Tendon vibration during submaximal isometric strength and postural tasks.

To establish the effect of tendon vibration (TV) on steady submaximal strength and static balance control, 29 women performed isometric plantarflexions at 10, 20, 30 and 50% of MVC and postural tasks of increasing difficulty (Normal Quiet Stance, NQS, Sharpened Tandem Stance and One Leg Stance) with vibrators (80 Hz) applied to the Achilles tendon. Both tasks were performed under four conditions (10 s each): eyes open, eyes closed, eyes closed with TV, eyes open with TV. During the isometric tasks, the application of TV increased the plantarflexion torque at 20 and 30% of MVC, accompanied by a greater (p < 0.001) activation of agonists (Medial Gastrocnemius, MGAS and Soleus, SOL), as well as a greater (p < 0.05) coactivation of the antagonist (tibialis anterior, TA). Non-significant differences were observed at 10 and 50% of MVC. During posture, TV resulted in a backward displacement of the Centre of Pressure only for NQS and an increase (p < 0.01) in EMG activity of the ankle muscles in all balance tasks. The MGAS contraction during TV application was confirmed (by ultrasonography) at ten participants by an increase of the pennation angle and decrease of its fascicle length (p < 0.05). The vibration-induced muscle excitation and accompanying increase in motor output, probably due to excitatory Ia afferent input, was confirmed during strength and postural tasks. However, motor output attenuates when the magnitude of central drive to the ankle muscles increases.

Vibration effects on static balance and strength.

The purpose of this study was to investigate the effects of a vibration training protocol and a conventional strength training program consisting of similar exercises on knee extensors and flexors strength and postural sway in middle-aged women. 38 women were randomly assigned into a Vibration Group (n=12, static and dynamic exercises on a vibration plate, frequency: 15-25 Hz, amplitude: 2-12.8 cm), a Strength Group (n=16, same exercises without vibration) and a Control Group (n=10). Both experimental groups trained for 12 weeks (3 sessions/w). Static balance was assessed in 3 tasks of increasing difficulty: Normal Quiet Stance, Sharpened Tandem, and One-Legged Stance. Postural sway was evaluated using the Centre of Pressure variations in the Anterior/Posterior and Medio/Lateral direction. Eccentric and concentric strength of knee extensors and flexors was recorded using a Cybex dynamometer. After vibration training, postural sway significantly decreased in both directions for the vibration group in all tasks (p<0.05), whereas no significant differences were observed for the other groups. Isokinetic strength significantly (p<0.05) increased for both experimental groups at selected angular velocities. It was concluded that side-alternating vibration could have beneficial effects on static balance control for middle-aged women. Gains in isokinetic strength were quite similar for both experimental groups.

The relationship between isokinetic knee extension and flexion strength with soccer kick kinematics: an electromyographic evaluation.

AIM: The purpose of this study was to investigate the relationship between isokinetic strength knee testing and soccer kick kinematics using electromyography (EMG). METHODS: Thirteen pubertal soccer players (age: 14.3+/-0.4 years) performed maximum instep soccer kicks, while knee angular position of the swinging leg was recorded using a twin-axis electrogoniometer. Bipolar surface EMG activity of the vastus medialis, vastus lateralis and biceps femoris (BF) muscles was recorded. The subjects also performed maximum knee extension and flexion efforts at concentric angular velocities of 1.04, 3.14 and 5.23 rad x s(-1) and eccentric angular velocities of 1.04 and 3.14 rad x s(-1). RESULTS: The correlation coefficients between isokinetic moments and knee angular velocity values during the kick ranged from 0.609 to 0.898 for concentric moments and from 0.431 to 0.612 for eccentric moments. Agonist EMG values during isokinetic tests ranged from 63.17+/-19.9% to 128.7+/-34.9% maximum voluntary contraction (MVC). Antagonist EMG levels ranged from 9.76+/-6.12% to 36.91+/-22.81% MVC. The corresponding EMG values during the soccer kick ranged from 12.78+/-6.8% to 122.34+/-61.5% MVC and increased as the foot approached the ball. CONCLUSION: Isokinetic tests at intermediate and fast angular velocities are adequate for monitoring strength training programs in soccer. However, muscle activation patterns differ between the two movements, especially those of the BF muscle.

Agonist and antagonist muscle activation during maximal and submaximal isokinetic fatigue tests of the knee extensors.

The purpose of this study was to examine the differences in electromyographic activity of agonist and antagonist knee musculature between a maximal and a submaximal isokinetic fatigue protocol. Fourteen healthy males (age: 24.3+/-2.5 years) performed 25 maximal (MIFP) and 60 submaximal (SIFP) isokinetic concentric efforts of the knee extensors at 60 degrees s(-1), across a 90 degrees range of motion. The two protocols were performed a week apart. The EMG activity of vastus medialis (VM), vastus lateralis (VL) and biceps femoris (BF) were recorded using surface electrodes. The peak torque (PT) and average EMG (aEMG) were expressed as percentages of pre-fatigue maximal value. One-way analysis of variance indicated a significant (p<0.05) decline of PT during the maximal (45.7%) and submaximal (46.8%) protocols. During the maximal test, the VM and VL aEMG initially increased and then decreased. In contrast, VM and VL aEMG continuously increased during submaximal testing (p<0.05). The antagonist (BF) aEMG remained constant during maximal test but it increased significantly and then declined during the submaximal testing. The above results indicate that agonist and antagonist activity depends on the intensity of the selected isokinetic fatigue test.

Changes in specific jumping performance after detraining period.

AIM: The purpose of this study was to examine the effects of a detraining period after specific plyometric training on vertical jump performance and mechanical properties of aponeurosis of medial gastrocnemius (MG). METHODS: Twenty participants were assigned into two groups and performed plyometric training on incline (IP, N.=10) and plane ground (PP, N.=10), respectively. Vertical jumping (squat jump [SJ], countermovement jump [CMJ], slow and fast drop jumps from 20 and 40 cm [DJ20, DJ40, DJf20, DJf40]) and aponeurosis strain of MG have been re-evaluated four weeks after the end of the training period. The electromyographic (EMG) activity of MG and tibialis anterior (TA) muscles and MG architecture were also recorded after a 4 week of detraining period. RESULTS: After detraining period, jumping height was decreased from 31.20±3.3 cm to 27.5±2 cm and from 31.8±2.9 cm to 29.5±1.9 cm for DJf20 and DJf40, respectively. For the IP group, these changes were accompanied by a significantly lower MG activity during the propulsion phase (from 0.93%±0.09 to 0.77±0.09% and from 0.89±0.09% to 0.60%±0.06 for DJf20 and DJf40, respectively). Similarly, strain was decreased from 22.7% (±0.05) to 16.3% (±0.05) after detraining period. CONCLUSION: These findings suggest that after four weeks of detraining, ankle muscle tendon complex properties withdraw to the pre-training values with lower performance, possible reflecting a different working length of the muscle.

Quantification of quadriceps and hamstring antagonist activity.

The coactivation of hamstrings and quadriceps, and its relation to knee joint stability and cruciate ligament loading, have been extensively examined over the last decades. The purpose of this review is to present findings on the quantification of antagonist activation around the knee. Coactivation of the quadriceps and hamstrings during many activities has been examined using electromyography (EMG). However, there are several factors that affect antagonist EMG activity, such as the type of muscle action, velocity of the movement, level of effort and angular position. Furthermore, the antagonist EMG can be affected by methodological factors which relate to the data recording, analysis and quantification of the signal. Research has demonstrated that the effect of cross-talk on the hamstrings and quadriceps antagonist EMG depends on electrode size and location, fat layer of the muscles and the technique used to reduce it. There is an inconsistency as to the method used to normalise antagonist EMG depending on the type of movement examined. This makes comparisons between studies difficult and, therefore, further research is recommended. The antagonist function is better represented when the antagonist moment exerted is known; however, the direct measurement of antagonist moments or forces is very difficult. Few studies have used mathematical models to determine the moment or force distribution around the knee, including antagonist forces. This can be attributed to the complexity of the anatomy and function of the knee joint. Despite this, in vivo and in vitro experiments have demonstrated that quadriceps contraction near full extension induces significantly higher anterior shear forces when compared with the forces exerted when the hamstrings act as antagonists, thus indicating the important role of antagonist activity in knee joint stability. However, the magnitude of this contribution to the force distribution around the knee in many activities is unclear.

Isokinetic eccentric exercise.

The development of active isokinetic dynamometers has allowed the assessment of muscular moment under eccentric activations that have different characteristics to concentric actions. It is well documented that at a given angular velocity the eccentric moment is greater than the corresponding concentric moment. The moment-velocity relationship under eccentric conditions has been investigated, with conflicting results. Particularly, eccentric moment was reported to remain similar to, or to increase or decrease with, increasing angular velocity. As with concentric actions, the reliability of isokinetic eccentric measurements is influenced by a number of factors such as gravity, preload force and testing position. The velocity-specific effects of eccentric training have not been extensively investigated. Based on current knowledge, eccentric exercise does not appear to be velocity-specific. Although the mode specificity of both concentric and eccentric exercises have been investigated, the resultant observations are conflicting. Eccentric training has been found to improve both concentric and eccentric strength: yet, it has also been reported to improve only concentric or eccentric strength. The reciprocal muscle group ratios under eccentric actions were found not to be influenced by angular velocity, but the significant role of the eccentric/concentric moment ratio of each muscle has not been examined thoroughly. It is well documented that eccentric activations are associated with delayed muscle soreness and muscle damage. A limited number of studies have reported that isokinetic eccentric efforts may result in a lower amount of muscle soreness compared with other exercise modalities. Isokinetic dynamometers provide some unique characteristics for rehabilitation applications. Examination of the clinical application of eccentric exercise is limited. Consequently, the use of this exercise modality in prevention and assessment of musculoskeletal injuries should be investigated further.

Muscle activation differences between eccentric and concentric isokinetic exercise.

PURPOSE: The purpose of this study was to compare electromyographic (EMG) activity and joint moment of agonists and antagonists between isokinetic eccentric and concentric knee muscle actions. METHODS: Twelve females (20.5 +/- 2.9 yr) performed maximum knee extension and flexion effort on a Biodex dynamometer isometrically and at concentric and eccentric angular velocities ranging from 30 degrees.s-1 to 150 degrees.s-1. EMG activity of vastus lateralis, rectus femoris, vastus medialis, and hamstrings was also recorded. The moment and agonist EMG values were normalized as a percentage of the maximum isometric values. The antagonist EMG was normalized as a percentage of the IEMG activity of the same muscle group when acting as agonist at the same angular velocity and angular position and taking into consideration the effects of muscle action. RESULTS: Three-way ANOVA designs indicated significantly greater normalized eccentric moments compared with concentric moments (P < 0.05), whereas the eccentric normalized integrated EMG (IEMG) of agonists and antagonists was significantly lower compared with the respective concentric IEMG values (P < 0.05). These differences were more evident at fast angular velocities. CONCLUSIONS: The present results demonstrate that neural activation and the resulting muscular action are different between isokinetic eccentric and concentric tests and depend also on the angular velocity of the movement. The antagonist IEMG activity is different depending on the muscle examined. The IEMG activity of the antagonists in this study indicate that the antagonist activity is an important factor that affects the resultant joint moment during isolated isokinetic maximum voluntary joint movements.

Gravitational moment correction in isokinetic dynamometry using anthropometric data.

The purpose of this study was to compare different methods of gravity correction in isokinetic dynamometry. The gravitational moment of the left leg-foot segment of 25 males was measured on a Biodex dynamometer at 30 degrees of knee flexion statically, and as the segment was passively falling from 0 degree to 90 degrees with a constant velocity of 0.035 rad.s-1. The tests were performed in a supine and a seated position. The gravitational moments were also estimated from anthropometric data and directly, using a reaction board method that was considered as the criterion measurement. Furthermore, the isokinetic maximum moment output of the knee flexors and extensors was measured and corrected for gravity using the above methods at angular velocities of 0.52 rad.s-1 and 2.62 rad.s-1 during eccentric, and at 0.52 rad.s-1, 2.62 rad.s-1, and 4.71 rad.s-1 during concentric activations. One-way repeated-measures analysis of variance tests revealed significant differences (P < 0.05) between the gravitational moments obtained on the isokinetic dynamometer and those estimated from anthropometric data or by the reaction board method. The maximum extensor and flexor moments were significantly different, corrected using the gravity correction methods on the dynamometer relative to the anthropometric model or the reaction board (P < 0.05). These results suggest that the determination of gravitational moments based on anthropometric data is more accurate than current gravity correction methods. This new proposed method should be implemented in isokinetic dynamometry applications.

The effects of the antagonist muscle force on intersegmental loading during isokinetic efforts of the knee extensors.

Hamstrings activation when acting as antagonists is considered very important for knee joint stability. However, the effect of hamstring antagonist activity on knee joint loading in vivo is not clear. Therefore, the purpose of this study was to examine the differences in antagonistic muscle force and their effect on agonist muscle and intersegmental forces during isokinetic eccentric and concentric efforts of the knee extensors. Ten males performed maximum isokinetic eccentric and concentric efforts of the knee extensors at 30 degrees s(-1). The muscular and tibiofemoral joint forces were then estimated using a two-dimensional model with and without including the antagonist muscle forces. The antagonist moment was predicted using an IEMG-moment model. The predicted antagonist force reached a maximum of 2.55 times body weight (BW) and 1.16 BW under concentric and eccentric conditions respectively. Paired t-tests indicated that these were significantly different (p<0.05). A one-way analysis of variance indicated that when antagonist forces are included in the calculations the patella tendon, compressive and posterior shear joint forces are significantly higher compared to those calculated without including the antagonist forces. The anterior shear force was not affected by antagonist activity. The antagonists produce considerable force throughout the range of motion and affect the joint forces exerted at the knee joint. Further, it appears that the antagonist effect depends on the type of muscle action examined as it is higher during concentric compared to eccentric efforts of the knee extensors.

Plantar pressure distribution during barefoot standing, walking and landing in preschool boys.

The purpose of this study was to examine the pressure distribution under the foot in preschool boys during standing, landing and walking tasks. Fourteen healthy boys (age 3.20+/-0.4 years) performed five activities (standing on one foot, standing on two feet, landing from a height onto one foot and both feet, and walking) on a Musgrave pressure platform system. The peak pressures were determined for eight areas of the foot. One-way analysis of variance (ANOVA) indicated that the total plantar pressures during landings were significantly higher compared to the overall plantar pressure developed during standing on one foot or two-feet and the contact phase of walking (P<0.05). A two-way ANOVA indicated that with the exception of the second to fifth toe area, the pressures were significantly higher elsewhere during landing and walking compared with the pressures during standing on two feet (P<0.05). Neither standing nor the stance phase of walking is fully representative of foot function in children.

The effects of fatigue on the resultant joint moment, agonist and antagonist electromyographic activity at different angles during dynamic knee extension efforts.

Examination of the effects of fatigue on antagonist function can provide information on the role of antagonists in limiting the resultant joint moment and stabilizing the knee. Therefore, the purpose of this study was to examine the moment, agonist and antagonist electromyographic (EMG) activity levels at different angular positions during an isokinetic muscular endurance knee extension test. Fifteen healthy males (age 22.6+/-1.9 yr) performed 34 maximal isokinetic concentric efforts of the knee extensors at 120 degrees s(-1). The EMG activity of vastus medialis and biceps femoris was recorded using surface electrodes. The motion ranged from 90 degrees to 0 degrees of knee flexion. The average moment and average EMG (AEMG) at 10-35 degrees, 36-55 degrees and 56-80 degrees angular position intervals were calculated for each repetition. Twenty eight efforts were further analysed. The moment of force demonstrated a decline of 70% at the end of the test. Two-way repeated measures analysis of variance tests indicated that this decline was significant (p < 0.05). No significant effects of angular position on fatigue moment characteristics were found. The agonist (vastus medialis) AEMG during the first repetition demonstrated a significant increase of 40-60% towards the middle part of the test (p < 0.05). In the second part of the test, the VM AEMG at longer muscle lengths was significantly higher compared to the initial efforts whereas the AEMG at short muscle lengths returned to initial values. The antagonist AEMG at all angular positions did not change significantly during the test. The decline in the resultant joint moment could be attributed to the effects of fatigue on the agonist muscle function. The agonist AEMG fatigue-patterns are dependent on the length of the muscle and may be due to alterations in the motor unit recruitment and/or activation failure in the quadriceps muscle. The biceps femoris maintains constant submaximal (21-33% of the maximum) AEMG activity which may play an important role in the stability of the knee joint. The contribution of antagonist activity to the resultant joint moment increases during the last part of an isokinetic concentric muscle endurance test.

The effects of normalization method on antagonistic activity patterns during eccentric and concentric isokinetic knee extension and flexion.

The purpose of this study was to compare different normalization methods of electromyographic (EMG) activity of antagonists during isokinetic eccentric and concentric knee movements. Twelve women performed three maximum knee extensions and flexions isometrically and at isokinetic concentric and eccentric angular velocities of 30 degrees .s(-1), 90 degrees .s(-1), 120 degrees .s(-1) and 150 degrees .s(-1). The EMG activity of the vastus lateralis, rectus femoris, vastus medialis and hamstrings was recorded. The antagonist integrated IEMG values were normalized relative to the EMG of the same muscle during an isometric maximal action (static method). The values were also expressed as a percentage of the EMG activity of the same muscle, at the same angle, angular velocity and muscle action (dynamic method) when the muscle was acting as an agonist. Three-way analysis of variance (ANOVA) designs indicated significantly greater IEMG normalized with the dynamic method compared to the EMG derived using the static method (P < 0.05). These differences were more evident at concentric angular velocities and at the first and last 20 degrees of the movement. The present findings demonstrate that the method of normalization significantly influences the conclusions on antagonistic activity during isokinetic maximum voluntary efforts. The dynamic method of normalization is more appropriate because it considers the effects of muscle action, muscle length and angular velocity on antagonist IEMG.

Effects of agonist and antagonist muscle fatigue on muscle coactivation around the knee in pubertal boys.

In many activities the knee joint flexes and extends actively with the involvement of both knee extensor and flexor muscle groups. Consequently the examination of the muscle activity during reciprocal movements may provide useful information on the function of these two muscle groups during fatigued conditions. Therefore, the purpose of this study was to examine the activity of antagonist muscles during a reciprocal isokinetic fatigue test of the knee extensors and flexors. Fifteen healthy pubertal males (age 13.8+/-0.8 years) performed 22 maximal isokinetic concentric efforts of the knee extensors at 60 degrees s(-1). The EMG activity of vastus medialis (VM), vastus lateralis (VL) and biceps femoris (BF) was recorded using surface electrodes. The motion ranged from 100 to 0 degrees of knee flexion. The average moment and average EMG (AEMG) at 10-30 degrees, 31-50 degrees, 51-70 degrees and 71-90 degrees angular position intervals were calculated for each repetition. Twenty efforts were further analyzed. Two-way repeated measures analysis of variance (ANOVA) tests indicated a significant decline of moment during the test (p<0.025). The VM and VL AEMG at longer muscle lengths increased significantly as the test progressed whereas the AEMG at short muscle lengths (10-30 degrees ) did not significantly change. The agonist AEMG of BF during the first repetition demonstrated a significant increase after the ninth repetition (p<0.025). The antagonist AEMG of all muscles did not change significantly during the test. These results indicate that there is consistent antagonist activity during both extension and flexion phases of an isokinetic reciprocal fatigue test. It can be concluded that during an isokinetic reciprocal fatigue test, both knee extensors and flexors are fatigued. However, this condition does not have a significant effect on the EMG patterns of these muscles when acting as antagonists during the test.

Muscle co-activation around the knee in drop jumping using the co-contraction index.

The purpose of this study was to examine the co-activation of the rectus femoris (RF) and biceps femoris (BF) during drop jumping exercises using the co-contraction index (CI). Ten trained male long jumpers performed drop jumps from 20 cm (DJ20), 40 cm (DJ40) and 60 cm (DJ60) on a force platform. Surface electromyographic (EMG) activity of the RF and BF, vertical ground reaction force data and knee joint angular displacement and angular velocities were recorded and normalized as percentage of maximum isometric values. The CI was calculated for the pre-contact, braking and propulsive phases of the jump using four methods: (1) by dividing the double integrated antagonistic activity by the sum of the RF and BF EMG; (2) by finding the amount of overlap between the linear envelopes of the agonist and antagonist muscles and dividing by the number of data points; (3) by calculating the co-contraction at any instant point of time; and (4) by dividing the BF integrated activity by the total registered muscle activity around the knee. The CI ranged from 13.03+/-9.33 to 70.80+/-25.81%, depending on the estimation method used. A two-way analysis of variance (ANOVA) indicated that the CI was not affected by drop jumping height (p>0.05) while it was significantly higher (p<0.05) in the pre-contact phase compared to the braking and propulsion phases. The CI can be useful when examining muscle co-activation using EMG measurements in drop jumps. However, the conclusions on muscle co-activation depend on the equation used to estimate CI and therefore a commonly accepted method is necessary.

Tibiofemoral joint forces during maximal isokinetic eccentric and concentric efforts of the knee flexors.

OBJECTIVE: The examination of muscular and tibiofemoral joint forces during maximal efforts of the knee flexors. DESIGN: The muscular and tibiofemoral joint knee forces during eccentric and concentric isokinetic efforts of the knee flexors were determined using a two-dimensional mathematical model. BACKGROUND: The examination of joint and muscle loading during isokinetic movements is important for the determination of safety of this exercise. METHODS: Ten healthy males performed three maximal isokinetic concentric and eccentric efforts of the knee flexors at angular velocities of 30 degrees s(-1), 90 degrees s(-1), 120 degrees s(-1) and 150 degrees s(-1). The muscular, tibiofemoral shear and compressive joint forces were determined using a two-dimensional model. RESULTS: The maximum muscular force ranged from 3.44 (Standard deviation, 1.32) times body weight to 6.19 (1.78) times body weight. The tibiofemoral compressive force ranged from 2.62 (1.17) times body weight to 5.89 (1.99) times body weight occurring at angles ranging from 0 degrees to 40 degrees of knee flexion. The posterior shear force ranged from 2.61 (1.33) times body weight to 3.89 (1.62) times body weight and was observed at angles ranging from 50 degrees to 80 degrees of knee flexion. Two-way analysis of variance designs indicated significant effects of type of muscle action and angular velocity on muscle and compressive forces (P<0.05). In contrast, the shear force was not affected by the type of muscle action or the angular velocity (P>0.05). CONCLUSIONS: Isokinetic efforts of the knee flexors induce high tibiofemoral joint forces, especially during high-speed eccentric tests.

In vivo determination of the patella tendon and hamstrings moment arms in adult males using videofluoroscopy during submaximal knee extension and flexion.

OBJECTIVE: The determination of patella tendon and hamstrings moment arms. DESIGN: The moment-arms were determined using a videofluoroscopy imaging method. BACKGROUND: The determination of hamstrings and quadriceps moment arms is important for the examination of muscle and joint forces in biomechanical applications. METHODS: Ten males performed one knee extension-flexion movement at a very slow (non-constant) angular velocity in front of the image intensifier screen of a videofluoroscopic system. The image of a calibration phantom with 2 cm squares was also recorded. Thirty five calibration points with known coordinates were digitized for the establishment of the polynomial equations that determined the coordinates of any point in the calibration phantom from its corresponding video coordinates. Fourteen control points were digitized and the difference between the actual and predicted coordinates was measured. The patella tendon and hamstrings (biceps femoris) moment arms to the tibiofemoral contract point were calculated. RESULTS: The mean error in the digitizing of the control points using the image deformation method ranged from 0.182 mm to 0.267 mm. The mean patella tendon moment arm ranged from 36.91 (SD 3.2) mm to 42.63 (SD 4.5) mm. The hamstrings moment arm ranged from 23.93 (SD 2.59) to 28.25 (SD 3.04) mm. CONCLUSION: The patellar tendon and hamstring moment-arms were determined using a videofluoroscopy method with acceptable accuracy. These parameters can be used for the biomechanical analysis of knee joint movements in two dimensional applications.

Resistive eccentric exercise: effects of visual feedback on maximum moment of knee extensors and flexors.

One of the most important features of isokinetic dynamometry is the accurate assessment of muscular function. One of the main factors affecting the accuracy of isokinetic parameters during maximum activation efforts is visual feedback. The purpose of this study was the examination of the effects of visual feedback on maximum moment measurements of the knee extensors and flexors during isokinetic eccentric activations. Twenty-five males performed five maximal efforts at angular velocities of 30 degrees/sec and 150 degrees/sec with and without visual feedback on a Biodex dynamometer. Visual feedback was provided as real time display of the moment output. A three-factor analysis of variance test revealed significant differences between the moments recorded with visual feedback and the nonvisual feedback maximum moments of knee extensors and flexors at both speeds. The mean extension peak moments at 30 degrees/sec and 150 degrees/sec under visual feedback condition were approximately 7.2 and 6.4% higher than the nonvisual feedback moments, respectively. The increase for the knee flexor moment was 8.7 and 9% for slow and fast speeds, respectively. These findings suggest that visual feedback can improve maximum eccentric output and should be provided during assessment of maximum eccentric strength on an isokinetic dynamometer.

Prediction of knee extensor and flexor isokinetic strength in young male soccer players.

STUDY DESIGN: Single group, cross-sectional study. OBJECTIVES: To measure various anthropometric and demographic variables in young male soccer players and to use these measurements to develop equations with which to predict the isokinetic, concentric, and eccentric moment of force. BACKGROUND: The development of equations that can predict isokinetic muscle strength from commonly measured subject characteristics can assist in the effective design of training and rehabilitation programs for athletic children. METHODS AND MEASURES: One hundred thirteen male soccer players (13.50 +/- 2.21 years of age) performed eccentric and concentric maximum efforts of the knee extensors and flexors at 60, 120, and 180 degrees/second. Stepwise regression tests were used to develop predictive equations using combinations of age, height, body mass, sexual maturation (Tanner) stage, percentage of body fat, and hours spent training per week. RESULTS: The results indicated a significant relationship of concentric and eccentric isokinetic strength for both knee extensors and flexors with 73-93% of the variance explained by using combinations of age, body mass, percentage of body fat, and hours training per week. Body mass and age were the main predictor variables under concentric conditions, whereas chronological age was not included in the models under eccentric conditions. CONCLUSIONS: The relation between isokinetic moment and multiple anthropometric and demographic variables depends on the type of muscle action examined. The results suggest that the equations we developed can be used to predict the isokinetic moment in trained, young soccer players. A cross-validation analysis is required to confirm the accuracy and the suitability of the equations developed in our study.

Combined effects of age and maturation on maximum isometric leg press strength in young basketball players.

AIM: Muscular strength of the leg extensor muscles in children can be affected by several factors such as age, sexual maturation, body mass and training status of the subjects. The purpose of the study was to examine maximal isometric strength characteristics of young male basketball players taking into consideration the combined effects of chronological age and sexual maturation. METHODS: One hundred and twenty male basketball players, aged from 12 to 17 years divided into 6 equivalent age subgroups performed maximum bilateral isometric leg press efforts. The parameters analysed were the maximal voluntary isometric force (MVC), relative strength (MVC/body mass and MVC/fat free mass), starting strength (F50: force exerted during the first 50 ms of the contraction) and speed strength index (the ratio of maximal force to time to attain maximal force). RESULTS: The results indicated that in almost all absolute force parameters, the 12-and 13-year olds demonstrated significantly (p<0.05) lower values compared with the 15(-1)6-and 17-years old groups. Age differences were also significant (p<0.05) when the effects of sexual maturation were taken into consideration in the statistical analysis but they were reduced when strength was adjusted for body mass. Finally, no significant differences for strength per unit of fat free mass were found (p>0.05). CONCLUSIONS: Maximum absolute strength of basketball players is significantly increased from 12 to 17 years and as sexual maturation stage increases. It also appears that body mass and fat free mass should be taken into consideration when examining age effects on strength in basketball players.