Comparing the effects of plyometric and isometric strength training on dynamic and isometric force-time characteristics

Abstract

The purpose of the study was to compare the change in dynamic and isometric force-time characteristics after plyometric (PLYO) or isometric strength training (ISO). Twenty-two endurance runners (age = 37 ± 6 years,stature = 1.71 ± 0.05 m, body mass = 62.7 ± 8.6 kg, weekly mileage = 47.3 ± 10.8 km) performed a countermovement jump (CMJ) and isometric mid-thigh pull (IMTP) test during pre- and post-tests. They were then randomly assigned to either PLYO or ISO group and completed 12 sessions of intervention over six weeks. The PLYO included drop jump, single leg bounding and split jump, and the ISO included IMTP and isometric ankle plantar flexion. Significant and large time x group interactions were observed for CMJ countermovement depth (P = 0.037, ƞ²p = 0.21) and IMTP and relative peak force (PF) (P = 0.030, ƞ²
p = 0.22). Significant and large main effects for time were observed in CMJ height, peak power, propulsive phase duration, countermovement depth, reactive strength index modified, IMTP PF and relative PF (P < 0.05, 0.20 ≤ ƞ²p ≤ 0.65). Effect for time showed small improvement in CMJ height for both PLYO (P < 0.001, d = 0.48) and ISO (P = 0.009,
d = 0.47), small improvement in CMJ PP in PLYO (P = 0.020, d = 0.21), large increase in countermovement depth (P = 0.004, d = 1.02) and IMTP relative PF (P < 0.001, d = 0.87), and moderate increase in propulsive phase duration (P = 0.038, d = 0.65) and IMTP PF (P < 0.001, d = 0.55) in ISO. There were large differences between groups for percentage change in countermovement depth (P = 0.003, d = 0.96) and IMTP relative PF (P = 0.047, d = 0.90). In conclusion, both PLYO and ISO improved CMJ jump height via different mechanisms, while only ISO resulted in improved IMTP PF and relative PF.

Full text

Biology of Sport, Vol. 39 No1, 2022 189
Plyometric vs isometric strength training
INTRODUCTION
Various forms of strength training including free weights, plyomet-
rics(PLYO) and isometric strength training (ISO) have been used with
the purpose of increasing force production to enhance athletic perfor-
mances[1,2]. The force producing capacity of amuscle is inuenced
by the muscle action (i.e. concentric, eccentric, isometric), due to
differences in neural activation[3,4]. Furthermore, each mode of
strength training has been shown to result in different magnitude of
adaptation to muscle hypertrophy, strength and power[1,2]. Although
eccentric strength training has been reported to induce enhanced
hypertrophic response as compared to other modes of strength train-
ing[2], comparison of strength increases remains controversial as
magnitude of adaptation resulting from each different mode of strength
training is dependent on the method of assessment (i.e. eccentric
training will induce greater increment in eccentric strength; concentric
training will induce greater increment in concentric strength; and ISO
Comparing the effects of plyometric and isometric strength
training on dynamic and isometric force-time characteristics
AUTHORS: Danny Lum1,2, Paul Comfort3,4, Tiago M. Barbosa2,5,6, Govindasamy Balasekaran2
1 Sport Science and Sport Medicine, Singapore Sport Institute, Singapore, Singapore
2 Physical Education and Sports Science, National Institute of Education, Nanyang Technological University,
Singapore, Singapore
3DirectorateofSport,Exercise,andPhysiotherapy,UniversityofSalford,Salford,GreaterManchesterM66PU,UK
4 Centre for Exercise and Sports Science Research, Edith Cowan University, Joondalup, WA 6027, Australia
5 Polytechnic Institute of Braganca, Braganca, Portugal
6 Research Centre in Sports, Health and Human Development (CIDESD), Vila Real, Portugal
ABSTRACT: The purpose of the study was to compare the change in dynamic and isometric force-time
characteristics after plyometric (PLYO) or isometric strength training (ISO). Twenty-two endurance runners
(age=37±6years,stature=1.71±0.05m,bodymass=62.7±8.6kg,weeklymileage=47.3±10.8km)
performedacountermovementjump(CMJ)andisometricmid-thighpull(IMTP)testduringpre-andpost-tests.
TheywerethenrandomlyassignedtoeitherPLYOorISOgroupandcompleted12sessionsofinterventionover
sixweeks.The PLYOincludeddropjump,singlelegboundingandsplitjump,andtheISOincludedIMTPand
isometric ankleplantarflexion.Significantandlargetimexgroupinteractionswereobserved forCMJ
countermovement depth (
P
=0.037,ƞ²p=0.21)andIMTPandrelativepeakforce(PF)(
P
=0.030,ƞ²p=0.22).
SignicantandlargemaineffectsfortimewereobservedinCMJheight,peakpower,propulsivephaseduration,
countermovementdepth,reactivestrengthindexmodied,IMTPPFandrelativePF(
P
<0.05,0.20≤ƞ²p≤0.65).
EffectfortimeshowedsmallimprovementinCMJheightforbothPLYO(
P
<0.001,
d
=0.48)andISO(
P
=0.009,
d
=0.47),smallimprovementinCMJPPinPLYO(
P
=0.020,
d
=0.21),largeincreaseincountermovement
depth (
P
=0.004,
d
=1.02)andIMTPrelativePF(
P
<0.001,
d
=0.87),andmoderateincreaseinpropulsive
phase duration (
P
=0.038,
d
=0.65)andIMTPPF(
P
<0.001,
d
=0.55)inISO.Therewerelargedifferences
betweengroupsforpercentagechangeincountermovementdepth(
P
=0.003,
d
=0.96)andIMTPrelative
PF(
P
=0.047,
d
=0.90).Inconclusion,bothPLYOandISOimprovedCMJjumpheightviadifferentmechanisms,
whileonlyISOresultedinimprovedIMTPPFandrelativePF.
CITATION: LumD,Comfort P,Barbosa TM,BalasekaranG.Comparing the effectsofplyometric and isometric
strengthtrainingondynamicandisometricforce-timecharacteristics.BiolSport.2022;39(1):189–197.
Received:2020-11-28;Reviewed:2021-01-09;Re-submitted:2021-01-21;Accepted:2021-02-01;Published:2021-03-07.
will induce greater increment in eccentric strength)[1], indicating that
adaptation is specic to the method of training.
Isometric strength training is characterised by the exertion of force
without external movement. Increases in strength associated with
this mode of training are dependent on several factors including the
joint angle at which training occurs, duration, intensity and rate at
which force is developed[1]. Researchers have also shown that ISO
results in the improvement of various sports related movements[5–8].
In addition, the results of arecent study indicated that the inclusion
of ISO to atraditional strength training intervention improved 3rep-
etition maximum squat performance to agreater magnitude than
traditional strength training alone in powerlifters (10.4% vs 3.5%)[9].
Together, the results of the aforementioned studies indicate that ISO
is aviable option to include in athletes’ training regimes to enhance
strength and dynamic performances.
Original Paper
DOI: https://doi.org/10.5114/biolsport.2022.103575
Key words:
Isometric mid-thigh pull
Countermovement jump
Peak power
Reactive strength index modied
Corresponding author:
Danny Lum
3Stadium Drive
Singapore 397630, Singapore
Tel.: +65 97290819
E-mail: dannylum82@gmail.com
ORCID:
Danny Lum
0000-0002-8908-3791
Paul Comfort
0000-0002-1131-8626
Tiago M. Barbosa
0000-0001-7071-2116
Govindasamy Balasekaran
0000-0001-6101-2695

190
Danny Lum et al.
2preliminary tests. This study was part of another study which aimed
to compare the effects of PLYO and ISO on endurance running per-
formance.
Participants
Sixteen male and six female endurance runners (n = 22,
age = 37± 6 years, stature = 1.71± 0.05 m, body
mass=62.7±8.6kg, weekly mileage=47.3±10.8km) were
recruited for participation in this study, with n=11 for each group.
Participants have been running more than 30km per week for the
last six months; and have not sustained any lower limb injury for the
last six months. Eight of the participants were participating in regu-
lar (2–3time per week) resistance training prior to the study while
the rest of the 14participants did not have any prior resistance
training experience. An equal number of participants with prior re-
sistance training experience were assigned to each training group.
The experiments reported in the manuscript were performed in
accordance with the ethical standards of the Helsinki Declaration
and that the participants signed an informed consent form. The study
received ethical approval from the institutional review board of the
local university.
Testing Procedures
Participants were requested to refrain from consuming alcohol and
caffeine, and from participating in intensive training sessions for
24hrs prior to all testing sessions. During the pre- and post-testing
sessions, participants completed the CMJ test and IMTP. All testing
sessions began with 5minutes of moderate intensity jogging on
amotorized treadmill, followed by lower body exercises including
body weight squat, single leg stiff leg deadlift, side lunges and calf
raises. One minute of recovery period was provided prior to com-
mencing the test for that day.
Countermovement Jump Test. The CMJ test was conducted prior
to the IMTP and was performed on dual force plates (Force Decks,
VALD Performance, FD4000, Queensland, Australia) sampling at
1000Hz. Participants were asked to keep their arms akimbo to
eliminate arm swing and maintain their back upright to reduce an-
gular displacement of the hips. Participants performed 3jumps,
separated by 30srest intervals. The commercially available For-
ceDecks software (VALD Performance, ForceDecks, Queensland,
Australia) was used to analyse and generate the CMJ variables using
conventional methods[22]. Participants were asked to stand as still
as possible for>1sprior to the commencement of the countermove-
ment. Take-off was dened as the time point at which the total
vertical force fell below the threshold of 20N[23]. Dependent vari-
ables included; jump height was calculated based on velocity of
centre of mass at take-off, using the impulse momentum relationship,
PF, and peak power (PP), time to take off (TTO), unweighting, brak-
ing and propulsion phase duration, countermovement depth and
reactive strength index modied (RSImod) obtained from highest
CMJ height were recorded and analysed. The PF and PP were
Plyometric training is another form of training used to enhance
force production characterised by ballistic movements that make use
of the stretch shortening cycle, whereby aconcentric muscle action
is enhanced by prior eccentric muscle action of the muscle, enhanc-
ing force production through both neurological potentiation and stor-
age and release of elastic energy[10–12]. This form of training in-
cludes jumping exercises that involves short (<250ms) (e.g.
hopping) or long (>250ms) (e.g. countermovement jump) ground
contact time[11], and is often included into strength training program
to improve rapid force production[2,10]. Plyometric training has
also been shown to benet various athletic performances[13–18].
To date, only two studies have compared the effects of PLYO and
ISO on neuromuscular adaptations[19,20]. It was reported that
ISO resulted in greater increases in tendon stiffness and isometric
force production, but lower improvements in jump height as compared
to PLYO. In addition, ISO only improved jump height of anon-coun-
termovement jump[19,20]. These ndings are in contrast with
ndings of other studies whereby ISO was shown to improve coun-
termovement jump (CMJ) height[5,7,8]. This may be attributable
to the fact that the ISO exercises used by Kubo etal.[20] was single
joint exercise and executed at submaximal intensity while the exer-
cises used by Bimson etal.[5] was executed at multiple joint angles,
and that used by Lum etal.[7] and Lum and Joseph[8] were multi-
joint exercise executed with maximal effort. In addition, despite show-
ing the difference in the effects of PLYO and ISO on jump perfor-
mances and morphological changes, Burgess etal.[19] and Kubo
etal.[20] did not provide data on the changes in force-time char-
acteristics which can provide practitioners with better understanding
and comparison of the adaptations to the two modes of strength
training. For example, acquiring information about countermovement
depth and time to take off (which include all phases of the movement;
unweighting, braking and propulsion phases) is important in under-
standing how achange in jump height is achieved[21]. Furthermore,
the study conducted by Burgess etal.[19] and Kubo etal.[20] used
participants who were not from athletic population, suggesting that
the results might not be applicable to individuals of higher training
status. In view of the gap in the literature, the purpose of this study
was to compare the change in dynamic and isometric force-time
characteristics after undergoing aperiod of either PLYO or ISO. It
was hypothesized that PLYO and ISO would result in similar improve-
ment in jump performance while ISO would result in greater improve-
ment in isometric strength measures.
MATERIALS AND METHODS
Experimental Procedure
Arandomized control trial research design was selected. Participants
were required to complete one preliminary testing session which
included CMJ and isometric mid-thigh pull (IMTP) test. Subsequent-
ly, participants were randomly assigned to either PLYO or ISO group.
Participants completed 6weeks of intervention training twice per
week. At the end of the intervention, participants repeated the

Biology of Sport, Vol. 39 No1, 2022
191
Plyometric vs isometric strength training
expressed normalized to body mass (e.g., PF / body mass). The
unweighting phase was identied as the onset of movement through
to the point at when negative velocity peaks (when force returns to
body mass). The braking phase was identied at the time between
peak negative velocity and returning to zero velocity (which corre-
sponds to the peak countermovement displacement), and propulsion
phase determined as the period when velocity exceeds 0.01m/s
through to take-off. The RSImod was obtained by dividing CMJ height
by CMJ TTO[24].
The IMTP was performed on the same dual force plates following
the procedure described by Comfort etal.[25]. Participants were
asked to adopt aposture that reected the start of the second pull
of the clean resulting in aknee exion angle of 125–145o and hip
exion angle of 140–150o stance measured using ahandheld goni-
ometer. Participants were required to fully extend the elbows, hold
on to the bar with hands strapped to the bar with lifting straps to
prevent grip from being alimiting factor. Upon the tester’s command,
participants were instructed to pull, by driving their feet into the oor,
‘as hard and fast as possible’. Participants had to maintain the ten-
sion for aperiod of 5s. Participants performed the IMTP twice, if
the PF was within 250Nbetween trials. Each attempt was sepa-
rated by a2min recovery period. The highest force generated during
IMTP was reported as the absolute PF[26]. Relative PF was
calculated by dividing the PF by participant’s body mass. In addition,
force at 100, 150 and 200ms (Force100, Force150 and Force200, re-
spectively) from the onset of pull were determined for each tri-
al[25,27]. The onset of pull was determined using an algorithm-
based analysis program (NMP Technologies LTD., London, UK) that
has been shown to produce high reliability[28].
Training
Participants were instructed to continue with their usual endurance
training but refrain from other forms of lower limb resistance training.
On all training sessions, participants were required to perform either
PLYO or ISO (Table 1) followed by 20min of treadmill running at
individual marathon pace.
Participants commence each session with 15min of warm up
including, jogging, lunges, squats and submaximal vertical jumps.
For PLYO, participants were instructed to jump to maximum height
for drop jump and split jump, and maximum distance for single
leg bounding, during each repetition. Participants were also in-
structed to minimise ground contact time for drop jump and single
leg bounding. For ISO, participants were instructed to exert maxi-
mum force as fast as possible and hold each repetition for 3sdu-
ration[7]. The IMTP was performed in the same position as during
the test. While during the isometric ankle plantar flexion,
TABLE 1. Plyometric and isometric strength training program.
Week PLYO ISO
Exercise xSets* xRepetitions Exercise xSets# xRepetitions#
1
40cm drop jump x3x5
Single leg bounding x3x5/side
Split Jump x3x5/side
Isometric ankle plantar exion x3x3
IMTP x3x3
2
40cm drop jump x4x5
Single leg bounding x4x5/side
Split Jump x4x5/side
Isometric ankle plantar exion x3x4
IMTP x3x4
3
50cm drop jump x4x5
† Single leg bounding x4x5/side
†Split Jump x4x5/side
Isometric ankle plantar exion x3x5
IMTP x3x5
4
50cm drop jump x4x5
† Single leg bounding x4x5/side
†Split Jump x4x5/side
Isometric ankle plantar exion x4x5
IMTP x4x5
5
60cm drop jump x4x5
†† Single leg bounding x4x5/side
††Split Jump x4x5/side
Isometric ankle plantar exion x4x5
IMTP x4x5
6
60cm drop jump x2x5
†† Single leg bounding x2x5/side
††Split Jump x2x5/side
Isometric ankle plantar exion x2x5
IMTP x2x5
Note: * Rest (passive) intervals between sets for Ply were 3minutes. # Rest (passive) intervals between sets and repetitions for Iso
were 3minutes and 2s, respectively. † Subjects held aweight plate on each hand that adds up to 5% of their body weight. ††
Subjects held aweight plate on each hand that adds up to 10% of their body weight.

192
Danny Lum et al.
RESULTS
The ICC and%CV data for all measured variables showed high repeat-
ability (Table 2). Test-retest data indicated ICC between 0.89–1.00
and %CV between 0.54–9.87 for all CMJ measures, and ICC between
0.94–1.00 and%CV between 1.51–6.47 for all IMTP measures.
Pre- and post-test results for all CMJ and IMTP measures are
displayed in Table 3and Table 4, respectively. Large time xgroup
interactions were observed in countermovement depth (P=0.037,
ƞ²p=0.21), IMTP PF (P=0.071, ƞ²p=0.22) and IMTP relative
PF (P=0.030, ƞ²p=0.22). Non-signicant yet moderate time
xgroup interactions were observed in CMJ PF, unweighting phase
duration, RSImod and Force150 (P>0.05, 0.03≤ƞ²p≤0.1).
Signicant large main effects for time were observed in CMJ height
(P<0.001, ƞ²p=0.65), CMJ PP (P=0.032, ƞ²p=0.21), propul-
sion phase duration (P=0.021, ƞ²p=0.24), countermovement
depth (P=0.014, ƞ²p=0.288), RSImod (P=0.022, ƞ²p=0.23),
IMTP PF (P<0.001, ƞ²p =0.53) and relative PF (P<0.001,
ƞ²p=0.53). While non-signicant but large effects were observed
for Force100 (P=0.073, ƞ²p=0.15) and Force200 (P=0.046,
ƞ²p=0.18). Non-signicant, but moderate main effect for time was
observed in CMJ PF (P=0.244, ƞ²p=0.07). The effect for time
showed signicant and small improvements in CMJ height for both
PLYO (P<0.001, d=0.48) and ISO (P=0.009, d=0.47). While
asignicant and small improvement in CMJ PP was observed in
PLYO only (P=0.018, d=0.31).
However, only ISO resulted in
asignicant and large increase in countermovement depth (P=0.004,
participants stood upright where the hips and knees were fully
extended, and ankle in 0o plantar exion. Abar was placed on the
shoulder and xed in position. Participants were required to max-
imally plantar ex the ankles while maintaining the extended hip
and knee positions.
Statistical Analyses
All tested variables are expressed by Mean (±1SD) and 95% of
condence intervals. Within session test-retest reliability was assessed
using two-way mixed intraclass correlation coefcients (ICC) and
coefcient of variation (%CV) for all measured variables. ICC values
were deemed as poor if ICC<0.50; moderate 0.50–0.74; good if
0.75–0.90; and excellent if ICC>0.90[29]. Acceptable within-
session variability was classied as<10%[30]. Mixed ANOVAs
(between- xwithin-participant analysis; 2training groups x2testing
times; P≤0.05) was performed for each variable. Effect size was
computed by partial eta-squared (ƞ²p) and deemed: without effect
if 0<ƞ²p≤ 0.01; small if 0.01<ƞ²p≤0.06; moderate if
0.06<ƞ²p≤0.14 and; large if ƞ²p>0.14[31]. All assumptions
to run ANOVAs have been checked beforehand, including normality
and sphericity. Degrees of freedom were corrected whenever spheric-
ity’s assumption was violated. Paired T-test was used to determine
if there was any change in test measures within group. Cohen’s dwas
calculated as standardized effect size for mean comparisons and
deemed: (i) trivial if d<0.20; (ii) small d0.20–0.49; (iii) moderate
if d0.50–0.80; and (iv) large if d>0.80[31].
TABLE 2. Reliability analysis of all measured variables.
ICC 95%CI %CV 95%CI
CMJ Height (cm) 1.00 0.99–1.00 1.20 1.00–1.50
CMJ PF (N ·kg-1) 0.98 0.96–0.99 2.83 2.32–3.65
CMJ PP (W ·kg-1) 1.00 0.99–1.00 1.65 1.36–2.13
CMJ TTO (s) 0.92 0.82–0.96 4.66 3.82–6.03
Unweighting Phase (s) 0.91 0.81–0.96 9.87 7.72–12.01
Braking Phase (s) 0.89 0.77–0.95 9.74 7.69–11.80
Propulsion Phase (s) 0.89 0.78–0.95 4.50 3.61–6.22
Countermovement Depth (cm) 0.93 0.85–0.96 3.48 2.77–4.75
RSImod (m ·s-1) 0.97 0.94–0.99 4.03 3.30–5.21
IMTP PF (N) 1.00 0.99–1.00 1.25 1.03–1.61
IMTP Relative PF (N ·kg-1) 1.00 0.99–1.00 1.25 1.03–1.61
Force100 (N) 0.97 0.94–0.99 5.36 4.28–7.29
Force150 (N) 0.98 0.95–0.99 4.71 3.76–6.39
Force200 (N) 0.98 0.95–0.99 4.58 3.66–6.22
Note: ICC=intraclass correlation coefcient, CI=condence interval, CV=coefcient of variation, CMJ=countermovement jump,
PF= peak force, PP=peak power, TTO=time to take off, Depth= countermovement depth, RSImod=reactive strength index
modied, IMTP=isometric mid-thigh pull, Force100=force at 100ms, Force150=force at 150ms, Force200=force at 200ms.

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TABLE 3. Analyses of countermovement jump measures.
CMJ
Height
(cm)
CMJ PF
(N · kg-1)
CMJ PP
(W ·kg-1)
CMJ TTO
(s)
Unweight-
ing Phase
(s)
Braking
Phase
(s)
Propulsion
Phase
(s)
Counter-
movement
Depth (cm)
RSImod
(m ·s-1)
PLYO
Pre 28.6 (6.3) 23.1 (1.8) 45.5 (7.2) 0.730
(0.067)
0.157
(0.046)
0.322
(0.043)
0.256
(0.028) 27.5 (5.8) 0.39 (0.08)
Post 31.5 (5.9) 23.2 (2.7) 47.7 (6.9) 0.737
(0.108)
0.139
(0.028)
0.329
(0.066)
0.269
(0.043) 27.9 (7.1) 0.44 (0.11)
(95%
CI) (2.0; 3.9) (-1.3; 1.1) (-4.0; -0.5) (-0.06; 0.07) (-0.052;
0.016)
(0.052;
-0.067)
(-0.010;
0.036) (-2.6; 3.5) (< -0.01;
0.09)
P < 0.001 0.822 0.018 0.922 0.259 0.790 0.207 0.759 0.053
d 0.48 0.04 0.31 0.11 0.47 0.13 0.36 0.06 0.52
ISO
Pre 28.6 (4.7) 23.9 (3.6) 44.4 (5.6) 0.754
(0.158)
0.137
(0.032)
0.352
(0.114)
0.266
(0.045) 27.1 (4.3) 0.39 (0.11)
Post 31.1 (5.8) 22.7 (2.2) 46.5 (8.8) 0.767
(0.117)
0.139
(0.038)
0.344
(0.088)
0.284
(0.039) 31.7 (4.7) 0.42 (0.12)
(95%
CI) (0.8; 4.3) (-0.4; 2.9) (-6.0; 1.7) (-0.05;
0.07) (-0.02; 0.03) (-0.062;
0.047)
(-0.001;
0.034) (-1.9; 7.3) (-0.02; 0.07)
P0.009 0.127 0.244 0.650 0.812 0.767 0.038 0.004 0.222
d 0.47 -0.4 0.28 0.14 0.06 0.08 0.65 1.02 0.26
Time
xGroup
Interaction
F0.194 2.179 0.002 0.069 1.246 0.166 0.111 5.047 0.597
P0.664 0.155 0.965 0.795 0.277 0.688 0.743 0.037 0.449
η2 p 0.01 0.10 <0.01 <0.01 0.06 0.001 <0.01 0.21 0.03
Time Main
Effect
F37.640 1.444 5.294 0.167 0.693 <0.001 6.263 7.384 6.123
P<0.001 0.244 0.032 0.687 0.415 0.996 0.021 0.014 0.022
η2 p 0.65 0.07 0.21 0.01 0.03 <0.01 0.24 0.28 0.23
Group
Main
Effect
F0.006 0.021 0.140 0.318 0.562 0.554 0.687 0.572 0.024
P0.939 0.886 0.712 0.579 0.462 0.465 0.417 0.459 0.879
η2 p <0.01 <0.01 <0.01 0.02 0.03 0.03 0.03 0.03 <0.01
Note: Δ=average change, CI=condence interval, CMJ=countermovement jump, PF=peak force, PP=peak power, TTO=time
to take off, Depth=countermovement depth, RSImod=reactive strength index modied.
TABLE 4. Analyses of isometric mid-thigh pull measures.
IMTP PF
(N)
IMTP Relative PF
(N ·kg-1)
Force100
(N)
Force150
(N)
Force200
(N)
PLYO
Pre 2112.5 (409.6) 32.8 (5.6) 1136.5 (236.5) 1362.0 (305.0) 1586.0 (338.2)
Post 2209.7 (448.7) 34.2 (5.1) 1194.9 (222.6) 1318.6 (448.4) 1697.0 (256.2)
(95% CI) (-35.4; 230.0) (-0.7; 3.4) (-62.9; 179.8) (-294.5; 207.8) (-49.0; 269.2)
P0.133 0.179 0.308 0.709 0.154
d 0.23 0.26 0.25 -0.11 0.37
ISO
Pre 2040.9 (389.9) 33.5 (3.8) 1095.8 (321.7) 1379.5 (408.2) 1582.1(441.5)
Post 2268.4 (440.4) 37.4 (5.1) 1181.4 (344.0) 1485.9 (440.5) 1713.5 (481.5)
(95% CI) (153.3; 301.6) (2.6; 5.3) (-32.9; 203.9) (-51.5; 264.4) (-65.5; 328.3)
P<0.001 <0.001 0.139 0.164 0.168
d 0.55 0.87 0.26 0.25 0.28
Time xGroup
Interaction
F3.642 5.487 0.127 1.266 0.035
P0.071 0.030 0.726 0.274 0.853
η2 p 0.15 0.22 <0.01 0.06 <0.01
Time Main Effect
F22.659 22.696 3.581 0.224 4.517
P<0.001 <0.001 0.073 0.641 0.046
η2 p 0.53 0.53 0.15 0.01 0.18
Group Main
Effect
F0.001 0.890 0.055 0.337 0.001
P0.971 0.357 0.818 0.568 0.971
η2 p <0.01 0.04 <0.01 0.02 <0.01
Note: Δ=average change, CI=condence interval, IMTP=isometric mid-thigh pull, Force100=force at 100 ms, Force150=force
at 150 ms, Force200=force at 200 ms.

194
Danny Lum et al.
Force150 (P=0.333, d=0.42) (Figure 2) were observed, with
ISO showing greater changes. However, non-signicant and small to
moderate differences in favour of PLYO for percentage change in all
CMJ measures except CMJ PP, was observed (P > 0.05,
0.22≤d≤0.58) (Figure 1).
DISCUSSION
This study compared the change in dynamic and isometric force-time
characteristics after undergoing aperiod of PLYO and ISO. Results
showed that both groups improved CMJ height, but only the ISO
group improved IMTP PF and relative PF. In addition, when percent-
age changes in CMJ measures were compared, there were only small
differences between groups except for countermovement depth, where
alarger increase was observed in ISO (ISO: 18.3% vs PLYO: 2.5%).
d=1.02), asignicant and moderate increase in propulsion phase
duration (P=0.038, d=0.65), asignicant and moderate improve-
ment in IMTP PF (P<0.001, d=0.55) and asignicant and large
improvement in IMTP relative PF (P<0.001, d=0.87).
Non-signicant and small group main effects were observed for
CMJ TTO, unweighting, braking and propulsion phase duration,
countermovement depth, IMTP relative PF and Force150 (P>0.05,
0.02≤ƞ²p≤0.04). Asignicant and large differences between groups
was observed for percentage change in countermovement depth
(P=0.003, d=0.96) (Figure 1), and relative PF (P=0.047,
d=0.90) (Figure 2), although anon-signicant yet large difference
in IMTP PF (P=0.061, d=0.84) and non-signicant and small
differences for unweighting phase (P=0.595, d=0.23) and pro-
pulsion phase (P=0.630, d=0.21) durations (Figure 1) and
FIG. 1. Percentage change in CMJ measures. ††Denotes signicant difference from PLYO (P<0.01).

Biology of Sport, Vol. 39 No1, 2022
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Plyometric vs isometric strength training
the CMJ action, which allowed for the improvement of task-specic
motor coordination in addition to muscular strength, hence, the im-
provement in CMJ height observed in ISO in the current study.
Despite the improvement in CMJ height observed in both groups,
there were only trivial to small changes to CMJ PF, PP and TTO.
Although minimal change in CMJ TTO was observed, there were
small reduction in unweighting phase duration and small increase
in propulsion phase duration in PLYO, amoderate increase in propul-
sion phase duration for ISO, with no change in braking phase dura-
tion for both groups. The increased propulsion phase duration would
have resulted in agreater propulsive impulse (force xtime) that re-
sulted in greater jump height in ISO. The lack of change in counter-
movement depth and small reduction in unweighting phase duration
in PLYO, and the lack for change in unweighting phase duration
despite the large increment in countermovement depth in ISO, indi-
cate that agreater unweighting net impulse was produced as com-
pared to pre-intervention. This eventually resulted in participants
producing similarly greater braking impulse as compared to pre-in-
tervention. The minimal change in breaking duration despite the
increased braking impulse indicate that greater rate of eccentric force
was applied. The moderate improvement in RSImod observed in
PLYO indicated that the greater amount of propulsive impulse could
have been partially contributed by the improved utilisation of the
stretch shortening cycle. Conversely, small change in RSImod was
However, when percentage changes in IMTP measures were com-
pared, there were large differences for PF (ISO: 11.1% vs PLYO:
4.8%) and relative PF (ISO: 11.5% vs PLYO: 4.6%). These ndings
show that PLYO and ISO resulted in similar improvement in jump
performance while ISO resulted in greater improvement in isometric
strength measures, thus, supported our hypothesis and the theory
of specicity.
The benet of PLYO on CMJ performance is well document-
ed[10,11,15,20,32]. Conversely, the effects of ISO on CMJ
performance remain controversial as some researchers have report-
ed no improvement[33,34] while others reported improve-
ments[5,7,8] in CMJ height after performing ISO. Furthermore,
previous studies that compared the neuromuscular adaptations be-
tween PLYO and ISO reported that ISO only resulted in improved
jump height of non-countermovement jumps[19,20]. It was sug-
gested that studies that reported no improvement in CMJ performance
after ISO were likely because ISO was performed using single joint
exercise and at single joint position; and ISO was not performed with
rapid and maximal effort[1]. Indeed, studies that have reported
improved CMJ, including the current study, have either performed
ISO at multiple joint positions[5] or performed ISO with multi-joint
exercise and rapid maximal contraction[7,8]. The performance of
ISO with multi-joint exercise and with rapid maximal contraction in
this study probably better mimicked the neuromuscular demands of
FIG. 2. Percentage change in IMTP measures. †Denotes signicant difference from PLYO (P<0.05).

196
Danny Lum et al.
observed in ISO indicating that the increase propulsive impulse was
more likely attributed to increased force production overtime due to
increased muscular strength and countermovement depth[35]. Based
on these ndings, improvement in CMJ height observed in PLYO and
ISO were due to different mechanisms.
Similar to previous studies, the current results showed greater
improvement in isometric PF and relative PF in ISO as compared to
PLYO[19,20]. In fact, it was previously reported that isometric peak
force did not change after undergoing aperiod of PLYO[36]. The
effect of ISO on improving isometric strength is well evident in the
literature, and is attributed to improved motor unit activation, ring
rate and synchronisation, muscle hypertrophy and tendon stiff-
ness[1,19,20,37]. This increased in ability to produce greater
force in the lower limb could be areason for the improved CMJ height
as it was previously reported that individuals were able to jump
higher by improving their muscular strength via strength training[36].
Although the neuromuscular adaptations attributed to the improved
isometric PF and relative PF observed in ISO are also evident in
PLYO[10,19,20,32], the lack of specicity in motor coordination
during training might be areason for the small improvement in IMTP
peak force and relative PF observed.
In contrast with previous studies that reported improved rate of
force development (RFD) after aperiod of ISO and PLYO[7,8,
19,36,38], the current results showed only small improvement
in RFD as reected by the small change in Force100, Force150 and
Force200. The interference effect of concurrent strength and endur-
ance training in this study could be areason for this discrepan-
cy[39,40]. Our participants continued with endurance run train-
ing while undergoing the intervention, but participants in studies
showing improved RFD did not perform concurrent strength and
endurance training[7,8,19,36,38]. Similar to the current nd-
ings, Häkkinen etal.[40] reported that participants who performed
concurrent strength and endurance training did not improve max-
imum RFD despite the improved isometric leg extension PF. As
adaptations to training differ according to specic mode of exercise,
the combined effect of strength and endurance training might have
resulted in certain degree of antagonism, leading to ablunted im-
provement in RFD[40].
Several limitations should be considered when interpreting the
current results. Firstly, the benets of ISO are dependent on the in-
tensity and rate of force developed during each contraction[1,7,38].
Therefore, participants’ compliance to perform each repetition with
maximal effort would greatly affect the magnitude of strength gain.
As force production was not measured during ISO, it was not known
if all participants had complied with the instructions given. Sec-
ondly, the intervention training was performed in concurrent with
endurance training, which might have induced an interference effect
and blunted the adaptations for RFD. Hence, the current results
might not be applicable to non-endurance sports athletes. Thirdly,
there was amixture of strength training experience among participants
in the current study. The results may differ if intervention was per-
formed by amore homogenous group of athletes. Future studies may
attempt to ll in these gaps.
CONCLUSIONS
In conclusion, both ISO and PLYO led to improved CMJ height via
different mechanisms. However, while ISO resulted in improved
maximum force production capability, this improvement was not
observed in PLYO. Finally, RFD was not improved in both training
groups. This was possibly due to interference effect from concurrent
strength and endurance training.
Acknowledgement
The authors declared that they have no conict of interest.
Conict of interest
The authors declared no conict of interest.
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