Potentiation and recovery following low- and high-speed isokinetic contractions in boys.

Abstract

The objective of this study was to examine the response and recovery to a single set of maximal, low and high angular velocity isokinetic leg extension-flexion contractions with boys. Sixteen boys (11-14 yrs) performed 10 isokinetic contractions at 60°.s-1 (Isok60) and 300°.s-1 (Isok300). Three contractions at both velocities, blood lactate and ratings of perceived exertion were monitored pretest and at 2, 3, 4, and 5 min of recovery (RI). Participants were tested in a random counterbalanced order for each velocity and recovery period. Only a single contraction velocity (300°.s-1 or 60°.s-1) was tested during recovery at each session to remove confounding influences between the recovery intervals. Recovery results showed no change in quadriceps' power at 300°.s-1, quadriceps' power, work and torque at 60°.s-1 and hamstrings' power and work with 60°.s-1. There was an increase during the 2 min RI in hamstrings' power, work and torque and quadriceps' torque with isokinetic contractions at 300°.s-1 suggesting a potentiating effect. Performance impairments during recovery occurred for the hamstrings torque at 60°.s-1 and quadriceps work with 300°.s-1. In conclusion, 10 repetitions of either low or high velocity isokinetic contractions (Isok60 or Isok300) resulted in full recovery or potentiation of most measures within 2 min in boys. The potentiation effect predominantly occurred following the hamstrings Isok300 which might be attributed to a greater agonist-antagonist torque balance and less metabolic stress associated with the shorter duration higher velocity contractions.

Full text

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136
Pediatric Exercise Science, 2011, 23, 136-150
© 2011 Human Kinetics, Inc.
Chaouachi, Haddad, Kaouech, and Chamari are with the Tunisian Research Laboratory, Sports Per-
formance Optimization, National Center of Medicine and Science in Sports (CNMSS), Tunis, Tunisia.
Castagna is with the School of Sport and Exercise Sciences, Faculty of Medicine and Surgery, Univer-
sity of Rome Tor Vergata, Rome, Italy. Wong is with the Dept. of Health and Physical Education, The
Hong Kong Institute of Education, Hong Kong, China. Behm is with the School of Human Kinetics and
Recreation, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada.
Potentiation and Recovery Following Low-
and High-Speed Isokinetic Contractions
in Boys
Anis Chaouachi and Monoem Haddad
National Center of Medicine and Science in Sports (CNMSS)
Carlo Castagna
University of Rome Tor Vergata
Del P. Wong
The Hong Kong Institute of Education
Fathi Kaouech and Karim Chamari
National Center of Medicine and Science in Sports (CNMSS)
David G. Behm
Memorial University of Newfoundland
The objective of this study was to examine the response and recovery to a single
set of maximal, low and high angular velocity isokinetic leg extension-flexion con-
tractions with boys. Sixteen boys (11–14 yrs) performed 10 isokinetic contractions
at 60°.s-1 (Isok60) and 300°.s-1 (Isok300). Three contractions at both velocities,
blood lactate and ratings of perceived exertion were monitored pretest and at 2,
3, 4, and 5 min of recovery (RI). Participants were tested in a random counter-
balanced order for each velocity and recovery period. Only a single contraction
velocity (300°.s-1 or 60°.s-1) was tested during recovery at each session to remove
confounding influences between the recovery intervals. Recovery results showed
no change in quadriceps’ power at 300°.s-1, quadriceps’ power, work and torque
at 60°.s-1 and hamstrings’ power and work with 60°.s-1. There was an increase
during the 2 min RI in hamstrings’ power, work and torque and quadriceps’

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Potentiation and Recovery Responses of Youth 137
torque with isokinetic contractions at 300°.s-1 suggesting a potentiating effect.
Performance impairments during recovery occurred for the hamstrings torque at
60°.s-1 and quadriceps work with 300°.s-1. In conclusion, 10 repetitions of either
low or high velocity isokinetic contractions (Isok60 or Isok300) resulted in full
recovery or potentiation of most measures within 2 min in boys. The potentiation
effect predominantly occurred following the hamstrings Isok300 which might be
attributed to a greater agonist—antagonist torque balance and less metabolic stress
associated with the shorter duration higher velocity contractions.
There are a number of important variables associated with resistance training
prescriptions including the intensity, volume or duration, frequency and recovery
intervals (RI; 1). The adult resistance training literature provides between set RI
recommendations based on phosphocreatine resynthesis (32), hormonal responses
(14), strength restoration (64), jump performance (47), and sprint time (15) and
other measures. Review articles and textbooks in general recommend 2–5 min of
recovery between resistance training sets for adults (1,69). However, there is very
little information in the literature regarding potentiation and fatigue during RI for
children and adolescents. A number of review articles, position stands and training
prescription papers recommend or suggest ranges of intensities and volumes of
resistance training for children and adolescents but do not mention recommended
RI (6,24,37,40,68).
There is a complex interplay between muscle fatigue and potentiation (3).
Potentiation can be defined as a temporary improvement in muscle performance
following a conditioning activity due to mechanisms either within the muscle or
neural factors (58). The ability to sustain forces can be viewed as a compromise
between fatigue-inducing impairments and neuromuscular strategies to enhance
or sustain performance (3). While the effect of an initial bout of high intensity
contractions has been shown to produce force facilitation or potentiation in some
adult studies (5,35,58), there are few studies examining recovery and potentia-
tion in children and adolescents. Jump performance has been enhanced following
dynamic warm-ups in high school female athletes (28), male adolescent athletes
(27) and elite youth soccer players (42). Both Faigenbaum et al. (26) and Duncan
and Woodfield (20) speculated that improvements in shuttle run and vertical jump
performance respectively may have been due to enhanced excitability of fast twitch
motor units. Muscle postactivation potentiation has been demonstrated in healthy
pre- and postpubertal children (9,44,45). The mechanisms of potentiation and fatigue
can occur simultaneously (51). A typical volume of resistance training which can
induce fatigue for an adult (7,57) could result in no appreciable signs of fatigue in
children or adolescents. Whether the response to a conditioning stimulus is fatigue
or potentiation is also dependent upon the trained state of the individual (58).
According to the concept of training specificity (4), for sports that involve
explosive and high speed contractions, training should involve similar contraction
velocities and intensities. As the few previous studies in children and adolescents
that have investigated RI used moderate or slower speed contractions, it is unknown
how youth respond to high and low speed, high intensity contractions. Youth are
reported to have a greater resistance to fatigue (19,52,53) and have been shown to
recover more rapidly than adults from three sets of a 10 repetition maximum load
(29) and repeated bouts of high intensity 120°.s-1 isokinetic contractions (19,70).
In fact, there are very few adult studies comparing the fatigue effects between high

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138 Chaouachi et al.
and low speed contractions. Celes et al. (16) reported greater losses in peak torque
and total work with 3 sets of 10; 60°.s-1 versus 300°.s-1 isokinetic knee extensions
in young men (~22 years). In a study using older men (60–74 years), there was no
difference in recovery of peak torque following 2 sets of 4 repetitions of 90°.s-1,
and 120°.s-1 isokinetic contractions (30, 60 and 90s recovery; 12).
Thus it was the objective of this study to investigate the extent of potentiation
or fatigue during the recovery of boys’ quadriceps and hamstrings following a
set of 10 repetitions of low and high velocity leg extension and flexion isokinetic
contractions. As the 60°.s-1 isokinetic contractions can generate greater torques over
longer durations than the 300°.s-1 contractions, it was hypothesized that the lower
speed contractions would provide greater force impairments during the recovery.
Methods
Subjects
Sixteen male competitive martial arts practitioners, between 11–14 years volun-
teered to participate in this study. Subject characteristics were 12.2 ± 0.9 years,
151.1 ± 8.6 cm, 42.5 ± 9.7 kg with mean body fat percentage and fat free mass
of 17.1 ± 4.2% and 34.9 ± 6.6 kg respectively. All of them had been practicing
their sport for at least 3 years with a mean training schedule of 6–8 hr per week. A
pubertal assessment was conducted using the composite score of the widely used
method of pubertal stage assessment described by Tanner (63). These data are in
respect to the available published data about puberty onset and development in
Tunisian schoolchildren aged between 8 and 16 years (66). In this regard, all the
study subjects ranged from stage 2 to stage 4 of Tanner score.
Following written consent and before experimental participation, volunteers
were examined by a physician in the National Center of Medicine and Science in
Sports, Tunis, and were assessed as having no injury to their lower limbs, orthopedic
limitations or illness that put them at risk while performing the testing exercises
or would influence performance or measurements. Body fat measurements were
conducted according to Deurenberg et al. (18) who reported similar prediction
errors between adults and young adolescents. The study was conducted according
to the Declaration of Helsinki and the protocol was fully approved by the Clinical
Research Ethics Committee and the Ethic Committee of the National Centre of
Medicine and Science of Sports of Tunis (CNMSS).
Experimental Protocol
Each subject completed 11 laboratory visits including a 2-part orientation session.
During the orientation phase, approximately 1 week before testing, each subject
was familiarized with the equipment (isokinetic dynamometer: Cybex NORM;
Henley Healthcare, Cybex International, Inc., Medway, MA) and procedures and
participants’ height, body mass, and skinfold thickness were measured. During this
sessions, subjects performed five submaximal and ten maximal isokinetic efforts
of the knee extensors and flexors at 300°.s-1 and 60°.s-1. The remaining 8 sessions
were completed during the course of the subsequent 16 days, so that approximately
48 hr separated each test-day.

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Potentiation and Recovery Responses of Youth 139
Sixteen boys were tested with 3 maximal isokinetic contractions at 60°.s-1
and 300°.s-1 before and following an intervention of 10 isokinetic contractions at
60°.s-1 or 300°.s-1 (Isok60 and Isok300, respectively). Ten isokinetic contractions
were chosen for their ecological validity as this number is within the recommended
range of repetitions (6–15 repetitions) reported in a number of position stands and
reviews for children’s resistance training (6,25,34,39,40,68). During each of the 8
follow-up sessions, subjects were tested at 300°.s-1 or 60°.s-1 for peak torque, mean
power, and total work during the recovery periods at 2, 3, 4 or 5 min to eliminate
the potentially confounding influence of the testing of various testing velocities on
subsequent recovery interval measures.
Ratings of perceived exertion (RPE) were monitored during the pretest, iso-
kinetic resistance protocols and RI. Blood was taken immediately before Isok60,
Isok300 and the RI tests for the measurement of blood lactate.
The athletes did not participate in their regular TKD training for 24 hr before
testing. This procedure was chosen to minimize any performance changes that
could occur over a longer time period and to ensure adequate functional recovery.
Each subject was tested at approximately the same time of day (within 2 hr) over
the 8 sessions to nullify diurnal variations.
Testing Protocol
At all testing sessions, each subject started the experimental trial by completing a
standardized procedure including a 5-min cycling warm-up at 70 W at 70 rpm on
a cycle ergometer before the initial isokinetic testing. As part of the warm-up, the
subjects sat on the Cybex Norm dynamometer (Cybex NORM; Henley Healthcare,
Cybex International, Inc., Medway, MA) and were secured to both the dynamometer
and the corresponding chair according to manufacturer’s specifications to minimize
extraneous movements and to maintain a constant hip joint angle (90°). Only the
dominant limb, determined from the kicking preference of the athlete, was tested.
After height, limb mass, gravity correction, and individual specific full range of
knee motion were recorded, subjects performed a set of four to five submaximal
leg extension and flexion contractions on the isokinetic dynamometer at 180°.s-1
as a specific warm-up. After a 3-min rest, the participants performed the pretest-
ing protocol which was randomly allocated consisting of three maximal intensity
leg extension and flexion repetitions on an isokinetic dynamometer at 300°.s-1 and
60°.s-1 respectively, with 3min rest between sets of testing contractions. Subjects
were instructed to not rest between the 3 repetitions. They were instructed to
continuously push the lever-arm through a full range of motion, as hard and as
fast as possible for three maximal efforts in both directions of the movement, with
extension always undertaken first. Three repetitions were chosen pretest and during
recovery as it is acknowledged in the literature that no more than 3–5 repetitions are
necessary when assessing strength (13). Following a 3-min rest period, the subjects
performed a resistance exercise protocol consisting of 10 maximum intensity, full
range of motion, isokinetic leg extension and flexion contractions at either 60°.s-1
(Isok60) or 300°.s-1 (Isok300) depending on the testing session day. This protocol
was selected due to its similarity to isokinetic strength testing protocols in the lit-
erature (46). The order of the isokinetic resistance protocol was randomized as was
the RI (2, 3, 4, and 5 min) for each testing session. After the isokinetic protocol, the

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140 Chaouachi et al.
testing protocol was the same as that used before the intervention. The participant’s
were verbally encouraged to produce maximal effort.
The parameters used for analysis were peak torque measured in Nm, mean
power and total work measured in Joules. Isokinetic measurements in children have
been reported to be reproducible (41). Table 1 illustrates the excellent isokinetic
measurement reliability attained in the current study. Between session reliability
was calculated by comparing multiple pretest measures.
Each young athlete was asked to state the respective RPE at the end of each
exercise set to ensure that the perceived effort was referred to this exercise-set only.
With every exercise set a copy of Borg’s CR10 scale (11) modified by Foster et al.
(33) was used to assist the young athletes in making their responses. The predictive
efficacy and validity of the Borg RPE has been demonstrated in children (22,23).
To assess blood lactate with the isokinetic resistance protocols, fingertip blood
samples were taken immediately before the isokinetic protocol and each RI. A blood
sample (5 µl) was obtained from the fingertip and analyzed for lactate concentration
(mmol·l-1) using portable blood lactate analyzer (LactatePro, Arkray, Tokyo, Japan).
Before each testing session and before each blood sampling, the lactate analyzer was
calibrated and used according to the manufacturer guidelines. The portable blood
lactate analyzer used in this study has been reported to be reliable and valid (50).
Table 1 Reliability Obtained From Preintervention Values of
Isokinetic Measures. The Parameters Used for Analysis Were Peak
Torque Measured in Nm, Mean Power and Total Work Measured in
Joules.
Velocity Muscle groupParameters PretestICC SEM
300°.s-1
Hamstring Peak torque 38.7 ± 11.20.9620.793
Mean power 102.1 ± 28.60.9552.534
Total work 47.8 ± 13.10.951 1.257
Quadriceps Peak torque 48.4 ± 12.10.928 1.990
Mean power 124.9 ± 34.80.9523.408
Total work 58.4 ± 15.40.9471.674
60°.s-1
HamstringPeak torque 60.4 ± 15.80.9910.306
Mean power 39.7 ± 10.30.9880.253
Total work 73.6 ± 20.10.9501.838
QuadricepsPeak torque 88.6 ± 19.20.9870.541
Mean power 57.3 ± 14.80.9800.553
Total work 99.6 ± 20.10.9940.280

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Potentiation and Recovery Responses of Youth 141
Statistical Analysis
The kinetic and RPE data were analyzed using 3 way repeated-measures ANOVA
(2 × 4 x 4; GB-Stat V. 7, Dynamic Microsystems, Maryland USA). Factors
included velocity of the isokinetic intervention contractions (Isok60 and Isok300),
pretest (separate pretests were performed with each of the 4 recovery intervals)
and recovery time (2, 3, 4, and 5 min RI). The preintervention and recovery test-
ing velocities (3 contractions each at 60°.s-1 and 300°.s-1) were not compared or
analyzed as a third factor in the ANOVA as it is well known that forces, work and
power outputs differ between 60°.s-1 and 300°.s-1 isokinetic contractions. Thus the
preintervention and recovery testing velocities were analyzed with separate 3 way
ANOVAs. The blood lactate data were analyzed using a repeated measures 2 way
ANOVA (2 × 2). Factors included velocity of contractions (Isok60 and Isok300)
and time (precontractions and recovery). If significant interactions were detected a
Bonferroni-Dunn’s correction procedure post hoc test was used. Significance was
considered to be achieved at p < .05. Effect sizes (ES = mean change / standard
deviation of the sample scores) were also calculated and reported (17). Data are
described as means ± SD (SD).
Results
Hamstrings Kinetics
Following the Isok300 intervention, there was a significant main effect for 300°.s-1
testing with 3.5%, 2.7% and 5.6% increases in hamstrings mean power (p = .006,
ES = 0.12), total work (p = .003, ES = 0.1) and peak torque (p < .0001, ES = 0.2)
respectively comparing the pretest to the combined recovery periods (Table 2).
After the Isok60 protocol, there were no main effects for hamstrings mean
power or total work for the 60°.s-1 testing. However, hamstrings peak torque
decreased by 3.4% when comparing the pretest to the combined recovery periods
for the 60°.s-1 isokinetic testing (p = .003, ES = 0.2). A main effect (p = .02, ES =
0.35) for RI showed an 11.4% increase in hamstrings peak torque from 2 (55.5 Nm
± 19.6) to 3 (61.8 Nm ± 17.6) minutes of recovery. There were no other significant
differences between the other recovery periods’ measurements.
Table 2 Illustrates the Significant (*) Main Effect for Time (Pre- to
Posttesting) With 300°.s-1 Testing With Hamstrings Mean Power
(p = 0.006), Total Work (p = 0.003) and Peak Torque (p < 0.0001)
Respectively.
Hamstrings
Mean Power
Hamstrings
Total Work
Hamstrings Peak
Torque (Nm)
Pretest98.6 ± 27.446.3 ± 13.738.7 ± 11.9
Posttests (data combined
over all recovery periods)
102.1 ± 29.5 * 47.6 ± 13.3 *40.9 ± 11.3*

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142 Chaouachi et al.
Quadriceps Kinetics
With the Isok60 intervention, there were no significant changes from 300°.s-1 pre-
test to recovery for mean power or between 60°.s-1 pretest and recovery for mean
power, total work or peak torque. The amount of total work by the quadriceps
decreased 2.6% (p = .003, ES = 0.1) when comparing the pretest to the combined
recovery 300°.s-1 testing.
Following the Isok300 intervention, there was a main effect (p = .002) for
300°.s-1 peak torque testing RI with the 2 min RI having 5.5%, 5.6%, 8.2% and
8.1% greater peak torque than the pretest, as well as 3 (ES = 0.2), 4 (ES = 0.3),
and 5 (ES = 0.3) minutes of recovery respectively (Figure 1).
Figure 1 — The figure illustrates a main effect (p = .002) for 300°.s-1 pretest and recovery
time (2–5 min) differences for quadriceps peak torque. Columns and bars represent means
and standard deviation respectively. Asterisks (*) indicate statistically significant (p = .002)
differences between pre- and postmeasures.
Blood Lactate
A significant (p < .0001) main effect for testing showed that RI (3.15 ± 0.9 mmol·l-
1) blood lactate values were 38.1% (ES = 1.4) greater than preintervention (2.28 ±
0.6 mmol·l-1) values. A second significant (p < .0001) main effect for contraction
velocity illustrated that the Isok60 (2.95 ± 0.9 mmol·l-1) blood lactate values were
18.9% (ES = 0.8) greater than the Isok300 (2.48 ± 0.6 mmol·l-1) values. Interaction
effects (p < .0001) indicated that overall recovery blood lactate values for both the
Isok60 and Isok300 exceeded the pretest values by 47.8% (ES = 1.65) and 27.9%
(ES = 1.07) respectively (Figure 2). Whereas there was no difference between the
pretest values, the blood lactate recovery values were significantly (p < .0001)
higher for the Isok60 than the Isok300 (Figure 2).

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Potentiation and Recovery Responses of Youth 143
Ratings of Perceived Exertion (RPE)
A main effect for testing indicated significantly (p < .0001) higher mean RPE scores
for the combined isokinetic protocols (Isok60 and Isok300 combined RPE: 3.9 ±
0.8) and recovery (RPE: 3.6 ± 0.7) periods compared with the pretests (RPE: 3.1
± 0.6). The isokinetic interventions (Isok60 and Isok300 combined) and RI RPE
scores exceeded the pretest scores by 24.7% (interventions: ES= 1.4) and 13.7%
(RI: ES = 0.89) respectively. The set of 10 isokinetic contractions’ RPE score
(Isok60 and Isok300 combined) was 9.8% (ES = 0.4) greater than the RI score.
Whereas the Isok60 intervention RPE was 48.5% higher (p < .0001) than the
Isok60 pretest contractions, the Isok300 intervention showed a nonsignificant 1.5%
higher RPE score than the Isok300 pretest contractions (Table 3). Other significant
(p = .0008) interactions indicated that RPE scores for all RI following the Isok60
were higher than the 2 min RI following the Isok300 intervention. The RPE scores
following the Isok60 RI were 25.1% (ES = 0.88), 16.3% (ES = 0.85), 17.5% (ES
= 1.25) and 15.3% higher than the 2 min RI following the Isok300 respectively.
There were no significant differences between the 3, 4, and 5 min RI following the
Isok300 and any of the recovery periods for the Isok60 (Table 3).
Figure 2 — The figure illustrates pretest and posttest differences between 60°.s-1 (Isok60)
and 300°.s-1 (Isok300) isokinetic contractions in blood lactate. The x-axis titles of pretest and
posttest 60 and 300 refer to Isok60 and Isok300 respectively. Columns and bars represent
means and standard deviation respectively. Asterisks (*) indicate statistically significant
differences between pre- and postmeasures. An addition (+) sign illustrates a significant
difference between the posttest measures at 60°.s-1 and 300°.s-1.

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144 Chaouachi et al.
Discussion
The most significant findings of this study were the potentiation of the boys’
hamstrings’ mean power, total work, and peak torque and quadriceps’ peak torque
following 10 maximal isokinetic repetitions at 300°.s-1 (Isok300). The present study
has also shown a lack of impairment during RI in quadriceps mean power at 300°.s-1,
quadriceps mean power, total work or peak torque at 60°.s-1 and hamstrings mean
power or total work with 60°.s-1 contractions.
In the current study, there was an increase of hamstrings mean power, total work
and peak torque and quadriceps peak torque (2 min recovery) following Isok300.
This potentiation may be attributed to a variety of neural responses. For example,
neural potentiation has been documented at the supraspinal level with motor evoked
potential facilitation occurring following varying durations and contraction intensities
(2). In that study the greatest potentiation was achieved with the shortest and strongest
contractions. At the level of the motoneuron, improved H-reflex amplitudes (36)
can persist for 10 min following conditioning contractions (67). At the muscle level,
increased muscle stiffness has been shown to persist for 90 min following 5 repetitions
of 8s contractions (65). Other peripheral factors such as increased sensitivity to Ca++
release or increases in muscle stiffness due to residual crossbridge attachments (59)
might also contribute to the facilitation or potentiation of contractions.
It is unlikely that the mechanisms associated with muscle postactivation
potentiation (PAP) played a significant role in the recovery potentiation. PAP
has been defined as an increase in the efficiency of a muscle or a decrease in the
energy needed to produce a submaximal force following a voluntary contraction
(3). Furthermore PAP can also positively affect rate of force development (58),
however as the velocity of movement was limited by the dynamometer, it would
not be a factor. As the current study’s recovery testing involved maximal contrac-
Table 3 Illustrates the Mean Ratings of Perceived Exertion (RPE)
Measures for Pretest, Intervention (Isok60 and Isok300) and
Recovery Tests. The Asterisk (*) Indicates that the Isok60 Pretest
RPE Measure was Significantly Lower than the Corresponding
Isok60 Intervention and Recovery Scores. The Omega (Θ) Symbol
Indicates Significant Differences Between the Single and Double
Omegas (Θ Θ) Measures.
Isok60 interventionIsok300 intervention
Pretest3.1 ± 0.7 *
3.1 ± 0.7 Θ Θ
Intervention (10 isokinetic
contractions)
4.6 ± 0.8 Θ
3.2 ± 0.5
2 min recovery
3.9 ± 0.9 Θ
3.1 ± 0.9 Θ Θ
3 min recovery
3.7 ± 0.7 Θ
3.7 ± 0.5
4 min recovery
3.7 ± 0.7 Θ
3.7 ± 0.8
5 min recovery
3.6 ± 0.4 Θ
3.4 ± 0.6

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Potentiation and Recovery Responses of Youth 145
tion forces, the PAP mechanisms (i.e., regulatory light chain phosphorylation) (62)
typically are only present with lower frequency muscle stimulation such as twitches,
unfused tetanus and submaximal voluntary contractions. Furthermore children are
reported to have lower proportional area of type II muscle fibers (10,53). As PAP
is reported to occur more predominantly in type II fibers (58), the mechanisms
underlying the improvement in recovery measures are more likely to be associated
with neural responses. However, if particular subjects did not produce maximal
exertion throughout the protocol, the increased muscle efficiency associated with
PAP could have reduced possible fatigue effects.
However, adult literature PAP studies have shown greater potentiation responses
in trained individuals (58). As the boys in this study were trained martial artists
who engaged regularly in power and strength activities, they may have been more
likely to emphasize potentiation rather than fatigue responses than untrained youth.
In the current study, there seemed to be a greater potentiation response by
the hamstrings compared with the primarily lack of change with the quadriceps
variables. Forbes et al. (31) reported a more equitable torque balance between ham-
strings and quadriceps in younger football players (under 12–16 years) as compared
with the greater predominance of quadriceps concentric torque with 17–18 year
old football players. They suggested that the hamstrings-quadriceps inequality with
17–18 year old players may be reflective of a limited focus on hamstrings training
or a greater focus on quadriceps training. They reported concentric hamstrings to
quadriceps peak torque ratios of 0.5–0.62. The young athletes in the current study
had even more balanced hamstrings to quadriceps concentric torque ratios of 0.8 for
the pretest 300°.s-1 isokinetic contractions and 0.67 for the 60°.s-1 contractions. In
the current study, the greater relative potentiation of the hamstrings may be related
to better hamstrings-quadriceps torque balance.
Other than RI hamstrings peak torque at 60°.s-1 and quadriceps total work
with 300°.s-1 isokinetic contractions, all other recovery measures following the
Isok60 and Isok300 showed full recovery by 2 min. However it could be argued
that there was a tendency for velocity specificity with augmented or potentiated
measures only occurring following the Isok300 (i.e., RI hamstrings mean power,
total work and peak torque and quadriceps’ peak torque at 300°.s-1). There was
no potentiation following Isok60. There is a physiological compromise or balance
between fatigue-inducing responses and neuromuscular strategies to enhance or
sustain performance (3). The 60°.s-1 contractions (Isok60) were of longer duration
(each repetition was approximately 1.5 s compared with ~0.3 s for the 300°.s-1
contractions). This increased duration and work involved a greater metabolic stress
as evidenced by the significantly greater blood lactate values as compared with the
Isok300. We would hypothesize that the greater work and duration-induced meta-
bolic disruptions with Isok60 and the subsequent metabolic stress counterbalanced
the neuromuscular mechanisms that might contribute to a potentiated response
during recovery. Future research must investigate different velocities of isokinetic
contractions with similar durations of work to determine specifically whether it
was the duration or velocity of the exercise that was the prime determinant of
performance facilitation or potentiation.
With the evidence of muscle stress (increased RPE and blood lactate), the lack
of change in RI quadriceps power at 300°.s-1, quadriceps power, total work or peak
torque at 60°.s-1 and hamstrings power or work with 60°.s-1 contractions would

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146 Chaouachi et al.
seem to indicate that the effects of potentiation and metabolic stress were relatively
balanced. Internal fatigue manifestations such as metabolic disruptions may be
experienced during repetitive contractions without a decrement in the targeted
force (3). This type of fatigue may be defined as an increase in the perceived effort
needed to exert a desired force and an eventual inability to produce this force (21).
Fatigue effects may have been more predominant with the decreases in the Isok60
RI hamstrings peak torque with 60°.s-1 isokinetic contractions and quadriceps total
work with 300°.s-1 contractions.
Although, most of the measured variables in the current study did not decrease
and some even increased following the exercise protocol there was conceivably
counterbalancing effects between potentiating and fatigue-inducing factors. Evi-
dence illustrating the presence of physical stress includes RPE scores that were
significantly higher during the Isok60, Isok300 and RI than the pretest; as well; the
RPE scores were significantly higher during the Isok60 and Isok300 than during
the RI. Blood lactate measures demonstrated that the Isok60 and Isok300 placed
a metabolic stress upon the participants’ muscles. The accumulation of blood
lactate during the recovery intervals is similar to the values reported in both adult
(56,61) and pubescent boys’ (49) resistance exercise. Thus, the ability to sustain
forces may be viewed as a compromise between a physical or metabolic stress and
neuromuscular strategies to enhance or sustain performance (3).
The lack of external manifestations of fatigue (with the exception of Isok60
hamstrings peak torque at 60°.s-1 and quadriceps total work with 300°.s-1 con-
tractions) exhibited throughout the RI may be attributed to a number of factors.
Although the intent of each participant was to contract maximally, the contraction
intensity and muscle activation may have been submaximal. Perry-Rana et al. (48)
had adult subjects perform 25 maximal intent, eccentric, isokinetic contractions
and illustrated that the average torque for the first contraction was approximately
75% of maximum and increased to approximately 85% of maximum by the end
of the set. Children do not activate their knee extensors to as great an extent as
adults (children ~65–70% (43,60) vs. adults ~85% (8)). Although the intent was
for maximal contractions, the possibly lower agonist activation levels of the boys
could result in less fatiguing submaximal contractions with the resultant response
being an increase in muscle activation and potentiation (3).
Other explanations for the lack of recovery impairments could be related to
lower anaerobic capacities (10), lactate concentrations (10,70), and muscle gly-
cogen levels (10), faster phosphocreatine resynthesis (53,55) and higher oxidative
capacities (55) of children and adolescents. Youth are also reported to have lower
proportional area of type II muscle fibers (10,53) which would provide greater
relative endurance capacities. Youth are reported to have a greater fatigue resistance
(19,52,53) and recover faster from high intensity exercise than adults due to their
lower maximal power output (30). They have been shown to recover more rapidly
from Wingate anaerobic tests (38), 3 sets of a 10 repetition maximum load (29)
and repeated bouts of high intensity 120°.s-1 isokinetic contractions (70). Similarly,
other studies using either isokinetic contractions (19) or sprints (54) have also
shown greater fatigue resistance in children and adolescents when compared with
adults. These findings may be a reflection of a lower reliance on glycolysis, more
rapid removal of metabolites and/or lower muscle activation levels. As the boys
in the current study were 11–14 years of age and transitioning from and through

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Potentiation and Recovery Responses of Youth 147
pubescence, the aforementioned metabolic factors may have ranged from child-like
to more adult responses dependent upon physiological maturation. However there
were no apparent or significant differences within the group. Finally the findings of
trained boys may not translate to untrained boys. A future study is needed examin-
ing both trained and untrained children subjected to similar durations of high and
low speed isokinetic contractions.
Conclusion
The present study showed that athletic boys were fully recovered (10 out of 12
measures) 2 min following 10 maximal repetitions using either 60°.s-1 or 300°.s-1
isokinetic contractions (Isok60 and Isok300). There was an indication for a velocity
and muscle specific potentiation effect during recovery following the hamstrings
Isok300 which might be attributed to a greater agonist—antagonist torque balance
in boys and less metabolic stress associated with the shorter duration higher velocity
contractions. Based on this study and previous research (6,29), it is recommended
that a rest interval of less than 1–2 min may be necessary to induce fatigue (i.e.,
decreases in force, work or power) in youth following a set of maximal intensity
resistance training. Conversely to improve subsequent performance, muscle poten-
tiation may be achieved two or more minutes following a set of maximal intensity
higher velocity resisted contractions.
Acknowledgments
This study was financially supported by the Tunisian Ministry of Scientific Research, Tech-
nology and Development of Competences, Tunisia. The authors would like to thank the staff
of the National Centre of Medicine and Science in Sports, as well as the young athletes for
their participation in this study. We especially thank Mme Touati Narjess and Mr Kasmi
Sofiene for their assistance with Cybex devise.
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