Simultaneous control of two rhythmical behaviors. II. Hindlimb walking with paw-shake response in spinal cat

The simultaneous control of the hindlimb paw-shake response and hindlimb walking at slow treadmill speeds (0.2-0.4 m/s) was examined in adult cats spinalized at the T12 level, 3-6 mo earlier. Paw shaking was elicited by either 1) application of adhesive tape or 2) water to the right hindpaw. To assess intralimb and interlimb coordination of the combined behaviors, activity from selected flexor and extensor muscles at the hip, knee, and ankle was recorded, and the kinematics of these joints were determined from high-speed cinefilm. When paw shaking was combined with hindlimb walking, the response in the stimulated limb was initiated during swing (F phase) of the step cycle. The onset of knee extensor activity provided the transition from the flexor synergy of swing to the mixed synergy of paw shake. At the end of the paw shake, an extensor synergy initiated the E-1 phase of swing, and the resultant joint motion was in-phase extension at the hip, knee, and ankle to lower the paw for contact with the treadmill belt. During the rapid (81 ms) paw-shake cycles, knee extensor and ankle flexor muscles exhibited single, coactive bursts that were reciprocal with coactive hip and ankle extensor bursts. This mixed synergy was reflected in the limb coordination, as knee flexion coincided with ankle extension and knee flexion coincided with ankle extension. Phasing of hip motions was variable, reflecting the role of the proximal in stabilization during paw shake (16). Although the number of paw-shake cycles combined during swing varied greatly from 2 to 14, average cycle periods, burst durations, and intralimb synergies were similar to those previously reported for spinal cats tested under conditions in which the trunk was suspended and hindlimbs were pendent (23, 27). For step cycles during which a long paw-shake response of 8-14 cycles occurred, swing duration of the shaking limb increased by 1 s, and during this prolonged interval, the contralateral hindlimb completed two support steps. Stance duration of the support steps was also prolonged. This adjustment maximized the duration of paw-contact and minimized any period of nonsupport by the contralateral hindlimb during paw shake. Completion of the paw-shake response was followed by either an alternating, or a nonalternating, gait pattern on the recovery steps. One spinal cat combined locomotion with short two-cycle paw-shake responses, and because the shortened response was limited primarily to the time ordinarily devoted to swing, interlimb adjustments were slight.(ABSTRACT TRUNCATED AT 400 WORDS)     

Intracellular analysis of reflex pathways underlying the stumbling corrective reaction during fictive locomotion in the cat

In cat and humans, contact between an obstacle and the dorsum of the foot evokes the stumbling corrective reaction (reflex) that lifts the foot to avoid falling. This reflex can also be evoked by short trains of stimuli to the cutaneous superficial peroneal (SP) nerve in decerebrate cats during the flexion phase of fictive locomotion. Here we examine intracellular events in hindlimb motoneurons accompanying stumbling correction. SP stimulation delivered during the flexion phase excites knee flexor motoneurons at short latency [minimum excitatory postsynaptic potential (EPSP) latency 1.8 ms; mean 2.7 ms]. Although a similar short latency excitation occurs in ankle extensors (mean latency, 2.8 ms), recruitment is delayed until successive shocks in the stimulus train overcome the locomotor-related hyperpolarization of ankle extensors. In ankle flexor motoneurons, SP stimulation evokes an inhibition (mean latency, 2.7 ms) that briefly reduces or stops their firing during the flexion phase. There is a phase-dependent modulation of SP-evoked EPSP amplitude as well as latency during locomotion. However, the more obvious change in SP reflex pathways with the onset of fictive locomotion is the reduced inhibition of ankle extensor motoneurons and the increased inhibition of ankle flexors. These results show that the characteristic pattern of hindlimb motoneuron activation during SP nerve-evoked stumbling correction results from 1) di- and trisynaptic excitation of knee flexor and ankle extensor motoneurons; 2) increased inhibitory postsynaptic potentials in ankle flexors and a suppression of inhibition in extensors, 3) sculpting of the short-latency SP postsynaptic effects by motoneuron membrane potential, and 4) longer latency excitatory effects that are likely evoked by lumbar interneurons involved in the generation of fictive locomotion.     

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.     

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)     

The effects of self-reinnervation of cat medial and lateral gastrocnemius muscles on hindlimb kinematics in slope walking

The aim of this study was to investigate the effects of self-reinnervation of the medial (MG) and lateral gastrocnemius (LG) muscles on joint kinematics of the whole hindlimb during overground walking on surfaces of varying slope in the cat. Hindlimb kinematics were assessed (1) with little or no activity in MG and LG (short-term effects of self-reinnervation), and (2) after motor function of these muscles was presumably recovered but their proprioceptive feedback permanently disrupted (long-term effects of self-reinnervation). The stance phase was examined in three walking conditions: downslope (−50%, i.e. −26.6°), level (0%) and upslope (+50%, +26.6°). Measurements were performed prior to and at consecutive time points (between 1 and 57 weeks) after transecting and immediately suturing MG and LG nerves. It was found that MG-LG self-reinnervation did not significantly change hip height and hindlimb orientation in any of the three walking conditions. Substantial short-term effects were observed in the ankle joint (e.g., increased flexion in early stance) as well as in metatarsophalangeal and knee joints, leading to altered interjoint coordination. Hindlimb kinematics in level and upslope walking progressed back towards baseline within 14–19 weeks. Thus in these two conditions the cats were walking without any detectable kinematic deficits, despite the absence of length feedback from two major ankle extensors. This was verified in a decerebrate preparation for four of the five cats. In contrast, ankle joint kinematics as well as interjoint coordination in downslope walking gradually progressed towards, but never reached their baseline patterns. The short-term effects can be explained by both mechanical and neural factors that are affected by the functional elimination of MG and LG. Permanent changes in kinematics during downslope walking indicate the importance of proprioceptive feedback from the MG and LG muscles in regulating locomotor activity of ankle extensors. Full recovery of hindlimb kinematics during level and upslope walking suggests that the proprioceptive loss is compensated by other sensory sources (e.g. cutaneous receptors) or altered central drive.     

Short-latency crossed inhibitory responses in extensor muscles during locomotion in the cat

During locomotion, contacting an obstacle generates a coordinated response involving flexion of the stimulated leg and activation of extensors contralaterally to ensure adequate support and forward progression. Activation of motoneurons innervating contralateral muscles (i.e., crossed extensor reflex) has always been described as an excitation, but the present paper shows that excitatory responses during locomotion are almost always preceded by a short period of inhibition. Data from seven cats chronically implanted with bipolar electrodes to record electromyography (EMG) of several hindlimb muscles bilaterally were used. A stimulating cuff electrode placed around the left tibial and left superficial peroneal nerves at the level of the ankle in five and two cats, respectively, evoked cutaneous reflexes during locomotion. During locomotion, short-latency ( approximately 13 ms) inhibitory responses were frequently observed in extensors of the right leg (i.e., contralateral to the stimulation), such as gluteus medius and triceps surae muscles, which were followed by excitatory responses ( approximately 25 ms). Burst durations of the left sartorius (Srt), a hip flexor, and ankle extensors of the right leg increased concomitantly in the mid- to late-flexion phases of locomotion with nerve stimulation. Moreover, the onset and offset of Srt and ankle extensor bursts bilaterally were altered in specific phases of the step cycle. Short-latency crossed inhibition in ankle extensors appears to be an integral component of cutaneous reflex pathways in intact cats during locomotion, which could be important in synchronizing EMG bursts in muscles of both legs.     

Adaptive control for backward quadrupedal walking. I. Posture and hindlimb kinematics

1. To gain new perspectives on the neural control of different forms of quadruped locomotion, we studied adaptations in posture and hindlimb kinematics for backward (BWD) walking in normal cats. Data from four animals were obtained from high-speed (100 fr/s) ciné film of BWD treadmill walking over a range of slow walking speeds (0.3-0.6 m/s) and forward (FWD) treadmill walking at 0.6 m/s. 2. Postural adaptations during BWD walking included flexion of the lumbar spine, compared to a relatively straight spine during FWD walking. The usual paw-contact sequence for FWD walking [right hindlimb (RH), right forelimb (RF), left hindlimb (LH), left forelimb (LF)] was typically reversed for BWD walking (RH, LF, LH, RF). The hindlimbs alternated consistently with a phase difference averaging 0.5 for both forms of walking, but the phasing of the forelimbs was variable during BWD walking. 3. As BWD walking speed increased from 0.3 to 0.6 m/s, average hindlimb cycle period decreased 21%, stance-phase duration decreased 29%, and stride length increased 38%. Compared to FWD walking at 0.6 m/s, stride length was 30% shorter, whereas cycle period and stance-phase duration were 17% shorter for BWD walking. For both directions, stance occupied 64 +/- 4% (mean +/- SD) of the step cycle. 4. During swing for both forms of walking, the hip, knee, and ankle joints had flexion (F) and extension (E1) phases; however, the F-E1 reversals occurred earlier at the hip and later at the knee for BWD than for FWD walking. At the ankle joint, the ranges of motion during the F and E1 phases were similar for both directions. During BWD walking, however, the knee flexed more and extended less, whereas the hip flexed less and extended more. Thus horizontal displacement of the limb resulted primarily from hip extension and knee flexion during BWD swing, but hip flexion and knee extension during FWD swing. 5. At the knee and ankle joints, there were yield (E2) and extension (E3) phases during stance for both forms of walking; however, yields at the knee and ankle joints were reduced during BWD walking. At the hip, angular motion was unidirectional, as the hip flexed during BWD stance but extended during FWD stance. Knee extension was the prime contributor to horizontal displacement of the body during BWD stance, but hip extension was the prime contributor to horizontal displacement during FWD stance. 6. Our kinematic data revealed two discriminators between BWD and FWD walking.(ABSTRACT TRUNCATED AT 400 WORDS)     

Ageing effects on knee and ankle joint angles at key events and phases of the gait cycle

The objective of this research was to determine whether joint angles at critical gait events and during major energy generation/absorption phases of the gait cycle would reliably discriminate age-related degeneration during unobstructed walking. The gaits of 24 healthy adults (12 young and 12 elderly) were analysed using the PEAK Motus motion analysis system. The elderly participants showed significantly greater single (60.3% versus 62.3%, p < 0.01) and double ( p < 0.05) support times, reduced knee flexion (47.7 degrees versus 43.0 degrees , p < 0.05) and ankle plantarflexion (16.8 degrees compared to 3.3 degrees , p = 0.053) at toe off, reduced knee flexion during push-off and reduced ankle dorsiflexion (16.8 degrees compared to 22.0 degrees , p < 0.05) during the swing phase. The plantarflexing ankle joint motion during the stance to swing phase transition (A2) for the young group (31.3 degrees ) was about twice ( p < 0.05) that of the elderly (16.9 degrees ). Reduced knee extension range of motion suggests that the elderly favoured a flexed-knee gait to assist in weight acceptance. Reduced dorsiflexion by the elderly in the swing phase implies greater risk of toe contact with obstacles. Overall, the results suggest that joint angle measures at critical events/phases in the gait cycle provide a useful indication of age-related degeneration in the control of lower limb trajectories during unobstructed walking.     

Mechanics of slope walking in the cat: quantification of muscle load, length change, and ankle extensor EMG patterns

Unexpected changes in flexor-extensor muscle activation synergies during slope walking in the cat have been explained previously by 1) a reorganization of circuitry in the central pattern generator or 2) altered muscle and cutaneous afferent inputs to motoneurons that modulate their activity. The aim of this study was to quantify muscle length changes, muscle loads, and ground reaction forces during downslope, level, and upslope walking in the cat. These mechanical variables are related to feedback from muscle length and force, and paw pad cutaneous afferents, and differences in these variables between the slope walking conditions could provide additional insight into possible mechanisms of the muscle control. Kinematics, ground reaction forces, and EMG were recorded while cats walked on a walkway in three conditions: downslope (-26.6 deg), level (0 deg), and upslope (26.6 deg). The resultant joint moments were calculated using inverse dynamics analysis; length and velocity of major hindlimb muscle-tendon units (MTUs) were calculated using a geometric model and calculated joint angles. It was found that during stance in downslope walking, the MTU stretch of ankle and knee extensors and MTU peak stretch velocities of ankle extensors were significantly greater than those in level or upslope conditions, whereas forces applied to the paw pad and peaks of ankle and hip extensor moments were significantly smaller. The opposite was true for upslope walking. It was suggested that these differences between upslope and downslope walking might affect motion-dependent feedback, resulting in muscle activity changes recorded here or reported in the literature.     

A role for hip position in initiating the swing-to-stance transition in walking cats

In this investigation, we obtained data that support the hypothesis that afferent signals associated with hip flexion play a role in initiating the swing-to-stance transition of the hind legs in walking cats. Direct evidence came from observations in walking decerebrate cats. Assisting the flexion of the hip joint during swing advanced the onset of activity in ankle extensor muscles, and this advance was strongly correlated with a reduction in the duration of hip flexor muscle activity. The hip angle at the time of onset of the flexion to extension transition was similar during assisted and unassisted steps. Additional evidence for the hypothesis that sensory signals related to hip flexion are important in regulating the swing-to-stance transition came from four normal animals trained to walk in a variety of situations designed to alter the coordination of movements at the hip, knee, and ankle joints during the swing phase. Although there were exceptions in some tasks and preparations, the angle of the hip joint at the time of onset of extensor activity was generally less variable than that of the knee and ankle joints. We also found no clear relationships between the angle of the limb and body axes, or the length of the limb axis, and the time of onset of extensor activity. Finally, there were no indications that the stretching of ankle extensor muscles during swing was a factor in regulating the transition from swing-to-stance.     

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)     

Intralimb coordination of the paw-shake response: a novel mixed synergy

Intralimb coordination of the paw-shake response (PSR) was studied in five normal and eleven spinal adult cats. Representative extensor and flexor muscles that function at the hip, knee, and ankle joints were recorded, and in six spinal cats the kinematics of these joints were determined from high-speed cinefilm. The PSR was characterized uniquely by mixed (flexor-extensor) synergies. Knee extensor (VL) and ankle flexor (TA) coactivity constituted one synergy, while the second synergy included hip extensors (GM, BF), knee flexors (BF, LG), and ankle extensor (LG). Joint displacements reflected the mixed synergy. Motions at the knee and ankle were out of phase, while motions at the hip were in phase with movements of the knee. Electromyographic burst durations and onset latencies were similar for normal and spinal cats, and in all cycles of a given PSR, the recruitment pattern was consistent for all muscles, except VL. High variability and missing bursts marked the activity of VL in some spinal cats. In PSRs with missing VL bursts, oscillations at the knee joint were not coordinated with cyclic actions at the hip and ankle. From the kinematic records three distinct phases of the PSR were identified: start-up consisted of the initial four to six cycles during which hip, knee, and ankle actions progressively became organized; steady-state included the middle three to five cycles that were characterized by consistent displacement at all three joints; and slow-down comprised the last three to four cycles during which the rate of oscillations slowed, and joint excursions decreased. During steady-state cycles, muscle contractions acted to reverse joint motions at the knee and ankle joints. Thus, knee and ankle extensor recruitment coincided with joint flexion, while joint flexors were recruited during joint extension. Muscle activity at the hip, however, was in phase with displacement. While neural input to muscle is consistent throughout the three phases of the PSR, segment motions can become progressively organized during start-up to achieve stable oscillations. Whether the PSR attains steady-state or not may hinge on the sensitive interplay that occurs between muscle activities and intersegmental mechanical interactions. That kinetic interplay is detailed in the following paper.     

Ageing effects on knee and ankle joint angles at key events and phases of the gait cycle

The objective of this research was to determine whether joint angles at critical gait events and during major energy generation/absorption phases of the gait cycle would reliably discriminate age-related degeneration during unobstructed walking. The gaits of 24 healthy adults (12 young and 12 elderly) were analysed using the PEAK Motus motion analysis system. The elderly participants showed significantly greater single (60.3% versus 62.3%, p < 0.01) and double ( p < 0.05) support times, reduced knee flexion (47.7° versus 43.0°, p < 0.05) and ankle plantarflexion (16.8° compared to 3.3°, p = 0.053) at toe off, reduced knee flexion during push-off and reduced ankle dorsiflexion (16.8° compared to 22.0°, p < 0.05) during the swing phase. The plantarflexing ankle joint motion during the stance to swing phase transition (A₂) for the young group (31.3°) was about twice ( p < 0.05) that of the elderly (16.9°). Reduced knee extension range of motion suggests that the elderly favoured a flexed-knee gait to assist in weight acceptance. Reduced dorsiflexion by the elderly in the swing phase implies greater risk of toe contact with obstacles. Overall, the results suggest that joint angle measures at critical events/phases in the gait cycle provide a useful indication of age-related degeneration in the control of lower limb trajectories during unobstructed walking.     

Prolonged quadriceps activity following imposed hip extension: a neurophysiological mechanism for stiff-knee gait?

The biomechanical characteristics of stiff knee gait following neurological injury include decreased knee flexion velocity at toe-off, which may be due to exaggerated quadriceps activity. The neuromuscular mechanism underlying this abnormal activity is unclear, although hyperexcitable heteronymous reflexes may be a source of impaired coordination. The present study examines the contribution of reflex activity from hip flexors on knee extensors following stroke and its association with reduced swing-phase knee flexion during walking. Twelve individuals poststroke and six control subjects were positioned in supine on a Biodex dynamometer with the ankle and knee held in a static position. Isolated hip extension movements were imposed at 60, 90, and 120 degrees /s through a 50 degrees excursion to end-range hip extension. Reflexive responses of the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) were quantified during and after the imposed hip rotation. Gait analysis was also performed for all subjects in the stroke group. In subjects with stroke, imposed hip extension evoked a brief reflexive response in the quadriceps, followed by a heightened level of sustained activity. The initial response was velocity dependent and was larger in the stroke group than in the control group. In contrast, the prolonged response was not velocity dependent, was significantly greater in the VL and RF in subjects with stroke, and, importantly, was correlated to decreased swing-phase knee flexion. Hyperexcitable heteronymous connections from hip flexors to knee extensors appear to elicit prolonged quadriceps activity and may contribute to altered swing-phase knee kinematics following stroke.     

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)     

Motor patterns of a fast voluntary postural task in man: trunk extension in standing

The co-ordination between muscles controlling the primary movement and associated postural adjustments during fast trunk extension movements was studied in six male subjects. Myoelectrical activity (EMG) was recorded from antagonistic muscle pairs at the trunk, hip, knee and ankle. Horizontal displacements of the upper trunk, hip and knee were recorded with an opto-electronic system (Selspot). A backward displacement of the trunk was accompanied by a forward displacement of the hip and knee. In general, the trunk started to move 30 ms before the hip and knee. Muscle activity first appeared in the ankle extensors (soleus/gastrocnemius) up to 150 ms prior to onset of prime mover muscles (trunk extensor, erector spinae; and hip extensors/knee flexors, hamstrings). This pre-activation was seldom followed by any detectable ankle joint movement. Prime mover muscles were activated simultaneously followed by the hip extensor gluteus maximus. Time to activation of muscles braking the movement (rectus abdominis, rectus femoris and vastus lateralis) was correlated with the amplitude of the primary movement (r = 0.63 0.75, P less than 0.01). Onset of activity in vastus lateralis was highly correlated with the amplitude of the forward displacement of the knee (r = 0.93, P less than 0.01). An associated postural adjustment appeared as an ankle flexion accompanied by activity in the ankle flexor tibialis anterior, and often also in the ankle extensors. This co-activation of antagonistic ankle muscles can under certain conditions have interesting functional implications for the control of posture.     

Reflexes induced by nerve stimulation in walking cats with implanted cuff electrodes

Summary Neural cuffs, implanted around various hindlimb nerves (sural, common peroneal, posterior tibial), were used to deliver brief stimulus trains to unrestrained cats walking on a treadmill. The resulting perturbations of the step cycle were evaluated by analyzing the EMG bursts recorded from the ankle extensors and by high speed cinematography. It was found that relatively weak stimulation (1.4 to 2 X T) of the posterior tibial nerve was very effective in eliciting a prolongation of the flexion phase provided the stimuli were applied just prior to the expected onset of the ankle extensor EMG burst. This ipsilateral hyperflexion was correlated with a prolongation of the contralateral extension. The same stimuli applied during stance usually evoked a yielding of the stimulated leg and a prolongation of the ongoing contralateral stance. In addition to these flexor and extensor reflex effects, it was found that low threshold stimulation of the sural and common peroneal nerves resulted in a powerful reflex activation of the ankle extensors. In contrast, stimulation of the gastrocnemius-soleus nerve (a muscle nerve) produced no discernible behavioral effects, even for stimuli at 3 X T, indicating that the observed reflexes are probably mediated by cutaneous afferents. The results were largely confirmed in experiments using the same cuffs implanted in spontaneously walking premammillary cats.     

A knee and ankle flexing hybrid orthosis for paraplegic ambulation

Limitations of mechanical walking orthoses for paraplegics are high energy consumption and upper limb loading. Flexing of the knee during swing phase has been used as a means of attempting to reduce these. It has been found that this has little effect because using knee flexion results in no change in the compensatory mechanisms required for swing foot clearance. This is because knee flexion can result in an increase in effective leg length, i.e. hip to toe distance. A combination of knee flexion and ankle dorsiflexion during swing phase is suggested as a means of reducing compensatory mechanisms. To examine this hypothesis, an orthosis incorporating knee and ankle flexion was constructed. The design used a novel mechanism to link the motion of the knee to that of the ankle, and also used functional electrical stimulation. Two spinal cord-injured subjects were trained to use the orthosis in two configurations. The first configuration used knee flexion and ankle dorsiflexion and the second configuration used knee flexion alone. Kinematic data were obtained to measure the compensatory mechanisms used during gait. The results showed that a combination of knee flexion and ankle dorsiflexion during swing phase resulted in a reduction in compensatory mechanisms when compared with knee flexion alone.     

Improvement in step clearance via calf muscle stimulation

The aim is to study the influence of electrically stimulated calf muscles on the effectiveness of the swinging leg movement. The study is carried out with a group of patients with incomplete spinal cord injuries both under stationary conditions and during cruth-assisted walking. Before stimulation is applied to the ankle plantar flexors, the knee extensors are inactivated. In each cycle, after ankle plantar flexor stimulation, peroneal stimulation is started, triggering the flexion reflex. From a biomechanical point of view. functional electrical stimulation (FES) of the ankle plantar flexors results in increased ground clearance of the lower extremity. Additionally, the FES-assisted lifting of the heel results in the elimination of extensor tone and thus shortens the swing time.     

Proprioceptive modulation of hip flexor activity during the swing phase of locomotion in decerebrate cats

This study examined the influence of proprioceptive input from hip flexor muscles on the activity in hip flexors during the swing phase of walking in the decerebrate cat. One hindlimb was partially denervated to remove cutaneous input and afferent input from most other hindlimb muscles. Perturbations to hip movement were applied either by 1) manual resistance or assistance to swing or by 2) resistance to hip flexion using a device that blocked hip flexion but allowed leg extension. Electromyographic recordings were made from the iliopsoas (IP), sartorius, and medial gastrocnemius muscles. When the hip was manually assisted into flexion, there was a reduction in hip flexor burst activity. Conversely, when hip flexion was manually resisted or mechanically blocked during swing, the duration and amplitude of hip flexor activity was increased. We also found some specificity in the role of afferents from individual hip flexor muscles in the modulation of flexor burst activity. If the IP muscle was detached from its insertion, little change in the response to blocking flexion was observed. Specific activation of IP afferent fibers by stretching the muscle also did not greatly affect flexor activity. On the other hand, if conduction in the sartorius nerves was blocked, there was a diminished response to blocking hip flexion. The increase in duration of the flexor bursts still occurred, but this increase was consistently lower than that observed when the sartorius nerves were intact. From these results we propose that during swing, feedback from hip flexor muscle afferents, particularly those from the sartorius muscles, enhances flexor activity. In addition, if we delayed the onset of flexor activity in the contralateral hindlimb, blocking hip flexion often resulted in the prolongation of ipsilateral flexor activity for long periods of time, further revealing the reinforcing effects of flexor afferent feedback on flexor activity. This effect was not seen if conduction in the sartorius nerves was blocked. In conclusion, we have found that hip flexor activity during locomotion can be strongly modulated by modifying proprioceptive feedback from the hip flexor muscles.