Rest insertion combined with high-frequency loading enhances osteogenesis.

Mechanical loading can significantly affect skeletal adaptation. High-frequency loading can be a potent osteogenic stimulus. Additionally, insertion of rest periods between consecutive loading bouts can be a potent osteogenic stimulus. Thus we investigated whether the insertion of rest-periods between short-term high-frequency loading bouts would augment adaptation in the mature murine skeleton. Right tibiae of skeletally mature (16 wk) female C57BL/6 mice were loaded in cantilever bending at peak of 800 microepsilon, 30 Hz, 5 days/wk for 3 wk. Left tibiae were the contralateral control condition. Mice were randomly assigned into one of two groups: continuous high-frequency (CT) stimulation for 100 s (n = 9), or 1-s pulses of high-frequency stimuli followed by 10 s of rest (RI) for 100 s (n = 9). Calcein labels were administered on days 1 and 21; label incorporation was used to histomorphometrically assess periosteal and endosteal indexes of adaptation. Periosteal surface referent bone formation rate (pBFR/BS) was significantly enhanced in CT (>88%) and RI (>126%) loaded tibiae, relative to control tibiae. Furthermore, RI tibiae had significantly greater pBFR/BS, relative to CT tibiae (>72%). The endosteal surface was not as sensitive to mechanical loading as the periosteal surface. Thus short-term high-frequency loading significantly elevated pBFR/BS, relative to control tibiae. Furthermore, despite the 10-fold reduction in cycle number, the insertion of rest periods between bouts of high-frequency stimuli significantly augmented pBFR/BS, relative to tibiae loaded continually. Optimization of osteogenesis in response to mechanical loading may underpin the development of nonpharmacological regiments designed to increase bone strength in individuals with compromised bone structures.     

Differential effects of jump versus running exercise on trabecular architecture during remobilization after suspension-induced osteopenia in growing rats

High-impact exercise is considered to be very beneficial for bones. We investigated the ability of jump exercise to restore bone mass and structure after the deterioration induced by tail suspension in growing rats and made comparisons with treadmill running exercise. Five-week-old male Wistar rats (n = 28) were randomly assigned to four body weight-matched groups: a spontaneous recovery group after tail suspension (n = 7), a jump exercise group after tail suspension (n = 7), a treadmill running group after tail suspension (n = 7), and age-matched controls without tail suspension or exercise (n = 7). Treadmill running was performed at 25 m/min, 1 h/day, 5 days/wk. The jump exercise protocol consisted of 10 jumps/day, 5 days/wk, with a jump height of 40 cm. Bone mineral density (BMD) of the total right femur was measured by dual-energy X-ray absorptiometry. Three-dimensional trabecular bone architecture at the distal femoral metaphysis was evaluated using microcomputed tomography. After 5 wk of free remobilization, right femoral BMD, right hindlimb muscle weight, and body weight returned to age-matched control levels, but trabeculae remained thinner and less connected. Although both jump and running exercises during the remobilization period increased trabecular bone mass, jump exercise increased trabecular thickness, whereas running exercise increased trabecular number. These results indicate that restoration of trabecular bone architecture induced by jump exercise during remobilization is predominantly attributable to increased trabecular thickness, whereas running adds trabecular bone mass through increasing trabecular number, and suggest that jumping and running exercises have different mechanisms of action on structural characteristics of trabecular bone.     

Muscle dynamics differences between legs in healthy adults

Differences in muscle dynamics between the preferred and nonpreferred jumping legs of subjects in maximal, explosive exercise were examined. Eight subjects performed nonfatiguing bouts of single-legged drop jumps and rebound jumps on a force sledge apparatus. Measures of flight time, reactive strength index, peak vertical force, and vertical leg-spring stiffness were obtained for 3 drop jumps and 3 rebound jumps on both legs. Subjects utilized a stiffer leg spring and a more explosive jumping action in the nonpreferred leg when performing a cyclical rebound jumping task in comparison to a single drop jump task (observed through differences in vertical leg-spring stiffness, peak vertical force, and reactive strength index, p < 0.05). The preferred leg performed equally well in both tasks. Between-leg analysis showed no differences in dependent variables between the preferred and the nonpreferred leg in the rebound jumping protocol. However, the drop jump protocol showed significant performance differences, with flight time and reactive strength index greater in the preferred leg than the nonpreferred leg (p < 0.05). We hypothesize that, throughout the lifespan, both legs are equally trained in cyclical rebound jumping tasks through running. However, because a preferred leg must be selected when performing any one-off, single-legged jump, imbalances in this specific task develop over time with consistent selection of a preferred jumping leg. The data demonstrate that the rebound jump protocol is representative of the symmetrical mechanics of forward running and that leg-spring stiffness is modulated depending on the demands of the specific task involved. Strength and conditioning practitioners should give careful consideration to appropriate jump protocol selection and should exercise caution when comparing laboratory results to data gathered in field testing.     

Jump exercise during remobilization restores integrity of the trabecular architecture after tail suspension in young rats.

Three-dimensional trabecular architecture was investigated in the femora of tail-suspended young growing rats, and the effects of jump exercise during remobilization were examined. Five-week-old male Wistar rats (n = 35) were randomly assigned to five body weight-matched groups: tail-suspended group (SUS; n = 7); sedentary control group for SUS (S(CON); n = 7); spontaneous recovery group after tail suspension (S+R(CON), n = 7); jump exercise group after tail suspension (S+R(JUM); n = 7); and age-matched control group for S+R(CON) and S+R(JUM) without tail suspension and exercise (S(CON)+R(CON); n = 7). Rats in SUS and S(CON) were killed immediately after tail suspension for 14 days. The jump exercise protocol consisted of 10 jumps/day, 5 days/wk, and jump height was 40 cm. Bone mineral density (BMD) of the femur and three-dimensional trabecular bone architecture at the distal femoral metaphysis were measured. Tail suspension induced a 13.6% decrease in total femoral BMD (P < 0.001) and marked deterioration of trabecular architecture. After 5 wk of free remobilization, femoral BMD, calf muscle weight, and body weight returned to age-matched control levels, but trabeculae remained thinner and less connected. On the other hand, S+R(JUM) rats showed significant increases in trabecular thickness, number, and connectivity compared with S+R(CON) rats (62.8, 31.6, and 24.7%, respectively; P < 0.05), and these parameters of trabecular architecture returned to the levels of S(CON)+R(CON). These results indicate that suspension-induced trabecular deterioration persists after remobilization, but jump exercise during remobilization can restore the integrity of trabecular architecture and bone mass in the femur in young growing rats.     

Effect of different rest intervals after whole-body vibration on vertical jump performance

Whole-body vibration (WBV) may potentiate vertical jump (VJ) performance via augmented muscular strength and motor function. The purpose of this study was to evaluate the effect of different rest intervals after WBV on VJ performance. Thirty recreationally trained subjects (15 men and 15 women) volunteered to participate in 4 testing visits separated by 24 hours. Visit 1 acted as a familiarization visit where subjects were introduced to the VJ and WBV protocols. Visits 2-4 contained 2 randomized conditions per visit with a 10-minute rest period between conditions. The WBV was administered on a pivotal platform with a frequency of 30 Hz and an amplitude of 6.5 mm in 4 bouts of 30 seconds for a total of 2 minutes with 30 seconds of rest between bouts. During WBV, subjects performed a quarter squat every 5 seconds, simulating a countermovement jump (CMJ). Whole-body vibration was followed by 3 CMJs with 5 different rest intervals: immediate, 30 seconds, 1 minute, 2 minutes, or 4 minutes. For a control condition, subjects performed squats with no WBV. There were no significant (p > 0.05) differences in peak velocity or relative ground reaction force after WBV rest intervals. However, results of VJ height revealed that maximum values, regardless of rest interval (56.93 ± 13.98 cm), were significantly (p < 0.05) greater than the control condition (54.44 ± 13.74 cm). Therefore, subjects' VJ height potentiated at different times after WBV suggesting strong individual differences in optimal rest interval. Coaches may use WBV to enhance acute VJ performance but should first identify each individual's optimal rest time to maximize the potentiating effects.     

Comparison of the symptoms of exercise-induced muscle damage after an initial and repeated bout of plyometric exercise in men and boys.

The purpose of this study was to compare symptoms of exercise-induced muscle damage after an initial and repeated bout of plyometric exercise in men and boys. Ten boys (9-10 yr) and 10 men (20-29 yr) completed two bouts of eight sets of 10 plyometric jumps, 2 wk apart. Perceived soreness (0-10, visual analog scale), isometric strength of the quadriceps at six knee flexion angles, and countermovement jump and squat jump height were assessed before and at 30 min, 24 h, 48 h, and 72 h after each bout. All variables followed the expected patterns of change in men, with soreness peaking at 24-48 h (5.8 +/- 1.7) and decrements in muscle function peaking at 30 min after the first bout (73-85% of baseline scores). Symptoms remained for 72 h after the first bout in men. In boys, symptoms were much less severe and peaked at 30 min (visual analog scale = 2.1 +/- 1.8, functional decrements 87-92% of baseline) and, with the exception of soreness, returned to baseline after 24 h. After the second bout of plyometric exercise, the level of soreness and decrements in countermovement jump, squat jump, and isometric strength were lower, although the effect was stronger in men, in all cases. The results of this study suggest that although children may experience symptoms of muscle damage after intensive plyometric exercise, they are much less severe. A prior bout of plyometric exercise also appears to provide children with some protection from soreness after a subsequent bout of plyometric exercise. Explanations for milder symptoms of exercise-induced muscle damage in children include greater flexibility leading to less overextension of sarcomeres during eccentric exercise, fewer fast-twitch muscle fibers, and greater and perhaps more varied habitual physical activity patterns.     

The optimal complex training rest interval for athletes from anaerobic sports

Complex training research has indicated that 3-4 minutes may be an optimum intracomplex rest interval. The purpose of this study was to determine if a heavy resistive exercise causes performance enhancement of a slow stretch-shortening cycle exercise and if there is an optimal rest interval. Eighteen subjects performed countermovement jumps (CMJs) before and after a 5 repetition maximum back squat lifting protocol. This procedure was repeated 4 times over 2 days using rest intervals of 30 seconds and 2, 4, and 6 minutes. Flight time and peak ground reaction force (GRF) were the dependent variables. All jumps were performed on a specially constructed sledge and force platform apparatus. Repeated measures analysis of variance found a significant reduction in flight time at the 30-second and 6-minute interval (p < 0.05). No significant difference was found between men and women. Only the men showed an enhancement in jump performance after the 4-minute interval. The improvement window was different for each subject, and an analysis of the greatest increase and decrease in flight time and peak GRF was conducted, showing a significant decrease for men and women and a significant increase in flight time for men and peak ground reaction force for women. The results suggest that complex training can benefit or inhibit CMJ performance depending on the rest interval. The individual determination of the intracomplex rest interval may be necessary in the practical setting.     

Rest-inserted loading rapidly amplifies the response of bone to small increases in strain and load cycles.

We hypothesized that a 10-s rest interval (at zero load) inserted between each load cycle would increase the osteogenic effects of mechanical loading near previously identified thresholds for strain magnitude and cycle numbers. We tested our hypothesis by subjecting the right tibiae of female C57BL/6J mice (16 wk, n = 70) to exogenous mechanical loading within a peri-threshold physiological range of strain magnitudes and load cycle numbers using a noninvasive murine tibia loading device. Bone responses to mechanical loading were determined via dynamic histomorphometry. More specifically, we contrasted bone formation induced by cyclic vs. rest-inserted loading (10-s rest at zero load inserted between each load cycle) by first varying peak strains (1,000, 1,250, or 1,600 micro epsilon) at fixed cycle numbers (50 cycles/day, 3 days/wk for 3 wk) and then varying cycle numbers (10, 50, or 250 cycles/day) at a fixed strain magnitude (1,250 micro epsilon). Within the range of strain magnitudes tested, the slope of periosteal bone formation rate (p.BFR/BS) with increasing strain magnitudes was significantly increased by rest-inserted compared with cyclical loading. Within the range of load cycles tested, the slope of p.BFR/BS with increasing load cycles of rest-inserted loading was also significantly increased by rest-inserted compared with cyclical loading. In sum, the data of this study indicate that inserting a 10-s rest interval between each load cycle amplifies bone's response to mechanical loading, even within a peri-threshold range of strain magnitudes and cycle numbers.     

Protection against muscle damage following fifty drop jumps conferred by ten drop jumps

This study investigated whether 10 drop jumps (DJs) would confer protective effect against muscle damage and soreness in a subsequent bout of 50 DJs. Sixteen men were randomly placed into either a group performing 1 set of 10 DJs followed by 5 sets of 10 DJs (10-50, n = 8) or another group performing 2 bouts of 5 sets of 10 DJs (50-50, n = 8) separated by 2 weeks. The DJs were performed from a box height of 0.6 m, with a 10-second interval between jumps and a 1-minute rest between sets. Jump height, peak vertical ground reaction force, ground contact time, and heart rate during DJs were measured, and blood lactate concentration was assessed before and immediately after DJs. Changes in maximal isometric (ISO) and isokinetic concentric torque (CON), vertical jump, muscle soreness, and plasma creatine kinase activity before, immediately after, and at 1, 24, 48, and 72 hours following exercise were compared between groups for the first and second bouts and between the bouts by a 2-way repeated-measures analysis of variance. Changes in ISO, CON, vertical jump, and muscle soreness were significantly (p < 0.05) smaller for 10 DJs compared with 50 DJs; however, no significant differences in the measures between groups were evident following the second bout. The changes in the measures following 50 DJs in the 10-50 group were significantly (p < 0.05) smaller than those following the first bout of the 50-50 group. These results suggest that 10 DJs and 50 DJs conferred the same magnitude of protective effect against muscle damage by 50 DJs.     

Effect of low-repetition jump training on bone mineral density in young women.

The hypothesis of the present study was that low-repetition and high-impact training of 10 maximum vertical jumps/day, 3 times/wk would be effective for improving bone mineral density (BMD) in ordinary young women. Thirty-six female college students, with mean age, height, and weight of 20.7+/-0.7 yr, 158.9+/-4.6 cm, and 50.4+/-5.5 kg, respectively, were randomly divided into two groups: jump training and a control group. After the 6 mo of maximum vertical jumping exercise intervention, BMD in the femoral neck region significantly increased in the jump group from the baseline (0.984+/-0.081 vs. 1.010+/-0.080 mg/cm2; P<0.01), although there was no significant change in the control group (0.985+/-0.0143 vs. 0.974+/-0.134 mg/cm2). And also lumbar spine (L2-4) BMD significantly increased in the jump training group from the baseline (0.991+/-0.115 vs. 1.015+/-0.113 mg/cm2; P<0.01), whereas no significant change was observed in the control group (1.007+/-0.113 vs. 1.013+/-0.110 mg/cm2). No significant interactions were observed at other measurement sites, Ward's triangle, greater trochanter, and total hip BMD. Calcium intakes and accelometry-determined physical daily activity showed no significant difference between the two groups. From the results of the present study, low-repetition and high-impact jumps enhanced BMD at the specific bone sites in young women who had almost reached the age of peak bone mass.     

Effect of static stretching with different rest intervals on muscle stiffness

The aim of the study was to investigate the effect of static stretching (SS) with different rest intervals on muscle stiffness. Fifteen healthy males participated in the study. Four bouts of thirty-second SS for the gastrocnemii were performed at the maximal dorsiflexion using dynamometer with two different rest intervals between stretches, namely 0 s (R0) and 30 s (R30). Each participant underwent both stretching protocols at least 48 h apart in a random order. Between each bout of SS, the ankle was moved to 20°-plantar-flexion in 3 s, held for each rest interval time, and then returned to the stretching position in 3 s. The shear elastic modulus of the medial gastrocnemius was measured before (PRE) and immediately after (POST) four bouts of SS to assess muscle stiffness of the medial gastrocnemius. Two-way repeated measures analysis of variance (protocol × time) indicated a significant interaction effect on the shear elastic modulus. The shear elastic modulus significantly decreased after SS in both protocols [R0, PRE: 11.5 ± 3.3 kPa, POST: 10.0 ± 2.6 kPa, amount of change: 1.6 ± 0.9 kPa (13.0 ± 5.2%); R30, PRE: 11.0 ± 2.8 kPa, POST: 10.2 ± 2.1 kPa, amount of change: 0.8 ± 1.3 kPa (6.0 ± 10.4%)]. Furthermore, the SS with 0-s rest interval induced greater decrease in shear elastic modulus when compared to SS with 30-s rest interval (p = 0.023). Thus, when performing SS to decrease muscle stiffness, rest intervals between stretches should be minimized.     

The influence of resting period length on jumping performance

The purpose of this study was to determine a resting interval between countermovement jumps (i.e., volleyball spikes) that allows the maintenance of maximal jumping performance. Ten male volleyball players (1.85 +/- 0.05 m, 77.2 +/- 10.6 kg, 21.6 +/- 5.3 years) performed 6 experimental jumping sessions. In the first and sixth sessions, maximal countermovement jump height was measured, followed by submaximal countermovement jumps to the point of volitional fatigue. The number of countermovement jumps was used as a reference to test the effect of rest period between volleyball spikes. From the second to fifth experimental sessions, 30 maximal volleyball spikes were performed with different resting periods (i.e., 8, 14, 17, and 20 seconds) followed by countermovement jumps. Between the 15th and 30th spikes, the blood lactate concentration and heart rate were measured. Because the performance on the first and sixth sessions was the same, no training effects were noticed. During the 8-second resting interval set, the lactate concentration increased significantly between the 15th and 30th spikes (i.e., from 3.37 +/- 1.16 mmol to 4.94 +/- 1.49 mmol); the number of countermovement jumps decreased significantly after spikes compared to those performed without a previous effort (i.e., from 23 +/- 7 jumps to 17 +/- 9 jumps); and these variables were significantly correlated (r = -0.7). On the other hand, the lactate concentration and number of countermovement jumps were stable across the other resting intervals, without a heart rate steady state. The results indicate that an adequate resting period between spikes allowed participants to achieve a lactate steady state in which the performance was maintained during the exercise. These findings show that resting intervals between 14 and 17 seconds, typical during volleyball matches, are indicated to use in volleyball spike drills due to their capacity to maintain maximal jumping performance.     

Comparison of creatine monohydrate and carbohydrate supplementation on repeated jump height performance

Creatine monohydrate (CrMH) supplementation aids the ability to maintain performance during repeated bouts of high-intensity exercise, including jump performance. However, carbohydrate supplementation may also provide similar benefits and is less expensive. This study compared the effects of an energy-free placebo, 2 different caloric concentrations of carbohydrate drinks, and a CrMH supplement on repeated jump heights. Sixty active males (mean age, 22 +/- 3.2 years) performed 2 sets of countermovement static jump height tests (10 jumps over 60 seconds) separated by 5 days to determine the differential effects of the placebo, carbohydrate, and CrMH on jump height sustainability over 10 jumps. Subjects were randomly assigned to groups (15 subjects per group) to receive daily doses (x5 days) of carbohydrate drinks containing 100 or 250 kilocalories (kcal), a 25-g CrMH supplement, or an energy-free placebo. After 5 days, the CrMH group experienced a significant weight gain (+1.52; +/-0.89 kg, p < 0.01), while the other groups did not. The 2 levels of carbohydrate and CrMH supplements were all significantly better at sustaining jump height than the energy-free placebo over the final 3-4 jumps. The 250-kcal carbohydrate-supplemented group experienced a level of benefit (p < 0.01) that was at least equal to that of the CrMH group (p < 0.05), suggesting that the higher dose of carbohydrate was as effective as CrMH in maintaining repeated bouts of high-intensity activity as measured by repeated static jumps. Given the equivalent performance improvement and the absence of weight gain, the carbohydrate supplementation could be considered the preferred option for weight-conscious power athletes involved in activities that require repeated- motion high-intensity activities.     

Bones benefits gained by jump training are preserved after detraining in young and adult rats.

We investigated the osteogenic responses to jump training and subsequent detraining in young and adult male rats to test the following hypotheses: 1) jump training has skeletal benefits; 2) these skeletal benefits are preserved with subsequent detraining throughout bone morphometric changes; and 3) there are no differences between young and adult rats during detraining in terms of the maintenance of exercise-induced changes. Twelve-week-old (young) and 44-wk-old (adult) rats were divided into the following four groups: young-sedentary, young-exercised, adult-sedentary, and adult-exercised. The exercised groups performed jump training (height = 40 cm, 10 jumps/day, 5 days/wk) for 8 wk followed by 24 wk of being sedentary. Tibial bone mineral content and bone mineral density in vivo significantly increased with jump training, and the effects were maintained after detraining in both the young and adult exercised groups, although the benefits of training became somewhat diminished. After 24 wk of detraining, the beneficial effects of training on bone mass and strength were preserved and associated with morphometric changes, such as periosteal perimeter, cortical area, and moment of inertia. There were no significant age-exercise interactions in such parameters, except for the periosteal perimeter. These results suggest that there are few differences in bone accommodation and maintenance by training and detraining between young and adult rats.     

Response of ventilatory and lactate thresholds to continuous and interval training

The purpose of this study was to evaluate the effects of continuous and interval training on changes in lactate and ventilatory thresholds during incremental exercise. Seventeen males were assigned to one of three training groups: group 1:55 min continuous exercise at approximately 50% maximum O2 consumption (VO2max); group 2: 35 min continuous exercise at approximately 70% VO2max; and group 3: 10 X 2-min intervals at approximately 105% VO2max interspersed with rest intervals of 2 min. All of the subjects were tested and trained on a cycle ergometer 3 day/wk for 8 wk. Lactate threshold (LT) and ventilatory threshold (VT) (in addition to maximal exercise measures) were determined using a standard incremental exercise test before and after 4 and 8 wk of training. VO2max increased significantly in all groups with no statistically significant differences between the groups. Increases (+/- SE) in LT (ml O2 X min-1) for group 1 (569 +/- 158), group 2 (584 +/- 125), and group 3 (533 +/- 88) were significant (P less than 0.05) and of the same magnitude. VT also increased significantly (P less than 0.05) in each group. However, the increase in VT (ml O2 X min-1) for group 3 (699 +/- 85) was significantly greater (P less than 0.05) than the increases in VT for group 1 (224 +/- 52) and group 2 (404 +/- 85). For group 1, the posttraining increase in LT was significantly greater than the increase in VT (P less than 0.05). We conclude that both continuous and interval training were equally effective in augmenting LT, but interval training was more effective in elevating VT.(ABSTRACT TRUNCATED AT 250 WORDS)     

Selected contribution: acute cellular and molecular responses to resistance exercise.

Training protocols apply sequential bouts of resistance exercise (RE) to induce the cellular and molecular responses necessary to produce compensatory hypertrophy. This study was designed to 1) define the time course of selected cellular and molecular responses to a single bout of RE and 2) examine the effects of interbout rest intervals on the summation of these responses. Rat muscles were exposed to RE via stimulation of the sciatic nerve in vivo. Stimulated and control muscles were obtained at various time points post-RE and analyzed via Western blot and RT-PCR. A single bout of RE increased intracellular signaling (i.e., phosphorylations) and expression of mRNAs for insulin-like growth factor-I system components and myogenic markers (e.g., cyclin D1, myogenin). A rest interval of 48 h between RE bouts resulted in much greater summation of myogenic responses than 24- or 8-h rest intervals. This experimental approach should be useful for studying the regulatory mechanisms that control the hypertrophy response. These methods could also be used to compare and contrast different exercise parameters (e.g., concentric vs. eccentric, etc.).     

High-impact exercise and growing bone: relation between high strain rates and enhanced bone formation.

We investigated whether high-impact drop jumps could increase bone formation in the middiaphyseal tarsometatarsus of growing rooster. Roosters were designated as sedentary controls (n = 10) or jumpers (n = 10). Jumpers performed 200 drop jumps per day for 3 wk. The mechanical milieu of the tarsometatarsus was quantified via in vivo strain gauges. Indexes of bone formation and mechanical parameters were determined in each of twelve 30 degrees sectors subdividing the middiaphyseal cortex. Compared with baseline walking, drop jumping produced large peak strain rates (+740%) in the presence of moderately increased peak strain magnitudes (+30%) and unaltered strain distributions. Bone formation rates were significantly increased by jump training at periosteal (+40%) and endocortical surfaces (+370%). Strain rate was significantly correlated with the specific sites of increased formation rates at endocortical but not at periosteal surfaces. Previously, treadmill running did not enhance bone growth in this model. Comparing the mechanical milieus produced by running and drop jumps revealed that jumping significantly elevated only peak strain rates. This further emphasized the sensitivity of immature bone to high strain rates.     

Enhancement of fat metabolism by repeated bouts of moderate endurance exercise.

This study compared the fat metabolism between "a single bout of prolonged exercise" and "repeated bouts of exercise" of equivalent exercise intensity and total exercise duration. Seven men performed three trials: 1) a single bout of 60-min exercise (Single); 2) two bouts of 30-min exercise, separated by a 20-min rest between exercise bouts (Repeated); and 3) rest. Each exercise was performed with a cycle ergometer at 60% of maximal oxygen uptake. In the Single and Repeated trials, serum glycerol, growth hormone, plasma epinephrine, and norepinephrine concentrations increased significantly (P<0.05) during the first 30-min exercise bout. In the Repeated trial, serum free fatty acids (FFA), acetoacetate, and 3-hydroxybutyrate concentrations showed rapid increases (P<0.05) during a subsequent 20-min rest period. During the second 30-min exercise bout, FFA and epinephrine responses were significantly greater in the Repeated trial than in the Single trial (P<0.05). Moreover, the Repeated trial showed significantly lower values of insulin and glucose than the Single trial. During the 60-min recovery period after the exercise, FFA, glycerol, and 3-hydroxybutyrate concentrations were significantly higher in the Repeated trial than in the Single trial (P<0.05). The relative contribution of fat oxidation to the energy expenditure showed significantly higher values (P<0.05) in the Repeated trial than in the Single trial during the recovery period. These results indicate that repeated bouts of exercise cause enhanced fat metabolism compared with a single bout of prolonged exercise of equivalent total exercise duration.     

Evaluation of plyometric intensity using electromyography

The purpose of this study was to investigate the motor unit activation of the quadriceps (Q), hamstring (H), and gastrocnemius (G) muscle groups during a variety of plyometric exercises to further understand the nature of these exercises. Twenty-three athletes volunteered to perform randomly ordered plyometric exercises, thought to cover a continuum of intensity levels, including two-foot ankle hops; 15-cm cone hops; tuck, pike, and box jumps; one- and two-leg vertical jump and reach; squat jumps with approximately 30% of their 1RM squat load; and 30- and 61-cm depth jumps. Integrated electromyographic data were analyzed for each exercise using a one-way repeated-measures ANOVA. Results revealed significant main effects for the Q when all subjects are analyzed, as well as for separate analysis of men, women, subjects with vertical jumps greater than 50 cm, and those with vertical jumps less than or equal to 50 cm (p < or = 0.05). Significant main effects were also found for the G muscle group in the analysis of all subjects, as well as for men and subjects with vertical jumps greater than 50 cm (p < or = 0.05). No significant main effects were found for the H muscle group. Pairwise comparisons revealed a variety of differences among plyometric exercises. In some cases, plyometrics previously reported to be of high intensity, such as the depth jump, yielded relatively little motor unit recruitment compared with exercises typically thought to be of low intensity. Results can assist the practitioner in creating plyometric programs based on the nature of the motor unit recruitment.     

Breath holding during intense exercise: arterial blood gases, pH, and lactate

Seven healthy endurance-trained [maximal O2 uptake (VO2max) = 57.1 +/- 4.1 ml.kg-1.min-1)] female volunteers (mean age 24.4 +/- 3.6 yr) served as subjects in an experiment measuring arterial blood gases, acid-base status, and lactate changes while breath holding (BH) during intense intermittent exercise. By the use of a counterbalance design, each subject repeated five intervals of a 15-s on:30-s off treadmill run at 125% VO2max while BH and while breathing freely (NBH). Arterial blood for pH, PO2, PCO2, O2 saturation (SO2) HCO3, and lactate was sampled from a radial arterial catheter at the end of each work and rest interval and throughout recovery, and the results were analyzed using repeated-measures analysis of variance. Significant reductions in pHa (delta mean = 0.07, P less than 0.01), arterial PO2 (delta mean = 24.2 Torr, P less than 0.01), and O2 saturation (delta mean = 4.6%, P less than 0.01) and elevations in arterial PCO2 (delta mean = 8.2 Torr, P less than 0.01) and arterial HCO3 (delta mean = 1.3 meq/l, P = 0.05) were found at the end of each exercise interval in the BH condition. All of the observed changes in arterial blood gases and acid-base status induced by BH were reversed during the rest intervals. During recovery, significantly (P less than 0.025) greater levels of arterial lactate were found in the BH condition.(ABSTRACT TRUNCATED AT 250 WORDS)