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EFFECT OF CONCURRENT ENDURANCE AND CIRCUIT RESISTANCE TRAINING SEQUENCE ON MUSCULAR STRENGTH AND POWER DEVELOPMENT MOKTAR CHTARA,1 ANIS CHAOUACHI,2 GREGORY T. LEVIN,3 MUSTAPHA CHAOUACHI,1 KARIM CHAMARI,2 MOHAMED AMRI,4 AND PAUL B. LAURSEN3 1

Institute of Sport and Physical Education, Ksar Said, Tunis, Tunisia; 2Research Unit, Evaluation, Sport, Sante´, National Centre of Medicine and Science in Sport, Tunis, Tunisia; 3School of Exercise, Biomedical and Health Sciences, Edith Cowan University, Joondalup, Western Australia, Australia; 4Nutrition Physiology Laboratory, Faculty of Sciences, Tunis, Tunisia

ABSTRACT Chtara, M, Chaouachi, A, Levin, GT, Chaouachi, M, Chamari, K, Amri, M, Laursen, PB. Effect of concurrent endurance and circuit resistance-training sequence on muscular strength and power development. J Strength Cond Res 22: 1037–1045, 2008—The purpose of this study was to examine the influence of the sequence order of high-intensity endurance training and circuit training on changes in muscular strength and anaerobic power. Forty-eight physical education students (ages, 21.4 6 1.3 years) were assigned to 1 of 5 groups: no training controls (C, n = 9), endurance training (E, n = 10), circuit training (S, n = 9), endurance before circuit training in the same session, (E+S, n = 10), and circuit before endurance training in the same session (S+E, n = 10). Subjects performed 2 sessions per week for 12 weeks. Resistance-type circuit training targeted strength endurance (weeks 1–6) and explosive strength and power (weeks 7–12). Endurance training sessions included 5 _ 2max (Vo _ 2max) repetitions run at the velocity associated with Vo _ 2max; for a duration equal to 50% of the time to exhaustion at Vo _ 2max. Maximal recovery was for an equal period at 60% Vo strength in the half squat, strength endurance in the 1-leg half squat and hip extension, and explosive strength and power in a 5-jump test and countermovement jump were measured preand post-testing. No significant differences were shown following training between the S+E and E+S groups for all exercise tests. However, both S+E and E+S groups improved less than the S group in 1 repetition maximum (p , 0.01), right and left 1-leg half squat (p , 0.02), 5-jump test (p , 0.01), peak jumping force (p , 0.05), peak jumping power (p , 0.02), and peak jumping height (p , 0.05). The intrasession sequence did not influence the adaptive response of muscular strength and

Address correspondence to Anis Chaouachi, [email protected] 22(4)/1037–1045 Journal of Strength and Conditioning Research Ó 2008 National Strength and Conditioning Association

explosive strength and power. Circuit training alone induced strength and power improvements that were significantly greater than when resistance and endurance training were combined, irrespective of the intrasession sequencing.

KEY WORDS plyometric training, combined training, interval training, interference effect

INTRODUCTION

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oncurrent strength and endurance training has received considerable attention in the literature (20,22,29). One major concern that has arisen from this body of knowledge is that the benefits incurred from resistance training in isolation may be compromised when strength and endurance training are performed concurrently (4,28). Several possible reasons have been suggested to explain this interference phenomenon (4,8,15,21,23,24,30,32), one of which is the sequencing and order effect of the training (15,20). A number of different sequencing and order effects have been implemented in previous research, including endurance training before strength training (38), strength training before endurance training (26,38) as well as the scheduling of both types of training on the same day (15,16,37) or on alternate days (4,17,19,37). Only 2 studies, however, have directly investigated the effect of the sequencing order during the same training session on strength and endurance adaptations (14,20). Using mostly sedentary female subjects, these studies showed that the intrasession sequencing had a negligible effect on changes in strength (14,20). The influence of the intrasession sequencing of strength and endurance training in male subjects, however, remains unknown. Previous work that examined intrasession sequencing and order effects employed continuous submaximal endurance protocols as well as the traditional type of weight training (14,20). Neither of these studies employed high-intensity interval training as a means of endurance training or resistance-type circuit training to enhance strength. Since it is well established that high-intensity interval training is an VOLUME 22 | NUMBER 4 | JULY 2008 |

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Intrasession Concurrent Training effective means of endurance training (29) and circuit training is a useful method to improve both strength and cardiovascular performance, it is important to understand how these 2 types of training interact when performed concurrently in a single session. The aim of the current study was to examine the influence of the intrasession sequencing of concurrent strength and endurance training (i.e., strength training before endurance training or endurance training before strength training in the same session) on strength, explosive-strength, and power development in males. Based on previous findings, we hypothesized that maximal strength, strength endurance, and explosive strength and power would increase in all training groups that performed strength training. Further, it was hypothesized that the 2 concurrent training groups would have smaller gains in strength and power than the group performing only resistance-type training.

METHODS Experimental Approach to the Problem

This study used a repeated-measures design to determine the magnitude of change in strength, explosive strength and power, and strength endurance after a 12-week training program. Further, changes in body composition are also reported. The training effect on each dependent variable was measured for each of the 5 different groups in the study: E (n = 10) high-intensity endurance run training; S (n = 9) strength circuit training; E+S (n = 10) endurance training before strength training, S+E (n = 10) strength training before endurance training, and C (n = 9) nontraining controls. Subjects

Forty-eight male physical education students volunteered to participate in the study. Participants were not involved in any organized sports activity, but were required to perform an average of 15 hours per week of physical activity as part of their university degree requirements. The average (SD) age, height, mass, and percentage body fat of the group were 21.4 (1.3) years, 178.2 (5.7) cm, 72.1 (6.3) kg, and 14.7 (3.0)%, respectively. The study was approved by the National University Ethical Committee, and all participants gave their written informed consent before the start of the study. Procedures

Participants were assigned to 1 of 5 groups based on initial test results. Group allocation was designed in such a way that any initial differences between groups in the dependent variables of muscular strength, explosive strength, and power were minimized. Training was performed twice per week for 12 weeks. Each subject performed the same evaluation protocol before and after the training period. The testing protocol included anthropometric measurements as well as strength and power assessments. Subjects were requested to refrain from intense activity in the 24-hour period before each

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test. The effect of these training programs on endurance performance parameters has recently been published elsewhere (13). Anthropometric Measurements

Height and body mass were measured using a stadiometer to the nearest 0.5 cm and balance weighing scales (accurate to 0.1 kg), respectively. Skinfold thickness was measured to the nearest 0.2 mm at 4 predetermined sites (biceps, triceps, subscapular, and suprailiac) using Harpenden skinfold calipers (Lange, Cambridge, MA). Skinfold thickness was then used to calculate the percentage of body fat using the techniques of Durnin and Womersley (18). All measurements were taken on the right side of the body by the same tester. Physical Measurements

The pre- and posttesting measurements were conducted on 2 different days separated by a minimum of 72 hours. The variables tested on day 1 of the pre- and posttesting sessions included a 1-leg half squat and a hip extension test for both the right and left legs. Day 2 of the testing consisted of a countermovement jump (CMJ) test, 5-jump distance test (5-J), and a 1 repetition maximum (1RM) half squat test. Subjects completed a standardized warm-up before each of the testing sessions including 10 minutes of cycling on a stationary cycle ergometer (Monark, Stockholm, Sweden) at 60 revmin21 and a series of dynamic stretches for 5 minutes. 1RM Half Squat Strength. 1RM half squat strength was recorded as the maximal weight subjects were able to rise in a half squat (;90° angle in the knee joint between the femur and tibia) as described by McMillan et al. (34). After the general warm-up, subjects performed a specific warm-up using 50% (10 reps), 75% (6 reps), and 85% (3 reps) of their estimated 1RM. Following this subsequent warm-up, the subjects’ resistance was fixed at a critical value of 5% below the expected 1RM and was gradually increased after each successful performance until failure. The interval between each trial was ;2 minutes. 1RM was normally achieved within 3 to 5 attempts. 1-Leg Half Squat. Subjects performed a maximum number of 1-leg half squats with a load corresponding to one-fourth of their body mass (35). The movements were performed at a constant rate of 30 half squats per minute using a metronome set at 60 beepsmin21. Subjects were required to be in the top or bottom position at each beep, and the test was stopped once this rate could no longer be maintained. Hip Extension. This exercise was performed with subjects lying supine and supported with dense foam at the shoulders and feet so that they remained 20 cm above ground level. Subjects then raised and lowered themselves a maximum number of times at the same rate described for the 1-leg half squats until they could no longer continue at the set tempo. Testing was performed with a load corresponding to 10% of

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TABLE 1. Outline of the circuit training program. Strength endurance

Week interval 1 2 3 4 5 6

4316–18 30:30 5316–18 5318–20 4320–22 40:20 5320–22 5322–24

(10) (10) (10) (20) (20) (20)

4316–18 5316–18 5318–20 4322–24 5322–24 5324–26

4316–18 5316–18 5318–20 4322–24 5322–24 5324–26

(5kg) (5kg) (5kg) (10kg) (10kg) (10kg)

4322–26 5322–26 5324–28 4326–30 5326–30 5328–32

4314–16 5314–16 5316–18 4318–20 5318–20 5320–24

Explosive strength and power Hopping Single-leg Hurdle S3R hops jumps S3R (d) S3R (h)

Drop jumps S3R (h) 7 8 9 10 11 12

(25) (25) (25) (25) (25) (25)

Hip extension, S3R (%)

Back extension, S3R

Arm flexion, S3R

Walking lunges, S3R (%)

1-leg half squats, S3R (%)

435–8 (0.30 m) 30:30 535–8 (0.30 m) 535–8 (0.45 m) 436–9 (0.45 m) 40:20 536–9 (0.60 m) 537–10 (0.60 m)

43(16–20) 53(16–20) 53(16–20) 43(24–30) 53(24–30) 53(24–30)

(0.50 (0.50 (0.60 (0.60 (0.60 (0.70

m) m) m) m) m) m)

43(30–36) 53(30–36) 53(36–42) 43(48–56) 53(48–56) 53(56–64)

(10) (10) (10) (20) (20) (20)

Single-leg bounds S3R (d)

43(20–24m) 53(20–24m) 53(24–30m) 43(36–45m) 53(36–45 m) 53(42–48 m)

43(20–24m) 53(20–24m) 53(24–30m) 43(36–45m) 53(36–45m) 53(42–48m)

Abdominal S3R 4322–26 5322–26 5324–28 4326–30 5326–30 5328–32 Multijumps S3R (d) 43(18–22m) 53(18–22m) 53(20–24m) 43(30–36m) 53(30–36m) 53(36–42m)

Sets (S) and repetitions (R) for all exercises are outlined. Percentages of body mass loads for 1-leg half squats, hip extension, and walking lunges exercises are outlined. Height (h) or distance (d) of the hurdle and the plinth, single-leg hops, single-leg bounds, and multijumps.

each subject’s body mass secured to their chest by their own arms (35). Countermovement Jump. Subjects performed 3 CMJ tests on a force platform (9281 C; Bioware, Kistler, Switzerland) with a 2-minute recovery period between each. Subjects were instructed to jump for maximal height keeping their hands on their hips during the jump in order to reduce any contribution from the upper limbs. No degree of knee angle restriction was

enforced during the eccentric phase of the CMJ. On completion of the 3 CMJ tests, the best jump height was recorded for analysis (31,39). Peak jumping force (Fpeak), peak jumping power (Wan), and peak jumping height (Hpeak) were recorded.

5-Jump Distance Test. The 5-J involved the subject attempting to cover the greatest horizontal distance possible by performing a series of 5 forward jumps with alternate left and right foot contacts (9). Subjects were allowed 3 trials, with the best result recorded for TABLE 2. Pre and 12-week post measurements of body mass and percentage of analysis. Results were obtained body fat in the various training groups of the present study. using a measuring tape to measure the total distance covPre Post ered from the edge of the toes at Group Body mass (kg) % Body fat Body mass (kg) % Body fat take off, to the edge of the heel at landing. E 75.1 6 6.2 14.9 6 1.9 74.9 6 6.5 13.1 6 1.7**

S S+E E+S C

68.9 73.9 70.7 71.5

6 2.9 6 6.3 6 6.6 6 3.0

14.2 6 2.2 14.9 6 4.2 14.7 6 2.4 14.6 6 4.0

69.8 6 3.1** 75.0 6 5.8** 71.8 6 6.6** 72.7 6 3.0

12.9 6 2.3** 12.7 6 2.8** 12.5 6 1.8** 14.6 6 4.4

E+S = endurance followed by strength training; S+E = strength followed by endurance training; E = endurance training only; S = strength training only; C = nontraining controls. **Significant difference: p , 0.01 between pre- and posttraining values.

Training Programs

The E and S training groups trained on Mondays and Thursdays, and the E+S and S+E groups trained on Tuesdays and Fridays. Subjects were allowed to drink ad libitum during all training sessions. VOLUME 22 | NUMBER 4 | JULY 2008 |

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Intrasession Concurrent Training 20 m. The training session included 5 high-intensity inter_ 2max each folvals, run at Vo lowed by a period of active running recovery performed at _ 2max. The duration 60% of Vo of each interval and recovery period was prescribed based on one-half of the individual’s Tmax. Pacing was controlled by the assessors who would sound Figure 1. Pre- and 12-week post scaled 1 repetition maximum scores (kgmb20.67) in the various training a signal; subjects were required groups. E+S = endurance followed by strength training; S+E = strength followed by endurance training; to be near each reference mark E = endurance training only; S = strength training only; C = nontraining controls. *Significant difference, p , 0.05. **Significant difference, p , 0.01. Improved significantly more than E, E+S, S+E, and C. ‡Improved significantly when the signal was sounded. more than E and C. The duration of the work periods was increased by 5% when the heart rate measured Endurance Training (E). In order to prescribe endurance at the end of an entire session was .10 beatsmin21 lower training, subjects performed 2 additional tests before training. than the heart rate at the end of the first interval performed at A Vam-eval track test was performed to determine maximal that particular intensity. _ 2max) (11), and a test to measure time to aerobic speed (Vo exhaustion (Tmax) at the maximal aerobic speed (7) was Circuit Training (S). The circuit training program consisted of performed 1 week later. Endurance training was carried out four 3-week periods (Table 1). Periods 1 and 2 focused on a 200-m outdoor synthetic track with landmarks every on strength endurance, and periods 3 and 4 focused on explosive strength and power. During the second week of each period, the demands of the training sessions were increased by decreasing the amount of TABLE 3. Maximal number of repetitions for the 1-leg half squat and hip extension pre- and 12 weeks post-training. rest between sets. Exercises included total and segmentary 1-leg half squat (RM) movements of the upper limbs, trunk, and lower limbs. Exercise Right Left Hip extension (RM) intensity was individualized by E Pre 52.8 6 7.9 52.9 6 8.2 23.1 6 2.3 instructing subjects to perform a Post 55.3 6 7.9 56.2 6 8.1 25.0 6 2.4 determined maximum number D% 4.7** 6.2** 8.2** of repetitions per set (Table 1). S Pre 51.1 6 6.9 51.8 6 13.1 22.8 6 3.3 Post 66.7 6 8.2 68.0 6 11.5 31.4 6 1.9 The maximum number of repD% 30.4**§ 31.3**§ 38.0**‡ etitions during the work period S+E Pre 49.6 6 9.6 52.6 6 9.4 23.7 6 2.3 that each subject could perform Post 61.4 6 8.2 64.9 6 9.3 33.8 6 2.7 ** was established by an individuD% 23.8**‡ 23.4**‡ 42.6 ‡ alized test before the start of the E+S Pre 51.3 6 4.7 52.1 6 9.2 22.5 6 3.5 Post 62.3 6 6.7 62.5 6 8.1 31.1 6 1.8 training program. An assessor D% 21.4**‡ 20.0**‡ 38.2**‡ was always present to verbally C Pre 51.7 6 7.7 53.8 6 5.8 21.9 6 4.4 encourage participants to perPost 52.7 6 9.1 55.3 6 6.0 22.9 6 4.4 form to their maximum ability D% 1.9 2.9 4.6* during the work periods and to Interaction, F = 36.92** F = 21.90** F = 13.4** df 4, 43 complete all 5 sets. The strength training session lasted approxiE+S = endurance followed by strength training; S+E = strength followed by endurance mately 30 minutes, excluding training; E = endurance training only; S = strength training only; C = nontraining controls. *Significant difference: p , 0.05. a 15- to 20-minute warm-up. **Significant difference: p , 0.01. Recovery between circuits was §Improved significantly more than E, E+S, S+E, and C. set at 2 minutes. Throughout ‡Improved significantly more than E and C. the training period, progressions of the exercises were

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delineate the differences between the groups. Significance was set at an a level of 0.05, and all statistical analyses were conducted using the statistical package for the Social Sciences (SPSS, Version 13.0; SPSS Inc., Chicago, IL). Reliability of variables was assessed using a 2-way average measure of the intraclass correlation coefficient. Effect size estimates for main comparisons were obtained from our experimental data, so that post hoc statistical power could be computed; power values ranged from 0.75 to 0.80 for the group sample sizes obtained (at an a level of 0.05).

RESULTS Body composition and body mass results for the pre- and posttraining measurements are presented in Table 2. All experimental groups showed a signifFigure 2. Pre- and 12-week post measurements of 5-jump distance test (m) (A); peak-jumping force (Nkg20.67) icant decrease in their body fat (B); peak jumping power (Wkg2 0.67) (C). E+S = endurance followed by strength training; S+E = strength percentage following the exerfollowed by endurance training; E = endurance training only; S = strength training only; C = control group. *Significant difference, p , 0.05. **Significant difference, p , 0.01. §Improved significantly more than E, E+S, cise training intervention [E+S S+E, and C. ‡Improved significantly more than (E) and (C). *Improved significantly more than C. (215.0%), S+E (214.8%), E (212.1%), and S (29.2%)], while the control group did not (C, 0.0%). Increases in body achieved by increasing hurdle height (0.50 to 0.70 m) and mass occurred in every group [E+S (+1.6%), S+E (+1.5%), plinth (0.30 to 0.60 m) as well as increasing the length of the S (+1.3%), and C (+1.7%)] except the endurance training jumps and bounds (Table 1). During the bounding and group (20.3%). drop-jump exercises, subjects were instructed to minimize Peak force and peak power were analyzed for absolute and ground contact time. allometrically scaled (Nkg20.67 and Wkg20.67) values since research has suggested that allometric scaling is more Combined Training (E+S and S+E). The combined training suitable for comparative analysis when body mass is groups performed both the endurance and circuit training considered (12,25). programs in a single session. The only difference between the Maximal strength (1RM) increased significantly (p , 0.01) 2 training groups was the order in which they executed the for all groups (S [+17.0%], S+E [+12.2%], E+S [+10.6%], E training, either endurance training before circuit or circuit [+6.2%], and C [+5.6%]; Figure 1). Significant (group 3 time) training before endurance. A 15-minute recovery period interaction effects were also observed (p , 0.001). Signifiseparated the training sessions. cantly greater increases in 1RM were seen following circuit Statistical Analyses training (groups S, S+E, and E+S) compared with the A paired-samples Student t-test identified differences endurance training (E) group and nontraining control (C) between the initial and final values of a variable in the same group (Figure 1). Further, the circuit training (S) group group. A 2-way (group 3 time) repeated-measures analysis increased 1RM significantly more than both the E+S and of variance was used to determine training-related effects in S+E groups (p , 0.01; Figure 1). Allometric scaling resulted each of the dependent variables over time. When a significant in similar changes in strength (S [+16.8%], S+E [+13.3%], main effect was identified, Scheffe´ post hoc tests were used to E+S [+12.2%], E [+8.0%], and C [+5.3%]). VOLUME 22 | NUMBER 4 | JULY 2008 |

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TABLE 4. Test-retest reliability of strength and explosive strength and power tests. Criterion measures

ICC

a

CV %

1RM 1-leg half squat right 1-leg half squat left Hip extension CMJ 5-J

0.93 0.89 0.87 0.89 0.93 0.91

0.93 0.89 0.87 0.89 0.93 0.91

2.4 3.1 3.0 3.3 2.6 2.8

ICC = intraclass correlation coefficient; a = Cronbach’s a reliability coefficients; CV = coefficient of variation; CMJ ¼ countermovement jump; 5-J ¼ 5-jump test.

Hip extension and 1-leg half squat strength endurance followed a trend similar to the 1RM results (Table 3). Significant (group 3 time) interaction effects were also observed (Table 3). All groups that performed strength training showed increases in strength endurance (S) more than the endurance-only (E) and control (C) groups. Furthermore, the circuit-only group (S) improved significantly more than the concurrent training groups (S+E and E+S) for the 1-leg half squat, but not hip extension (Table 3). Significant (group 3 time) interaction effects were observed in scaled peak force (p , 0.001), scaled peak power (p , 0.001), peak height jumped (p , 0.001), and in the 5-J (p , 0.001). Groups performing circuit training improved significantly more than the control or endurance training– only groups in allometrically scaled peak force (S [+14.7%], S+E [+11.6%], E+S [+9.6%], E [+5.4%], and C [+1.2%]), scaled peak power (S [+8.7%], S+E [+5.7%], E+S [+5.6%], E [+3.2%], and C [+0.1%]), peak height jumped (S [+7.0%], S+E [+3.3%], E+S [+3.3%], E [+1.7%], and C [+0.2%]) and in the 5-J (S [+9.2%], S+E [+6.5%], E+S [+5.7%], E [+3.4%], and C [+1.6%]) (Figure 2). Similar to other measurements, the S group outperformed the concurrent (E+S and S+E) groups in the 5-J (p , 0.01), in peak-jumping force (p , 0.05), in peak-jumping explosive strength and power (p , 0.02), and in peak jumping height (p , 0.05) (Figure 2). The test-retest interclass correlation coefficient and the reliability coefficient for all tests are presented in Table 4.

DISCUSSION The main findings of the present study were that a) for individuals not accustomed to regular resistance or endurance training, the intrasession sequencing of combined highintensity endurance and circuit resistance training did not influence the adaptive response of maximal muscular strength, explosive strength, and power and that b) increases in strength and power were significantly greater in those performing only circuit resistance training compared with

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those performing concurrent circuit resistance and endurance training, irrespective of the intrasession sequencing order. As mentioned, the intrasession sequencing order of a 12-week, low-frequency, concurrent circuit resistance training and endurance training program did not influence the change in maximal muscular strength, explosive strength, and power. The improvements shown in maximal muscular strength, strength endurance, and explosive strength and power were similar in both the E+S and S+E groups (Table 1, Figures 1 and 2). These findings are in agreement with those of previous studies in the area (14,20). The 10–12% increase in 1RM half squat strength found with circuit training is similar to the 12–14% increase in leg press strength found by Collins and Snow (14). However, Gravelle and Blessing (20) showed increases in leg press strength of 26.6% and 27.4% for S+E and E+S groups, respectively. These larger gains in strength compared with the present study and with the study of Collins and Snow (14) may be attributable to the type of endurance training employed (rowing versus running), the type of strength training employed (strength only), and other methodological variances. Indeed, the volume, intensity, and frequency of training may all play a role in influencing the degree of incompatibility observed between strength and endurance training (8,21,24,32). The frequency of concurrent strength and endurance training has been shown to influence the adaptive response. When the frequency of concurrent strength and endurance training is high (4–6 days per week), a reduced improvement in muscular strength has been observed (17,24,28). However, when the training frequency of concurrent strength and endurance training is low (2–3 days per week), maximal strength during both short-term (,12 weeks) (3,32) and long-term (21) training periods (.20 weeks) has been shown to increase at a rate similar to that of strength training alone. This suggests that a low-frequency approach to concurrent strength and endurance training is appropriate when improvements in strength are desired. In contrast to this, the present study found that despite performing lowfrequency training, maximal strength increased significantly more in the circuit training–only group (S, +17%) compared with the E+S (+10.6%) and S+E (+12.2%) groups (Figure 1). Since there was no difference in the work performed during the circuit training, the reduced improvements in strength displayed in the concurrent training groups may be attributable to the endurance component supplemented into the training program. This suggests that high-intensity interval training may compromise strength adaptations when concurrent training is performed, irrespective of the sequencing of this training. Strength measured during the 1RM increased in all groups, including increases of 6.2% for the E group and 5.6% for the C group. An increase in maximal strength is not uncommon after endurance training in isolation, and similar findings have been reported previously (27,36). However, the increase in maximal strength for the control group (+5.6%) suggests that

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Journal of Strength and Conditioning Research there is a variance of ;6% for this test. Since the difference between maximal strength gains in the circuit training–only group (17.0%) and the concurrent groups was greater than (E+S, 10.6%) or approaching this 6% variance (S+E, 12.2%), it can be concluded that when high-intensity interval and circuit resistance training are performed in a single session, interference of strength adaptation occurs. Changes in explosive strength and power among the groups paralleled the changes in muscular strength (Figure 2). In contrast, Gravelle and Blessing (20) found that peak anaerobic power measured during a Wingate test improved only in the S+E group and not in the E+S group or S-only group. In this study, however, the S+E group initially possessed a lower anaerobic ability and thus a greater capacity for improvement (20). In the current study, initial levels of explosive strength and power were similar in all training groups, making all groups equal with respect to their potential for improvement. Our findings suggest that explosive strength and power adaptations are unaffected when resistance and endurance training are performed concurrently in a single session (1). A possible reason for the difference in the explosive power shown between the S, S+E, and E+S groups in the current study may have been the sequencing of the 2 different types of circuit training. The first 6 weeks of the circuit training program were designed to enhance muscular strength, whereas the last 6 weeks was designed to develop explosive strength and power (Table 1). However, no testing occurred in the midpoint period of this study, which restricts our ability to determine the contribution of each period of training to the overall results achieved. However, Bell et al. (2,5) previously showed that strength adaptations can be maintained for a sustained period when a new training stimulus is introduced. This finding suggests that once the training program was altered from strength to explosive strength and power, the S group may have already achieved greater increases in strength and been better able to use these gains and neuromuscular adaptations to significantly improve their explosive strength and power results when compared to the concurrent groups. Hennessy and Watson (22) reported that 18 weeks of continuous and high-intensity run training caused a deterioration of vertical jump performance, a measure commonly used to assess anaerobic power. These authors proposed that the endurance training reduced the capacity of the neuromuscular system to rapidly generate force (22). The findings of the present study are in agreement with these findings in that the high-intensity endurance run training performed by the concurrent training groups resulted in less of an increase in explosive strength and power compared with the strength-only training group. Future research should attempt to examine mechanistically why this may occur. To the best of our knowledge, the present study is the first to examine the influence of the intrasession strength and endurance training sequencing order on changes in strength

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endurance. The results of the changes in strength endurance after concurrent strength and endurance training were equivocal. The 1-leg half squat showed larger increases in the strength training group compared with the concurrent strength and endurance training groups, while no difference was found between the strength-only and concurrent training groups for the increase attained in hip extension (Table 3). These differences may be due to the ability of the endurance component (running) of the concurrent training to assist with enhancing hip extension strength endurance and the development of the core stabilizers, which would have also been recruited during both running and the hip extension test. Irrespective of the sequencing order or whether endurance training was performed, circuit-type strength training proved to be an effective means of increasing strength endurance. Few studies have assessed strength endurance after strength training programs, and those that have have done so to test the continuum theory that light loads and a high number of repetitions are most beneficial for increasing strength endurance, whereas heavy loads and fewer repetitions are more beneficial for increasing maximal strength (10,33). The concurrent training groups in the present study completed 12 weeks of high-intensity endurance running sessions, reaching 100% of maximal aerobic speed, defined as _ 2max (6) as well as strength the minimum speed that elicits Vo training sessions that included 6 weeks of strength endurance training and 6 weeks of explosive strength and power development in circuit-training format. While one might hypothesize that the first activity performed would result in some residual fatigue experienced during the second activity, thereby reducing the quality of that session (15), evidence of this was not shown in the present study. Both the S+E and E+S groups maintained equal training intensities and as a result made similar improvements in strength, power, and strength endurance. As previously mentioned, however, this study did not assess these strength and power variables throughout the training period, and therefore the time course for adaptation within these groups remains unknown. Further research in this area is required to investigate the time course of these changes with concurrent training programs.

PRACTICAL APPLICATIONS The present study has shown that a 12-week, low-frequency, resistance-type circuit training program resulted in significant improvements in muscular strength, explosive strength and power, and strength endurance. Therefore, circuit type programs that use individualized intensities to ensure maximal effort over a short period are beneficial training strategies for improving overall strength. Second, when this training was combined with high-intensity endurance training, strength and explosive strength and power were still increased, but not to the same extent. Previous research had suggested that low-frequency endurance training does VOLUME 22 | NUMBER 4 | JULY 2008 |

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Intrasession Concurrent Training not compromise strength improvements, and, therefore, there is a possibility that high-intensity interval training is more likely to be counterproductive to strength and power adaptations when concurrent training is being performed. Last, the intrasession order of strength and endurance training resulted in no significant differences between these two conditions. This finding suggests that there is no advantage to performing either strength or endurance training before the other when both types of training are performed in a single session. However, if the development of strength and power is the priority of the program, then concurrent training in a single session is not advised.

ACKNOWLEDGMENTS This study was financially supported by the Ministe`re de la Recherche Scientifique, de la Technologie et du De´veloppement des Compe´tences, Tunisia.

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