Effects of Isometric Scaling on Vertical Jumping Performance

Jump height, defined as vertical displacement in the airborne phase, depends on vertical takeoff velocity. For centuries, researchers have speculated on how jump height is affected by body size and many have adhered to what has come to be known as Borelli’s law, which states that jump height does not depend on body size per se. The underlying assumption is that the amount of work produced per kg body mass during the push-off is independent of size. However, if a big body is isometrically downscaled to a small body, the latter requires higher joint angular velocities to achieve a given takeoff velocity and work production will be more impaired by the force-velocity relationship of muscle. In the present study, the effects of pure isometric scaling on vertical jumping performance were investigated using a biologically realistic model of the human musculoskeletal system. The input of the model, muscle stimulation over time, was optimized using jump height as criterion. It was found that when the human model was miniaturized to the size of a mouse lemur, with a mass of about one-thousandth that of a human, jump height dropped from 40 cm to only 6 cm, mainly because of the force-velocity relationship. In reality, mouse lemurs achieve jump heights of about 33 cm. By implication, the unfavourable effects of the small body size of mouse lemurs on jumping performance must be counteracted by favourable effects of morphological and physiological adaptations. The same holds true for other small jumping animals. The simulations for the first time expose and explain the sheer magnitude of the isolated effects of isometric downscaling on jumping performance, to be counteracted by morphological and physiological adaptations.     

Trunk position influences joint activation pattern and physical performance during vertical jumping

Eight well-trained males carried out squat jump and countermovement jump with large (SJL and CMJL) and with small (SJS and CMJS) range of motion to study the influence of trunk position on joint recruitment pattern and jumping height. The main criteria in SJS and CMJS were to maintain trunk in near vertical position during execution. Joint angles, activation time, time at maximum joint velocity for ankle joint, knee joint and hip joint, vertical propulsion time and jumping height were determined using film analysis. The joint activation followed proximal to distal pattern in CMJL, SJL and CMJS, but the pattern was reversed in SJS. The ratio of active state and vertical propulsion time was similar for all joints (63.1 and 72.8%) in CMJL, SJL and CMJS except in SJS where the ratio was significantly less for hip (46.9%) and knee (51.9%). The difference between CMJL and SJL in jumping height was 6.9 ± 2.8 cm which is significantly less than that between CMJS and SJS (14.5 ± 5.3 cm). We concluded that knee joint and hip joint muscles could not contribute to the positive work during the push-off phase when the range of motion is small, the trunk is vertical and the activation level of the muscles is low.     

Locomotor Adaptation versus Perceptual Adaptation when Stepping Over an Obstacle with a Height Illusion

Background

During locomotion, vision is used to perceive environmental obstacles that could potentially threaten stability; locomotor action is then modified to avoid these obstacles. Various factors such as lighting and texture can make these environmental obstacles appear larger or smaller than their actual size. It is unclear if gait is adapted based on the actual or perceived height of these environmental obstacles. The purposes of this study were to determine if visually guided action is scaled to visual perception, and to determine if task experience influenced how action is scaled to perception.

Methodology/Principal Findings

Participants judged the height of two obstacles before and after stepping over each of them 50 times. An illusion made obstacle one appear larger than obstacle two, even though they were identical in size. The influence of task experience was examined by comparing the perception-action relationship during the first five obstacle crossings (1–5) with the last five obstacle crossings (46–50). In the first set of trials, obstacle one was perceived to be 2.0 cm larger than obstacle two and subjects stepped 2.7 cm higher over obstacle one. After walking over the obstacle 50 times, the toe elevation was not different between obstacles, but obstacle one was still perceived as 2.4 cm larger.

Conclusions/Significance

There was evidence of locomotor adaptation, but no evidence of perceptual adaptation with experience. These findings add to research that demonstrates that while the motor system can be influenced by perception, it can also operate independent of perception.     

Orderly recruitment among motoneurons supplying different muscles.

Virtually all movements involve the recruitment of motor units from multiple muscles. Given the functional diversity of motor units (motoneurons and the muscle fibers they supply), the effective production of specific movements undoubtedly depends upon some principle(s) to organize the ensemble of active motor units. The principle acting to organize the recruitment of motor units within muscles is the size principle, whereby the first motor units to be recruited have the smallest values for axonal conduction velocity and contractile force, and are the slowest to contract and fatigue. Here we consider the possibility that the size principle applies in the recruitment of motor units across muscles, i.e., that regardless of their muscles of origin, active motor units are recruited in rank order, for example, from low to high conduction velocity. The benefits of orderly recruitment across muscles could be similar to the acknowledged advantages of orderly recruitment within muscles. One benefit is that the neural process involved in organizing active motor units would be simplified. In a muscle-based scheme, the size principle would organize only those motor units within individual muscles, leaving the nervous system with the additional task of coordinating the relative activities of motor units from different muscles. By contrast, in an ensemble-based scheme, orderly recruitment of all motor units according to the size principle would automatically coordinate motor units both within and across motor nuclei. Another potential benefit is the provision for movements with smooth trajectory, the result of interleaving the divergent torque contributions made by motor units from muscles that differ in their orientations about joints. Otherwise, if order were restricted within muscles, the torque trajectory of a joint would change unevenly as participating muscles begin contracting at different times and grade activity at different rates. These considerations support speculation that motor units recruited from co-contracting muscles are collectively recruited according to the size principle.     

Optimizing the Distribution of Leg Muscles for Vertical Jumping

A goal of biomechanics and motor control is to understand the design of the human musculoskeletal system. Here we investigated human functional morphology by making predictions about the muscle volume distribution that is optimal for a specific motor task. We examined a well-studied and relatively simple human movement, vertical jumping. We investigated how high a human could jump if muscle volume were optimized for jumping, and determined how the optimal parameters improve performance. We used a four-link inverted pendulum model of human vertical jumping actuated by Hill-type muscles, that well-approximates skilled human performance. We optimized muscle volume by allowing the cross-sectional area and muscle fiber optimum length to be changed for each muscle, while maintaining constant total muscle volume. We observed, perhaps surprisingly, that the reference model, based on human anthropometric data, is relatively good for vertical jumping; it achieves 90% of the jump height predicted by a model with muscles designed specifically for jumping. Alteration of cross-sectional areas—which determine the maximum force deliverable by the muscles—constitutes the majority of improvement to jump height. The optimal distribution results in large vastus, gastrocnemius and hamstrings muscles that deliver more work, while producing a kinematic pattern essentially identical to the reference model. Work output is increased by removing muscle from rectus femoris, which cannot do work on the skeleton given its moment arm at the hip and the joint excursions during push-off. The gluteus composes a disproportionate amount of muscle volume and jump height is improved by moving it to other muscles. This approach represents a way to test hypotheses about optimal human functional morphology. Future studies may extend this approach to address other morphological questions in ethological tasks such as locomotion, and feature other sets of parameters such as properties of the skeletal segments.     

Scaling and jumping: gravity loses grip on small jumpers

There are several ways to quantify jumping performance, a common definition being the height gained by the body's centre of mass (CM) in the airborne phase. Under this definition, jump height is determined by take-off velocity. According to the existing literature on jumping and scaling, take-off velocity, and hence jumping performance is independent of size because the energy that differently sized geometrically scaled jumpers can generate with their muscles is proportional to their mass. In this article it is shown, based on a simple energy balance, that it is incorrect to presume that jump height does not depend on size. Contrary to common belief, size as such has does have an effect on take-off velocity, putting small jumpers at a mechanical advantage, as is shown analytically. To quantify the effect of size on take-off velocity, a generic jumper model was scaled geometrically and evaluated numerically. While a 70-kg jumper took off at 2.65 m/s and raised its CM by 0.36 m after take-off, a perfectly geometrically similar jumper of 0.7 g reached a take-off velocity of 3.46 m/s and raised its CM by 0.61 m. The reason for the better performance of small jumpers is their higher efficacy in transforming the energy generated by the actuators into energy due to vertical velocity of the CM. Considering the ecological and evolutionary relevance of different definitions of jump height, size-dependent efficacy might explain why habitual jumping is especially prominent among small animals such as insects.     

Young horse response on changing distance in free jumping combination

The aim of the present study was to verify the influence of distance between obstacles in combination for free jumping test on linear and temporal kinematic parameters of the jump. Investigated groups of halfbred stallions being prepared for 100 days performance test (two groups, 36 horses in total) were filmed on different distances between main doublebarre obstacle and last cross-pole in the jumping lane. Both groups of horses were filmed during their regular work in the same training centre 1 week before performance test. Jumping parameters were obtained on the same size of the obstacle. Data were analysed separately for both groups by analysis of variance. On the basis of the conducted study, it is possible to conclude that in the range of the most popular free jumping distance horses may use different jumping techniques to clear the jump. The shorter distances between last two obstacles in the jumping lane in the range of 6.8 to 7.1 m stimulate higher jumps; however, the reaction of horses was not exactly the same for all measured jumping parameters.     

Multichannel thin-film electrode for intramuscular electromyographic recordings.

It is currently not possible to record electromyographic (EMG) signals from many locations concurrently inside the muscle in a single wire electrode system. We developed a thin-film wire electrode system for multichannel intramuscular EMG recordings. The system was fabricated using a micromachining process, with a silicon wafer as production platform for polyimide-based electrodes. In the current prototype, the flexible polymer structure is 220 microm wide, 10 microm thick, and 1.5 cm long, and it has eight circular platinum-platinum chloride recording sites of 40-microm diameter distributed along the front and back surfaces with 1,500-microm intersite spacing. The system prototype was tested in six experiments where the electrode was implanted into the medial head of the gastrocnemius muscle of rabbits, perpendicular to the pennation angle of the muscle fibers. Asynchronous motor unit activity was induced by eliciting the withdrawal reflex or sequential crushes of the sciatic nerve using a pair of forceps. Sixty-seven motor units were identified from these recordings. In the bandwidth 200 Hz to 5 kHz, the peak-to-peak amplitude of the action potentials of the detected motor units was 75 +/- 12 muV and the root mean square of the noise was 1.6 +/- 0.4 muV. The noise level and amplitude of the action potentials were similar for measures separated by up to 40 min. The experimental tests demonstrated that thin film is a promising technology for a new type of flexible-wire intramuscular EMG recording system with multiple detection sites.     

Evolution of jumping capacity in Tropidurinae lizards: does habitat complexity influence obstacle‐crossing ability?

Jumping performance is relevant for lizards in many ecological contexts and might be favoured during the colonization of structurally complex habitats. Although ground-dwelling lizards use jumps to overcome small obstacles in their natural environments, jumping capacity has been mostly studied in arboreal species. Here, we analysed the evolution of jumping behaviour and performance in lizards from eight ground-dwelling species of Tropidurinae attempting to cross obstacles of different heights in a jumping track, both when undisturbed and under continuous stimulation. To establish ecological correlates with habitat complexity, individuals from two contrasting Brazilian habitats, the arid Caatingas (sand species) and the savannah-like Cerrados (rock species), were compared. Rock species jumped more often and crossed higher obstacles than sand ones in both tests, and performed more vertical than horizontal jumps. Although sand species performed less jumps, they were more successful at crossing the obstacles presented in comparison with rock species. Phylogenetic analyses confirmed these findings and demonstrated a large divergence in jumping capacity between sister-species from different habitats. Therefore, the differences in propensity and endurance for jumping activity appear to be independent of phylogenetic relationships in Tropidurinae and likely reflect an adaptation to the contrasting environments inhabited. The ecological implications of these findings are discussed.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 393–402.     

Preschool-aged children’s jumps: Imitation performances

Imitative behavior underlaid by perception and action links during children’s development in complex locomotor skills has been the object of relatively few studies. In order to explore children’s motor coordination modes, 130 children divided into five age groups from 3.5 to 7.5 years were instructed to imitate jumping tasks in spontaneous motor situation and in various imitative contexts by an adult providing verbal orders and gestural demonstrations. Their conformity to the model, stability and variability scores were coded from a video analysis when they performed jumps with obstacles. To evaluate their postural-motor control level, the durations of the preparatory phase and jumping flights were also timed. Results showed that all age groups generated the demonstrator’s goal but not necessarily the same coordination modes of jumping. In imitation with temporal proximity, the model helped the youngest age groups to adopt his coordination modes and stabilized only the oldest age groups’ performances starting from 5.5 years old, without effect on learning imitation. Differences between the youngest and oldest children in the jump duration suggested that the reproduction of a complex motor activity such as jumping with a one foot take-off would require resolution and adjustment of main postural stability.     

Anti-predator defence drives parallel morphological evolution in flea beetles

Complex morphological or functional traits are frequently considered evolutionarily unique and hence useful for taxonomic classification. Flea beetles (Alticinae) are characterized by an extraordinary jumping apparatus in the usually greatly expanded femur of their hind legs that separates them from the related Galerucinae. Here, we examine the evolution of this trait using phylogenetic analysis and a time-calibrated tree from mitochondrial (rrnL and cox1) and nuclear (small subunits and large subunits) genes, as well as morphometrics of femora using elliptic Fourier analysis. The phylogeny strongly supports multiple independent origins of the metafemoral spring and therefore rejects the monophyly of Alticinae, as defined by this trait. Geometric outline analysis of femora shows the great plasticity of this structure and its correlation with the type and diversity of the metafemoral springs. The recognition of convergence in jumping apparatus now resolves the long-standing difficulties of Galerucinae-Alticinae classification, and cautions against the value of trait complexity as a measure of taxonomic significance. The lineage also shows accelerated species diversification rates relative to other leaf beetles, which may be promoted by the same ecological factors that also favour the repeated evolution of jumping as an anti-predation mechanism.     

Properties of twitch motor units of the ankle extensor muscles in the bullfrog Rana catesbeiana

A study of the mechanical properties of the twitch motor units in the ankle extensor muscles of bullfrogs was undertaken to expand our view of the diversity of motor unit properties among vertebrates. Two muscles were chosen that represent a wide range of extensor function: the plantaris longus (PL) is a large muscle providing most of the force for ankle extension in hopping and swimming, and the tibialis posticus (TP) is relatively small and may act as an ankle stabilizer or be primarily postural in function. Both muscles have highly fatigable motor units, but also some (especially in TP) low or non-fatigable ones. Mean tetanic tensions of motor units in both muscles are relatively large as compared with those of mammals but are especially large in PL, No clear correlations were found between contraction times and either motor unit tetanic tensions or fatigability, nor did the motor units fall into clearly defined types based on any functional parameters. Overall contraction and relaxation times are slow compared with those of mammals and are somewhat slow compared to those of other frogs; unlike results from earlier studies, the large units of PL are slower than the small units of TP. This results in PL units reaching fused tetani at lower stimulus frequencies. The twitch/tetanus and force/frequency ratios in PL motor units are much larger than those of TP, giving PL units greater relative forces at lower stimulus rates. These results are discussed in the context of motor unit function. © 1994 Wiley-Liss, Inc.     

Analyzing variable behavioral contingencies: Are certain complex skills homologous with locomotion?

This paper considers behavioral contingencies that change as a function of time, of the individual's own behavior (as in locomotion and reading), of the behavior of other parties or of interactions with them. A detailed analysis of locomotion and of reading out loud shows that the behavioral contingencies for these are virtually the same. The terrain being traversed and the locomotion behavior involved are shown to be analogous to a segment of text being read and the articulation of the words. In both cases, successive upcoming segments are perceived and processed, and during the processing phases, motor behavior is formulated. In both, the smooth concatenation of the motor phases for successive segments requires buffering. Both involve corrective or digressive actions in response to obstacles or unanticipated stimuli encountered. Both involve looking ahead at the upcoming segment and processing it while the motor phase of the prior segment is still in progress. For both, the size, entropy, familiarity, and other attributes of the upcoming segment are parameters of the performance. It is suggested that locomotion has similar parallels with certain other complex skills, such as listening, copying, receiving Morse code, simultaneous interpreting, and certain types of performance, and may therefore be their phylogenetic prototype and biological homologue.     

Gear ratios at the limb joints of jumping dogs

An increase in gear ratio of the limb extensor muscles during joint extension has been suggested to be a mechanism that facilitates optimal power production by skeletal muscles. The objectives of this study were to: (1) measure gear ratios at the wrist, elbow, shoulder, ankle, knee, and hip joints of jumping dogs, (2) compute the work performed by each of these joints, and (3) measure muscle shortening velocity for a joint exhibiting an increasing gear ratio during joint extension. The gear ratio out-lever was computed by dividing the ground reaction force (GRF) moment by the GRF, whereas the in-lever was directly measured as the perpendicular distance from the joint center to the line of action of the extensor muscle. In addition, changes in fascicle length were measured from the vastus lateralis muscle using sonomicrometry. Of the joints examined, only the gear ratios at the shoulder and knee joints increased during jumping in a manner that could facilitate peak power production of actively shortening muscles. The vastus lateralis was found to shorten at an average velocity of 3.20 muscle lengths per second. This is similar to estimates of shortening velocity that produce peak muscular power in mammals the size of dogs. Additionally, the knee extensors were found to produce a large proportion (26.6%) of the positive external work of the limbs. These observations suggest that dynamic gearing in jumping dogs may allow the extensor muscles of the knee joint to shorten in a way that maximizes their power production.     

Blue foxes’ motivation to gain access to solid floors and the effect of the floor material on their behaviour

Farmed blue foxes are willing to work to gain access to a sand floor from a wire mesh floor. It is not clear whether the foxes work for the sand floor because of its solidity or because it enables them to perform certain behaviours, e.g. exploration and digging. Here, we measured blue foxes’ motivation to gain access from a wire mesh floor to a floor with 15–30 cm deep sand, a floor with 3–4 cm deep sand, a solid concrete floor, and another wire mesh floor. In addition, we analysed the foxes’ behaviour on these floor materials. Seven male blue foxes were trained and tested in self-constructed operant apparatuses. In an apparatus, the fox could move a bottomless test cage from a wire mesh floor to a neighbouring, alternative floor material for a 4-min visit by pressing a lever in the test cage for a fixed number of times (Fixed Ratio, FR). The foxes worked for each floor material for 12 days. In each daily test session, the foxes were exposed to work on one of the four workloads (FR 6, 12, 24, 48), for 3 h. The behaviour of the foxes was analysed during the 4-min visits on each floor material. The results showed that there was no difference between the floor materials, either in the demand elasticity of the fragment of the demand curve (ranging from −0.46 to −0.33), or in the intensity of the demand. However, the foxes’ behaviour varied between the floor materials. More digging, play, rooting (exploration with the muzzle), and vole jumping were observed on the floor materials with sand, than on the concrete floor and the wire mesh floor. Both the presence and the depth of the sand layer stimulated these behaviours. It is concluded that juvenile blue foxes do not value solid floor materials more than a wire mesh floor. However, the sand floor stimulates more digging, play, vole jumping, and exploration than the concrete floor or wire mesh floor. Furthermore, the depth of sand may be an important factor in eliciting these behaviours. Access to a floor material with sand may improve the welfare of farmed blue foxes by providing the possibility to perform species-specific behaviours.     

The jump of the click beetle (Coleoptera, Elateridae)—a preliminary study

A preliminary account is given of the jump of the click beetle, Athous haemorrhoidalis (F.). The jump is normally made from an inverted position. It involves a jack-knifing movement whereby a prosternal peg is slid very rapidly down a smooth track into a mesosternal pit. The muscles which produce this movement are allowed to build up tension by a friction hold on the dorsal side of the peg. The anatomy of this jumping mechanism is briefly described. Ciné recording showed that the jump was usually nearly vertical and could exceed 0.3m in height; the beetle normally rotated several times head over tail during a jump. The jump was produced by a very rapid upwards movement of the beetle's centre of gravity during the jack-knifing action. In a typical jump, a 4 × 10⁻⁵ kg beetle could be subjected to an upwards acceleration of 3800 m/s⁻² (380 g). The minimum work done and the power output of the muscles causing jumping have been calculated. A simple mechanical model has been constructed to simulate a jump, and several possible ways in which the jumping mechanism could operate have been discussed.     

Local innervation patterns of the metathoracic flexor and extensor tibiae motor neurons in the cricket Gryllus bimaculatus

To elucidate neural mechanisms underlying walking and jumping in insects, motor neurons supplying femoral muscles have been identified mainly in locusts and katydids, but not in crickets. In this study, the motor innervation patterns of the metathoracic flexor and extensor tibiae muscles in the cricket, Gryllus bimaculatus were investigated by differential back-fills and nerve recordings. Whereas the extensor tibiae muscle has an innervation pattern similar to that of other orthopterans, the flexor has an innervation unique to this species. The main body of the flexor muscle is divided into the proximal, middle and distal regions, which receive morphologically unique terminations from almost non-overlapping sets of motor neurons. The proximal region is innervated by about 12 moderate-sized excitatory motor neurons and two inhibitory neurons while the middle and distal regions are innervated by three and four large excitatory motor neurons, respectively. The most-distally located accessory flexor muscle, inserting on a common flexor apodeme with the main muscle, is innervated by at least four small excitatory (slow-type) and two common inhibitory motor neurons. The two excitatory and two inhibitory motor neurons that innervate the accessory flexor muscle also innervate the proximal bundles of the main flexor muscle. This suggests that the most proximal and distal parts of the flexor muscle participate synergistically in fine motor control while the rest participates in powerful drive of tibial flexion movement.     

Maturation of the flight motor pattern without movement inManduca sexta

The development of the flight motor pattern was studied by recording acutely with fine wire electrodes inserted in the thoracic muscles of pharate moths of known age and by recording chronically for up to 8 days with implanted electrodes. Externally visible morphological characteristics by which the age of a pharateManduca sexta can be established were identified (Table 1). Bouts of activity lasting approximately 30 min to 2 h and alternating with inactive periods of similar duration were recorded as early as the ninth day after pupation and on all successive days until early on the day of eclosion, typically 19 days after pupation (Figs. 1,5). During the 3 days preceding the day of eclosion a rhythmic flight motor pattern was produced (Fig. 2). The rhythmic activity ceased 5^1/2–10^1/2 h before eclosion and only an occasional, large potential change was recorded from the thoracic muscles during this time (Fig. 3). During the 3 days of rhythmic activity the percent-age of time that the animal was active did not change (Fig. 4). The flight motor pattern matured, in that the cycle-time decreased and became less variable (Fig. 6). The approximate flight phase relationship between an elevator muscle and the dorsal longitudinal depressor muscle did not become less variable as the cycle-time improved. The flight motor pattern produced by pharate moths caused neither movement of the scutum nor an increase in thoracic temperature in marked contrast to the consequences of adult motor activity (Fig. 7). Intracellular recording from the dorsal longitudinal muscle of pharate moths 20–30 h before eclosion showed that, after repeated stimulation of the motor nerve at 2/s, only small junctional potentials were elicited (Fig. 8). A burst of 6 stimuli at 50/s elicited 2–5 active membrane responses and a contraction. These observations explain the absence of thoracic movement in immature animals producing the flight motor pattern and the presence of movement in immature animals stimulated to eclose. They also show that the neuromuscular junction matures rapidly during the day before eclosion.     

The jump of Petrobius (Thysanura, Machilidae)

The jump of the sea-shore bristletails, Petrobius brevistylis and P. maritimus , has been investigated. A brief study of jumping distance and orientation showed that jumping behaviour was partly random, particularly with regard to the direction of a jump. A high speed cine camera was used to analyse both forward and backward jumps. The jumping action consists of a tail beat which reflects the strong depression of the posterior abdomen. This swings up the anterior part of the body, including the centre of gravity. The model suggested to explain this action proposes two movements spreading from either end of the bristletail: 1. A straightening of the abdomen starting posteriorly, 2. An increasing ventral curvature of the thorax starting anteriorly. Movement 1 will account for the raising of the centre of gravity, and the rapidity of movement 2 will determine the final direction of the jump.
The muscular basis of these movements has been discussed using the data of Manton (1972), Barlet (1967) and Bitsch (1973). A catapult system was proposed which depended on the dynamic balance between two abdominal muscular systems. It was suggested that the dorsal longitudinal muscles were antagonised by the deep oblique muscles. A short study of the energetics of jumping was made in order to estimate the times needed for sufficient energy to be stored for particular jumps. These times were then compared with known times spent by Petrobius in the cocked (energy storing) pre-jump position.     

Sensitivity of the cross-correlation between simulated surface EMGs for two muscles to detect motor unit synchronization.

The purpose of the study was to evaluate the use of cross-correlation analysis between simulated surface electromyograms (EMGs) of two muscles to quantify motor unit synchronization. The volume conductor simulated a cylindrical limb with two muscles and bone, fat, and skin tissues. Models of two motor neuron pools were used to simulate 120 s of surface EMG that were detected over both muscles. Short-term synchrony was established using a phenomenological model that aligned the discharge times of selected motor units within and across muscles to simulate physiological levels of motor unit synchrony. The correlation between pairs of surface EMGs was estimated as the maximum of the normalized cross-correlation function. After imposing four levels of motor unit synchrony across muscles, five parameters were varied concurrently in the two muscles to examine their influence on the correlation between the surface EMGs: 1) excitation level (5, 10, 15, and 50% of maximum); 2) muscle size (350 and 500 motor units); 3) fat thickness (1 and 4 mm); 4) skin conductivity (0.1 and 1 S/m); and 5) mean motor unit conduction velocity (2.5 and 4 m/s). Despite a constant and high level of motor unit synchronization among pairs of motor units across the two muscles, the cross-correlation index ranged from 0.08 to 0.56, with variation in the five parameters. For example, cross-correlation of EMGs from pairs of hand muscles, each having thin layers of subcutaneous fat and mean motor unit conduction velocities of 4 m/s, may be relatively insensitive to the level of synchronization across muscles. In contrast, cross-correlation of EMGs from pairs of leg muscles, with larger fat thickness, may exhibit a different sensitivity. These results indicate that cross correlation of the surface EMGs from two muscles provides a limited measure of the level of synchronization between motor units in the two muscles.