Gait-related motor patterns and hindlimb kinetics for the cat trot and gallop

To assess speed- and gait-related changes in semitendinosus (ST) activity, EMG was recorded from three cats during treadmill locomotion. Selected step cycles were filmed, and hip and knee joint kinematics were synchronized with EMG records. Swing-phase kinetics for trot and gallop steps at 2.25 m/s were compared for gait-related differences. Also, swing kinetics for different gallop forms were compared. With few exceptions, ST-EMG was characterized by two bursts for each step cycle; the first preceded paw off (STpo), and the second preceded paw contact (STpc). The two-burst pattern for the walk was defined by a high-amplitude STpo burst and a brief, low-amplitude STpc burst; at the slowest walk speeds, the STpc burst was occasionally absent. For the trot, the STpo burst was biphasic, with a brief pause just after paw off. With increasing walk-trot speeds, the duration of both bursts (STpo, STpc) remained relatively constant, but recruitment increased. Also, the onset latency of the STpo burst shifted, and a greater proportion of the burst was coincident with knee flexion during early swing. At the trot-gallop transition, there was an abrupt change in the two-burst pattern, and galloping was characterized by a high-amplitude STpc burst and a brief, low-amplitude STpo burst. At the fastest gallop speeds, the STpo burst was often absent, and the reduction in or elimination of the burst was associated with a unique pattern of swing phase kinetics at the knee. Knee flexion during the gallop swing was sustained by two inertial torques related to hip linear acceleration (HLA) and leg angular acceleration (LAA); correspondingly, muscle contraction was unnecessary. Conversely, knee flexion at the onset of the trot swing relied on a flexor muscle torque at the knee acting with an inertial flexor torque (LAA). Rotatory and transverse gallops at 4.0 m/s had similar swing phase kinetics and ST-EMG. Gait-related changes in ST-EMG, particularly at the trot-gallop transition, are not congruent with neural models assuming that details of the ST motor pattern are produced by a spinal CPG. We suggest that motor patterns programed by the spinal CPG are modulated by input from supraspinal centers and/or motion-related feedback from the hindlimbs to provide appropriate gait-specific activation of the ST.     

Adaptive control for backward quadrupedal walking. II. Hindlimb muscle synergies

1. To compare the basic hindlimb synergies for backward (BWD) and forward (FWD) walking, electromyograms (EMG) were recorded from selected flexor and extensor muscles of the hip, knee, and ankle joints from four cats trained to perform both forms of walking at a moderate walking speed (0.6 m/s). For each muscle, EMG measurements included burst duration, burst latencies referenced to the time of paw contact or paw off, and integrated burst amplitudes. To relate patterns of muscle activity to various phases of the step cycle, EMG records were synchronized with kinematic data obtained by digitizing high-speed ciné film. 2. Hindlimb EMG data indicate that BWD walking in the cat was characterized by reciprocal flexor and extensor synergies similar to those for FWD walking, with flexors active during swing and extensors active during stance. Although the underlying synergies were similar, temporal parameters (burst latencies and durations) and amplitude levels for specific muscles were different for BWD and FWD walking. 3. For both directions, iliopsoas (IP) and semitendinosus (ST) were active as the hip and knee joints flexed at the onset of swing. For BWD walking, IP activity decreased early, and ST activity continued as the hip extended and the knee flexed. For FWD walking, in contrast, ST activity ceased early, and IP activity continued as the hip flexed and the knee extended. For both directions, tibialis anterior (TA) was active throughout swing as the ankle flexed and then extended. A second ST burst occurred at the end of swing for FWD walking as hip flexion and knee extension slowed for paw contact. 4. For both directions, knee extensor (vastus lateralis, VL) activity began at paw contact. Ankle extensor (lateral gastrocnemius, LG) activity began during midswing for BWD walking but just before paw contact for FWD walking. At the ankle joint, flexion during the E2 phase (yield) of stance was minimal or absent for BWD walking, and ankle extension during BWD stance was accompanied by a ramp increase in LG-EMG activity. At the knee joint, the yield was also small (or absent) for BWD walking, and increased VL-EMG amplitudes were associated with the increased range of knee extension for BWD stance. 5. Although the uniarticular hip extensor (anterior biceps femoris, ABF) was active during stance for both directions, the hip flexed during BWD stance and extended during FWD stance.(ABSTRACT TRUNCATED AT 400 WORDS)     

Functionally complex muscles of the cat hindlimb

Similarities between the muscle synergies associated with the flexion reflex and locomotion in reduced preparations have suggested that spinal circuits subserving these two motor tasks might share common interneurons. To test this hypothesis in functionally complex muscles, we studied the interaction between low-threshold cutaneous afferents and the locomotor central pattern generator (CPG) during treadmill locomotion in awake, intact cats. Electrical stimuli were delivered via implanted nerve cuff electrodes at all phases of locomotion, and EMGs were recorded from fourteen intramuscular subregions in eight bifunctional thigh muscles (adductor femoris, biceps femoris, caudofemoralis, gracilis, semimembranosus, semitendinosus, tensor fasciae latae, and tenuissimus). In addition, the EMG patterns recorded during locomotion were compared with those recorded during two other centrally driven rhythmical behaviors, scratching and paw shaking, to determine whether the functional relationships among these intramuscular subregions were fixed or task dependent. Four of the five broad, bifunctional muscles studied (biceps femoris, gracilis, semimembranosus, and tensor fasciae latae) had functional subunits that could be differentially activated in one or more of the three movements studied; adductor femoris was consistently uniformly activated despite its distributed skeletal attachments. The pattern of recruitment of the intramuscular functional subunits was movement-specific. The locomotor CPG and cutaneous reflex pathways both similarly subdivided some bifunctional muscles, but not others, into intramuscular subregions. The results of the present study confirm that some combinations of muscle subregions and cutaneous nerves constitute simple reciprocal categories of flexors and extensors, as described originally by Sherrington (1910). "Typical" low threshold excitatory or inhibitory reflex responses were produced in muscles or muscle subregions that were recruited as "net" flexors of extensors, respectively. However, muscles with complex activation patterns during walking often had very individualized, complex reflex responses during locomotion that did not conform to the background locomotion synergies. All of the reflex responses observed were mediated by low threshold cutaneous afferents. These data indicate that there are multiple, low threshold, excitatory and inhibitory cutaneous reflex pathways that have highly specialized connections with flexor and extensor muscles and even their intramuscular subregions. It is also clear that the premotoneuronal circuits mediating these cutaneous reflex effects are not necessarily synonymous with those of the locomotor CPG. These two systems do interact powerfully, however, suggesting some convergence. The nature of the convergence between the CPG and the many independent subsets of spinal interneurons mediating cutaneous reflexes is specialized and muscle subregion-specific.     

Functionally complex muscles of the cat hindlimb

Several cat hindlimb muscles that exhibit differential activation (activity that is restricted to a specific region of muscle) during natural movements were studied to determine the possible roles of 1) non-uniform distribution of histochemically-identified muscle fiber-types (semitendinosus, ST; tibialis anterior, TA) or 2) mechanical heterogeneity (biceps femoris, BF; tensor fasciae latae, TFL). Using chronic recording techniques, electromyographic (EMG) activity was recorded from multiple sites of each muscle during treadmill locomotion, ear scratch, and paw shake. Standard histochemical analysis was performed on each muscle to determine fiber-type distribution. The histochemically regionalized muscles (ST and TA) were differentially active during slow locomotion; the deep regions (high in type I [SO] fibers) were active, but the superficial regions (high in type IIB [FG] fibers) were inactive. Vigorous movements (fast locomotion, ear scratch, paw shake) produced additional, synchronous activation of the superficial regions. In all movements, ST and TA activation patterns were consistent with the existence of identically timed synaptic inputs to all motoneurons within each motoneuron pool, resulting in an orderly recruitment of each whole pool. The differential activation recorded from ST and TA during slow locomotion was presumably a consequence of the non-uniform distribution of the different muscle fiber types. In contrast, differential activation of the histochemically nonregionalized, mechanically heterogeneous muscles (BF and TFL) resulted from non-synchronous activation of different muscle regions. The selective activation of BF or TFL compartments was indicative of differential synaptic inputs to, and selective recruitment of, subpopulations of the motoneuron pool, with each motoneuron subpopulation exclusively innervating physically separate regions of the muscle consistent with the regions defined by the neuromuscular territories of the major nerve branches supplying each muscle. Individual neuromuscular compartments of BF and TFL differ in their mechanical arrangements to the skeleton and in their contribution to mechanical action(s) at the hip and knee joints. Selective neural activation of mechanically distinct compartments within a mechanically heterogeneous muscle can provide highly advantageous mechanical "options" for animals that perform kinematically diverse movements. With regard to EMG recording techniques, the results of this study emphasize the need for carefully chosen EMG sampling sites and the value of knowing the muscle histochemistry, neuromuscular and musculoskeletal anatomy and possible mechanical functions prior to recording EMG.     

Stumbling corrective reaction: a phase-dependent compensatory reaction during locomotion.

1. Tactile stimuli to the paw consisting of a stick making contact or an air puff aimed at the dorsum were used to study the phasic influence of locomotor activity on the reflex pattern elicited in extensor and flexor muscles and on the induced compensatory movements in intact cats. The resulting movements and reflex pattern are called "stumbling corrective reactions." 2. The basic reflex pattern and movements of the stumbling corrective reaction are: a) if the stimulus occurs during the swing phase, a short-latency activation of the flexor muscles, which induces an additional flexion of the limb lifting the paw over the obstacle; b) if the stimulus occurs during the support phase, an inhibition followed by an excitation of the extensor muscles, which neither increase nor decrease the extension. However, the stimulus evokes an increased flexor activity in the succeeding swing phase, which induces a brisker flexion. 3. Tactile stimuli to the proximal part of the limb or to the belly in front of the knee evoked the same type of phase-dependent compensatory reactions. Such reactions would presumably be beneficial for the animal to avoid high obstacles that impede movement. 4. A nonnoxious electrical stimulus (typically 2 mA; 1 ms) applied to the dorsum of the paw was used to study systematically the reflex pattern of the stumbling corrective reaction. Two pathways were defined to the knee flexor (semitendinosus). One early burst was evoked at about 10 ms and one later at about 25 ms after the stimulus. Short inhibitory pathways and longer excitatory pathways (20-50 ms) projected to the extensor nuclei. A short-latency (10 ms) excitatory pathway to the ankle extensor (lateral gastrocnemius) was also activated. 5. A painful electrical stimulus applied to the dorsum of the paw evoked large flexor responses during the whole step cycle. During the support phase the locomotion was disrupted as the supporting limb was withdrawn. 6. The results demonstrate that intact cats are able to compensate rapidly for unpredicted perturbations and that the reflex pattern and the induced corrective movements are adapted to the locomotor activity so that functionally meaningful movements are evoked in each phase of the step cycle. 7. The evoked reflexes and their modulation are consistent with those previously found in chronic spinal cats during walking and in paralyzed spinal cats performing "fictive locomotion." It is suggested that the same spinal pathways are used, and that they are controlled by the spinal "locomotor generator."     

A correlogram analysis of the activity in the rostral ventromedial medulla of awake rats and in rats anesthetized with ketamine or pentobarbital following the administration of morphine

The present study was designed to determine the relative contribution of the gastrocnemius muscle to isometric plantar flexor torque production at varying knee angles, while investigating the activation of the gastrocnemius muscle at standardised non-optimal lengths. Voluntary plantar flexor torque, supramaximally stimulated twitch torque and myoelectric activity (EMG) from the triceps surae were measured at different knee angles. Surface and intra-muscular EMG were recorded from the soleus muscle and the medial and lateral heads of the gastrocnemius muscle in 10 male subjects. With the ankle angle held constant, knee angle was changed in steps of 30° ranging from 180° (extended) to 60° (extreme flexion), while voluntary torque from a 5-s contraction was determined at 10 different levels of voluntary effort, ranging from 10% of maximal effort to maximal effort. To assess effort, supramaximal twitches were superimposed on all voluntary contractions, and additionally during rest. Maximal plantar flexor torque and resting twitch torque decreased significantly in a sigmoidal fashion with increasing knee flexion to 60% of the maximum torque at 180° knee angle. For similar levels of voluntary effort, the EMG root mean square (RMS) of gastrocnemius was less with increased knee flexion, whereas soleus RMS remained unchanged. From these data, it is concluded that the contribution of gastrocnemius to plantar flexor torque is at least 40% of the total torque in the straight leg position. The decrease of gastrocnemius EMG RMS with decreasing muscle length may be brought about by a decrease in the number of fibres within the EMG electrode recording volume and/or impaired neuromuscular transmission.     

Scratch responses in normal cats: hindlimb kinematics and muscle synergies

1. Scratch responses evoked by a tactile stimulus applied to the outer ear canal were characterized in nine adult cats. Chronic electromyographic (EMG) electrodes were surgically implanted in selected flexor and extensor muscles of the hip, knee, and ankle joints to determine patterns of muscle activity during scratching. In some trials EMG records were synchronized with kinematic data obtained by digitizing high-speed ciné film, and in one cat, medial gastrocnemius (MG) tendon forces were recorded along with EMG. For analysis the response was divided into three components: the approach, cyclic, and return periods. Usually scratch responses were initiated with the cat in a sitting position, but in some trials the animal initiated the response from a standing or lying posture. 2. During the approach period the hindlimb ipsilateral to the stimulated ear was lifted diagonally toward the head by a combination of hip and ankle flexion with knee extension. Hindlimb motions during the approach period were associated with sustained EMG activity in hip-flexor, knee-extensor (occasionally), and ankle-flexor muscles. Initial hindlimb motions were typically preceded by head movements toward the hindpaw, and at the end of the approach period, the head was tilted downward with the stimulated pinna lower than the contralateral ear. During the return period movements were basically the reverse of the approach period, with the hindpaw returning to the ground and the head moving away from the hindlimb. 3. During the cyclic period the number of cycles per response varied widely from 1 to 60 cycles with an average of 13 cycles, and cycle frequency ranged from 4 to 8 cycles/s, with a mean of 5.6 cycles/s. During each cycle the paw trajectory followed a fairly circular path, and the cycle was defined by three phases: precontact, contact, and postcontact. On average the contact phase occupied approximately 50% of the cycle and was characterized by extensor muscle activity and extension at the hip, knee, and ankle joints. The hindpaw contacted the pinna or neck at the base of the pinna throughout the contact phase, and paw contact typically resulted in a rostral motion of the head as the hindlimb extended. 4. The postcontact phase constituted approximately 24% of scratch cycle and was usually initiated by the onset of knee flexion. Ankle and then hip flexion followed knee flexion, and flexor muscles were active during the postcontact phase as the paw was withdrawn from the head. The precontact phase constituted approximately 26% of scratch cycle and was initiated by knee joint extension and knee-extensor activity.(ABSTRACT TRUNCATED AT 400 WORDS)     

The role of cutaneous afferents in controlling locomotion evoked by epidural stimulation of the spinal cord in decerebrate cats

The effects of the cutaneous input on the formation of the locomotor pattern in conditions of epidural stimulation of the spinal cord in decerebrate cats were studied. Locomotor activity was induced by rhythmic stimulation of the dorsal surface of spinal cord segments L4-L5 at a frequency of 3-5 Hz. Electromyograms (EMG) recorded from the antagonist muscles quadriceps, semitendinosus, tibialis anterior, and gastrocnemius lateralis were recorded, along with the kinematics of stepping movements during locomotion on a moving treadmill and reflex responses to single stimuli. Changes in the pattern of reactions observed before and after exclusion of cutaneous receptors (infiltration of lidocaine solution at the base of the paw or irrigation of the paw pads with chlorothane solution) were assessed. This treatment led to impairment of the locomotor cycle: the paw was placed with the rear surface downward and was dragged along in the swing phase, and the duration of the stance phase decreased. Exclusion of cutaneous afferents suppressed the polysynaptic activity of the extensor muscles and the distal flexor muscle of the ipsilateral hindlimb during locomotion evoked by epidural stimulation of the spinal cord. The effects of exclusion of cutaneous afferents on the monosynaptic component of the EMG response were insignificant.     

Single joint perturbation during gait: neuronal control of movement trajectory

The aim of this study was to investigate the effect of single joint displacement on the pattern of leg muscle electromyographic (EMG) activity during locomotion. For the first time, unilateral rotational hip or knee joint displacements were applied by a driven orthotic device at three phases of swing during locomotion on a treadmill. The response pattern of bilateral leg muscle activation with respect to the timing and selection of muscles was almost identical for displacements of upper (hip joint) or lower (knee joint) leg. The leg muscle EMG responses were much stronger when the displacement was directed against the physiological movement trajectory, compared with when the displacement was reinforcing, especially during mid swing. It is suggested that these response patterns are designed to restore physiological movement trajectory rather than to correct a single joint position. Displacements released at initial or terminal swing, assisting or resisting the physiological movement trajectory, were followed by similar and rather unspecific response patterns. This was interpreted as being directed to stabilise body equilibrium.     

Stumbling corrective reaction during fictive locomotion in the cat

An obstacle contacting the dorsal surface of a cat's hind foot during the swing phase of locomotion evokes a reflex (the stumbling corrective reaction) that lifts the foot and extends the ankle to avoid falling. We show that the same sequence of ipsilateral hindlimb motoneuron activity can be evoked in decerebrate cats during fictive locomotion. As recorded in the peripheral nerves, twice threshold intensity stimulation of the cutaneous superficial peroneal (SP) nerve during the flexion phase produced a very brief excitation of ankle flexors (e.g., tibialis anterior and peroneus longus) that was followed by an inhibition for the duration of the stimulus train (10-25 shocks, 200 Hz). Extensor digitorum longus was always, and hip flexor (sartorius) activity was sometimes, inhibited during SP stimulation. At the same time, knee flexor and the normally quiescent ankle extensor motoneurons were recruited (mean latencies 4 and 16 ms) with SP stimulation during fictive stumbling correction. After the stimulus train, ankle extensor activity fell silent, and there was an excitation of hip, knee, and ankle flexors. The ongoing flexion phase was often prolonged. Hip extensors were also recruited in some fictive stumbling trials. Only the SP nerve was effective in evoking stumbling correction. Delivered during extension, SP stimulus trains increased ongoing extensor motoneuron activity as well as increasing ipsilateral hip, knee, and ankle hindlimb flexor activity in the subsequent step cycle. The fictive stumbling corrective reflex seems functionally similar to that evoked in intact, awake animals and involves a fixed pattern of short-latency reflexes as well as actions evoked through the lumbar circuitry responsible for the generation of rhythmic alternating locomotion.     

Vertical perturbations of human gait: organisation and adaptation of leg muscle responses

During the last several years, evidence has arisen that the neuronal control of human locomotion depends on feedback from load receptors. The aim of the present study was to determine the effects and the course of sudden and unexpected changes in body load (vertical perturbations) on leg muscle activity patterns during walking on a treadmill. Twenty-two healthy subjects walking with 25% body weight support (BWS) were repetitively and randomly loaded to 5% or unloaded to 45% BWS during left mid-stance. At the new level of BWS, the subjects performed 3–11 steps before returning to 25% BWS (base level). EMG activity of upper and lower leg muscles was recorded from both sides. The bilateral leg muscle activity pattern changed following perturbations in the lower leg muscles and the net effect of the vertical perturbations showed onset latencies with a range of 90–105 ms. Body loading enhanced while unloading diminished the magnitude of ipsilateral extensor EMG amplitude, compared to walking at base level. Contralateral leg flexor burst activity was shortened following loading and prolonged following unloading perturbation while flexor EMG amplitude was unchanged. A general decrease in EMG amplitudes occurred during the course of the experiment. This is assumed to be due to adaptation. Only the muscles directly activated by the perturbations did not significantly change EMG amplitude. This is assumed to be due to the required compensation of the perturbations by polysynaptic spinal reflexes released following the perturbations. The findings underline the importance of load receptor input for the control of locomotion.     

Locomotor and reflex adaptation after partial denervation of ankle extensors in chronic spinal cats

This work investigates the capacity of the spinal cord to generate locomotion after a complete spinal section and its ability to adapt its locomotor pattern after a peripheral nerve lesion. To study this intrinsic adaptive capacity, the left lateral gastrocnemius-soleus (LGS) nerve was sectioned in three cats that expressed a stable locomotion following a complete spinal transection. The electromyograph (EMG) of multiple hindlimb muscles and reflexes, evoked by stimulating the left tibial (Tib) nerve at the ankle, were recorded before and after denervation during treadmill locomotion. Following denervation, the mean amplitude of EMG bursts of multiple hindlimb muscles increased during locomotion, similar to what is found after an identical denervation in otherwise intact cats. Reflex changes were noted in ipsilateral flexors, such as semitendinosus and tibialis anterior, but not in the ipsilateral knee extensor vastus lateralis following denervation. The present results demonstrate that the spinal cord possesses the circuitry necessary to mediate increased EMG activity in multiple hindlimb muscles and also to produce changes in reflex pathways after a muscle denervation. The similarity of changes following LGS denervation in cats with an intact and transected spinal cord suggests that spinal mechanisms play a major role in the locomotor adaptation.     

Cat hindlimb motoneurons during locomotion. III. Functional segregation in sartorius

Cat sartorius has two distinct anatomical portions, anterior (SA-a) and medial (SA-m). SA-a acts to extend the knee and also to flex the hip. SA-m acts to flex both the knee and the hip. The objective of this study was to investigate how a "single motoneuron pool" is used to control at least three separate functions mediated by the two anatomical portions of one muscle. Discharge patterns of single motoneurons projecting to the sartorius muscle were recorded using floating microelectrodes implanted in the L5 ventral root of cats. The electromyographic activity generated by the anterior and medial portions of sartorius was recorded with chronically implanted electrodes. The muscle portion innervated by each motoneuron was determined by spike-triggered averaging of the EMGs during walking on a motorized treadmill. During normal locomotion, SA-a exhibited two bursts of EMG activity per step cycle, one during the stance phase and one during the late swing phase. In contrast, every recorded motoneuron projecting to SA-a discharged a single burst of action potentials per step cycle. Some SA-a motoneurons discharged only during the stance phase, whereas other motoneurons discharged only during the late swing phase. In all cases, the instantaneous frequencygram of the motoneuron was well fit by the rectified smoothed EMG envelope generated by SA-a during the appropriate phase of the step cycle. During normal locomotion, SA-m exhibited a single burst of EMG activity per step cycle, during the swing phase. The temporal characteristics of the EMG bursts recorded from SA-m differed from the swing-phase EMG bursts generated by SA-a.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that popliteal fat provides damping during locomotion in the cat

Current models and concepts of motor control represent the limb as a neuro-musculoskeletal system and rarely include other potentially important supporting tissues such as fascia and adipose tissue. It is possible that a normal complement of adipose tissue could contribute to the viscoelastic properties of supporting limbs and enhance stability during locomotion. The purpose of this study was to determine if the popliteal fat pad plays a role in locomotion in the cat. It is hypothesized that the fat pad limits flexion and reduces angular acceleration of the included hip, knee and ankle joints in the sagittal plane throughout the step cycle. 3D kinematics from 3 spontaneously locomoting decerebrate cats both before and after lipectomy were recorded during treadmill walking. Four time points throughout the step cycle were chosen for angular acceleration analysis: mid-stance, paw off, mid-swing and peak deceleration at the end of the re-extension of the knee. Significant increases in maximum angular acceleration for the hip, knee and ankle joints at these time points were observed. No significant increase in range of motion was found across all 3 included angles after lipectomy. Therefore, the hypothesis that the popliteal fat pad acts to decrease the angular acceleration is supported by these findings. The data indicate that the popliteal fat pad contributes to the damping component of the viscoelastic properties of the limb. These results may be applied to models of the hindlimb and knowledge of the effects of obesity on movement.     

Locomotor deficits in the mutant mouse,Lurcher

Summary The effect of total Purkinje cell degeneration on treadmill locomotion was studied in the cerebellar mutant mouse Lurcher. Other movements such as swimming and scratchting were also studied in order to evaluate the cerebellar control of rhythmic actions. Cinematographic and electromyographic recordings were taken from normal and Lurcher mice that were subsequently perfused to obtain a Purkinje cell count. Walking deteriorated progressively and was clearly abnormal in 30 day old Lurchers with 90% Purkinje cell degeneration. In adult Lurcher mice in which Purkinje cells were totally absent, walking was characterized by short steps with exaggerated hindlimb flexion in the swing phase. Also, both the interlimb step ratio, defined as the step length of the reference limb divided by the step length of the opposite limb, and the interlimb coupling, defined as the temporal relation of one footfall with respect to the footfall of another limb, varied more than in normal mice. Furthermore, the locomotion of Lurcher mice displayed increased vertical displacement of the hip and an inability to produce continuous step cycles without stumbling. Both the EMG onset relative to foot contact and the EMG burst duration were highly variable, and a greater overlap in the activities of antagonist muscles at the transition from ankle extension to flexion was evident. Although both walking and swimming involve cyclical limb movements, the disorganization of the cycle and the irregular EMG pattern seen in the Lurcher during walking were not observed during swimming. Furthermore, scratching was well executed in the Lurcher mice. However, a consistently higher tonic extensor activity at the ankle appeared during walking, swimming and scratching. These results suggest that, in contrast to swimming and scratching, the requirements of walking depend to a greater degree on a functional cerebellar cortex for successful performance.     

Phase-dependent electromyographic activity of the lower-limb muscles of a patient with clinically complete spinal cord injury during orthotic gait

We examined the lower-limb electromyographic (EMG) activity from a patient with clinically complete spinal cord injury during orthotic gait. A newly developed gait orthosis was used to obtain bipedal locomotion. The surface EMG data during the gait together with the biomechanical variables were collected by way of a radio EMG system. A cyclic EMG activation pattern corresponding to the gait cycles were observed in each of the paralyzed lower-limb muscles during the orthotic gait. Although the EMG activation did not seem to contribute toward generating the gait, it showed some similarities to that of the infant stepping or immature gait. These results might be regarded as one of the indirect pieces of evidence that suggest the existence of a spinally originating motor mechanism underlying human locomotion. Received: 5 September 1997 / Accepted: 1 December 1997     

Disynaptic excitation from the medial longitudinal fasciculus to lumbosacral motoneurons: modulation by repetitive activation,descending pathways,and locomotion

Summary Short-latency excitatory postsynaptic potentials (EPSPs) evoked by stimulation in the medial longitudinal fasciculus (MLF) were recorded intracellularly from motoneurons in the cat lumbosacral spinal cord. Monosynaptic and disynaptic EPSPs occurred in most flexor and extensor motoneurons studied. These EPSPs resulted from the activation of fast (> 100 m/s) descending axons from the MLF to the spinal cord. Several features distinguished monosynaptic and disynaptic MLF EPSPs. Disynaptic EPSPs exhibited temporal facilitation during short trains of stimulation, whereas monosynaptic EPSPs did not. Disynaptic EPSPs, but not monosynaptic EPSPs, were also facilitated by stimulation of the pyramidal tract and the mesencephalic locomotor region. However, disynaptic MLF EPSPs exhibited little or no facilitation when conditioned by short-latency cutaneous pathways. During fictive locomotion, the amplitude of disynaptic MLF EPSPs was modulated, with maximal amplitudes during the step cycle phase when the recorded motoneuron was active, resulting in reciprocal patterns of modulation of flexors and extensors. No comparable change was seen in the amplitude of monosynaptic MLF EPSPs during fictive stepping. These data suggest that the central pattern generator for locomotion modulates disynaptic MLF excitation at a premotoneuronal level in a phase-dependent manner. The effects of lesions made in the MLF and thoracic cord suggest that the interneurons in the disynaptic pathway from the MLF to motoneurons reside in the lumbosacral cord.     

Lumbar back muscle activity during locomotion: effects of voluntary modifications of normal trunk movements

The mechanisms of adaptation of the trunk to changed mechanical conditions were studied during locomotion in man. The myoelectrical (EMG) activity in lumbar back muscles and the movements of the trunk were recorded in nine healthy subjects during walking and running on a motor-driven treadmill. Two different types of voluntary modifications of the movement pattern were used: (1) The trunk was kept in an extreme forward or backward tilted position. In both these situations the basic EMG pattern with two periods of activity per stride cycle was maintained during walking, whereas a major shift relative to the stride cycle (25% of the stride cycle duration) occurred in running with the trunk tilted backwards. The synchrony of the back muscle activation at both sides increased when locomotion was performed with the trunk tilted forwards. The relative duration of the EMG bursts was similar to normal locomotion and corresponded to 15-26% of the stride cycle duration in walking and 23-37% in running. (2) In the other type of modification the subjects were instructed to exaggerate the angular trunk movements either in the sagittal or in the frontal plane. The basic EMG pattern and phase relationships remained in most cases unchanged. One exception was running with exaggerated lateral movements, in which only one period of back muscle activity per stride cycle was observed. The relative duration of the bursts was longer in trials with exaggerated trunk movements as compared to normal locomotion. In walking and running with the trunk tilted forwards or backwards the lumbar back muscles were not always involved as prime movers of the trunk. This was in contrast to the more dynamic situations, in which the back muscle activity appeared to be directly involved in braking and reversing the exaggerated trunk movements.     

Simultaneous control of two rhythmical behaviors. I. Locomotion with paw-shake response in normal cat

We investigated the ability of normal cats, trained to maintain a constant position while walking on a treadmill, to combine the paw-shake response with quadrupedal locomotion. Hindlimb paw-shake responses were elicited during walking after the right hindpaw was wrapped with tape. To assess intralimb and interlimb coordination of the combined behaviors, electromyographic (EMG) recordings from forelimb extensor muscles and from selected flexor and extensor muscles at the three major hindlimb joints were correlated with joint motion by using high-speed, cinefilm analysis. When paw shaking was combined with walking, the response occurred during the swing phase of the taped hindlimb. To accommodate the paw-shake response, swing duration of the shaking hindlimb and of the homolateral forelimb increased and was followed by a brief recovery step. Concurrently, to compensate for the response, stance durations of the contralateral forelimb and hindlimb increased. The magnitude of these adjustments in interlimb coordination was influenced by the number of paw-shake cycles, which ranged from one to four oscillations. Transitions between the muscle synergies for the paw-shake response and swing were smooth in the shaking limb. Early in the swing phase, when the flexor muscles were still active (F phase), the paw shake was initiated by an early onset of knee extensor activity, which preceded extensor activity at the hip and ankle. This action provided a transition from the general reciprocal synergy between flexor and extensor muscles of locomotion to the mixed synergy that is typical of the paw shake (30). Following the last paw-shake cycle, an extensor synergy initiated the E-1 phase of swing, and the resultant joint motion was in-phase extension of the hip, knee, and ankle to lower the paw for stance. Average cycle period and burst duration for muscles participating in the paw-shake response were similar to those reported for normal cats assuming a standing posture (28, 30). The average number of paw-shake cycles, however, decreased from eight to three when the response occurred during walking, suggesting that the response was truncated to provide for continued locomotion. Further, hip motion was variable when the paw shake was combined with swing, and sometimes the hip failed to oscillate and its trajectory was similar to that of an unperturbed swing phase. When hip joint oscillations occurred during the paw-shake response, they were in-phase with ankle motions.(ABSTRACT TRUNCATED AT 400 WORDS)