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EFFECTS OF INERTIAL SETTING ON POWER,FORCE,
WORK,AND ECCENTRIC OVERLOAD DURING FLYWHEEL
RESISTANCE EXERCISE IN WOMEN AND MEN
LUIS M. MARTINEZ-ARANDA
1
AND RODRIGO FERNANDEZ-GONZALO
1,2
1
Muscle and Exercise Physiology Laboratory, Department of Physiology and Pharmacology, Karolinska Institutet, Stockholm,
Sweden; and
2
Radiobiology Unit, Laboratory of Molecular and Cellular Biology, Institute for Environment, Health and Safety,
Belgian Nuclear Research Center, SCKCEN, Mol, Belgium
ABSTRACT
Martinez-Aranda, LM and Fernandez-Gonzalo, R. Effects of
inertial setting on power, force, work and eccentric overload
during flywheel resistance exercise in women and men. J
Strength Cond Res 31(6): 1653–1661, 2017—Exercise load
is a key component in determining end-point adaptations to
resistance exercise. Yet, there is no information regarding the
use of different inertia (i.e., loads) during isoinertial flywheel resis-
tance exercise, a very popular high-intensity training model. Thus,
this study examined power, work, force, and eccentric overload
produced during flywheel resistance exercise with different iner-
tial settings in men and women. Twenty-two women (n=11)
and men (n= 11) performed unilateral (in both legs) isolated
concentric (CON) and coupled CON and eccentric (ECC) exer-
cise in a flywheel knee extension device employing 6 inertias
(0.0125, 0.025, 0.0375, 0.05, 0.075, 0.1 kg$m
22
). Power
decreased as higher inertias were used, with men showing
greater (p#0.05) decrements than women (236 vs. 229%
from lowest to highest inertia). In contrast, work increased as
higher inertias were employed, independent of sex (p#0.05;
;48% from lowest to highest inertia). Women increased CON
and ECC mean force (46–55%, respectively) more (p#0.05)
than men (34–50%, respectively) from the lowest to the highest
inertia evaluated, although the opposite was found for peak force
data (i.e., peak force increased more in men than in women as
inertia was increased). Men, but not women, increased ECC
overload from inertia 0.0125 to 0.0375 kg$m
2
. Although esti-
mated stretch-shorting cycle use during flywheel exercise was
higher (p#0.05) in men (6.6%) than women (4.9%), values
were greater for both sexes when using low-to-medium inertias.
The information gained in this study could help athletes and
sport and health professionals to better understand the impact
of different inertial settings on skeletal muscle responses to
flywheel resistance exercise.
KEY WORDS isoinertial resistance exercise, stretch-shortening
cycle, training optimization
INTRODUCTION
Flywheel iso-inertial resistance exercise (RE) was
first introduced as a countermeasure for the dele-
terious effects of microgravity on skeletal muscle
(4). Nowadays, flywheel RE is a very popular RE
model in elite sports (12,13,40), rehabilitation, and injury
prevention programs (2,15,32). In addition, flywheel RE
has emerged as a novel conditioning routine for recreational
practitioners and the aging population (5). In contrast to tra-
ditional constant-load RE where maximal activation is only
required at the “sticking point” of the concentric (CON)
action (26), the flywheel technology offers accommodated
and unlimited resistance during coupled CON and eccentric
(ECC) muscle actions using the inertia of a rotating flywheel.
Consequently, the loading stimulus has been described as
more optimal during flywheel RE compared with conven-
tional RE (29). This is supported by data showing that force,
power, and increases in muscle mass and neural activation are
typically greater after flywheel RE than after conventional RE
(14,15,21,22,28,29). For example, after 5 weeks of flywheel
RE, muscle volume increased by 6 vs. 3% increment after 5
weeks of traditional weight stack training (27).
The superior adaptations induced by flywheel RE are
explained, at least in part, by the maximal nature of the
stimulus throughout the entire CON action and the possibility
to generate even greater peaks of force during the ECC phase
of the movement (i.e., ECC overload) (39). In addition, the
powerful stretch reflex produced in the ECC–CON transition
during flywheel RE may also play an important role explain-
ing the robust training adaptations induced by this exercise
regimen. Indeed, other training methods, such as plyometric
training, use the energy stored during the ECC phase to
potentiate the performance of a subsequent CON action
(i.e., stretch-shortening cycle; SSC) (41). To date however,
Address correspondence to Dr. Luis M. Martinez-Aranda,
luismanuel6049@gmail.com.
31(6)/1653–1661
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Ó2016 National Strength and Conditioning Association
VOLUME 31 | NUMBER 6 | JUNE 2017 | 1653
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the role of the SSC has not been evaluated in RE using fly-
wheel technology.
Exercise-induced muscle adaptations (e.g., hypertrophy or
power) can to some extent be manipulated toward the desired
outcome by modifying exercise execution (i.e., speed) and/or
load (16,24), at least in exercise routines not calling for mus-
cular failure (25). In supporting this concept, earlier studies
reported that high-load, low-speed RE boosted hypertrophic
adaptations, whereas low-load, high-speed RE was found to
be a better stimulus for power gains (3,33). In the particular
case of flywheel RE, however, all repetitions should be per-
formed at maximal intensity, which translates into maximal
possible speed during exercise execution. Yet, there is no infor-
mation regarding the impact of altering the inertia (i.e., load-
ing stimulus) in the adaptive response to flywheel RE.
Studies using flywheel RE have typically employed
flywheels with inertias ranging from 0.11 kg$m
2
(22) to
0.036 kg$m
2
(15). Even though all repetitions will be exe-
cuted with maximal voluntary effort, lower inertia allows for
more rapid muscle actions, whereas high inertia slows down
the exercise execution. These differences in movement
velocity will impact the power, force, and work produced
during flywheel RE and may consequently influence adap-
tations to chronic training. Given the great amount of ath-
letes, conditioning professionals and researchers employing
this RE paradigm, studies assessing power, force, and work
produced during flywheel RE using different inertias are
warranted. The information gained from such studies could
aid fine-tuning and personalizing flywheel RE training pro-
tocols for a wide range of populations, from elite athletes to
patients suffering from various diseases.
Although RE-induced muscle adaptations occur in both
women and men, there is no consensus about the different/
equal magnitude of such adaptations across sexes (1,18,34,37).
When employing flywheel RE, hypertrophic adaptations
have been reported to be similar across sexes, yet gains in
maximal strength and power at high loads may be somewhat
greater from men than for women (14). Therefore, any effort
to refine flywheel RE protocols should include the analysis of
potential sex differences.
The main purpose of this study was to analyze force,
power, work, and ECC overload generated during knee
extension flywheel RE with 6 different inertias in women
and men. In addition, differences in force production during
coupled ECC–CON and isolated CON flywheel RE were
addressed to indirectly analyze the SSC use. We hypothesized
that force, power, and work would differ across sexes and
inertias, and that isolated CON actions would call for lower
force production than coupled ECC–CON muscle actions,
underlining the importance of the SSC during flywheel RE.
METHODS
Experimental Approach to the Problem
Participants performed maximal unilateral (in both legs)
isolated CON and coupled CON–ECC tests in a flywheel
knee extension device using 6 different inertias, i.e., 0.0125,
0.025, 0.0375, 0.05, 0.075, 0.1 kg$m
2
. Force, power, and work
produced were measured, and ECC overload calculated
thereafter. In addition, force during isolated CON exercise
was also assessed. Before any test using the flywheel knee
extension device, participants completed 2 familiarization
sessions to ensure appropriate technique during tests. All
tests were preceded by a standardized warm-up and per-
formed at the same time of the day (61 hour). Verbal
encouragement was provided by research staff during all
tests. Real-time feedback on force and knee angle was pro-
vided during familiarization sessions.
Subjects
Twenty-two subjects (11 women; 32.1 64.8 years, 166.2 6
5.5 cm, 57.9 67.8 kg, and 11 men; 35.4 613.0 years, 177.5 6
6.3 cm, 75.4 610.4 kg) with no previous muscle joint or
bone injury for the past 6 months volunteered for the study.
Sample size calculations indicated that for an expected dif-
ference of 50% in power produced by men vs. women using
flywheel RE (14) and a 25% difference in power generated
using inertia 0.05 vs. 0.075 kg$m
2
(23), 10 subjects per group
ensured a statistical power of ;0.80. Subjects were healthy
and moderately active individuals, engaged in 2–4 days per
week of vigorous (1.9 61.0 h$wk
21
) or moderate (2.0 6
1.5 h$wk
21
) exercise. All subjects were requested to avoid
strenuous activities and lower-limb RE at least 48 hours
before any test. A period of .48 hours was required between
test sessions. Information about the study purposes and
potential risks associated with the experiments were ex-
plained to all subjects before obtaining their written
informed consent to participate. The study protocol was
approved by the Regional Ethical Review Board in Stock-
holm (#2014/2174-31/1).
Equipment
All tests were performed on a seated knee extension flywheel
device (YoYo Technology Inc., Stockholm, Sweden) (39),
equipped with a force sensor (100 Hz; Model 276A,
K-Toyo, Korea). During coupled CON–ECC actions, by
knowing the inertia used, power (during CON actions)
and total work (CON + ECC) were calculated for each
repetition by measuring rotational velocity with the aid
of a magnetic encoder system and associated software
(BlueBrain, nHance, Stockholm, Sweden). Knee joint
angular position was measured using electro-goniometry
(MuscleLab). Machine settings were individually accom-
modated for each subject during familiarization and then
maintained throughout all tests. Thighs, hip, and chest
were fixed to the machine using straps. For the dynamic
tests, i.e., isolated CON and coupled CON–ECC tests, the
flywheel knee extension device was equipped with wheels
providing different inertia (load), corresponding to 0.0125,
0.025, 0.0375, 0.05, 0.075, 0.1 kg$m
2
. The order of domi-
nant vs. nondominant leg was randomized in a counterbal-
ance manner for all tests. The order of inertias employed
Effects of Inertia in Flywheel Resistance Exercise
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during CON and CON–ECC exercise was randomized for
each subject during familiarization and maintained
through all tests. The mean value of 3 repetitions (2 for
isometric tests) in each leg in a particular test was consid-
ered for further analysis.
Maximal Isometric Torque
Maximal unilateral isometric torque of quadriceps femoris
muscle was measured in both legs at 1208knee extension.
During 5 seconds, the subject was requested to push, trying
to extend the knee, as hard as possible against the crossbar of
the knee extension device, which had been adjusted and
fixed in the desired position (i.e., 1208knee flexion). Two
attempts were performed for each limb. An additional
attempt was carried out if trials differed .5% for a given
leg. The best score in a 1-second window defined peak iso-
metric torque (21,22). A recovery of 2.5 minutes was allowed
between tests in the same leg.
Isolated CON Flywheel Test
Unilateral isolated CON mean torque was measured in both
legs in the flywheel knee extension device using the 6 inertial
settings previously described. Subjects performed 1 set of 2
CON actions for each leg and inertia, with 2 minutes of
recovery between legs and 4 minutes rest between tests in
the same leg. Starting from a completely static position, the
subject was requested to push as hard as possible from 908
knee flexion to full extension (1808). After a 10-second rest
period, a second repetition was carried out.
Coupled CON–ECC Flywheel Test
Subjects performed 6 sets of 3 maximal coupled CON–
ECC unilateral repetitions for both legs in the flywheel
knee extension ergometer with 2-minute recovery between
legs and 4-minute rest between sets in the same leg. Each
set was carried out with an inertia corresponding to 0.0125,
0.025, 0.0375, 0.05, 0.075 or 0.1 kg$m
2
. After an initial, sub-
maximal repetition to initiate the flywheel movement, the
subject was instructed to push with maximal effort, and
therefore as fast as possible, through the entire CON action
(i.e., from 908knee flexion to full extension). Upon reaching
full extension, the flywheel strap rewound because of iner-
tial forces, which initiated the ECC muscle action. To pro-
duce ECC overload, subjects were requested to resist gently
during the first third of the ECC action and then to apply
maximal breaking force to stop the movement at about 908
knee flexion (39) (Figure 1). Then, the next CON action
was immediately initiated. The ECC overload was calcu-
lated in both absolute (Nm = ECC peak force 2CON peak
force) and relative values (ECC peak force 3100/CON
peak force 2100). The SSC during flywheel RE employing
different inertias was estimated as follow: (CON force dur-
ing coupled CON–ECC 3100/CON force during isolated
CON 2100). In addition, the coupling time between ECC
and CON actions was calculated in the final 158of the ECC
phase and the initial 158of the CON action.
Statistical Analyses
Data are presented as mean 6standard deviation (SD). Sta-
tistical analyses were performed using SPSS v.21 (SPSS Inc.,
Chicago, IL). Data distribution was examined for normality
using the Shapiro–Wilk test. A reliability analysis (intraclass
correlation coefficient; ICC) was carried out for all outcome
measures to determine whether randomized order of the
inertias had any impact on the results. Differences between
dominant vs. nondominant legs in women and men were
analyzed employing a 1-way ANOVA. Isometric values were
analyzed using 1-way ANOVA (women vs. men). A 2-way
ANOVA (inertia 3sex) was used to examined work, power,
CON and ECC peak and mean force values, isolated CON
and ECC overload during flywheel RE. A 3-way ANOVA
(inertia 3action 3sex) was employed to analyzed potential
differences in CON force production during isolated CON
vs. coupled ECC–CON actions (i.e., estimated SSC use).
When significant interactions were found, simple effect tests
were employed, and the false discovery rate procedure was
used to compensate for multiple post hoc comparisons (11).
The significance level was set at 5% (p#0.05). Effect sizes
(ES) were calculated as follow: ([mean A 2mean B]/SD A)
(31). Interpretation of the magnitude of the ES was per-
formed as follow: ,0.35, 0.35–0.8, 0.8–1.5, .1.5 for trivial,
small, moderate, and large, respectively (31).
RESULTS
The reliability analysis showed no impact of the order of
inertias on the data recorded, as indicated by ICC values
.0.9. Preliminary analysis showed no significant differences
(p.0.05) between dominant vs. nondominant legs in any of
the variables measured, and therefore this variable was not
considered for further analysis. A significant (p,0.0005)
main effect of sex was found in all variables analyzed, except
for ECC overload in relative values (%) (see below). Thus,
men showed greater absolutes values compared with women
in maximal isometric torque (211.3 639.0 vs. 120.4 639.9
Nm; F= 58.3, p,0.0005) (ES = 2.33) and in all variables
measured during isolated CON and CON–ECC flywheel
tests (p,0.0005). Given that sex differences in absolute
values were so evident, they are not indicated in tables and
figures unless specifically stated.
Coupled CON–ECC Flywheel RE
There was an inertia 3sex interaction for power data (F=
10.2; p,0.0005). Thus, power values in men decreased to
a greater extent across the different inertias used when com-
pared with women (Figure 2A). The percentage of power
loss between the lowest and the highest inertia for men and
women was 36.1% (ES = 0.97) and 29.1% (ES = 0.68),
respectively. In addition, overall power values were 43.7%
lower in women than in men (main effect of sex; F= 20.9,
p,0.0005).
There was no inertia 3sex interaction for work output
during flywheel RE. However, there was a main effect of
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inertia (F= 124.3, p,0.0005) because of greater work
values in both men (ES = 1.04) and women (ES = 1.23) as
inertia was increased (Figure 2B). In addition, there was
a main effect of sex (F= 24.1, p,0.0005). Thus, men pro-
duced more work than women in all inertias analyzed.
There was an inertia 3sex interaction for both mean and
peak force during CON and ECC actions (Frange = 3.4–6.4;
p#0.05; Table 1). Thus, greater CON and ECC force values
were obtained as inertia was increased (ES .0.9 for men
and women in both CON and ECC actions). Interestingly,
women were able to increased CON mean force more than
men in relative terms (46 vs. 34%) from the lowest to the
highest inertia employed. Sim-
ilarly, ECC mean force
increased more in women than
in men from the lowest to the
0.075 kg$m
2
inertia (55 vs.
50%). However, increments in
peak force as inertia was
increased were greater in men
than women for both CON
(ES = 1.01 for men and 0.72
for women) and ECC actions
(ES = 1.05 for men and 0.64
for women) (Table 1). Overall,
men had greater peak and
mean CON and ECC force val-
ues in absolute terms than
women for a given inertia
(main effect of sex; F range =
39.6–44.1; p,0.0005). An
interaction inertia 3action
(F= 45.6; p,0.0005) was
found for coupling time in
ECC and CON actions. Thus,
the time to complete the first 158of the CON action
increased more than the time employed to perform the last
158of the ECC phase as inertia increased (Table 1) (ES =
4.71 for men and 3.60 for women in CON phase; ES = 3.71
for men and 3.33 for women in ECC phase). In addition,
men had overall lower values of both ECC (16%) and CON
(15%) coupling time compared with women.
During coupled CON–ECC flywheel RE, ECC actions
showed higher peak force than CON actions in all inertias
(inertia 3action interaction; F= 2.9; p= 0.015) (see example
in Figure 1), independently of sex. When analyzing this dif-
ference (i.e., ECC overload), there was an inertia 3sex
Figure 1. Example of 1 set of 3 repetitions of coupled concentric (CON) and eccentric (ECC) flywheel resistance
exercise using inertia 0.075 kg$m
2
.
Figure 2. Power (A) and work (B) data across inertias in women and men. Significant effects (p#0.05): a, inertia 3sex interaction; b, main effect of sex; c,
main effect of inertia. Significant post hoc differences: *(p#0.05), and **(p,0.01) vs. previous (lower) inertia. Data as mean 6standard error of the mean.
Effects of Inertia in Flywheel Resistance Exercise
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TABLE 1. Force (Nm), coupling time (s), and stretch-shortening cycle use (%) during flywheel resistance exercise employing different inertias.*†
Inertia in kg$m
2
0.0125 0.025 0.0375 0.05 0.075 0.1
CON peakz§k
Men 122.0 625.3 146.3 631.5¶ 152.5 636.0¶ 155.9 640.2 158.2 638.3 161.1 638.7
Women 76.5 617.6 90.9 623.9¶ 92.7 626.0 95.3 627.6 101.1 630.4¶ 98.3 630.0
CON meanz§k
Men 80.8 620.5 97.3 621.7¶ 102.0 624.6¶ 104.4 627.2# 107.9 629.1¶ 108.7 629.5
Women 44.6 613.9 56.5 618.0¶ 59.4 618.5¶ 61.0 620.4 64.3 620.6¶ 65.3 620.9
ECC peakz§k
Men 142.9 627.9 173.1 635.4¶ 186.8 648.5¶ 187.0 647.8 193.2 650.0 192.7 647.3
Women 93.4 616.0 109.5 626.2¶ 111.9 631.0 111.6 633.6 117.6 634.6 114.3 632.2
ECC meanz§k
Men 91.0 617.0 122.3 625.3¶ 132.8 633.6¶ 134.1 635.2 137.2 639.5 134.5 636.2
Women 52.9 614.3 72.1 622.5¶ 79.2 625.6¶ 78.4 624.7 82.1 626.5 78.4 624.3
Isolated CON meanz§k
Men 77.1 616.3 90.1 621.0¶ 93.3 620.0¶ 98.4 625.0¶ 102.7 626.2¶ 106.0 626.4¶
Women 43.4 612.1 53.1 615.3¶ 55.6 616.6# 58.3 617.3# 61.9 619.4¶ 63.7 619.9
Coupling time ECC§k**††
Men 0.25 60.04 0.27 60.04# 0.32 60.06¶ 0.35 60.07¶ 0.45 60.05¶ 0.51 60.07¶
Women 0.30 60.05 0.32 60.06 0.38 60.07¶ 0.44 60.08¶ 0.52 60.09¶ 0.60 60.09¶
Coupling time CON§k**††
Men 0.25 60.04 0.30 60.04¶ 0.35 60.06¶ 0.40 60.08¶ 0.50 60.06¶ 0.58 60.07¶
Women 0.30 60.04 0.35 60.06¶ 0.41 60.06¶ 0.49 60.08¶ 0.58 60.10¶ 0.66 60.10¶
SSCk
Men 4.4 610.2 9.1 613.1 9.7 613.7 7.4 615.5 5.6 613.1 3.1 615.4
Women 3.8 618.6 6.5 611.9 7.5 612.4 4.7 612.3 4.1 68.3 3.1 612.9
*CON = concentric; ECC = eccentric; SSC = stretch-shortening cycle expressed in relative values (FL CON mean 3100/isolated CON 2100).
†Data as mean 6SD.
zSignificant effects (p#0.05): inertia 3sex interaction.
§Significant effects (p#0.05): main effect of sex.
kSignificant effects (p#0.05): main effect of inertia.
¶Significant post hoc differences: p,0.01 vs. immediately previous (lower) inertia.
#Significant post hoc differences: p#0.05.
**Significant effects (p#0.05): main effect of action.
††Significant effects (p#0.05): inertia 3action interaction (ECC–CON).
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interaction (F= 2.4; p= 0.04), for ECC overload (expressed
as %). Thus, in relative terms (%), men tended to increase
ECC overload as inertia was increased from 0.0125 to
0.0375 kg$m
2
, whereas women showed decreased ECC
overload from the lowest to the 0.05 kg$m
2
inertia
(Figure 3A). The highest ECC overload value was 22% for
men (inertia 0.0375 kg$m
2
) and 25% for women (inertia
0.0125 kg$m
2
) (Figure 3A). When analyzed in absolute val-
ues (i.e., Nm), there was an inertia 3sex interaction (F= 2.9;
p#0.05) because of increased ECC overload in men from
the 0.0125 to the 0.0375 kg$m
2
inertia (ES = 0.77). This
response was not found in women, where ECC overload
remained practically unchanged across inertias (Figure 3B).
The highest value of ECC overload (Nm) was 35.0 Nm
(inertia 0.075 kg$m
2
) and 19.2 Nm (inertia 0.0375 kg$m
2
)
for men and women, respectively (Figure 3B).
Isolated CON and SSC Use During Flywheel RE
An interaction inertia 3sex (F= 4.0; p= 0.002) was found
for force produced during isolated CON action. Thus, force
values increased to a greater extent in women (47%; ES =
1.02) than in men (37%; ES = 1.09) from the lowest to the
highest inertia (Table 1).
There was an inertia 3sex interaction (F= 4.9; p,
0.0005) for estimated SSC use during flywheel RE. Although
men had overall greater values for SSC use in all inertias
(except 0.1 kg$m
2
), both sexes showed higher SSC use dur-
ing exercise employing low-medium inertias (i.e., 0.025 and
0.0375 kg$m
2
). The inertia inducing greater SSC use was
0.0375 kg$m
2
for both men (9.7%) and women (7.5%) (Table
1).
DISCUSSION
This study analyzed power, work, force, and ECC overload
produced during knee extension flywheel RE using 6
different inertial settings. In addition, potential differences
across sexes were assessed, and SSC use was estimated. In
agreement with the hypothesis, there were important differ-
ences in force, power, and work across inertias used, and
between men and women. We also report that performing
RE using this particular technology allows for a substantial
SSC use, which was maximized by using medium inertias in
both sexes.
Despite the visible and evident differences in movement
velocity during flywheel RE employing different inertial
settings, this is the first investigation reporting the power,
work, and force produced across a wide range of inertias.
Given that these RE variables could affect muscle and
functional adaptations to chronic training (16,24,35), the
data presented here could aid in fine-tuning exercise proto-
cols employing flywheel RE. From the existing literature, we
were only able to identify 3 investigations where an inertia
selection process was carried out before commencing a fly-
wheel RE training period (12,13,40). In these studies, the
inertia selection was rather simplistic, comparing the maxi-
mal power developed across 2 different inertial settings. Our
results showing decreased power as inertia increased may
indicate that other variables apart from power (i.e., work
output) should be considered when selecting the best inertia
for a particular purpose.
Across the inertial settings analyzed, power values were
;44% lower in women than in men, confirming previous
reports employing traditional RE (9,20). Interestingly, power
across the different inertias used was also different between
men and women, with greater decrements from the lowest
to the highest inertia in men than in women. These results
are supported by previous investigations employing conven-
tional RE, where sex differences in power or peak velocity
between men and women were greater when light loads
Figure 3. Eccentric (ECC) overload expressed in relative (A) and absolute (B) values across inertias in women and men. Significant effects (p#0.05): a,
inertia 3sex interaction; b, main effect of sex; c, main effect of inertia. Significant post hoc differences: *(p,0.01) vs. previous (lower) inertia. Data as mean 6
standard error of the mean.
Effects of Inertia in Flywheel Resistance Exercise
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were employed, and differences decreased as resistance was
increased (9,30). Differences in power outcome during RE as
loads increase across sexes may be because of (a) a better
capacity of women to maintain movement velocity as inertia
increases, and/or (b) an inability of women, when compared
with men, to increase movement velocity with very light
inertias/loads, and/or (c) lean body mass differences across
sexes. Thus, although the mechanism(s) for power differen-
ces between men and women in light vs. high inertias/loads
is still unknown, it appears that sex is a variable to consider
when selecting the inertia to be employed during flywheel
RE. In addition, studies employing flywheel RE in women
and men should take into account potential differences in
muscle/lean body mass because this factor may help ex-
plaining some of the differences across sexes shown in the
current study.
In line with the general understanding of the force–
velocity relationship (17), force values during CON and
ECC actions increased during flywheel RE because inertia
increased in both men and women. Similarly, a recent study
using free weights also reported greater peak force values
during higher vs. lower loads (9). The authors used those
data to recommend higher loads to improve force pro-
ducing capacity (9). In the current study, the relative in-
crements in peak CON and ECC force were greater in men
when compared with women as inertia was increased. In
contrast, women increased CON and ECC mean force
more than men in relative terms, from the lowest to the
highest inertia. Therefore, our data indicate that men and
women may respond differently to inertia increments, with
men relying more on short and explosive moments of great
force production (increased peak force), whereas women
rather produce lower peak forces but they are able to
maintain force levels for a longer period within the muscle
action (increased mean force). The different response across
sexes in the coupling time of both ECC and CON actions is
another indication of the more explosive capacity of men
compared with women. These data seem to be supported
by previous research showing sex differences in skeletal
muscle structure (i.e., greater area of type I, slow, fatigue-
resistant fibers in women than men vs. greater type II, fast-
explosive fibers in men compared with women) (36). In
addition, the fact that men increased peak force more than
women because inertia/load was higher could explain, at
least partly, the greater gains in maximal force and peak
power at high loads in men than women previously re-
ported after flywheel RE (14).
The ECC overload that can be produced during flywheel
RE is a critical feature of the exercise model that has been
used to partly explain the greater muscle adaptations
induced by this exercise paradigm when compared with
conventional RE (27–29). Our results indicate that ECC
overload can be generated during knee extension flywheel
RE in all inertias analyzed, ranging from 17 to 25% (i.e., 17–
25% more peak force production during ECC compared
with CON), which confirms previous reports from our lab-
oratory (14). Given the greater capacity of the muscle to
produce force during ECC vs. CON actions (19,38), it seems
ECC overload is critical to offer an appropriate stimulus to
maximize neural drive and muscle use (7). The current data
indicate that men have a greater capacity than women to
generate ECC overload during flywheel RE. In women, the
greater ECC overload in relative terms occurred at the light-
est inertia employed (i.e., highest velocity), which confirms
results from earlier research showing women produced sig-
nificantly more ECC force, relative to CON, than men only
at very high movement velocities (8).
The SSC is often described as the ability to store energy
during the ECC muscle action to potentiate the subsequent
CON action (6). In a recent study, we inferred that flywheel
RE training could emphasize the stretch reflex and the SSC
use, which would boost neural adaptations after a period of
training (15). The current results showed lower force in iso-
lated CON compared with force during the CON phase in
coupled ECC–CON muscle actions. Although this has been
described before using traditional RE models (10), our re-
sults are the first indicating significant SSC use during fly-
wheel RE. Thus, the inertia 0.0375 kg$m
2
showed the
highest (estimated) SSC use independent of sex. The partic-
ular benefits that may be associated with such strategy, and
the magnitude of potential differences with other RE modes,
remain to be investigated.
In summary, this study assessed power, work, peak and
mean CON and ECC force, ECC overload, and the SSC use
during knee extension flywheel RE in men and women using
6 different inertial settings. Power decreased because higher
inertias were used and more so in men than in women. In
contrast, work increased because higher inertias were
employed independently of sex. Women increased CON
and ECC mean force more than men as greater inertias were
used. Yet, peak force increments were higher in men than in
women as inertia increased. Although men increased ECC
overload from inertia 0.0125 to 0.0375 kg$m
2
, ECC overload
was rather constant across the inertias analyzed in women.
Men produced slightly higher SSC than women, yet values
were greater for both sexes when using low-to-medium in-
ertias. The information gained by this study highlights that
manipulating the inertial setting during flywheel RE will
modify the stimulus imposed on the muscles. Future training
studies are necessary to elucidate whether differences in iner-
tial settings translate into different flywheel RE-induced
muscle adaptations.
PRACTICAL APPLICATIONS
Isoinertial flywheel resistance exercise is a time-effective
method to increase force, power, and muscle mass. Given
the extensive use of this training paradigm in elite sports,
rehabilitation and clinical settings, and among recreational
practitioners, we believe that current results will help
designing and fine-tuning new training programs employing
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flywheel RE. Indeed, this is the first study describing the
impact of inertia (i.e., load) on RE variables that could
influence end-point training adaptations. Considering the
power, work, ECC overload, and SSC use data, the inertia of
0.0375 kg$m
2
seems as an appropriate choice for general
conditioning purposes in both women and men. In contrast,
athletes looking for explosive adaptations may use lower
inertias calling for a shorter ECC–CON coupling time and
greater power production, whereas practitioners pursuing
greater work output during RE should employ higher iner-
tias. In addition, modifying the inertial setting during fly-
wheel RE may affect women and men differently in terms
of force and power produced, and ECC overload achieved.
ACKNOWLEDGMENTS
We thank M. V. Garnacho-Castan
˜o and M. Gimeno-Raga
for technical support during the initial part of the study. This
investigation was partly funded by T-O
¨Stiftelsen (#1301;
RF-G) and STROKE-Riksfo
¨rbundet (RF-G). The funding
agencies did not have any role in the experimental design,
or data collection, analysis, or interpretation, or manuscript
writing or submission. The authors declare no conflicts of
interest. The results of the present study do not constitute
endorsement of the product by the authors or the NSCA.
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