Electrical Activity and Relative Length Changes of Dog Limb Muscles as A Function of Speed and Gait

One generally thinks of muscles as force generating elements that shorten to bring about the observed movements of the limbs, body and trunk during normal locomotion. Recent studies of the mechanics and energetics of terrestrial locomotion point to the importance of muscular activity while muscles remain the same length or are being stretched (Cavagna, Saibene & Margaria, 1963, 1964; Heglund, 1979; Heglund et al. 1979). In this study we set out to analyse the relative importance of these different types of muscular activities in a series of extensors and flexors of the limbs as dogs increased speed within a gait and changed gait.

The relative importance of the different types of muscular activity should change as animals change gait due to changes which occur in the mechanics of locomotion. A walking animal pole-vaults over its rigid limb in a manner analogous to an inverted pendulum. There is an alternate exchange between kinetic energy of its centre of mass in the forward direction and gravitational potential energy as the animal rotates over its rigid limb. As much as 60– 70% of the absolute changes in kinetic and gravitational energy of the centre of mass within each stride are conserved by this inverted pendulum mechanism (Cavagna & Kaneko, 1977; Cavagna, Heglund & Taylor, 1977; Heglund, 1979). In order for this mechanism to operate, some extensors must be active with minimal changes in length to keep the joints rigid as the animal rotates over its limbs.

Trotting, hopping, and galloping seem to involve a spring-like mechanism. The animal simultaneously breaks its fall and decelerates during one part of the stride, and then reaccelerates and lifts its centre of mass during another part. Energy appears to be stored by stretching active muscles and tendons as the animal decelerates and breaks its fall and then recovered as it reaccelerates upwards and forwards (Alexander & Vernon, 1975 ; Cavagna et al. 1977; Heglund, 1979). Some extensor muscles at each of the limb joints must be active when the animal lands to allow elastic energy to be stored in the muscles and tendons.

Another possibility for storage and recovery of elastic strain energy in muscles and tendons occurs as the limbs are alternately accelerated and decelerated relative to the centre of mass. The kinetic energy contained in the accelerated limb could be stored in muscles and tendons as the limb slows and reverses direction. This type of storage and recovery of energy could occur at all speeds in the muscles which reverse the limb direction.

Although these ideas about muscular activity have not been tested directly, they are consistent with published data from kinematic and electromyographic studies during quadrupedal locomotion. A number of limb extensor muscles have been shown to be active for joint stabilization during walking in rats (Cohen & Gans, 1975), cats (Goslow, Reinking & Stuart, 1973; English, 1978; Rasmussen, Chan & Goslow, 1978), and dogs (Tokuriki, 1973a; Wentink, 1976). Also some limb flexor and extensor muscles become active prior to limb reversal (cf. Engberg & Lundberg, 1969; Goslow et al. 1973). The present study set out to test these ideas about muscle activity directly by simultaneously measuring electrical activity and length of a series of extensor and flexor muscles of the fore-and hindlimbs of dogs as a function of speed and gait. Use of a variable speed treadmill allowed for the analysis of a large number of successive and stable strides of select speeds and gaits.

We selected dogs as experimental animals because they are extremely tractable, and because we had made measurements of mechanical work, mechanical power output and metabolic power input (Cavagna et al., 1977; Heglund et al., 1979; Taylor, 1978, 1980) on individuals that were available for these studies. Two mongrel dogs (1 female, 1 male, Canis familiaris, 20 kg) were used in these experiments. They were fed commercially available diets supplemented with vitamins and calcium. The dogs were housed in large indoor cages with adjoining outdoor runs. Both dogs ran on a treadmill regularly over a period of several years before the experiments. Our measurements were made while the animals ran at a constant speed on a treadmill for nine speeds (3 within a walk, 3 within a trot, and 3 within a gallop).

We selected muscles for study that would test our ideas about holding the limb rigid during a walk, operating a limb-spring mechanism during a trot, and storing elastic energy during the end of the swing phase. Supraspinatus, triceps brachii (long and lateral heads), gluteus medius, vastus lateralis and gastrocnemius (medial and lateral heads) were chosen to study the possible role of joint extensors in the elastic storage of energy during locomotion. These particular muscles represent extensors of the shoulder, elbow, hip, knee and ankle respectively. Biceps brachii, brachialis and sartorius were selected to study the role of limb flexors during the swing phase. Latissimus dorsi and biceps femoris were chosen to study the possible role of elastic storage of energy in limb muscles that are capable of decelerating the limbs at the end of the swing phase.

A small patch of the dog’s skin was shaved over the muscles to be studied. The area was cleaned with alcohol, lidocaine hydrochloride was injected under the skin, and a small incision was made. The electrodes were implanted and sutured in place. The local anaesthesia was adequate to prevent discomfort to the animal. A ground electrode coated with EKG electrode jelly was taped to a shaved area at the base of the tail. Placement of the electrodes was verified before and after each experiment by back stimulation and palpation. Recordings were made from a minimum of six separate electrode placements within each muscle of each dog. Four to thirty-eight strides were analysed from each placement.

The electrical activity of the muscles was recorded from electrodes made of 51μ OD wire with a resistance of 700 Ω.m^{− 1} (Evanohm’s, Wilber B. Driver Co., Newark, NJ). The electrodes were constructed by placing a loop of wire through the barrel of a 23-gauge needle. The loop was cut and two millimetres of insulation were burned off the ends of the wire. The wire was bent backwards forming two hooks such that the bare ends were approximately four millimetres apart. The signals obtained from the electrodes were first amplified by a set of EMG preamplifiers attached to the dog. The signal was then carried by co-axial cable to Grass EMG amplifiers (Model P511) and filtered for 60 Hz noise. The output of these amplifiers was connected to a 4-channel Tektronix storage oscilloscope (Series 5000). The EMG signals were recorded using an FM tape recorder (Bell & Howell CPR 4010). The tape recordings were played back on a multichannel strip chart recorder for analysis (Brush Gould, Model 260).

The electrical activity of the muscles was analysed only for duration of activity, i.e. on and off. The means and ranges for on and off were determined for four to thirty-eight sequential strides in each experiment.

Sixteen millimetre cine films were synchronized with the EMG recordings using an Eclair GV-16 cine camera. The cine films were taken at framing rates of between 50 and 200 frames/s. The limb angles and limb excursions were analysed by projecting the cine film onto a ground glass screen, and tracing the dog’s outline frame by frame for a series of strides. A celluloid model of the major limb bones was constructed with movable joints. It was placed over the drawings to determine the limb angles. The angles were compared on ten separate drawings at each speed to determine their reproducibility. The values obtained using this method agreed well with independent measurements of shoulder angles made using high speed cineradiography where the bones could be visualized, eliminating the need for the celluloid model (pers. comm. F. A. Jenkins, Jr.). Relative muscle lengths were determined frame by frame during the stride by using a celluloid model on which the appropriate origin and insertion had been marked.

In the series of figures that follow, we present our measurements of electrical activity of muscles (EMGs) from the forelimb and hindlimb together with a measure of changes in muscle length. Measurements were made for three walking, three trotting, and three galloping speeds. Representative records are presented for one mid-speed within each gait. Means and ranges of the onset and termination of electrical activity are given. The angle of the joint around which a muscle operates is plotted for each of the muscles that acts only around a single joint. These angles allow one to determine whether the muscle-tendon unit was shortening, lengthening, or staying the same length during the time it was active. For those muscles that act around two joints it was necessary to plot change in muscle length directly to obtain this information.

The joint angles for both the shoulder and the elbow are given in Fig. 1 and the EMGs of the front limb muscles that we measured are given together with the appropriate angle or length in Figs. 2– 7. The joint angles for the hip, knee, and ankle are given in Fig. 8, and the EMGs of the hindlimb muscles that we measured are given together with the appropriate angle or length in Figs. 9– 13. The data for each muscle are preceded by a description of its anatomical position and a statement of its function as inferred from this position (after Miller, Evans & Christensen, 1979). Such a statement provides a basis for comparison with our results and emphasizes the value of the present approach for determining muscle function.

These are given in Fig. 1 for dogs walking (3· 3 km.h^{− 1}),; 2 km.h^{− 1}) and galloping (24· 1 km.h^{− 1}) on a treadmill. Shoulder flexion after foot-down and shoulder extension in the last half of the swing phase are characteristic of the walk and trot. Shoulder trotting (8· movements during galloping, however, contrast in both their temporal pattern and extent. In addition to the post-foot-down flexion and late swing phase extension movements of the slower gaits, extension during the second half of the stance and flexion in early swing also occur. Elbow flexion after foot lift-off and extension prior to foot down are characteristic of the swing phase at all gaits. In addition, a period of early stance phase elbow flexion occurs. This flexion is slight in the walk, more pronounced in the trot and distinct in the gallop.

This is a single-joint muscle arising from the supraspinous fossa and cranial border of the scapular spine (Fig. 2). This muscle inserts by a strong tendon on the cranial border of the greater tubercle of the humerus (see drawing, top of Fig. 2). Its function, inferred from its anatomical position, is extension of the shoulder joint and forward advancement of the limb. Our data suggest the following action based on electrical activity and changes in length of the muscle during a stride.

The muscle stabilized and prevented collapse of the shoulder joint when the limb was bearing weight. At a walking speed of 3· 2 km.h^{− 1} the muscle became active just after the foot touched the ground and remained active for 80% of the stance phase (or 54% of the total stride period). The shoulder flexed 15° during this period of electrical activity. Thus the muscle lengthened slightly while it was active. The pattern of activity was similar at different walking speeds.

The muscle stabilized, and prevented collapse of the shoulder joint when the limb was bearing weight. At a trotting speed of 8· 3 km.h^{− 1} the muscle became active just after the foot touched the ground and remained active for 66 % of the stance phase (or 33 % of the stride period). During the activity the shoulder flexed by 20°. The pattern of activity was similar as trotting speed increased from 6· 6 to 11 km.h^{− 1}

The muscle provided some stabilization of the shoulder joint and assisted in its extension during the stance phase. The muscle became active just after footdown and was stretched (allowing elastic storage of energy to take place), then it shortened and aided in shoulder extension at the end of the stance (providing propulsion and allowing for recovery of stored elastic energy). At a galloping speed of 24· 2 km.h^-1 the muscle was active for 65 % of the stance phase (or 18% of the total stride period). During the period of activity the shoulder alternately flexed 22° and extended 22°. The pattern of activity was similar as the animal doubled its speed within a gallop.

This is a single-joint muscle which arises by an aponeurosis from most of the tricipital line on the lateral surface of the humerus (Fig. 3). The muscle inserts by a broad tendon on the olecranon process of the ulna. Its function, inferred from its anatomical position, is extension of the elbow. Our data suggest the following action based on electrical activity and changes in length of the muscle during a stride.

The muscle was not active.

The muscle was active while being stretched when the dog landed (preventing collapse of the elbow joint and allowing elastic storage to take place). Then it shortened and assisted in elbow extension in late stance phase (providing propulsion and allowing for recovery of stored elastic energy). At a trotting speed of 8· 6 km.h^{− 1}, the lateral head of the triceps became active just as the foot touched the ground. It remained active for 64 % of the stance phase (33 % of the stride period). During this activity, the elbow alternately flexed 20° and then extended 12⁰. The pattern of activity was similar as trotting speed increased from 6· 1 11· 0 km.h^-1.

The muscle was active while being stretched when the dog landed (preventing collapse of the elbow joint and allowing elastic storage of energy to take place). In late stance the muscle shortened and assisted elbow extension (providing propulsion and allowing for recovery of stored elastic energy). At a galloping speed of 25 km.h^{− 1} the muscle became active just as the foot touched the ground and remained active for 70% of the stance phase (20% of the stride period). During activity, the elbow alternately flexed 35° and extended 15°. The pattern of activity was similar as the animal doubled its speed within a gallop.

This is a two-joint muscle which arises from the distolateral two-thirds of the caudal border of the scapula (Fig. 4). This muscle inserts by a robust, round tendon on the caudal olecranon process of the ulna. Its function, inferred from its anatomical position, is extension of the elbow. Our data suggest the following action based on electrical activity and changes in length of the muscle during a stride.

The muscle stabilized and prevented collapse of the elbow joint when the limb was bearing weight. At a walking speed of 3-3 km. h^-1 the long head of the triceps became active at or just before foot down. The muscle remained active about 69 % of the stance phase (or 45 % of the stride period). The muscle shortened by about 5 % during the time it was active. This shortening was due to a flexion of the shoulder joint which more than compensated for the flexion of the elbow (Fig. 1). The pattern of activity was the same at different walking speeds.

The muscle stabilized and prevented collapse of the elbow joint after the foot made contact with the ground, initiated elbow extension, and provided a propulsive force during the last part of the stance phase. At a trotting speed of 8· 2 km.h^{− 1}, the Muscle became active just before the foot touched the ground. The muscle remained active for about 47% of the stance phase or about 28% of the stride. The muscle shortened by about 7 % while it was active. This shortening was due first to a flexion of the shoulder joint (which more than compensated for the flexion of the elbow) and then an extension of the elbow (Fig. 1). The pattern of activity was similar as trotting speed increased from 6· 6 to 11· 0 km.h^{− 1} except for the period of activity before the foot touched the ground which increased from o to about 50 msec.

The muscle assisted elbow extension just before the foot contacted the ground, stabilized and prevented collapse of the elbow joint after the foot first made contact with the ground, and initiated elbow extension providing a propulsive force while the foot was in contact with the ground. At a galloping speed of 24· 1 km.h^{− 1}, the long head of the triceps became active just before the foot touched the ground and remained active for 70 % of the stance phase or 32 % of the stride. During the activity when the foot was on the ground, the muscle shortened by about 8%. After the foot touched the ground the elbow began to flex, but this was more than compensated for by flexion of the shoulder. The muscle continued its activity while the elbow began to extend, providing some propulsion (Fig. 1). The pattern of activity was similar as the animal doubled its speed within a gallop.

This originates as an aponeurosis from the lumbodorsal fascia of the caudal seven or eight thoracic vertebrae and all of the lumbar vertebrae (Fig. 5). In addition, muscular slips of origin arise from the proximolateral surface of the last two or three ribs. The insertion of this muscle is complex. In addition to a tendinous insertion at the teres tubercle on the medial side of the humerus, fascial slips of the muscle join the medial triceps and deep pectoralis. Its function, inferred from its anatomical position, is to draw the limb backwards and to pull the trunk forward over the supported limb. Our data suggest the following action based on electrical activity and changes in length of the muscle during a stride.

The muscle pulled the trunk forward over the supported limb and decelerated the forward movement of the free limb. The activity of this muscle was biphasic. The major period of activity occurred during the stance phase. At a walking speed of 3· 2 km.h^{− 1}, the muscle became active and began to pull the trunk forward over the supported limb after about 44 % of the stance phase was completed. It then remained active, shortened by 10 %, and provided propulsion for about 46 % of the stance phase. There was a second short burst of activity just before and after the foot touched the ground as the muscle decelerated the forward movement of the limb. This activity lasted for about 7% of the stride period. The muscle lengthened by about 10% as it decelerated forward movement of the free limb. The pattern was similar at all walking speeds.

The muscle pulled the trunk over the supported limb, decelerated the forward movement of the free limb (allowing elastic storage of kinetic energy) and reaccelerated the limb backwards (allowing recovery of the stored energy). The activity of the muscle was still biphasic during a trot. At a trotting speed of 8· 2 km.h^{− 1}, the muscle became active and began to pull the trunk forward over the supported limb for about 49 % of the stance phase (33 % of the total stride period). During the second period of activity, the muscle first decelerated the forward movement of the limb, and then initiated the acceleration of the limb in the opposite direction. This burst of activity lasted about 11 % of the total stride period. During a complete stride the muscle alternately shortened and lengthened by about 10%. The pattern of activity was similar at different trotting speeds.

The muscle pulled the trunk forward over the supported limb, accelerated the free limb backwards before and just after the foot touched the ground, and, during an additional third phase of activity added in the gallop, it supported the free limb during the initial part of the swing phase. At a galloping speed of 24· 1 km.h^{− 1}, the muscle became active and began to pull the trunk forward over the supported limb after about 50 % of the stance phase was completed. It then remained active for the remainder of the stance during the propulsive stroke. It continued its activity after the foot left the ground, supporting the free limb during the initial part of the swing phase. Sometimes there was a pause between the propulsive and support phases (Fig. 5) and at other times the activity was continuous. Both phases accounted for about 32% of the total stride period. The muscle shortened by about 40 % as it provided propulsion and its length was then unchanged while it supported the free limb. The pattern of activity was similar at different galloping speeds.

This is a two-joint muscle which originates by a strong tendon from the tuber scapula (coracoid process) (Fig. 6). Distally the tendon of insertion divides: the more robust of the two inserts on the ulna tuberosity and the other on the radial tuberosity. Its function, inferred from anatomical positions, is flexion of the elbow joint. Our data suggest the following action based on electrical activity and changes in length of the muscle during a stride.

The muscle stabilized and prevented collapse of the shoulder joint when the limb was bearing weight, and assisted in elbow flexion during the swing phase. At a walking speed of 3· 2 km.h^{− 1} the muscle became active after about 37 % of the stance phase was completed. It remained active for the remainder of the stance phase and 43 % of the following swing phase. The activity lasted 57 % of the stride period. The muscle remained about the same length while it was active during the stance phase and then shortened by about 15% during the swing phase as the elbow flexed (Fig. 1). The pattern of activity was similar at different walking speeds.

The muscle stabilized and prevented collapse of the shoulder joint when the limb was bearing weight. At a trotting speed of 8· 2 km. h^{− 1}, the muscle became active after about 46 % of the stance phase was completed and continued its activity for the remainder of the time the foot was on the ground and 7 % of the following swing phase (23 % of the total stride). The muscle lengthened by about 5 % during the stance phase due to extension of the elbow (Fig. 1). The pattern of activity was essentially the same at different trotting speeds.

The muscle stabilized and prevented collapse of the shoulder joint when the limb was bearing weight, and provided for elbow flexion during the swing phase. At a galloping speed of 24· 1 km.h^{− 1}, the muscle became active after about 56% of the stance phase was completed and continued its activity for the remainder of the time the foot was on the ground and 16% of the following swing phase (23 % of the total wride). The active muscle lengthened by about 10% during the late stance phase and then alternately shortened and lengthened by about 20 % during the swing phase dt^ to extension and flexion of the elbow (Fig. 1). The pattern of activity was similar at different galloping speeds.

This is a single-joint muscle which originates from the proximal part of the caudal surface of the humerus (Fig. 7). Distally the muscle has a dual insertion, some fleshy fibres end on the tendon of the m. biceps brachii coursing to the radial tuberosity, and a distinct tendon inserts on the ulnar tuberosity. Its function, inferred from its anatomical position, is flexion at the elbow. Our data suggest the following action based on electrical activity and changes in length of the muscle during a stride.

The muscle flexed the elbow joint during swing phase to decelerate an extension of the elbow prior to foot-down. At a walking speed of 3· 3 km.h^{− 1} the muscle became active about 30 msec after the foot left the ground and remained active for 87 % of the swing phase. The muscle was active for about 28 % of the stride period. The active muscle alternately lengthened and shortened as the elbow flexed and extended by about 50° during the swing phase. The pattern of activity was similar at different walking speeds.

The muscle flexed the elbow and maintained the joint in a flexed position prior to extension (little if any involvement in the control of extension during late swing phase). At a trotting speed of 8· 2 km.h^{− 1} the muscle became active about 21 msec after the foot left the ground and remained active for 61 % of the swing phase. The muscle was active for about 32 % of the stride period. The elbow flexed by about 60° during the first half of its activity and maintained this flexed position during the remainder of the time it was active. The pattern of activity was similar at different trotting speeds.

The muscle flexed the elbow (little if any involvement in control of extension). At a galloping speed of 24· 1 km.h^{− 1} the muscle became active 27 msec after the foot left the ground and remained active for 42 % of the swing phase. The muscle was active for 29 % of the total stride. The elbow flexed by about 66° and extended by about 10° during the time the muscle was active. The pattern of activity was similar at different galloping speeds.

These are given in Fig. 8 for dogs walking (3· 3 km.h^{− 1}), trotting (8· 2 km.h^{− 1}) and galloping (24· 1 km.h^{− 1}) on a treadmill. At all gaits the hip (iliofemoral) begins slight extension in late swing prior to foot-down. At foot-down, the hip generally stabilizes before it begins gradual extension during the remaining stance phase. When the foot is lifted off the ground, the hip begins flexion which continues until the late swing phase extension begins. Flexion-extension movements of the knee and ankle are essentially in phase for the three gaits. At foot-down, both joints flex in early stance before they begin extension coincident with propulsion. This early flexion (the ‘yield’) is slight in walking, moderate in the trot, and distinct in the gallop. Foot lift-off is characterized by knee and ankle flexion which continues to about mid-swing. Both joints extend in the second half of the swing in anticipation of foot-down.

This is a single-joint muscle of extensive origin from the lateral aspect of the ilium (wing; gluteal surface) (Fig. 9). The muscle inserts by a strong tendon on the free end of the greater trochanter, Its function inferred from its anatomical position is extension of the hip joint. Our data suggest the following action based on electrical activity and changes in length of the muscle during a stride.

The muscle extended the hip. At a walk of 3· 4 km.h^{− 1} the muscle became active just before the foot touched the ground and remained active for 62% of the stance phase (48 % of the stride period). The muscle shortened during the entire time it was active extending the hip by 30°. The pattern of activity was the same at different walking speeds.

The muscle extended the hip. At a trotting speed of 8· 5 km.h^{− 1} the muscle became active before the foot touched the ground and remained active for the first part of the stance phase. It was active for about 27 % of the stride period. The muscle shortened during the entire time it was active, extending the hip by about 15°. The pattern of activity was the same at different trotting speeds.

The muscle extended the hip. At a galloping speed of 24· 7km.h^{− 1} the muscle became active before the foot touched the ground and remained active for the first part of the stance phase. It was active for 25 % of the stride period. The muscle shortened during the entire time it was active, extending the hip by about 17°. The pattern of activity was the same at different trotting speeds.

This is a single-joint muscle arising on the craniolateral surface of the proximal fifth of the femur (Fig. 10). Distally this muscle’s tendon of insertion fuses to that of the m. rectus femoris, m. vastus medialis and the m. tensor fasciae latae; the common tendon incorporates the patella before continuing to insert on the tibial tuberosity. Its function, inferred from anatomical position, is extension of the knee. Our data suggest the following action based on electrical activity and changes in length of the muscle during a stride.

The muscle stabilized the knee during most of the stance phase, then shortened, providing forward propulsion. At a walking speed of 3· 1 km.h^{− 1} the muscle became active just prior to foot-down and remained active for 81 % of the stance phase (58% of the stride period). The muscle length was unchanged during most of its activity, stabilizing the knee, and then at the end of the stance phase it shortened and extended the knee by about 10°. The pattern was similar at all walking speeds.

The active muscle was first stretched after the foot touched the ground (allowing storage of elastic energy) and then shortened and extended the knee (allowing recovery of the stored elastic energy). At a trotting speed of 8· 1 km.h^{− 1} the muscle became active just before the foot touched the ground and remained active for 70% of the stance phase (32% of the stride period). The muscle stretched, then shortened as the knee alternately flexed and extended by 20°. The pattern of activity was the same at different trotting speeds.

The active muscle was first stretched after the foot touched the ground and remained active for 94 % of the stance phase (25 % of the stride period). The muscle alternately lengthened and shortened during activity as the knee flexed and extended by 35°. The pattern of activity was similar at all galloping speeds.

This is a two-joint muscle that arises from the medial supracondylar tuberosity of the femur. This muscle fuses with the lateral gastrocnemius through much of its length. Distally the tendon inserts on the tuber calcanei of the calcaneus. Its function, inferred from its anatomical position, is extension of the tarsal joint and flexion of the knee. Our data suggest the following action based on electrical activity and changes in length of the muscle during a stride.

The muscle extended and then stabilized the ankle during the stance phase. At a walking speed of 3· 2 km.h^{− 1}, the muscle became active at foot-down and remained active 81 % of the stance phase (58 % of the stride). Simultaneous knee and ankle flexion resulted in about a 9% active length change in early stance. Muscle length remained about the same during subsequent knee and ankle extension in the late stance. The pattern of activity was similar at all walking speeds.

The muscle supported the ankle during early stance and extended the ankle in late stance. At a trotting speed of 7-8 km.h^{− 1}, muscle activity commenced after foot-down and remained for 73 % of the stance phase (33 % of the stride). During the stance as the knee and ankle alternately flexed and extended the muscle actively shortened 15 % of its total length. The pattern was similar at other trotting speeds.

The muscle supported the ankle during early stance phase ankle flexion (allows elastic storage) and extended the ankle, allowing recovery of elastic storage energy, during late stance. At a gallop of 24 km.h^{− 1}, though muscle activity did not begin until after foot-down, the muscle was active during part of the very rapid flexion Bf the knee and ankle during the yield. The resulting activity of medial gastrocnemius while the muscle is stretched resulted in some elastic storage. The muscle was active 88 % of the stance phase (25 % of the stride). During the yield, the muscle was actively stretched 9% of its length which was followed by active shortening of 10% corresponding to rapid extension of the knee and ankle. The pattern was similar at other galloping speeds.

This is a two-joint muscle arising from the cranial ventral spine of the iliac crest as well as from the lumbodorsal fascia (Fig. 12). The muscle belly passes medially along the thigh to a heavy fascia just proximal to the patella. The fascia ties tightly to the tendons of the m. rectus femoris and m. vastus medialis. Its function, inferred from its anatomical position, is flexion of the hip, extension and adduction of the thigh. Our data suggest the following action based on electrical activity and changes in length of the muscle during a stride.

The muscle flexed the hip. At a walking speed of 2· 7 km.h^{− 1} the muscle became active before the foot left the ground and remained active for 73 % of the swing phase. The muscle was active for 27 % of the stride period. The hip flexed by about 46° while the muscle was active: thus the muscle shortened by about 28% during its activity. The pattern of activity was the same at different walking speeds.

The muscle flexed the hip. At a trotting speed of 8· 2 km.h^{− 1} the muscle became active after the foot left the ground, and remained active for 38% of the swing phase. The muscle was active for 28% of the stride period. The hip flexed by about 57° while the muscle was active. The muscle shortened by about 22 % during its activity. The pattern of activity was the same at different trotting speeds.

The muscle flexed the hip. At a galloping speed of 24· 2 km.h^{− 1} the muscle became active after the foot left the ground and remained active for 55 % of the swing phase. The muscle was active for 42 % of the stride. The hip flexed by about 50° while the muscle was active. The pattern of activity was similar at different galloping speeds.

This is a two-joint muscle (largest of the two heads) arising fleshy from the ventro-caudal end of the sacrotuberous ligament and the ischial tuberosity (Fig. 13). Distally the tendon of insertion appears as an aponeurosis, part of which inserts on the patella and part of which traverses to the tibial tuberosity. Two muscle lengths were analysed ; a patellar aspect and tibial aspect. Its function, inferred from its anatomical position, is extension of the hip and knee when the limb is bearing weight. The tibial part of the muscles raises the leg when the limb is not bearing weight and therefore can flex the knee under these circumstances. Our data suggest the following action based on electrical activity and changes in length of the muscle during a stride.

The muscle extended the hip after the foot touched the ground. At a walking speed of 3· 3 km.h^{− 1} the muscle became active just before the foot touched the ground and remained active for 69% of the stance phase (56% of the stride period). The patellar portion of the muscle shortened by about 20% while the muscle was active due primarily to hip extension. Simultaneous knee flexion resulted in about a 10% active shortening of the tibial portion. The pattern of activity was similar at all walking speeds.

The muscle decelerated the forward movement of the limb before the fool touched the ground, and extended the hip after the foot touched the ground. At a trotting speed of 8· 3 km.h^{− 1}, the muscle became active prior to foot-down (25 % of the swing phase) and remained active for about 46 % of the stance phase (34 % of the stride period). During the activity period prior to foot-down, the muscle decelerated the forward movement of the limb by slowing the rate of extension of the knee. Knee extension at this time resulted in stretch of the tibial portion of the active muscle (by about 10% of its length) allowing for elastic storage of kinetic energy. Both portions of the muscle actively shortened by about 10% after foot-down, providing forward propulsion and allowing recovery of the stored elastic energy. The stance phase shortening occurred when there was a simultaneous extension of the hip and flexion of the knee. However, flexion of the knee could occur passively due to the effects of gravity on the weighted limb. The pattern of activity was similar at all trotting speeds.

The muscle decelerated the forward movement of the limb and reaccelerated it in the opposite direction before the foot touched the ground and extended the hip after the foot touched the ground. At a galloping speed of 24· 8 km.h^{− 1}, the muscle became active well prior to foot-down (27 % of swing phase) and remained active for 51 % of the stance phase (33 % of total stride period). The deceleration of the limb was due mainly to slowing the rate of knee extension, and the reacceleration of the limb in the opposite direction was due to both flexion of the knee and extension of the hip. During the deceleration phase, the tibial aspect was being actively stretched by about 10%. After foot-down, the entire muscle shortened by about 15 %, due first to extension of the hip and flexion of the knee in early stance; then both portions continued shortening due to an extension of the hip which was more than sufficient to compensate a simultaneous extension of the knee in late stance. The pattern was similar at all galloping speeds.

In this analysis we set out to determine the relative importance of the three types of muscular activity (shortening, isometric and lengthing) as animals increase speed within a gait and as they change gait. In contrast to previous studies, we obtained a large number of synchronous joint angle and muscle activity measurements at a variety of speeds. These data were extremely reproducible because we used a treadmill where speeds could be replicated easily. Wetzel et al. (1975) compared limb joint angles and stride profiles of cats moving on a treadmill with cats moving overground. The analysis revealed the extension period of the swing phase to be significantly shorter for the treadmill cats than for the overground ones. The authors concluded the differences were due at least in part to the intimidating influence of the treadmill. We found that stride frequency, swing phase and support phase for our dogs were the same when they ran at the same speed along the ground and on the treadmill. This is probably the result of using dogs that were well trained for treadmill running.

We found that during a walk the electrical activity and length changes of a number of limb extensors are consistent with the interpretation that joint stabilization was a major function of these muscles during the period when the limb was bearing weight This type of activity is necessary for the animal to ‘pole-vault’ over its limbs and thereby store and recover gravitational potential energy. Both the supraspinatus of the shoulder and the long head of the triceps and biceps brachii of the elbow underwent only minor length change (≅ 5 %) during their stance phase activity. In the hindlimb, the vastus lateralis and medial gastrocnemius were also active for large parts of the stance with little length change. Thus we conclude that one important role of these joint extensors during the walk is to provide joint stabilization. Not all extensors need contribute to such stabilization as is evidenced by a lack of activity of lateral triceps in the walk. These data are essentially in agreement with published findings for the walking dog (Tokuriki, 1973 a; Wentink, 1976), cat (Engberg & Lundberg, 1969; English, 1978; Rasmussen et al., 1978), Virginia opossum (Jenkins & Weijs, 1979) and rat (Cohen & Gans, 1975).

We found that some of the limb extensors are active while being stretched prior to shortening during a trot and gallop as would be required for elastic storage of energy when the animal lands. For review of this concept as related to locomotion in general and limb mechanics specifically, see Cavagna et al. (1964, 1977) and Goslow et al. (1973). This type of activity increased dramatically with increasing speed (both in duration and in amplitude of the EMG signal). In the forelimb we found this pattern of EMG activity as the muscles were stretched for the supraspinatus and for the lateral head of the triceps. The long head of the triceps, which acts across two joints, actively shortened during all of its activity. Some hindlimb muscles that we sampled also were alternately stretched and then shortened while they were active. The vastus lateralis was stretched and then shortened while active both during trotting and galloping and this activity pattern was consistent over a range of speeds. Activity of the gastrocnemius while being stretched and then shortening was consistently observed at fast trot and galloping speeds, but this pattern was inconsistent at slower trotting speeds. It seems possible that this inconsistency was due to compartmentalization of activity within the muscle at the slower trotting speeds. Compartmentalization has been observed in architecturally complex muscles such as the mammalian masseter (Herring, Grimm & Grimm, 1979).

Similar patterns of activity (i.e. limb extensors active while being stretched and then shortening) have been observed for the dog by Tokuriki (19736, 1974) and for the cat by Engberg & Lundberg (1969), English (1978) and Rasmussen et al. (1978). It therefore seems likely that this ‘bounce’ spring function of extensors is probably general for quadrupedal mammals.

We also found a pattern of muscular activity in the limb flexors that would allow storage of kinetic energy at the end of the swing phase and recovery during the propulsive stroke. This was apparent in the latissimus dorsi during a walk and trot (but less clear in a gallop) and in the biceps femoris during a trot and a gallop. This type of activity was not observed in the biceps brachii, brachialis or satorius. Our data are consistent with those of Tokuriki (1973 a, b; 1974) for a dog and with those of English (1978) and Rasmussen et al. (1978) for the cat.

We conclude that our electrical activity and length changes of the muscle measurements are consistent with the ‘pole-vault’ and ‘spring mechanisms’ for energy conservation proposed from studies of the mechanics and energetics of locomotion (Cavagna et al. 1963, 1964; Alexander & Vernon, 1975; Cavagna et al. 1977; Taylor, 1980).

This work was supported by NFH grants no. AM 18140-01, 02, 03, 04.