Eleftherios Kellis

Quantification of Quadriceps and Hamstring Antagonist Activity

SummaryThe 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.

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.

Determining variables of plyometric training for improving vertical jump height performance: a meta-analysis.

Saez Saez de Villarreal, E, Kellis, E, Kraemer, WJ, and Izquierdo, M. Determining variables of plyometric training for improving vertical jump height performance: a meta-analysis. J Strength Cond Res 23(2): 495-506, 2009-Plyometric training improves vertical jump height (VJH). However, the effectiveness of plyometric training depends on various factors. A meta-analysis of 56 studies with a total of 225 effect sizes (ESs) was carried out to analyze the role of various factors on the effects of plyometrics on VJH performance. The inclusion criteria for the analysis were a) studies using plyometric programs for lower-limb muscles, b) studies employing true experimental designs and valid and reliable measurements, and c) studies including enough data to calculate ESs. Subjects with more experience in sport obtained greater enhancements in VJH performance (p < 0.01). Subjects in either good or bad physical condition benefit equally from plyometric work (p < 0.05), although men tend to obtain better power results than women after plyometric training (p < 0.05). With relation to the variables of performance, training volumes of more than 10 weeks and more than 20 sessions, using high-intensity programs (with more than 50 jumps per session), were the strategies that seemed to maximize the probability of obtaining significantly greater improvements in performance (p < 0.05). To optimize jumping enhancement, the combination of different types of plyometrics (squat jump + countermovement jump + drop jump) is recommended rather than using only 1 form (p < 0.05). However, no extra benefits were found to be gained from doing plyometrics with added weight. The responses identified in this analysis are essential and should be considered by strength and conditioning professionals with regard to the most appropriate dose-response trends for optimizing plyometric-induced gains.

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.

Isokinetic Eccentric Exercise

SummaryThe 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.

Ankle Sprain Injuries and Risk Factors in Amateur Soccer Players During a 2-Year Period

Background Although ankle sprains are common in soccer, the role of various risk factors in amateur soccer players is unclear. Purpose To identify the incidence of ankle sprain injuries, associated time loss of participation, and risk factors during two consecutive seasons in amateur players. Study Design Descriptive epidemiology study. Methods Of 336 athletes enrolled in the study, 312 male amateur soccer players were observed during a 2-year period. Ankle sprain injury incidents, participation time loss, injury mechanisms, ankle region injured, and other risk factors were recorded in games and practice sessions using questionnaires. Results During the study 208 ankle injuries were recorded, of which 139 were ankle sprains. These led to 975 sessions lost (on average, 7 lost sessions per injury). Most incidents (80.6%) were contact injuries, occurring mostly in defenders. Injury rates were equal between games and practice, while 61.1% of injuries were observed toward the end of each half of the game (P < .05). The injury incidence rate was higher during the first 2 months of the season as opposed to the last month (P < .05). Multinomial logistic regression showed that previous ankle sprain (P < .05) is a significant predictor of ankle sprain injury. Conclusion Ankle sprain injuries in amateur soccer players are primarily contact injuries, occurring mainly in defenders and during both games and practice. It appears that more injuries occur in players with previous ankle injury. Injury rates are higher toward the end of a game and chiefly occur during the first 2 months of the season.

The effects of antagonist moment on the resultant knee joint moment during isokinetic testing of the knee extensors

Abstract The purpose of this study was to examine the effects of moment of antagonistic muscle on the resultant joint moment during isokinetic eccentric and concentric efforts of the knee extensors. Ten males performed maximum eccentric and concentric knee extension and flexion efforts on a Biodex dynamometer at 0.52 rad · s−1 (30° · s−1). Electromyographic (EMG) activity of vastus medialis and biceps femoris (hamstrings) was also recorded. The antagonistic moment of the hamstrings was determined by recording the integrated EMG (iEMG)/moment relationship at different levels of muscle effort. The iEMG/moment curves were fitted using second-degree polynomials. The polynomials were then used to predict the antagonistic moment exerted by the hamstrings from the antagonist iEMG. The antagonistic moment had a maximum of 42.92 Nm and 28.97 Nm under concentric and eccentric conditions respectively; paired t-tests indicated that this was a significant difference (P < 0.05). These results indicate that the resultant joint moment of knee extensors is the result of both agonist and antagonist muscle activation. The greater antagonist muscle activity under concentric activation conditions may be partly responsible for the lower resultant joint concentric moment of knee extensors compared with the corresponding eccentric activation. The antagonist moment significantly affects comparisons between the isokinetic moments and agonist EMG and in vitro force measurements under different testing (muscle action and angular velocity) conditions.

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.

Effects of combined strength and kick coordination training on soccer kick biomechanics in amateur players

The aim of this study was to examine the effect of a soccer (strength and technique) training program on kinematics and electromyographic (EMG) muscle activity during a instep kick. Ten amateur soccer players (aged 19.9±0.4 years, body mass 74.8±9.1 kg, height 177.4±6.7 cm) constituted the experimental group (EG) whereas 10 players (age 21.6±1.3 years, weight 71.5±6.7 kg, height 175.2±3.4 cm) served as controls (CG). The EG followed a 10‐week soccer‐specific training program combining strength and technique exercises. All participants performed an instep soccer kick using a two‐step approach while three‐dimensional data and EMG from six muscles of swinging and support legs were recorded prior to and after training. Maximum isometric leg press strength, 10‐m sprint performance and maximum speed performance on a bicycle ergometer were also measured. Analysis of variance designs with repeated measures showed that the EG improved significantly (P<0.05) maximum ball speed, the linear velocity of the foot, ankle and angular velocity of all joints during the final phase of the kick. Training had insignificant effects on EMG values, apart from an increase in the averaged EMG of the vastus medialis whereas maximum isometric strength and sprint times significantly improved after training (P<0.05). The present results suggest that the application of the training programs using soccer‐specific strength exercises would be particularly effective in improving of soccer kick performance.

Effects of an intermittent exercise fatigue protocol on biomechanics of soccer kick performance.

The purpose of this study was to examine the effects of fatigue on biomechanical indices of soccer kick performance. Ten male amateur soccer players performed maximal instep kicks prior to, in the middle and after the implementation of a 90 min intermittent exercise protocol. Three‐dimensional data, ground reaction forces (GRFs) and segmental moments were measured during the kick while blood lactate and ammonia concentrations were monitored throughout the protocol. Analysis of variance designs with repeated measures indicated a significant increase in ammonia (P<0.01) and lactate levels (P<0.01) following fatigue. The GRFs and joint displacement curves during the kick remained unaltered after fatigue (P>0.01). However, post‐fatigue maximum angular velocity of the shank, the net moments acting on the shank and the resultant joint moments were significantly lower compared with the corresponding pre‐exercise values (P<0.01). The velocity of the ball was 24.69 m/s prior to the protocol and significantly decreased to 21.78 m/s after (P<0.01). Similarly, the ball/foot speed ratio significantly (P<0.01) declined from 1.40±0.12 (pre‐fatigue) to 1.33±0.18 (post‐fatigue). The present results suggest that an exercise protocol that simulates soccer game conditions results in significant impairment of soccer kick performance. This could be attributed to alterations of the function of the neuromuscular system and force generation capacity, which may have altered the mechanics of soccer kick performance.