Journal of Sports Sciences (1466447X)21(11)pp. 933-942
Fatigue represents a reduction in the capability of muscle to generate force. The aim of the present study was to establish the effects of exercise that simulates the work rate of competitive soccer players on the strength of the knee extensors and knee flexors. Thirteen amateur soccer players (age 23.3 ± 3.9 years, height 1.78 ± 0.05 m, body mass 74.8±3.6 kg; mean ± s) were tested during the 2000-2001 soccer season. Muscle strength of die quadriceps and hamstrings was measured on an isokinetic dynamometer. A 90 min soccer-specific intermittent exercise protocol, incorporating a 15 min half-time intermission, was developed to provide fatiguing exercise corresponding in work rate to a game of soccer. The exercise protocol, performed on a programmable motorized treadmill, consisted of the different intensities observed during soccer match-play (e.g. walking, jogging, running, sprinting). Muscle strength was assessed before exercise, at half-time and immediately after exercise. A repeated-measures analysis of variance showed significant reductions (P < 0.001) in peak torque for both the quadriceps and hamstrings at all angular velocities (concentric: 1.05, 2.09, 5.23 rad.s-1; eccentric: 2.09 rad.s-1). The peak torque of the knee extensors (KE) and knee flexors (KF) was greater before exercise [KE: 232±37, 182±34, 129±27, 219±41 N.m at 1.05, 2.09 and 5.23 rad.s-1 (concentric) and 2.09 rad.s-1 (eccentric), respectively; KF: 126±20, 112±19, 101±16, 137±23 N.m] than at half-time (KE: 209±45, 177±35, 125±36, 214±43 N.m; KF: 114±31, 102±20, 92±15, 125±25 N.m) and greater at half-time than after exercise (KE: 196±43, 167±35, 118±24, 204±43 N.m; KF: 104±25, 95±21, 87±13, 114±27 N.m). For the hamstrings: quadriceps ratio, significant changes were found (P<0.05) for both legs, the ratio being greater before than after exercise. For fast:slow speed and left:right ratios, no significant changes were found. We conclude that there is a progressive reduction in muscle strength that applies across a range of functional characteristics during exercise that mimics the work rate in soccer.
Cellular and Molecular Biology Letters (16891392)9(4 A)pp. 773-775
Ergonomics (00140139)48(11-14)pp. 1568-1575
Most soccer players have a favoured foot for kicking the ball, and it is believed that this preference may lead to an asymmetry in the strength and flexibility of the lower extremities. This study was designed to determine whether asymmetry in strength and flexibility are present in the legs of soccer players. Forty-one elite and sub-elite soccer players (age 23.4 ± 3.8 years; height 1.81 ± 0.06 m; body mass 81.7 ± 9.9 kg) were studied (data are presented as mean ± SD). The dynamic strength of knee flexors (hamstrings) and knee extensors (quadriceps) was measured using an isokinetic dynamometer at angular velocities of 1.05, 2.09, 5.23 rad/s (in a concentric mode) and 2.09 rad/s (in an eccentric mode). The concentric strength ratio (hamstringsconc/quadricepsconc) and the dynamic control ratio (hamstringsecc/quadricepsconc) were computed. Hip joint flexibility (in flexion) was measured using a goniometer. A significant difference between the preferred and non-preferred leg was found in the knee flexors at 2.09 rad/s (119 ± 22 versus 126 ± 24Nm; P < 0.05) and for the dynamic control ratio (0.79 ± 0.13 versus 0.84 ± 0.16Nm; P < 0.05). In both cases the knee flexors of the preferred leg were weaker than those of the non-preferred leg. A total of 28 of the 41 players (68%) had significant musculoskeletal abnormality (imbalance 410%) in one or more specific muscle groups. No significant differences were found in flexibility of the hip joint between the preferred and non-preferred leg (P < 0.05). It is concluded that the lower strength of the knee flexor muscl es of the preferred leg may be associated with the differential use of these muscle during the kicking action and thus constitutes a unique training effect associated with soccer. This in turn can lead to muscular imbalance which is generally regarded as an injury risk factor. © 2005 Taylor & Francis.
Journal of Electromyography and Kinesiology (18735711)16(3)pp. 257-263
Surface electromyography has been useful in comparing muscular activity among different sports movements and it is a valuable technique for evaluating muscle activation, co-ordination and fatigue. Since these important variables have not been investigated during the full game in soccer, the present study aimed to investigate the activity of major muscles of the lower extremity during a soccer-simulation fatiguing protocol. Ten amateur soccer players (age 21.40 ± 3.13 years; height 1.77 ± 0.06 m; mass 74.55 ± 8.5 kg) were tested. The exercise protocol, performed on a programmable motorised treadmill, consisted of the different intensities observed during soccer match-play (walking, jogging, running, sprinting). Electromyographic activity was recorded from the rectus femoris (RF), biceps femoris (BF), tibialis anterior (TA) and gastrocnemius (GC) muscles before exercise, at half-time and immediately after the 90-min exercise protocol. The EMG data were analysed using custom-written software to compute the root mean square (RMS) value over ten gait cycles. With regard to RF, BF and TA, a significant main effect (P < 0.05) was found for condition (pre-game, half-time and post-game), speed (6, 12, 15 and 21 km h-1) (P < 0.05) and interaction between condition and speed (P < 0.05). For GC, a significant effect was not found for condition or interaction between condition and speed, but a significant main effect (P < 0.001) was found for speed, with the RMS value increasing continually with increasing speed from 6 to 21 km h-1. The results indicated that after a simulation of the exercise intensity of soccer-play the EMG activity in major lower-limb muscles was less than before. This decrease indicated that prolonged intermittent exercise had an effect on muscle activity even when work-rate was sustained. © 2005 Elsevier Ltd. All rights reserved.
Mathematical modeling and optimization of snatch technique based on dynamic synthesis is the main purpose of this study. The barbell trajectory is proposed as the main index which was evaluated experimentally by several researchers who introduced optimum trajectory according to percentage of their owners' success. We believe that in optimizing the barbell trajectory we should consider the mechanical principles. Therefore we use a five-link biomechanical model to evaluate its behaviour and to predict the optimum barbell trajectory which minimizes the specific mechanical criterion by using genetic algorithm (GA) which is theoretically and empirically proven to provide a robust search in complex spaces, thereby offering a valid approach to problems requiring efficient and effective searches. Comparing the results of this model and experimental observations of other researchers, we show an improvement to introduce a good predictive model. Using this model can help the coaches to improve the performance of weightlifters.
Rahmani-nia f., F.,
Rahnama, N.,
Hojjati z., ,
Soltani b., Sport Sciences for Health (18247490)2(3)pp. 118-124
Six young obese females (20.5±1.22 years; BMI=34.5±4.3 kg/m2) participated in a resistance exercise (RE) protocol (12 exercises, 4 sets × 15 repetitions at 60% of 1RM), an aerobic exercise (AE) protocol (3 sets × 10 min ergometer cycling at 60% of maximal heart rate with 5 min of rest between sets), and a control session. Blood samples were collected before, and immediately after and 10 h after exercise. All protocols were done in follicular phase of the menstrual cycle. ANOVA (3×3) with repeated measure on exercise (3 levels) and time (3 levels) factors was used to determine the effects of exercise protocol, time and exercise protocol by blood sampling time interaction, triglycerides, HDL-cholesterol, LDL-cholesterol, and glucose concentrations. Immediately and 10 h after AE, serum leptin was significantly lower (p <0.05). Serum insulin after AE was lower than after RE and control sessions (p <0.05). Furthermore, no significant differences were found in serum leptin and insulin between RE and control sessions immediately after and 10 h after exercise. Blood glucose, triglycerides, total cholesterol, HDL-cholesterol and LDL-cholesterol were unchanged in both exercise protocols. In conclusion, AE resulted in a serum leptin reduction and suppressed the circadian rhythm of serum insulin when sampled immediately and 10 h after exercise; instread, RE did not result in serum leptin or insulin changes. © Springer-Verlag Italia 2008.
Sport Sciences for Health (18247490)3(3)pp. 57-64
The purpose of this study was to determine the effect of sprint exercise and a detraining period on lipid peroxidation (malondialdehyde, MDA) and response of antioxidant system (ferric reducing ability of plasma (FRAP), uric acid, bilirubin and total protein). Forty-two male rats were divided randomly into two groups: sprint group (experimental) (n = 24) and control group (n = 18, without any training programme). The experimental group trained for 12 weeks, 3 times per week, with given intensities. Five trained rats experienced the detraining, so the effects of detraining were also studied (from the 8th to 12th weeks). Data were analysed using a two-way repeated ANOVA. A significant difference between the two groups was observed for MDA (p = 0.022), FRAP (p = 0.005) and bilirubin (p = 0.002) but for total protein and uric acid no significant change occurred. In the experimental group, a significant difference was found through various assessments for MDA (p = 0.001), FRAP (p = 0.001), bilirubin (p = 0.008) and uric acid (p = 0.012). It can be concluded that this protocol of sprint exercise training causes adaptation in the antioxidant system and lipid peroxidation, but by detraining these results will be reversed. © Springer-Verlag Italia 2008.
