39,43,44
When comparing adaptations in muscle volume between
isometric training variations, several patterns emerged,
conforming to accepted dynamic training principles. Of
the studies comparing isometric training at differing joint
angles (Table 1), only three evaluated muscle volume or
thickness.30-32 All three studies found that isometric train-
ing at long muscle lengths (LMLs) was superior to equal
volumes of training at short muscle lengths (SMLs) for in-
creasing muscle size.30-32 These findings are not surprising
as a large portion of the existing literature has demonstrated
that dynamic training through a large range of motion is
beneficial when hypertrophy is desired.70-72 Additionally,
contractions at LML tend to produce higher quantities of
muscle damage, likely by altering the joint moment arm
and increasing mechanical tension when compared to a
SML.73 Contractions at LML also result in greater blood
flow occlusion, rates of oxygen consumption, and metabo-
lite buildup when compared to SML contractions.49 These
metabolic factors are well established to contribute to mus-
cular hypertrophy.74,75
While volume‐equated isometric training leads to greater
improvements in hypertrophy when performed at LMLs,30,32,33
the magnitude of hypertrophy was not significantly different
in any of the seven included studies investigating/reporting
training intensity.13,30-32,34,43,44 Interestingly, the pooled data
of included study outcomes suggest that training intensity
has a small effect on hypertrophy and explains little of the
variation in hypertrophic adaptation (Figure 3). For example,
Kubo et al13 compared the effects of load‐equated isomet-
ric contractions held for short (~1 second) or long (20 sec-
onds) periods of time. While both long‐ and short‐duration
Study, quality Subjects Intervention
Morphological and neural adaptations
(P < 0.05, ES ≥ 0.50)
Performance effect
(P < 0.05, ES ≥ 0.50)
Young, McDonagh, & Davies
(1985)65
12/20
Healthy
M = 4
20.5 y
Isometric plantar flexion
LI = 30% MVIC (7‐15 × 60 s)
HI = 100% MVIC (3‐s contrac-
tions)
HI, 5 wk; and LI, 8 wk, 7/wk
LI:
↑MVIC (3.3%/wk)
↑MVIC (30.2%, ES = 2.22)
↑Fatigue index (19.4%,
ES = 1.72)
HI:
↑MVIC (5.5%/wk
↑MVIC (21.2%, ES = 1.67)
ES, effect size (Cohen’s d); HI, high intensity; LI, low intensity; MI, medium intensity; MVIC, maximal voluntary isometric contraction.
*Denotes P > 0.05.
TABLE 2 (Continued)

8
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ORANCHUK et Al.
TABLE 3 Contraction intent
Study, year
(quality) Subjects Intervention
Morphological and neural adaptations
(P < 0.05, ES ≥ 0.50)
Performance effect
(P < 0.05, ES ≥ 0.50)
Balshaw, Massey,
Maden‐Wilkinson,
Tillin, & Folland (2016)
43
(15/20)
Healthy, untrained
M = 43
Isometric knee extension
MST = 1‐s build to 75% of MVIC, hold
for 3 s (40 contractions)
EST = rapidly built to ≥80% of MVIC
and hold for 1 s (40 contractions)
12 wk, 3/wk
MST:
↑Muscle volume (8.1%, ES = 0.50)
↑EMG at MVIC (27.8%, ES = 0.67)
↑EMG 0‐150 ms (14.3%, ES = 0.36)
EST:
↑EMG 0‐100 and 0‐150 ms (12.5%‐31.3%,
ES = 0.26‐0.67)
MST:
↑MVIC (23.4%, ES = 1.19)
↑Force at 150 ms (12.1%, ES = 0.74)
EST:
↑MVIC (17.2%, ES = 1.24)
↑Force at 50, 100, and 150 ms
(14.4%‐32.6%, ES = 0.65‐1.06)
Maffiuletti & Martin
(2001)48
(17/20)
Healthy untrained
M = 21
Isometric knee extension
RC = 4 s to reach MVIC
BC = 1 s to reach MVIC
7 wk, 3/wk
RC:
↓VL EMG
BC:
↑Peak twitch (29.8%)
↓Contraction time
↓Maximal twitch relaxation
RC:
↑MVIC at 55°, 65° (15.7%) and 75°
↑Eccentric torque at 60° s−1 (15.6%)
↑Concentric torque at 60 and 240° s−1
BC:
↑MVIC at 55°, 65° (27.4%) and 75°
↑Eccentric torque at 60° s−1 (18.3%)
↑Concentric torque at 60 and 240° s−1
Massey, Balshaw,
Maden‐Wilkinson,
Tillin, & Foland
(2018)67
(18/20)
Healthy untrained
M = 42
MST = 25 ± 2 y
EST = 25 ± 2 y
CON = 25 ± 3 y
Isometric knee extension
MST = 1‐s build to 75% of MVIC, hold
for 3 s (~10 contractions)
EST =rapidly built to ~80% of MVIC
(~10 contractions)
12 wk, 3/wk
MST:
↑Muscle volume (8.1%, ES = 0.47)
↑VL aponeurosis area (5.9%, ES = 0.34)
↑Tendon stiffness (14.3%, ES = 0.79)
↑Young’s modulus (14.4%, ES = 0.60)
↑Tendon‐aponeurosis stiffness (22.7%, ES = 0.54)
EST:
↑VL aponeurosis area (4.4%, ES = 0.38)
↓Tendon CSA (2.8%, ES = 0.31)
↓Tendon elongation (11%, ES = 0.75)
↑Tendon stiffness (19.9%, ES = 0.95)
↓Tendon strain (11.8%, ES = 0.56)
↑Young’s modulus (21.1%, ES = 1.13)
↑Tendon‐aponeurosis elongation (16%, ES = 1.0)
MST:
↑MVIC (23.6%, ES =1.17)
EST:
↑MVIC (16.7%, ES =1.23)
Tillin & Folland (2014)47
(12/20)
Healthy, recreationally
active male university
students
N = 19
MST = 20.9 ± 1.1 y
EST = 20.2 ± 2.4 y
Isometric knee extension
MST = 1‐s build to 75% of MVIC, hold
for 3 s (10 contractions)
EST = rapidly built to ≥90% of MVIC
and hold for 1 s (10 contractions)
4 wk, 4/wk
MST:
↑M‐wave at MVIC (28.1%, ES = 1.28)
↓%EMG at 50 and 150 ms (11.7%‐22.1%,
ES = 0.59‐0.79)
EST:
↑M‐wave at 50 and 100 ms (25%‐42%,
ES = 0.95‐1.05)
MST:
↑MVIC (20.5%, ES = 1.46)
↑MVIC at 50, 100, and 150 ms
(3.09%‐7.39%, ES = 0.084‐0.52)
EST:
↑MVIC (10.6%, ES = 0.56)
↑MVIC at 50, 100, and 150 ms
(13.1%‐53.7%, ES = 0.96‐1.2)
(Continues)

|
9
ORANCHUK et Al.
contractions led to small, but significant increases in mus-
cle thickness, there was little difference (P > 0.05) between
groups (7.6%, ES = 0.38, P = 0.023% vs 7.4%, ES = 0.36,
P = 0.018).13 Similarly, Kanehisa et al44 employed ten weeks
of volume‐equated isometric training at either low (60%) or
high (100%) intensity. While both low‐ and high‐intensity
training programs significantly increased triceps brachii hy-
pertrophy, there was no statistical between‐group difference
(P = 0.061) in anatomical cross‐sectional area (low: 12.1%,
ES = 1.72 vs high: 17.1%, ES = 1.65).44 However, high‐in-
tensity training had a greater effect on muscle volume than
the lower intensity (12.4%, ES = 0.28% vs 5.3%, ES = 0.26;
P = 0.039) despite nearly identical effect sizes.44 These find-
ings are in close agreement with recent studies and meta‐
analyses that concluded that hypertrophic adaptations are
similar if total load is equated and training intensity is >20%
of maximal voluntary contraction.76,77
When the training volume is not equated between groups,
it seems higher volumes are better for inducing muscular
hypertrophy, regardless of contraction intensity. Meyers39
compared low (3 × 6 seconds MVIC)‐ and high (20 × 6 sec-
onds MVIC)‐volume isometric training of the elbow flexors.
Following the six‐week intervention, the high‐volume train-
ing program resulted in significantly greater improvements in
muscle girth compared to the low‐volume group (P < 0.05).
Similarly, Balshaw et al43 and Massey et al67 compared “max-
imal strength” (40 × 3 seconds contractions, 75% of MVIC)
and “explosive” (40 × 1 seconds contractions, 80% of
MVIC) isometric training. Following the 12‐week interven-
tions, the “maximal strength” training groups experienced
significant improvements in quadriceps muscle volume
(8.1%, ES = 0.50, P = 0.001), whereas the “explosive”
training groups (2.6%, ES = 0.17‐0.26, P = 0.195‐0.247)
did not.43 Furthermore, the difference between groups was
statistically significant (P < 0.05).43,67 Interestingly, Schott,
McCully, and Rutherford34 found that long‐duration (4 × 30
second MVIC) contractions resulted in greater hypertrophic
adaptations when compared to short (4 sets × 10 × 3 second
MVIC)‐duration contractions despite total time‐under‐ten-
sion being equated between groups. Following 14 weeks, the
long‐duration contraction group significantly (P = 0.022)
improved vastus lateralis anatomical cross‐sectional area
at the proximal (10.1%) and distal (11.1%) portions of the
femur, whereas no significant hypertrophic adaptations were
observed in the short‐duration group (P > 0.05).34 Schott,
McCully, and Rutherford’s34 findings are somewhat sur-
prising as both groups underwent the same time‐under‐ten-
sion. However, sustained contractions are known to restrict
blood flow, reduce muscle oxygen saturation, and increase
metabolite concentrations in the muscle78,79 stimulating hy-
pertrophy via multiple local and systemic mechanisms.74,75
Additionally, muscle contractions at LML consume more
Study, year
(quality) Subjects Intervention
Morphological and neural adaptations
(P < 0.05, ES ≥ 0.50)
Performance effect
(P < 0.05, ES ≥ 0.50)
Williams (2011)66
(15/20)
Healthy, untrained
university students
M = 11
F = 12
Ramp = 9
Ballistic = 8
22.8 y
Isometric knee extension
RC = 4 s to reach MVIC
BC = 1 s to reach MVIC
6 wk, 3/wk
RC:
↑Ramp VA (7.7%, ES = 1.99)
↑Ballistic VA (8.3%, ES = 1.75)
*↑150 ms VA (9.82%, ES = 0.74)
BC:
↑Ramp VA (4.1%, ES = 1.07)
↑Ballistic VA (7.9%, ES = 1.50)
↑150 ms VA (31.6%, ES = 1.84)
RC:
↑Ramp MVIC (20%, ES = 1.95)
↑Ballistic MVIC (17.8%, ES = 1.56)
*↑150 ms force (14.3%, ES = 1.10)
BC:
↑Ramp MVIC (15.7%, ES = 0.75)
↑Ballistic MVIC (18.9%, ES = 0.88)
↑150 ms force (48.8%, ES = 3.66)
BC, ballistic contraction; ES, effect size (Cohen’s d); EST, explosive strength training; MST, maximal strength training; MVIC, maximal voluntary isometric contraction; RC, ramp contraction; VA, voluntary activation.
*Denotes P > 0.05.
TABLE 3 (Continued)

10
|
ORANCHUK et Al.
TABLE 4 Other independent variables
Study, quality Subjects Intervention
Morphological and neural adaptations
(P < 0.05, ES ≥0.50)
Performance effect
(P < 0.05 and/or ES ≥0.50)
Kubo, Kanehisa, & Fukunaga
(2001)13
14/20
Healthy, untrained
M = 8
22.6 y
Isometric knee extension
SC = 3 × 50 rapid contractions
LC = 4 × 20 s
70% MVIC
12 wk, 4/wk
SC:
↑Muscle volume (7.4%, ES = 0.36)
*↑Tendon stiffness (17.5%, ES = 0.57)
↑Elastic energy (25.6%, ES = 1.85)
LC:
↑Muscle volume (7.6%, ES = 0.38)
↑Tendon stiffness (57.3%, ES = 1.38)
↑Elastic energy (12%, ES = 0.58)
SC:
↑MVIC (49%, ES = 2.47)
LC:
↑MVIC (41.6%, ES = 2.21)
Meyers (1967)39
13/20
Healthy university
students
M = 29
Isometric elbow flexion
LV = 3 × 6 s
HV = 20 × 6 s
100% MVIC
6 wk, 3/wk
LV:
↑Muscle girth at 170° in trained arm
HV:
↑Muscle girth at 170° in trained and untrained
arm
↑Muscle girth at 90° in trained arm
LV:
↑MVIC at 170° (15.4%, ES = 0.93)
*↑Muscle endurance (49.7%, ES = 0.71)
HV:
↑MVIC at 170° (15.5%, ES = 0.46)
*↑MVIC at 90° (9%, ES = 0.50)
↑Muscle endurance (42.7%, ES = 0.67)
Schott, McCully, & Rutherford
(1995)34
10/20
Healthy, untrained
M = 1
F = 6
22.7 y
Isometric knee extension
SC = 4 × 10 × 3 s
LC = 4 × 30 s
70% of MVIC
14 wk, 3/wk
LC:
↑Muscle ACSA at lower (11.1%) and upper
(10.1%) femur
SC:
↑MVIC (31.5%)
↑Concentric torque at 120 and 180° s−1
(11.3%‐11.6%)
LC:
↑MVIC at 90° (54.7%)
Ullrich, Holzinger, Soleimani,
Pelzer, Stening, & Pfeiffer
(2015)69
16/20
Healthy, active
university students
F = 10
24.4 ± 3.2 y
Isometric knee extension
TP limb = 3 wk 60%, 4 wk 80%,
3 wk 60%, 2 wk 80% of MVIC
DUP limb = Alternating
sessions at 60% and 80% of
MVIC in one limb
16 wk, 2/wk
TP:
↑Thigh circumference (6.2%, ES = 0.45)
↑VL thickness at 25%, 50%, and 75% muscle
length (15.5%‐18.5%, ES = 0.98‐1.23)
↑VL fascicle length (13.7%, ES = 1.17)
↑MVIC EMG (45%)
DUP:
↑Thigh circumference (5.0%, ES =0.37)
↑VL thickness at 25%, 50%, and 75% muscle
length (12.4%‐19.7%, ES = 0.72‐1.01)
↑VL fascicle length (14.2%, ES = 0.90)
↑MVIC EMG (46%)
TP:
↑MVIC (24%)
↑Concentric torque at 60° s−1 (19%)
DUP:
↑MVIC (23%)
↑Concentric torque at 60° s−1 (15%)
(Continues)

|
11
ORANCHUK et Al.
oxygen,49 which may in part explain the advantage of LML
training when muscular hypertrophy is the primary goal.
4.1.2
|
Muscle architecture
Unlike muscle volume, which is highly dependent on total
training volume, there are demonstrable differences between
contraction type and alteration in fascicle length and penna-
tion angle.80 To date, very few studies have compared the
effect of isometric resistance training variations on muscle ar-
chitecture; of those that have, results are equivocal. Noorkoiv,
Nosaka, and Blazevich32 compared isometric training at SML
(38.1 ± 3.7° knee flexion) and LML (87.5 ± 6° knee flexion).
Interestingly, the vastus lateralis fascicle length at the mid‐
portion of the femur significantly increased following SML
(5.6%, ES = 0.63, P = 0.01), but not LML (3.8%, ES = 0.34,
Study, quality Subjects Intervention
Morphological and neural adaptations
(P < 0.05, ES ≥0.50)
Performance effect
(P < 0.05 and/or ES ≥0.50)
Waugh, Alktebi, De Sa, & Scott
(2018)68
14/20
Healthy, physically
active
M = 8
F = 10
30.1 ± 7.9 y
Isometric plantar flexion
SR = 3 s between reps
LR = 10 s between reps
90% MVIC
12 wk, 3/wk
SR:
↑Echo‐type II (collagen reorganization)
SR & LR:
↑Stiffness
↑Tendon stress
↑Young’s modulus
↓Strain %
↓Tendon elongation
SR & LR:
↑MVIC
DUP, daily undulating periodization; ES, effect size (Cohen’s d); HV, high volume; LC, long contraction; LR, long rest; LV, low volume; MVIC, maximal voluntary isometric contraction; SC, short contraction; SR, short rest; TP,
traditional periodization.
*Denotes P > 0.05.
TABLE 4 (Continued)
FIGURE 2 Isometrically trained joint angle and hypertrophic
adaptations (N = 3)
FIGURE 3 Isometric training intensity and hypertrophic
adaptations (multiple comparison, N = 9)

12
|
ORANCHUK et Al.
P = 0.20) training.32 Conversely, LML (5.8%, ES = 0.33,
P = 0.02) significantly (P = 0.01) outperformed SML train-
ing (−1.1%, ES = 0.04, P > 0.05) for increasing distal fasci-
cle length of the same muscle.32 Furthermore, LML training
resulted in greater (P < 0.01) physiological cross‐sectional
areas in three of the four quadriceps heads, whereas the SML
training did not (P > 0.05).32 Only one other isometric train-
ing comparison study reported meaningful shifts in muscle
architecture and found that vastus lateralis pennation angle
increased following LML (10.6%, ES = 0.45, P = 0.038), but
not SML training (6.5%, ES = 0.46, P = 0.076).30 However,
Alegre et al30 only measured the vastus lateralis pennation
angle at the midpoint of the femur and potentially missed out
on possible adaptations at the distal portion of the muscle.
4.1.3
|
Tendon morphology
The primary function of the tendon is to transfer forces be-
tween bone and muscle, facilitating joint motion.5 Although
originally assumed to be inert, tendinous structures can expe-
rience adaptations and are capable of significant architectural
adaptations from habitual loading and injury.3-5,81-83 Injured
tendons tend to be less stiff, despite increased thickness84 due
to a shift in viscoelastic properties.5 Additionally, tendinopa-
thy negatively affects tendon structure, leading to increased
vascularization and overall thickness.5,84 Although long‐
term alteration in tendon morphology is minimal in healthy,
mature human tissue,5 tendons can increase in stiffness to
optimize the time and magnitude of force transmission be-
tween muscle and bone.3,4,82 Conversely, healthy increases
in tendon thickness and stiffness in response to exercise have
been found to be region specific and may have rehabilita-
tive, pre‐habilitative, and performance benefits.3,4,20,81,82 For
instance, heavy (resistance) training can lead to an increase
in maximal muscular force and rate of force development
by increasing tendon stiffness, thus reducing the electrome-
chanical delay.5,83,85 Additionally, increased tendon stiffness
through chronic loading can be due to increased tendon CSA
without alterations in viscoelastic properties, potentially im-
proving safety when performing ballistic movements.5 While
widely used in rehabilitation settings, there is a general lack
of information regarding what isometric training variables
are important for triggering specific tendinous adaptations.
Of the studies included in this review, only six directly
assessed tendon structure or function. Two studies compared
contraction intensity,41,42 with others examining the effects of
contraction length,13 intent,67 rest periods,68 and joint angle.31
Arampatizis et al41,42 compared 14‐week training programs
consisting of volume‐equated isometric plantar flexion at
low (~55%) or high (~90%) intensities. Both investigations
found increased Achilles tendon CSA and stiffness following
high‐intensity (17.1%‐36%, ES = 0.82‐1.57, P < 0.05), but
not low‐intensity (−5.2% to 7.9%, ES = 0.26‐0.37, P > 0.05)
training.41,42 Furthermore, tendon elongation under stress (an
indication of elasticity) increased following low‐intensity
(14.0%‐16.1%, ES = 0.56‐0.84, P > 0.05), but not high‐inten-
sity (−1.4% to 3.9%, ES = 0.06‐0.20, P > 0.05) training.41,42
Additionally, the included studies only compared isometric
training at ~55 and 90% of MVIC which leaves a large range
of potential intensities. However, previous interventions
have reported large increases (17.5%‐61.6%, ES = 0.57‐4.9,
P < 0.05) in tendon stiffness following training between 70%
and 100% of MVIC.11,13,85 Therefore, it might be that a mini-
mum intensity of ~70% MVIC is required to induce meaning-
ful changes in tendon thickness and stiffness.
While only a single study has examined the effect of iso-
metric training at different muscle lengths on tendon adapta-
tion,31 the results tend to support a paradigm of LML training
being superior to SML training. Kubo et al31 trained the knee
extensors at either 50° or 90° of flexion and observed a sig-
nificantly greater increase in tendon stiffness (P = 0.021) fol-
lowing LML (50.9%, ES = 1.22, P = 0.014), when compared
to SML training (6.7%, ES = 0.26, P = 0.181). Similarly, dis-
tal tendon and deep aponeurosis elongation decreased follow-
ing LML training (−14%, ES = 0.62, P = 0.034), whereas the
SML group experienced a trivial increase (3.9%, ES = 0.15,
P > 0.05). When comparing isometric contraction duration
and tendon adaptations, only a single study exists.13 While
both long (57.3%, ES = 1.38, P = 0.003) and short (17.5%,
ES = 0.57, P = 0.217) contraction durations increased tendon
stiffness, a significant between‐group difference was reported
(P = 0.045).13 Additionally, no significant differences in ten-
don elongation were present in either long (−2.2%, ES = 0.19,
P > 0.05)‐ or short (4.1%, ES = 0.29, P > 0.05)‐contraction‐
duration groups. Similarly, calculated elastic energy absorp-
tion increased in both long (12%, ES = 0.58, P = 0.007)‐ and
FIGURE 4 Isometric training intensity and force production
(N = 3)

|
13
ORANCHUK et Al.
short (25.7%, ES = 1.85, P = 0.002)‐duration groups with
no significant difference between groups (P = 0.056) despite
large differences in percent change and effect sizes along with
a relatively low P‐value. While the total time‐under‐tension
was equalized between groups, the one‐second duration of the
short contraction group meant that a larger relative propor-
tion of each effort would be spent building isometric force.
Therefore, the maximal‐force time‐under‐tension was not
equalized.13 Similar to muscle tissue, tendon adaptations are
responsive to chronic changes in total mechanical load3,86,87;
therefore, the potentially greater load in the long contraction
group could explain the discrepancy in tendinous adaptations.
Massey et al67 were the only researchers comparing con-
traction intent on morphological tendon adaptations. Both
“maximal strength training” and “explosive strength train-
ing” produced significant improvements in vastus lateralis
aponeurosis area (5.9%, ES = 0.34% vs 4.4%, ES = 0.38),
Young’s modulus (14.4%, ES = 0.60% vs 21.1%, ES = 1.13),
and tendon stiffness (14.3%, ES = 0.79% vs 19.9%,
ES = 0.95).67 However, only the “explosive strength training”
group experienced significant increases in tendon‐aponeuro-
sis complex elongation (16%, ES = 1.0 vs −2.96, ES = 0.10)
and decreased tendon CSA (−2.8%, ES = 0.31% vs 0.41%,
ES = 0.03), tendon elongation (−11%, ES = 0.75% vs
−4.95%, ES = 0.27), and tendon strain (−11.8%, ES = 0.56
vs −4.17, ES = 0.19).67 Therefore, intent and rate of contrac-
tion appear to be an important training consideration. Lastly,
Waugh et al68 compared load‐equated isometric plantar flex-
ions with intra‐contraction rest periods of 3 or 10 seconds.
While there were differences (P > 0.05) in type I and type II
collagen (factors in fiber reorganization),88,89 there were no
between‐group discrepancies (P > 0.05) in any other depen-
dent variables following the 14‐week intervention.68 These
data support a paradigm of a threshold intensity for mechani-
cal loading to achieve tendon adaptations.86,87
4.2
|
Neurological adaptations
Of the 23 studies included in this review, 12 directly measured
neural function.13,30-32,37,38,43,47,48,65,66,68 Of these 12 studies, it
is notable that one did not report any neurological data in their
results,68 while two reported no significant changes follow-
ing training, regardless of the condition.13,65 When examining
electromyography (EMG) amplitude assessed through EMG,
a clear trend existed between the studies comparing isomet-
ric training at different muscle lengths. Electromyographic
amplitude tends to increase by larger magnitudes and over
a larger range of joint angles following LML training, com-
pared to training at SML. For example, Bandy and Hanten38
examined isometric knee extension training at SML (30°),
medium muscle length (MML; 60°), and LML (90°), assess-
ing EMG amplitude at seven joint angles from 15° to 105°
of flexion. Medium‐to‐large (ES = 0.74‐2.28) improvements
at six joint angles were observed following LML training,
whereas MML and SML training only improved EMG activ-
ity at five (ES = 0.36‐2.26), and four (ES = 0.87‐1.65) of the
assessed joint angles, respectively.38 Similarly, Kubo et al31
observed larger increases in EMG activity at all measured
angles following LML (7%‐8.8%, ES = 0.45‐0.72) compared
to SML (3.1%‐7.5%, ES = 0.25‐0.44) training. Conversely,
Alegre et al30 reported an increase in EMG amplitude in
favor of the SML training group, the only investigation to
do so. Although the magnitude of increases in EMG ampli-
tude was medium‐large, the changes were limited to 50‐60°
(ES = 0.77, P = 0.205) and 60‐70° (ES = 1.0, P = 0.36) of
knee flexion during isokinetic knee extensions.30 These find-
ings are consistent with the findings of other investigations in
that alterations in EMG amplitude are most specific at shorter
muscle lengths.37,71,72
All four studies comparing the effects of isometric train-
ing with different contraction intents (ballistic vs ramp) as-
sessed neurological and neuromuscular adaptations via EMG
and peripheral nerve stimulation interpolated twitch.43,47,48,66
As expected, adaptations were specific to the intent utilized
in training. For example, Balshaw et al43 examined the effects
of 12 weeks of “maximal strength training” (1‐second build
to ~75% of MVIC and maintain for 3 seconds), with “explo-
sive strength training” (rapid build to ≥90% of MVIC and
maintain for 1 second). The improvements in EMG ampli-
tude at MVIC were larger (ES = 0.36, P = 0.370) following
“maximal strength training” (27.8%, ES = 0.67, P < 0.001)
compared to “explosive strength training” (19.1%, ES = 0.44,
P = 0.099). Conversely, “explosive strength training”
(31.3%, ES = 0.67, P = 0.003) increased EMG activity to a
greater (P < 0.001) degree during the 0‐ to 100‐ms and 0‐
to 150‐ms period of muscle contraction compared to “max-
imal strength training” (14.3%, ES = 0.36, P = 0.009).43
Additionally, only the rapid contraction group significantly
increased EMG amplitude in the first 100 ms of muscle con-
traction (12.5%, ES = 0.26, P = 0.048).43 Similarly, previous
investigations examining contraction intent found greater
improvements in EMG amplitude during MVIC with MST
(1.28%‐7%/week, ES = 0.06‐0.33/week) when compared
to EST (0.68%‐1.31%/week, ES = 0.18‐0.25/week).47,48,66
Furthermore, participants training with a ballistic intent
(1.04%‐10.5%/week, ES = 0.26‐0.31/week) achieved greater
improvement in EMG amplitude during the initial 150 ms of
maximal contraction when compared to MST (2.93%‐5.53%/
week, ES = 0.03‐0.07/week).43,47,48,66 These findings support
the principle of training specificity as only the groups who
intended to produce force quickly improved in that regard.
4.3
|
Performance enhancement
Isometric training is commonly prescribed in rehabilitation
settings, or early in physical preparation plans as a means

14
|
ORANCHUK et Al.
to increase neuromuscular, musculo‐skeletal, and proprio-
ceptive function. It is thought that the aforementioned im-
provements will later transfer to dynamic performance once
specific movement patterns are integrated into the physi-
cal preparation plan. Despite existing literature reporting
benefits of isometric training on multi‐joint dynamic per-
formance,11,85,90 none of the studies included in the current
review included dynamic multi‐joint assessments.
4.3.1
|
Isometric peak force
Only four studies included in the present review directly com-
pared MVIC production between group training at different
intensities.44-46,65 Isometric peak force is considered a highly
reliable measure, with a growing body of research reporting
the validity of isometric assessments for assessing health and
athletic performance.28,91 While training specificity is a major
factor in performance improvements, if MVIC force is the de-
sired outcome there does not appear to be a clear advantage to
training at high or low intensities (Figure 4). Szeto et al46 was
the only study that reported statistically significant improve-
ments in MVIC force in some, but not all training groups.
Szeto et al46 had subjects train their knee extensors at 25%,
50%, or 100% of MVIC. Following 15 sessions over three
weeks, the group training at 25% did not experience statisti-
cally significant strength improvements despite medium ef-
fect sizes (22.3%, ES = 0.61, P = 0.085).46 Conversely, large
and statistically significant improvements were observed
when training at 50% (31.3%, ES = 1.14, P = 0.002) and
100% (45.7%, ES = 1.44, P = 0.013) of MVIC.46 However,
time‐under‐tension, not total load, was equalized between
groups, meaning that the 50% training group produced twice
as much total force as the 25% group. While no data about
fatigue are presented, it could be hypothesized that the group
training with maximal effort underwent significantly greater
loading than the other groups.46 Additionally, the inclusion of
a perceived effort or fatigue scale may have been valuable.
A clear pattern can be observed when comparing max-
imal force production following training at different mus-
cle lengths. Despite LML resulting in greater hypertrophic
adaptations, there is no difference in maximal force pro-
duction at the trained joint angle between SML and LML
interventions when analyzing the seven studies that directly
compared joint angles (Data S4).31,32,35,37,38,40,64 However,
transfer to non‐trained joint angles is much lower following
SML training. For example, Bandy and Hanten,38 Bogdanis
et al,64 Kubo et al,31 and Thepaut‐Mathieu, van Hoecke,
and Maton37 all trained participants at different muscle
lengths and measured MVIC at numerous joint angles pre‐
and post‐training. Bandy and Hanten38 observed signifi-
cant (P < 0.05) improvements at four, five, and seven of
the tested joint angles following SML, MML, and LML,
respectively. Bogdanis et al64 reported increased MVIC at
two of the assessed joint angles following SML training
(22%‐57.4%, ES = 0.88‐2.41), while the LML group im-
proved in all six angles (~12.3%). Similarly, the SML group
in Kubo et al’s31 investigation significantly (P < 0.05) im-
proved MVIC at five angles, while the LML group expe-
rienced significantly improved force production at eight
of the tested angles. Interestingly, Thepaut‐Mathieu, Van
Hoecke, and Maton37 found that their LML group signifi-
cantly (P < 0.05) improved at four angles, compared to two
and five angles in the SML and the MML group, respec-
tively. These data suggest that LML and MML isometric
resistance training is superior to SMLs when the aim is to
improve force throughout a range of motion.
4.3.2
|
Length‐tension
The length‐tension relationship, typically assessed by isomet-
ric or isokinetic contractions, is defined as the muscle length
or joint angle at which peak force/torque is produced.92 Many
studies have demonstrated acute optimal angle/length shifts
toward longer muscle lengths following concentric, isomet-
ric, and eccentric exercise.73,93-98 Additionally, eccentric
resistance training and training over a larger range of mo-
tion are well established for increasing the optimal angle
long‐term.70,95 It is plausible that the same relationship exists
between muscle length and a shift in the optimal angle fol-
lowing isometric contractions. However, only a single study
included in this review reported the angle of peak isokinetic
torque,30 while another examined optimal angle through
an isometric leg press.64 Alegre et al30 observed a shift of
11° (14.6%, ES = 1.1, P = 0.002) toward longer muscle
lengths following eight weeks of training at LML, whereas
the SML group experienced a shift of 5.3° (7.3%, ES = 0.91,
P = 0.039) in the opposite direction. Likewise, Bogdanis
et al64 reported a decrease in optimal angle following SML
training (−9.7%, ES = 1.77) while the optimal angle was
maintained in the LML group. While length‐tension curve
shifted toward the angle of training in several other studies,
none were significant or altered the angle at which maximal
isometric force was produced.30 While a very limited sample,
the report of Alegre et al30 is unsurprising given that isometric
exercise at LMLs is preferable to SMLs for acutely altering
the length‐tension relationship.99 Finally, it should be noted
that no included study reported any significant differences in
isometric or isokinetic length‐tension curves between group
training with different intensities, contraction intents, or any
other independent variable.
4.3.3
|
The rate of force development
The rate of force development (RFD) is an important meas-
urement in sports performance, as force application in many
activities occurs over short time periods.14,100-102 Therefore,

|
15
ORANCHUK et Al.
while peak force is a valid and highly reliable means of
broadly monitoring neuromuscular function, rapid force pro-
duction characteristics are equally valuable and more specific
to the execution of explosive tasks.2,100-103 Unfortunately, only
three training studies examining different contraction intents
reported RFD variables.43,47,48 Regardless, all three studies
reported that isometric training with an “explosive” or “ballis-
tic” intent was superior to ramping contractions for improving
rapid force production.43,47,66 These findings align with the
previously discussed alterations in EMG amplitude between
contraction intents. For example, Williams66 compared the ad-
aptations following ballistic or ramp isometric training. While
the ramp group experienced larger improvements in MVIC
(ramp, 17.8%‐20%, ES = 1.56‐1.95, P = 0.0008 vs ballistic,
15.7%‐18.9%, ES = 0.75‐0.88, P = 0.0036), only the ballis-
tic training group significantly improved voluntary activation
(31.6%, ES = 1.84, P = 0.0096) and force at 150 ms (48.8%,
ES = 1.29, P = 0.0074).66 Similar findings are reported by
Balshaw et al43 and Tillin and Folland47 where only the bal-
listic training groups significantly (P < 0.05) improved force
at 50 and 100 ms (Table 3). These findings are not surpris-
ing, as several researchers have reported increased rapid force
and power production, driven heavily by neurological altera-
tions.104-106 Additionally, there is evidence to suggest that the
intent of movement may be of similar value to actual external
contraction velocity when improving RFD characteristics.107
4.3.4
|
Dynamic performance
The transferability of isometric resistance training to dy-
namic performance is questionable, despite specific iso-
metric assessments closely relating to sports performance.91
Likewise, the degree of transference of isokinetic contraction
to real‐world movements has yet to be elucidated fully.24,26,27
Regardless, isokinetic testing provides a valuable means of
assessing dynamic performance. Five studies utilized isoki-
netic assessments with three comparing various trained
joint angles30,40,48 and two studies comparing contraction
intent48 or length of contraction, respectively.34 Maffiuletti
and Martin48 reported similar improvements in eccentric
torque at 60° s−1 and concentric torque at slow (60° s−1) and
faster (120° s−1) angular velocities regardless of contraction
intent. When comparing isometric training at different mus-
cle lengths, Alegre et al30 and Noorkoiv et al33 observed sig-
nificant (P < 0.05) improvements after training at LML, but
not SML in concentric torque at 60 and 30° s−1, 60, 90, and
120° s−1, respectively, despite no significant differences in
MVIC improvements between groups. Conversely, Lindh40
reported that neither SML nor LML training groups improved
isokinetic torque at 180° s−1 while both groups significantly
(P < 0.01) improved peak torque at 30° s−1. Finally, Bogdanis
et al64 observed similar improvements in one repetition max-
imum squat (9.6%, ES = 0.61% vs 11.9%, ES = 0.64) and
countermovement jump height (7.2%, ES = 0.66% vs 8.4%,
ES = 0.51) following SML and LML leg press training, re-
spectively. One possible explanation for these findings is that
the LML training groups in Alegree et al30 and Noorkoiv et
al33 experienced larger hypertrophic adaptations than the cor-
responding SML participants. Unfortunately, neither Lindh40
nor Bogdanis et al64 assessed morphological adaptations,
making further analysis difficult.
4.4
|
Applications
While the direct transfer of isometric resistance training to dy-
namic movements is questionable, physiological adaptations
such as increased muscle mass and improved tendon quali-
ties are beneficial in a variety of contexts. There is a well‐
established relationship between muscle mass, strength, and
functional performance in a variety of activities and popula-
tions.108-110 While it may require specific training in a move-
ment to optimize neuromuscular performance,71,111 it is clear
that producing and maintaining muscle mass and strength
should be a priority for athletes and special populations alike.
For this reason, isometric contractions are regularly used in
rehabilitation programs and during specific training phases
where dynamic contractions may be contraindicated.
The long‐held belief that isometric resistance training
should occur at the most important angle present in a dy-
namic activity holds true112-115 as the largest improvements in
neuromuscular function occur at the trained angle.31,32,37,38,40
However, large neurological discrepancies exist between
isometric and dynamic movements25 suggesting that static
training may not be an effective strategy for directly improv-
ing sports performance and should be primarily employed to
alter morphology. Therefore, isometric training should occur
predominantly at relatively LMLs as there is a clear advan-
tage for improving muscle volumes (Figure 2) and strength
throughout a range of motion.30-33,37,38 Additionally, large
increases in tendon stiffness following LML have been re-
ported, which would likely reduce electromechanical delay
and therefore improve RFD.5,31,116 Furthermore, LML iso-
metric training may have beneficial effects on the length‐ten-
sion relationship,30 although greater evidence is needed to
solidify optimal angle as a key variable in performance and
injury prevention.92 Similarly, architectural qualities of mus-
cle may underpin the length‐tension relationships. However,
Alegre et al30 observed no significant (P > 0.05) shift in fas-
cicle length regardless of training angle, while Noorkoiv et
al32 reported conflicting findings depending on which quad-
riceps head was evaluated. Therefore, isometric resistance
training, regardless of muscle length, appears unlikely to ef-
ficiently lengthen muscle fascicles.
Training intensity is a key variable prescribed in intelligently
designed resistance training programs. Evidence suggests that
high‐intensity resistance training is superior for improving force

16
|
ORANCHUK et Al.
production.45,76,117 However, the studies cited in this review show
a questionable relationship between intensity and force produc-
tion adaptations (Figure 4).13,30-32,34,43,44,46,65 Consistent with
recent original research and meta‐analyses, isometric training
intensity does not appear to affect hypertrophic adaptations.76,77
While the lack of relationship between contraction intensity and
force production is somewhat surprising, previous literature
has reported that submaximal intensities can produce similar
strength improvements when taken to failure, or when the vol-
ume is equated between groups.77,118 These findings suggest that
isometric training intensity is not important when aiming to im-
prove force production or alter muscle morphology. Therefore,
increasing contraction durations,34 increasing total volume, or
shifting to longer muscle lengths30-32,38,40 is likely more efficient
means of progressing isometric resistance training if strength
and muscle size are a priority. Conversely, high‐intensity (≥
70% of MVIC) isometric contraction exclusively produced in-
creased tendon thickness and stiffness.41,42 As overly compliant
tendons are often an issue in untrained and injured populations,
progressively increasing intensity during isometric contractions
may be a safe and efficient means of preparing tendinous tissue
for future dynamic loading.12,82 Additionally, sports requiring a
high degree of reactive strength require relatively stiff tendinous
structures to optimize performance.90,119,120
Isometric training, like other modes of resistance exer-
cise, should be executed in a way that most closely relates
to the primary outcome goal. When muscular hypertrophy
or maximal force production is the priority, the evidence
demonstrates that there is little difference between contrac-
tions completed with a ballistic or a gradual ramp to the pre-
scribed intensity.43,47,48,66 However, if rapid force production
takes precedence, as it would in several sports, then isometric
contractions should be performed as such.43,47,66 Conversely,
ballistic contractions may be contraindicated or cause exces-
sive pain in rehabilitative or special populations,20 despite
potential to provide unique morphological tendon adapta-
tions.67 Therefore, while ballistic contractions offer unique
neuromuscular benefits, sustained contractions generally
offer similar or greater morphological adaptations that are
likely of interest to a wider variety of trainee.43,48,66
4.5
|
Limitations and directions for
future research
While trends, or lack thereof, are evident in many of the key
independent variables discussed in the current review, sev-
eral limitations exist. While the widely homogeneous pop-
ulations inter‐ and intra‐study allowed for simple analysis,
none of the included studies utilized special populations such
as patients with tendon disorders, high‐performance athletes,
or experienced resistance trainees. Researchers and practi-
tioners alike need to be cognizant of this limitation if wishing
to generalize findings. Similarly, very few of the included
studies examined the effect of isometric training on dynamic
performance, and only one utilized closed‐chain or functional
performance tasks in their testing batteries. Finally, while 26
studies were included, the large variety of independent and
dependent variables made extensive inter‐study analysis dif-
ficult and hence definitive conclusions problematic.
While the limitations present are broad, several directions
for interesting future research exist. Isometric resistance train-
ing is often utilized by strength and conditioning coaches early
in a training plan with the intent of preparing muscle and ten-
don morphologies for future dynamic loading. However, to
the authors’ knowledge, no published studies have examined
the effect of a proceeding isometric training phase on dynamic
or ballistic training periods despite a rise in popularity with
this approach.14 On a related note, a limited number of studies
have examined isometric training with free‐weights. Isometric
contraction intensity does not play a large role in driving mor-
phological or neuromuscular adaptations, and total volume is
likely a more important variable. However, resistance training
modes have specific load cutoff points for altering tissue or
neural properties.1,10 As such, future studies should aim to es-
tablish approximate weekly loading guidelines for a variety of
populations, muscle groups, and dependent variables. Another
interesting direction is determining whether isometric train-
ing can improve dynamic muscular endurance. Unfortunately,
only a single included study evaluated fatigue,65 and no stud-
ies examined fatigue during dynamic or stretch‐shortening
cycle activities such as cycling or running.
Another avenue for research geared toward rehabilitative
populations is a multivariate examination of contraction in-
tensity and joint angles. Physical therapists often prescribe
isometric training as a means to stimulate morphological ad-
aptations and improve neuromuscular function while tightly
maintaining a pain‐free range of motion. Anecdotally, ther-
apists often limit isometric contractions to moderate joint
angles as the increased ligament strain and pressure synon-
ymous with maximal contraction intensities at large degrees
of joint flexion may cause unwanted pain and inhibition.15,16
However, training at LML is superior to SML training for
producing morphological and neuromuscular adaptations.
Therefore, it would be fascinating to compare the effects of
submaximal isometric training at LMLs with maximal iso-
metric training at SMLs. As previously mentioned, the body
of literature examining the characteristics of “pushing,”
“holding,” and “quasi” isometric actions is growing.54-60,78
However, there is a paucity of long‐term experimental studies
examining these isometric contraction subsets.
5
|
PERSPECTIVES
Despite a relatively limited quantity of studies to base con-
clusions upon, specificity of training applies to isometric

|
17
ORANCHUK et Al.
resistance training as it does to traditional dynamic resistance
training. Therefore, isometric training should be prescribed
in line with the primary outcome goals. Training at LML and
with sustained contractions has been found to be beneficial
for improving muscle morphology, while high‐intensity con-
tractions (>70% MVIC) are likely required to substantially
improve tendon structure and function (eg, tendon stiffness).
Similarly, ballistic intent has been found to improve rapid
force production even though movement velocity is zero.
Finally, a greater number of studies, with a broader applica-
tion of isometric training variations, are needed to determine
optimal applications for altering the morphology and improv-
ing dynamic performance in athletic, rehabilitative, and spe-
cial populations alike.
ACKNOWLEDGEMENTS
Dustin J. Oranchuk, Adam G. Storey, André R. Nelson,
and John B. Cronin declare that they have no conflicts of
interest relevant to the content of this review. No funding
was received for this review that may have affected study
design, data collection, analysis or interpretation of data,
writing of this manuscript, or the decision to submit for pub-
lication. Dustin J. Oranchuk was supported by the Auckland
University of Technology’s Vice Chancellors Doctoral
Scholarship.
CONFLICT OF INTEREST
None.
ORCID
Dustin J. Oranchuk https://orcid.
org/0000-0003-4489-9022
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SUPPORTING INFORMATION
Additional supporting information may be found online in
the Supporting Information section at the end of the article.
How to cite this article: Oranchuk DJ, Storey AG,
Nelson AR, Cronin JB. Isometric training and long‐
term adaptations: Effects of muscle length, intensity,
and intent: A systematic review. Scand J Med Sci
Sports. 2019;00:1–20. https://doi.org/10.1111/
sms.13375