Lower extremity dexterity is associated with agility in adolescent soccer athletes.

Agility is important for sport performance and potentially injury risk; however, factors affecting this motor skill remain unclear. Here, we evaluated the extent to which lower extremity dexterity (LED) and muscle performance were associated with agility. Fourteen male and 14 female soccer athletes participated. Agility was evaluated using a hopping sequence separately with both limbs and with the dominant limb only. The LED test evaluated the athletes' ability to dynamically regulate foot-ground interactions by compressing a spring prone to buckling with the lower limb. Muscle performance included hip and knee isometric strength and vertical jump height. Correlation analyses were used to assess the associations between muscle performance, LED, and agility. Multiple regression models were used to determine whether linear associations differed between sexes. On average, the female athletes took longer to complete the agility tasks than the male athletes. This difference could not be explained by muscle performance. Conversely, LED was found to be the primary determinant of agility (double limb: R(2) = 0.61, P < 0.001; single limb: R(2) = 0.63, P < 0.001). Our findings suggest that the sensorimotor ability to dynamically regulate foot-ground interactions as assessed by the LED test is predictive of agility in soccer athletes. We propose that LED may have implications for sport performance, injury risk, and rehabilitation.

Electromyographic activity of cat hindlimb flexors and extensors during locomotion at varying speeds and inclines.

Electromyographic activity (EMG) was used to determine how hindlimb muscle activation patterns are modified as speed and incline of locomotion are varied in treadmill-trained cats. EMG was recorded using chronically implanted i.m. electrodes from the soleus, medial gastrocnemius, gluteus medius, and tibialis anterior muscles of adult cats during treadmill locomotion at a range of speeds and inclines. The patterns of changes in EMG activity at varying speeds and inclines were similar in all cats. Across speeds, the integrated EMG per step decreased for the soleus but remained constant for the other muscles. The integrated EMG per step was elevated in all muscles at higher inclines. Generally, with increased speed or incline the mean EMG per step was elevated in the medial gastrocnemius, gluteus medius, and tibialis anterior, the largest increase seen in the medial gastrocnemius. Soleus mean EMG per step remained unchanged with increased speed, but showed an absolute increase at the higher inclines. The integrated EMG per minute was always highest for the soleus followed by the medial gastrocnemius, and always lowest for the tibialis anterior. At the faster speeds, the 'on-time' increased in the tibialis anterior and decreased in the other muscles. These data suggest that the number of motor units activated and/or their firing frequencies increased in the medial gastrocnemius and the gluteus medius during locomotion at faster speeds or larger inclines, while relatively little change occurred in the soleus and tibialis anterior. These data also suggest that while there is considerable modulation of the level and duration of excitation of the extensor motor pools there is relatively little modulation of the flexor motor pools to adjust for both the speed and the incline of locomotion.

Weight-bearing hindlimb stepping in treadmill-exercised adult spinal cats.

Hindlimb locomotion on a motor-driven treadmill was studied in 5 cats spinalized at a low thoracic level adults. Six months after surgery, the cats were anesthetized and implanted for electromyographic (EMG) and force recordings in hindlimb muscles. For the last 5 months of the spinalization period, the hindlimbs of each cat were exercised daily for 30 minutes on a treadmill. Data were collected during hindlimb locomotion on a treadmill across the entire range of speeds each cat could accommodate. All trials were filmed (100 frames/s) for kinematic analysis. EMG data were recorded from the soleus (Sol), medial gastrocnemius (MG), tibialis anterior (TA) and extensor digitorum longus (EDL). Forces were recorded in vivo from the Sol and MG tendons. All cats could sustain full weight-bearing stepping without the need for mechanical stimulation of the tail. Although the general stepping pattern of the spinal cats was remarkably similar to that of normal cats, several key differences were identified. Compared to normal cats, the adult spinal cats walked at a lower range of speeds and exhibited a longer swing phase duration. The Sol produced forces and displayed activation periods comparable to those observed in normal cats. The MG of adult spinal cats, however, produced lower forces and had a later onset of activation in comparison to normal cats. Each of the muscles in all spinal cats exhibited tremor during stepping. These results suggest that there were limitations in the activation levels of some hindlimb flexor and extensor muscles during treadmill locomotion. These data further suggest that, in normal cats, accommodation to treadmill speed is accomplished by modulating supraspinal input to the lumbar spinal cord while leaving many of the timing details to be regulated by lumbar spinal networks.

Biomechanical factors associated with shoe/pedal interfaces. Implications for injury.

The principal demand on the body during cycling is on the lower extremities as they are responsible for producing a majority of the energy imparted to the bike. As a result the legs, due to high reactive forces between the foot and pedal, experience high loads on the joints. These loads may adversely affect joint tissues and contribute to overuse injuries, e.g. knee pain. The mechanical link between the leg and the bike is the shoe/pedal interface. This transmission site, by design, can either create smooth transfer of energy or abnormally high repetitive loads which are potentially injurious to the body. Incidence of lower extremity injury in cycling is high, and historically biomechanical analyses of this activity have focused their attention on either the rider or the bike, but not the link between the two. Recently, pedal designs have changed in response to complaints of sore knees with the development of pedals allowing varying degrees of float. This form of transmission is intended to enhance power transfer from rider to bike as well as minimise trauma to the legs by permitting the foot to rotate during the pedalling cycle in a toe-in/heel-out or heel-in/toe-out movement pattern. Recent evidence suggests this type of pedal design does reduce trauma and maintains power output. This article reviews common lower extremity overuse injuries and biomechanical factors during the pedalling cycle with the primary focus on the shoe/pedal interface. We will summarise information available on lower extremity kinematics and kinetics as well as recent data specifically related to shoe/pedal interface kinetics, evaluation of different pedal types-specifically comparison between clipless 'fixed' and clipless 'float' systems-and discuss their resultant effect on lower extremity dynamics and their implications for injury.

Biomechanics of sprint running. A review.

Understanding of biomechanical factors in sprint running is useful because of their critical value to performance. Some variables measured in distance running are also important in sprint running. Significant factors include: reaction time, technique, electromyographic (EMG) activity, force production, neural factors and muscle structure. Although various methodologies have been used, results are clear and conclusions can be made. The reaction time of good athletes is short, but it does not correlate with performance levels. Sprint technique has been well analysed during acceleration, constant velocity and deceleration of the velocity curve. At the beginning of the sprint run, it is important to produce great force/power and generate high velocity in the block and acceleration phases. During the constant-speed phase, the events immediately before and during the braking phase are important in increasing explosive force/power and efficiency of movement in the propulsion phase. There are no research results available regarding force production in the sprint-deceleration phase. The EMG activity pattern of the main sprint muscles is described in the literature, but there is a need for research with highly skilled sprinters to better understand the simultaneous operation of many muscles. Skeletal muscle fibre characteristics are related to the selection of talent and the training-induced effects in sprint running. Efficient sprint running requires an optimal combination between the examined biomechanical variables and external factors such as footwear, ground and air resistance. Further research work is needed especially in the area of nervous system, muscles and force and power production during sprint running. Combining these with the measurements of sprinting economy and efficiency more knowledge can be achieved in the near future.

Tendon force measurements and movement control: a review.

Knowledge of the mechanical and electrical output from skeletal muscle is of interest to investigators from several disciplines including physiology, biomechanics, neuroscience, orthopedics, and physical rehabilitation. Estimates of muscle output (i.e., force) have generally been made using indirect calculations. Forward solution (e.g., EMG) and optimization models have recently been developed using a wide variety of input parameters to estimate force output of individual muscles. These estimates, however, have lacked comparison values necessary for validation. In vivo measurements of muscle force have been made in both animals and humans using a "buckle" type tendon transducer surgically implanted on the tendons of the muscles under study. Investigations utilizing these transducers have addressed a wide range of questions regarding muscle function. This review examines the use of this technology and discusses the significance of the future use of "buckle" transducers in studies exploring load sharing among muscles and in the validation of existing models that estimate muscle force.

Mechanical energy management in cycling: source relations and energy expenditure.

Conservation of energy suggests that during cycling the constrained lower extremity is capable of delivering energy to the bicycle without expending energy to move the limbs. The purpose of this study was to characterize the management of mechanical energy during cycling and, specifically, to evaluate the potential for system energetic conservatism. Mechanical energy contributions derived from lower extremity energy sources were computed for 12 experienced male cyclists riding at five combinations of cadence and power output. The knee joint dominated (> 50%) in contributing to system energy and a moderate amount of energy was derived from hip joint reaction forces (> 6%). Energy generations and dissipations at the sources were sensitive to power output and, within the range of conditions studied, insensitive to cadence. Two energy models estimated mechanical energy expenditure under hypothetical single-joint and multijoint muscle operating conditions. When multijoint muscles were incorporated into the energy management analysis, a significant reduction in mechanical work relative to the single-joint muscle operation occurred. Energy savings associated with multijoint muscle energy transfers were enhanced at higher bicycle power levels, suggesting that conservation of mechanical energy is plausible given appropriate actions of two-joint muscles.

Kinetic and kinematic factors involved in the execution of front aerial somersaults.

The purpose of this investigation was to study the variation in ground reaction forces during take-off and to calculate the angular impulse produced about the center of mass by these forces during execution of a front aerial somersault. Additionally, several kinematic variables were discussed in relation to performance. Nine high level female gymnasts volunteered as subjects. Ground reaction force (GRF) patterns were monitored by a Kistler force platform and the selected kinematic variables were calculated from high speed film synchronized with the force records. Total body center of mass position was determined and subsequently used in the calculation of angular impulse during rear, double, and front leg support. While results showed similar angular impulse patterns across our subject population, a variety of kinetic and kinematic variables were presented in order to more completely discuss the components of angular impulse development during the take-off phase of a front aerial somersault. For example, while the mean angular impulse was 61.7 Nms (+/- 13.1), a surprising finding was the contribution made to this impulse by the rear leg, 70.9% (+/- 7.2).

Phasic relations in 90 degree abduction-adduction of the arm: the ARIMA representation.

Myoelectric signals (MES's) from the medial deltoid, posterior deltoid and medial trapezius were analyzed during 90 degree abduction-adduction cycles of arm movement. Two healthy males were utilized as subjects. The MES's records were divided into sub-sections of 50 ms of duration, and each segment was described by a suitable model. The models were based on autoregressive integrated moving average (ARIMA) processes. Several distinct phases were discerned within a given movement cycle, each phase being associated with a distinct type of ARIMA process. Phasic relations among the three muscles were then revealed. A simple test was suggested to detect the onset of muscle activity in real-time situations.

Knee flexor moments during propulsion in cycling--a creative solution to Lombard's Paradox.

The function of two joint muscles in the human lower extremity was studied during a cycling task with efficiency of their action discussed in light of Lombard's Paradox. Special pedals were designed to monitor reaction forces parallel to the sagittal plane of the body. Net moments of force about the hip, knee and ankle and EMG patterns in selected lower extremity muscles were recorded in five subjects pedalling against a constant load. The most original aspect of this study was the clear difference in hip and knee action during the propulsive phase of the pedalling cycle. A knee flexor moment was consistently observed in all subjects starting approximately half way through the propulsive phase of crank rotation (0-180 degrees) and presented as a creative solution to Lombard's Paradox.

In vivo moment arm calculations at the ankle using magnetic resonance imaging (MRI).

In vivo moment arm lengths for the Achilles tendon and tibialis anterior (TA) were determined in 10 adult male subjects. Moment arms were measured as the perpendicular distance between the joint center of rotation (CR) and the center of the muscle's tendon on a series of sagittal plane magnetic resonance images. The first set of calculations used a fixed CR and the second a moving CR. The position of the CR was determined using a modification of the graphical method of Reuleaux. For both moving and fixed CR conditions, moment arms increased by approximately 20% for the Achilles tendon and decreased by approximately 30% for the TA when the ankle moved from maximum dorsiflexion to maximum plantarflexion. Moment arms averaged 3.1% greater for the Achilles tendon and 2.5% greater for the TA when calculated using a fixed CR. These data suggest that the averaged moment arm lengths for the Achilles tendon and the TA were relatively unaffected by the use of a fixed vs moving CR.

A comparison of the triceps surae and residual muscle moments at the ankle during cycling.

The rigid linked system model and principles of inverse dynamics have been widely used to calculate residual muscle moments during various activities. EMG driven models and optimization algorithms have also been presented in the literature in efforts to estimate skeletal muscle forces and evaluate their possible contribution to the residual muscle moment. Additionally, skeletal muscle-tendon forces have been measured, directly, in both animals and humans. The purpose of this investigation was to calculate the moment produced by the triceps surae muscles and compare it to the residual muscle moment at the ankle during cycling at three power outputs (90, 180 and 270 W). Inferences were made regarding the potential contribution made by each triceps surae component to the tendon force using EMG and muscle-tendon length changes. A buckle-type transducer was surgically implanted on the right Achilles tendon of one male subject. Achilles tendon forces measured in vivo were multiplied by their corresponding moment arms to yield the triceps surae moment during the three working conditions. Moment arm lengths were obtained in a separate experiment using magnetic resonance imaging (MRI). Pedal reaction forces, body segment accelerations (determined from high speed film), and appropriate mass parameters served as input to the inverse solution. The triceps surae moment was temporally in phase with and consistently represented approximately 65% of the residual muscle moment at the ankle. These data demonstrate the feasibility of using implanted transducers in human subjects and provide a greater understanding of musculoskeletal mechanics during normal human movements.

Is coordination of two-joint leg muscles during load lifting consistent with the strategy of minimum fatigue?

The purpose of this study was to examine if strong correlations reported for a back lift task between activity (EMG) of two-joint rectus femoris (RF), hamstrings (HA), and gastrocnemius (GA) and the difference in the joint moments could be predicted by minimizing an objective function of minimum fatigue. Four subjects lifted barbell weights (9 and 18 kg) using a back lift technique at three speeds normal, slow, and fast. Recorded ground reaction forces and coordinates of the leg joints were used to calculate the resultant joint moments. Surface EMG of five muscles crossing the knee joint were also recorded. Forces of nine muscles were calculated using static optimization and a minimum fatigue criterion. Relationships (i) (RF EMG-HA EMG) vs (knee moment hip moment) and (ii) GA EMG vs. (ankle moment knee moment) were closely related (coefficients of determination were typically 0.9 and higher). Qualitatively similar relationships were predicted by minimizing fatigue. Gastrocnemius and hamstrings had the agonistic action at both joints they cross during load lifting, and their activation and predicted forces increased with increasing flexion knee moments and extension ankle and hip moments. The rectus femoris typically had the antagonistic action at the knee and hip, and its activation and predicted force were low. Patterns of predicted muscle forces were qualitatively similar to the corresponding EMG envelopes (except in phases of low joint moments where accuracy of determining joint moments was presumably poor). It was suggested that muscle coordination in load lifting is consistent with the strategy of minimum muscle fatigue.

Correlation of myoelectric activity and muscle force during selected cat treadmill locomotion.

The purpose of this investigation was to associate the force produced by the cat medial gastrocnemius (MG) during unrestrained treadmill locomotion with the corresponding myoelectric signals (MES's). An intervention analysis based on the autoregressive-integrated moving average (ARIMA) process was performed on records obtained at treadmill speeds of 0.67 ms-1 and 2.24 ms-1. Results indicate that the pattern of MG myoelectric activity during a single step cycle may be divided into two parts. The primary burst (E1 burst) of activity occurs before foot contact and represents an energy build up which is related to a major part of the MG force monitored at the tendon. During stance, a second burst of activity is depicted by an almost critically damped second order system, and is responsible for the residual tension observed. Thus in vivo forces can be linked to MES's provided that phase differences between electrical and mechanical responses are taken into account.

Relationship between ankle muscle and joint kinetics during the stance phase of locomotion in the cat.

The purpose of this study was to examine the relationship between internal force production in selected skeletal muscles and the externally calculated joint moment during overground locomotion in the adult cat. Hindlimb segments were modelled as a linked system of rigid bodies and a generalized muscle moment (GMM), the sum over all active and passive tissues acting about the joint, was calculated using principles of inverse dynamics. Moments produced by individual muscles were calculated using tendon transducers implanted in freely moving cats and muscle moment arm information. Results indicated that the externally measured variables of peak ground reaction force and joint position were equally important to the determination of peak ankle GMM. Examination of peak moments revealed that increases in peak ankle GMM were met by increases in medial (MG) and lateral (LG) gastrocnemius output. Peak soleus (SOL) moments did not change significantly as a function of peak ankle GMM. The role of the plantaris (PLT) was less clear, with peak moments increasing significantly as a function of peak ankle GMM in one cat. All four ankle extensors were important to the attainment of peak ankle GMM early in stance. Subsequently, SOL and PLT contributed substantially to the ankle GMM throughout stance, LG moments declined to near zero, soon after peak ankle GMM; and MG moments demonstrated a substantial but more gradual decline. The relative contributions of these individual muscles to the ankle GMM were supported by their respective architecture, uniarticular versus multiarticular function, and physiological profiles.

A technique for estimating mechanical work of individual muscles in the cat during treadmill locomotion.

Mechanical work, the product of force and length change, was assessed in selected hindlimb extensors of two adult cats during three different speeds of unrestrained treadmill locomotion. Forces were measured using implanted transducers placed on the soleus (SOL) and medial gastrocnemius (MG) tendons. A three dimensional technique of muscle length estimation using high speed cinematography was found preferable to either two dimensional or trigonometric measurements derived from anatomical and kinematic parameters. Length excursions increased in both muscles as treadmill speed increased. However, at all speeds of locomotion, the uniarticular SOL exhibited a greater range of motion than the biarticular MG. Increases in treadmill speed resulted in higher peak forces in the MG and constant or slightly lower peak forces in the SOL. These speed-dependent changes in length and force resulted in higher total positive work, lower total negative work, and higher net work for both muscles with increasing speeds. These data illustrate the importance of three-dimensional kinematics in determining changes in muscle length and describe the relative force and work changes in a slow and fast ankle extensor with changes in speed of locomotion.

Mechanical output of the cat soleus during treadmill locomotion: in vivo vs in situ characteristics.

To study the mechanical output of skeletal muscle, four adult cats were trained to run on a treadmill and then implanted under sterile conditions and anesthesia with a force transducer on the soleus tendon and EMG electrodes in the muscle belly. After a two-week recovery period, five consecutive step cycles were filmed at treadmill speeds of 0.8, 1.3 and 2.2 m s-1. Locomotion data in vivo included individual muscle force, length and velocity changes and EMG during each step cycle. Data for an average step cycle at each speed were compared to the force-velocity properties obtained on the same muscle under maximal nerve stimulation and isotonic loading conditions in situ. Results indicate that the force and power generated at a given velocity of shortening during late stance in vivo were greater at the higher speeds of locomotion than the force and power generated at the same shortening velocity in situ. Strain energy stored in the muscle-tendon unit during the yield phase in early stance is felt to be a major contributor to the muscle's enhanced mechanical output during muscle shortening in late stance.

Kinematic analysis of human upper extremity movements in boxing.

Using two synchronized cameras, four experienced boxers were filmed while they threw a series of punches at a practice bag. Three-dimensional (3D) coordinates of each boxer's shoulder, elbow, wrist, and glove were used to estimate linear and angular kinematics of the upper extremity. Average velocities at contact ranged from 5.9 to 8.2 m/s with peak velocities of 6.6 to 12.5 m/s reached 8 to 21 ms prior to hand/grove contact with the bag. Significant differences in shoulder and wrist velocitie, elbow angle excursions, and elbow angular velocities were seen when comparing hooks and jabs. Few differences were evident when comparing the kinematics of gloved versus bare-handed punches. Results are significant in providing kinematic data characteristic of experienced performers, which may be used in a kinetic model of punch impact and its relationship to potential injury mechanisms.

Three-dimensional cinematographic analysis of water polo throwing in elite performers.

Thirteen members of the United States Men's Water Polo Team were filmed using two synchronized cameras while shooting at a goal. Three-dimensional (3D) coordinates of the throwers' shoulder, elbow, wrist, and the ball were used to estimate elbow angle, elbow angular velocity, and ball velocity at release. Ball release velocities ranged from 14.5 to 25.8 m/sec, with peak elbow angular velocities averaging 1137 degrees/sec. Peak elbow angular velocity typically was reached just prior (x = 28 msec) to release as the elbow approached full extension. Results are significant in establishing the efficacy of 3D techniques in evaluating throwing mechanics and may prove useful in: identifying characteristics of superior performers, assessing differences in throwing technique between injured and non-injured populations, and providing a kinematic data base for further studies of throwing kinetics and potential injury mechanisms.

EMG profiles of lower extremity muscles during cycling at constant workload and cadence.

Limited conclusions concerning the variability in EMG patterns during cycling can be made from available data in the literature because of methodological differences which include electrode placement and experimental design. The purpose of this study was to monitor EMG signals from ten lower extremity muscles over a large number of pedalling cycles in experienced cyclists at constant workload and cadence. Variability across subjects was evaluated by calculating the coefficient of variation (CV) at 10% intervals of the pedalling cycle. Within subject EMG patterns were very consistent within a single trial. The single-joint hip and knee extensors (gluteus maximus, vastus medialis, and vastus lateralis) had the lowest CV values (less than 30%). This low variability appears to support their role as power generators. Variability was generally higher in the hamstring muscles with two biceps femoris patterns emerging despite relatively similar experimental conditions. EMG signals from surface and fine wire electrodes for the hamstring muscles were compared for possible contribution to the discrepancies in the EMG profiles. Fine wire EMG data were quite similar to those obtained using surface electrodes, indicating that crosstalk had minimal effect, in general, on the hamstring signals. The tibialis anterior, gastrocnemius, and soleus muscles displayed fairly repeatable patterns, with variability highest in the first 20% of the pedalling cycle for all muscles studied.