Plyometric Training Does Not Affect Central and PeripheralMuscle Fatigue Differently in Prepubertal Girls and Boys

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Plyometric Training Does Not Affect Central and Peripheral Muscle Fatigue Differently in Prepubertal Girls and Boys

Article in Pediatric Exercise Science · November 2010

DOI: 10.1123/pes.22.4.547 · Source: PubMed

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547 Pediatric Exercise Science, 2010, 22, 547-556

Plyometric Training Does Not Affect Central and Peripheral Muscle Fatigue Differently in Prepubertal Girls and Boys

Albertas Skurvydas and Marius Brazaitis Lithuanian Academy of Physical Education

The aim of the study was to evaluate the effect of plyometric training (PT) on central and peripheral (muscle) fatigue in prepubertal girls and boys. The boys (n = 13, age 10.3 ± 0.3 years) and girls (n = 13, age, 10.2 ± 0.3 years) performed continuous 2-min maximal voluntary contractions (MVCs) before and after 16 high-intensity PT sessions. PT comprised two training sessions per week of 30 jumps in each session with 20 s between jumps. The greatest effect of PT was on excitation–contraction coupling, (twitch force increased by 323% in boys and 21% in girls) and height of a counter–movement jump (increased by 37% in boys and 38% in girls). In contrast, the quadriceps voluntary activation index, central activation ratio, and MVC did not change significantly after PT. The thickness of the quadriceps muscle increased by 9% in boys and 14% in girls after PT. In conclusion, boys and girls demonstrated similar changes in indicators of central fatigue (50–60% decrease) and peripheral fatigue (45–55% decrease) after MVC before and after PT.

Regular participation in physical activity during childhood is regarded as an important lifestyle factor for improving musculoskeletal health, fitness, and body composition (6,10,29). Recent reports show that short-term (7–10 months) high- impact jumping exercise during prepuberty has a persistent effect over and above the effects of normal growth and development (10,18). Short-term (10 weeks) plyometric training (PT) increases jumping performance in prepubescent boys (5,17). Despite these findings, however, the effects of PT on voluntary and electrically induced muscle performance in prepubertal children are not clear.

Exercise-induced fatigue caused by limitations in either the skeletal muscles or the nervous system (8). The terms peripheral fatigue and central fatigue are used to discriminate between these two possible sites of muscle fatigue (2,16). It is now clear that peripheral fatigue occurs during many types of muscle exercise, but increasing evidence indicates that central fatigue also contributes (8). A recent study using interpolated tetani showed that 8–12-years-old children have a significant deficit in voluntary activation of the quadriceps muscle (27). Our earlier data showed no significant difference in voluntary activation of the quadriceps muscle

Skurvydas and Brazaitis are with the Dept. of Applied Physiology and Physiotherapy, Lithuanian Academy of Physical Education, Kaunas, Lithuania.

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between 12–14-years-old boys and adult men, although we also found that children are more susceptible to central fatigue than are adults (30).

There is some evidence that neuromuscular fatigue in adults is sex dependent (13,14,33), and that women exhibit less neuromuscular fatigue than men. The main cause of this difference is thought to reside in the periphery and not in the central nervous system. However, it is unclear whether this sex difference in neuromuscular fatigue appears at prepubertal age, and whether the effect of PT on neuromuscular fatigue is sex dependent. If the main reason why men’s muscles are more liable to fatigue than women’s reflects differences in fiber-type composition (33), it follows that girls’ muscles should be more resistant to fatigue than boys’ muscle. Our objec- tive was to extend the work of Wüst and colleagues (33) by studying sex differences in muscle fatigue in prepubertal children and the effects of PT plyometric training on central and peripheral (muscle) fatigue.

Materials and Methods

Subjects

We studied 13 boys (age, 10.3 ± 0.3 years, height, 1.46 ± 0.08 m, mass, 41.2 ± 8.3 kg) and 13 girls (age, 10.2 ± 0.3 years, height, 1.43 ± 0.08 m, mass, 37.2 ± 6.3 kg) in experimental group and 10 boys (age, 10.3 ± 0.3 years, height, 1.45 ± 0.07 m, mass, 42.9 ± 6.4 kg) in control group. The subjects of control group participated in all experiments as well as subjects of experimental group, but subjects of control group did not go in for training jumping fitness. This study was approved by the local Ethics Committee. A written informed consent was obtained from the adults, the children’s parents, and children. The children were recruited from a local school.

All participants took part in classes of physical activity twice per week, and could be considered as physically active. However none of the volunteers had specialized in any form of sports training. Boys and girls have not yet developed secondary sex characteristics. This period of development is referred to as preadolescence (6).

Plyometric Training

Children in the experimental group performed PT twice per week for 2 months for a total of 16 training sessions. In every session, the children performed 30 counter- movement jumps (CMJs) on a contact platform with 20 s of rest between each jump.

The children were instructed to jump up as high as possible using an arm swing, i.e., the hands were not positioned on the waist to eliminate arm swing. After each jump, the children were informed of the height of the CMJ and were motivated to perform each jump as high as possible. The H of the CMJ was measured before and after the 16 PT sessions using a multicomponent Kistler force plate (Kistler;

Type 9286A, John Glenn Drive, Amherst). The H of CMJ was calculated by the formula: H (cm) = 11.226 × Tf², where Tf = flight time (s; 4).

Isometric Torque and Electrical Stimulation

The isometric torque of knee extensor muscles was measured using an isokinetic dynamometer (System 3; Biodex Medical Systems, Shiley, New York). The subjects sat upright in the dynamometer chair with the knee joint positioned at 120

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Central and Peripheral Fatigue in Girls and Boys 549

degrees angle (180 degrees—full knee extension). The equipment and procedure for electrical stimulation were essentially the same as previously described (26).

Direct muscle stimulation was applied using two carbonized rubber electrodes, covered with a thin layer of electrode gel (ECG-EEG Gel; Medigel, Modi’in, Israel).

One of the electrodes (6 × 11 cm) was placed transversely across the width of the proximal portion of the quadriceps femoris. Another electrode (6 × 20 cm) covered the distal portion of the muscle above the patella. A standard electrical stimulator (MG 440; Medicor, Budapest, Hungary) was used. The electrical stimulation was delivered in square-wave pulses, 0.5 ms in duration. The tolerance of volunteers to electrical stimulation was assessed on a separate occasion, and only participants who showed good compliance with the procedure were recruited for the study.

The intensity of electrical stimulation was selected individually by applying single stimuli to the muscles tested. During this procedure the current was increased until no increment in single twitch torque could be detected by an additional 10%

increase in current strength.

Ultrasonography: Quadriceps Muscle Thickness

Ultrasonograph (SonoSite; TITAN HST/10–5, Wilbury, Hitchin), using a 5-MHz linear-array probe was applied to obtain axial-plane images of the quadriceps muscle. All measurements were performed on the right leg after the subjects rested in the supine position for 20 min to allow fluid shifts to occur. Measurements were taken in the supine position with the subject in a relaxed state. To measure the thickness of the m. quadriceps femoris, measurements were taken at five points, which were chosen after drawing an imaginary line between the upper edge of the kneecap and spina iliaca anterior superior every 5 cm, i.e., 5, 10, 15, 20, and 25 cm from the upper edge of the kneecap. To compare results, the measurements were taken at symmetrical points on both legs. Muscle thickness was calculated using the values in the cm scale on the monitor screen. The mean value of five measurements was used for analysis.

Experimental Procedures

Before beginning the tests of muscle performance, the thickness of the quadriceps muscle was measured in the experimental and control groups. The children then performed a warm-up comprising 10 squat-stands and 5 min of running in place at an intensity that elicited a heart rate (HR) of 130–150 beats per minute (about 70% of maximum HR). HR was measured with a Polar HR recorder (S-625×, Polar Electro, Kempele, Finland). The H of the control jump was then measured; each subject performed 3–5 CMJs, with 20 s between each jump, and the best attempt was recorded.

After 3 min of jumping, the child was positioned in the Biodex system 3 dynamometer chair and the stimulating electrodes were placed on the right leg.

Single stimuli were applied every 30 s at a progressively higher intensity until the required current was reached; 4–5 stimuli were usually delivered. After a 5-min rest, the force-generating capacity of the quadriceps muscle was assessed by applying 1-s trains of electrical stimuli at 1, 10, 20, 50 and 100 Hz; an interval of 2–3 s was needed to change the stimulation frequency. After a 5-min rest, three 5-s MVCs were obtained with a 2-min rest between each. At ~3 s of the MVC, a 250-ms test

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train of stimuli at 100 Hz (TT100Hz) was superimposed on the voluntary contraction. The same TT100Hz was repeated 1–2 s after the MVC. These TT100Hz contractions were used to assess the voluntary activation of the knee extensors. A 5-min rest was allowed after the third MVC, and the experiment was terminated after a 2-min MVC. The TT100Hz was superimposed on the contraction at ~3, 29, 59, 89, and 119 s. At ~30, 60, 90, and 120 s, the knee extensors were relaxed for 2–3 s, and TT100Hz was delivered using the same protocol as used after the brief MVC in the first part of the experiment.

The amplitude of the superimposed tetani was calculated from the baseline, which was estimated as the average torque over 1 s just before the stimulation. The superimposed TT100Hz produced measurable torque increments in all subjects.

The sensitivity of the Biodex System 3 in torque measurements is ± 1.36 Nm.

For the voluntary activation index (VA), the TT100Hz torque of the relaxed muscles was used as the control torque, and the following formula was applied: VA (%) = [1—(superimposed TT100Hz torque / control TT100Hz torque)] × 100 (30).

Voluntary and electrically induced muscle performances were tested 2–3 days before the start of the first PT and 2–3 days after the final PT session in experimental group. Boys from the control group were tested twice separated by 8 weeks.

Statistics

Descriptive data are presented as means and SD. The effects of training (before vs.

after training) and gender (girls vs. boys) on neuromuscular contractile properties were assessed using two-way repeated measures analyses of variances (ANOVA).

If significant effects were found, post hoc testing was performed, applying paired t tests with a Bonferroni correction for multiple comparisons. Statistical significance of all tests was set at p < .05. Statistical power (SP) was calculated for all indicators.

Fatigue index (FI) of MVC, TT100Hz and VA were calculated. FI= (before 2-min of MVC– after 2-min of MVC) / before 2-min of MVC) × 100%.

Results

Electrically Induced Muscle Performance

Data on electrically induced muscle performance before and after the 8 weeks of PT are presented in Figure 1. In general, knee extension torque increased (p <

.0001) with the frequency of electrical stimulation, although the differences from before to after training were not significant, except for electrically induced torque at 1 Hz in boys. The twitch torque increased after PT by 323.2% ± 210.8% (p <

.001; SP > 99%) in boys and by 21.2% ± 48.2% (p > .05) in girls; the twitch torque did not change significantly in the control group. The twitch-to-tetanic ratio (100 Hz-induced force) increased from 0.054 ± 0.022–0.183 ± 0.029 (p < .001; SP >

99%) in boys and from 0.072 ± 0.047–0.075 ± 0.049 (p > .05) in girls after PT.

Voluntarily Induced Muscle Performance

The MVC did not change significantly after PT (Figure 2), whereas the H of the CMJ increased by 36.7% ± 11.7% (from 24.1 ± 3.8 cm to 32.8 ± 5.1 cm, p < .001;

SP > 99%) in boys and by 37.7% ± 12.8% (from 21.8 ± 3.3 cm to 29.9 ± 3.8 cm,

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Figure 2 — Knee extension torque during the continuous 2-min maximal voluntary con- traction before and after plyometric training. Values are means ± SD * p < .05, compared with the value at 3 s.

Figure 1 — Electrically induced quadriceps muscle isometric torque before and after plyo- metric training. Values are means ± SD * p < .05, compared between before and after training.

p < .001; SP > 99%) in girls. There was no significant difference in voluntary and electrically induced muscle performance between boys in the control and experi- mental groups before PT (p > .05). MVC, VA, and the H of the CMJ did not change significantly after 8 weeks in the control group (p > .05).

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Central and Peripheral Fatigue During the 2-Min MVC

Data on knee extension torque during the 2-min MVC are presented in Figure 2.

The torque decreased (p < .001; SP > 99%) during the prolonged MVC in both boys and girls; the changes from before to after PT were not significant. Data on the VA of the knee extensor muscles during the 2-min MVC are presented in Figure 3.

The VA at 3 s of the 2-min MVC before PT was 74.4% ± 17.5% in boys and 70.4%

± 17.2% in girls; the respective values after PT were 81.7% ± 15.4% and 76.9%

± 13.9%. The VA did not differ significantly between boys and girls, and did not change significantly after PT. The VA during the 2-min MVC was calculated every 30 s. The fatigue index of VA, an indicator of fatigue, decreased significantly (p

< .001; SP > 95%), after the 2-min MVC in boys and girls both before and after PT. The fatigue index of VA was 53.1% ± 16.2% before PT and 64.7% ± 22.4%

after PT in boys and 50.5% ± 20.1% before PT and 50.8% ± 17.4% after PT in girls. The fatigue index of VA did not differ significantly between boys and girls.

Peripheral fatigue (changes in TT100Hz; Figure 4) and central fatigue (changes in VA; Figure 3) did not change significantly after PT in girls and boys or after 8 weeks in the control group.

Changes in Muscle Thickness

After PT, the thickness of the quadriceps muscle increased significantly by 8.7%

± 7.9% in boys (from 2.5 ± 0.42 cm to 2.85 ± 0.43 cm, p < .05; SP = 68.7) and by 13.7% ± 8.9% in girls (from 2.07 ± 0.22 cm to 2.39 ± 0.26 cm, p < .05; SP = 74.1). The thickness of the quadriceps muscle in the control group (boys) was 2.44

± 0.43 cm before and 2.51 ± 0.43 cm after the 8 weeks (p > .05).

Figure 3 — Voluntary activation index of the quadriceps muscle during the 2-min of maxi- mal voluntary contraction of knee extensors before and after plyometric training. Values are means ± SD * p < .05, compared with the value at 3 s.

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Discussion

The main finding of our study was that the lack of significant difference between girls and boys in indicators of central and peripheral fatigue both before and after PT. On the basis of adult data (13,14,33), we expected that neuromuscular fatigue would be greater in boys than in girls. Several studies have demonstrated higher muscle glycolytic enzyme activities in men than in women (25), which is consistent with a lower proportion of slow, fatigue-resistant, type I fibers in men (24,28).

Contrary to our expectations, we found no significant differences between girls and boys in changes in muscle force–generating capacity. We found: a) that the H of the CMJ and thickness of the quadriceps muscle increased significantly in both boys and girls after PT and that the extent of these increases did not differ between boys and girls, and b) that PT significantly affected excitation–contraction coupling, as shown by the increase in twitch torque, only in boys.

PT affected voluntarily induced muscle performance both in girls and boys, as shown by the significantly increased H of the CMJ. The increases in twitch torque suggest possible adaptations in muscle excitation–contraction coupling (22). An increase in the twitch-to-tetanus ratio is associated with increasing slowness of the muscle. In general, the greatest changes after PT occur in the periphery, i.e., in excitation–contraction coupling. It is not clear, however, why the increase in twitch torque was much greater in boys than in girls after PT. We speculate that excitation–contraction coupling adapts faster in boys than in girls.

We believe that the specific characteristics of the PT training increased the H of the CMJ more than MVC. Venturelli et al. (32) showed that 12 weeks of Figure 4 — Fatigue index of maximal voluntary contraction (MVC) torque, and electro- stimulation-induced torque at 100 Hz for 250 ms (TT100Hz) during a 2-min MVC before and after plyometric training.

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sprint training in preadolescent soccer players increased maximal running speed but did not affect jumping height, possibly because of the specificity of training.

In a review, Markovic (19) showed that, in adults, vertical jump H increased by 6–10% after 16–20 PT sessions; this increase was only one third of that shown by the prepubertal children in our study. The 36% increase in the H of the CMJ in our study agrees with the data of Kotzamanidis et al. (17), although we used 16 PT sessions and they used 20 PT sessions. In the study of Kotzamanidis et al., the children performed 2–3 times as many jumps (60–100 jumps) in each PT session than in our study (30 jumps). We think that the large increase in the H of the CMJ in our study was because our children performed jumps every 20 s without experiencing fatigue, the children were told of the height of each jump, and they were encouraged to perform each jump as high as possible.

The H of the CMJ increased more than thickness of quadriceps muscle, which is consistent with the concept that, in children, increases in strength relate more to increased neuromuscular activation and coordination than muscle hypertrophy, although strength training in children results the increase in muscle mass (6,7,11,22).

This is consistent with our data. In adults, it is generally accepted that early increases in strength after strength training occur because of changes in neural mechanisms and not because of muscle hypertrophy (9). In our study the increase in thickness of quadriceps muscle was significant but the greater increase was in the H of the CMJ.

The data on voluntary activation of skeletal muscles in children are more controversial. Applying single twitches as interpolated contractions during MVC, two earlier studies showed an almost complete activation of the plantar flexor and knee extensor muscles in children and adolescent boys (1,3). However, assess- ments using single twitches can overestimate VA because of a low signal-to-noise ratio. We used interpolated tetani instead of twitches. As expected, the VA values were lower in our study than in the two earlier studies of children that used single twitches (1,3). Our VA values for boys were slightly higher than those reported in another study that used similar methods (~70–82% vs. ~68%, respectively; 27).

Irrespective of the PT, all volunteers showed a similar relative decrease in MVC torque at the end of the 2-min contraction. These torque changes were also similar to those reported previously for ankle dorsiflexors during a continuous MVC (16). Discussion of the ability to exercise at high intensity often focuses on the muscle properties (20,23). A lower glycolytic rate, which results in less lactate accumulation in skeletal muscles, is given as the major reason for lower power output, greater fatigue resistance, and faster recovery in children than in adults (12,15,21,26,31). However, in our study, the jumps in each PT session were performed every 20 s we did not expect the boys’ muscles to be more resistant to metabolic fatigue after the PT.

Conclusions

In conclusion, the main difference between girls and boys in neuromuscular adapta- tion after PT was that excitation–contraction coupling changed significantly only in boys. The changes in the H of the CMJ and muscle thickness from before to after PT did not differ between boys and girls. PT did not affect voluntary activation of the quadriceps muscle in girls or boys. In general, the indicators of central and peripheral fatigue did not differ significantly between girls and boys both before and after PT performed at maximal intensity.

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