The controversy over the influence of static tilt on soleus motoneuron excitability in man

Tendon reflexes for predicting movement recovery after acute spinal cord injury in humans

OBJECTIVE

Use the tendon reflex to examine spinal cord excitability after acute spinal cord injury (SCI), relating excitability findings to prognosis.

METHODS

We conducted repeated measures of reflex responses to mechanical taps at the patellar and Achilles tendons of the lower limbs, and the wrist flexor tendons of the upper limbs in persons with acute SCI, beginning as early as the day of injury. The single largest EMG response (peak-to-peak) for each site was recorded. Subjects were compared based on level of injury and final neurologic status of lower limb motor function (i.e. absence of any voluntary recruitment in a lower limb muscle: motor-complete; voluntary recruitment in 1 or more lower-limb muscles: motor-incomplete).

RESULTS

We studied 229 subjects with acute SCI. Persons with injury to the cervical or thoracic spinal cord and who were (or became) motor-incomplete showed large tendon responses, even at the time of initial evaluation. In combination with larger tendon response amplitudes, the presence of the 'crossed-adductor' response to patellar tendon taps at the acute stage was highly predictive of functional motor recovery following SCI. In marked contrast, tendon responses were small (e.g. < 0.1 mV) or absent in persons with acute, motor-complete injury (and which remained motor-complete), and the crossed-adductor response was never seen. Reflex amplitudes and the incidence of the crossed-adductor response increased somewhat over time in persons with motor-complete SCI, but did not approach the values seen in motor-incomplete subjects.

CONCLUSIONS

Taken together, tendon response amplitude and reflex spread were sensitive and specific indicators of preserved supraspinal control over lower limb musculature in subjects with acute SCI. A simple algorithm using these outcome measures predicted a 'motor-complete' status with 100% accuracy, and a motor-incomplete status with accuracy exceeding 91%.

Static and dynamic changes in body orientation modulate spinal reflex excitability in humans

In the present study, we investigated the modulation pattern of the soleus H reflex in healthy subjects in response to imposed static and dynamic changes in body angle, referenced to the vertical plane. Soleus H reflexes were recorded using conventional methods with subjects either supine or while they were erect. Changes in body angle were initiated with subjects lying supine on a tilt table. Table position was controlled via a motor and could move from the horizontal to the upright position and beyond. Elastic bands around the trunk (upper and lower part) and around the thigh and shank secured subjects' position. In the vertical position, the soleus H reflex exhibited a strong depression in all subjects tested, reaching amplitudes as low as 40+/-8.1% of the control reflex (Ho). With subjects supine, positioning the body at 10 degrees, 20 degrees, 40 degrees, 60 degrees, 90 degrees, -50 degrees and -20 degrees all resulted in a significant facilitation of the soleus H reflex. The reflex magnitude at these angles ranged from 140+/-17.2% to 180+/-10.9% of the Ho. Reflex facilitation was also observed following dynamic tilt of the body in the sagittal plane (at 1.8 degrees /s) with the H reflex reaching amplitudes as high as 300+/-18.3% of Ho. Our findings indicate that changes in body orientation induced a significant facilitation of the H reflex magnitude in soleus motoneurones that were essentially independent of angular change in body orientation or of movement direction. In addition, they highlight the potent modulatory effects that natural stimulation of the vestibular system can have on reflex excitability. The implications of our findings are discussed in relation to the maintenance of body posture.

Comparison of tibial nerve H-reflex excitability after cervical and lumbar spine manipulation

BACKGROUND

Previous investigations indicate that spinal manipulation leads to short-term attenuation of alpha-motoneuron excitability, when assessed by means of the Hoffmann reflex. Past studies, however, are limited to regional effects, such as lumbar manipulation effects on lumbar alpha-motoneuron activity.

OBJECTIVE

This study compared and contrasted the effects of cervical and lumbar spine manipulation on the excitability of the lumbar alpha-motoneuronal pool in human subjects without low back pain, and compared the effects of cervical (nonregional) and lumbar (regional) spinal manipulation on lumbar alpha-motoneuron pool excitability in healthy subjects. The specific aim of this study was to determine if the inhibitory effects on the lumbar alpha-motoneuron pool associated with spinal manipulation are limited to the specific region in which the manipulative procedure is applied, or if rostral (cervical) manipulation can also influence caudal (lumbar) motoneuron excitability.

METHOD

Thirty-six nonpatient human subjects were used to study the effect of cervical and lumbar spinal manipulation on the amplitude of the tibial nerve Hoffmann reflex, recorded from the gastrocnemius muscle. The Hoffmann reflex (H-reflex) technique allows for an indirect index of motoneuron pool excitability by means of peripheral nerve Ia-afferent fiber stimulation. Reflexes were recorded before and after spinal manipulative procedures.

RESULTS

Lumbar spinal manipulation, as measured by amplitude changes of the tibial nerve H-reflex, attenuated lumbar alpha-motoneuronal activity. Suppression of motoneuronal excitability was significant (P <.05) but transient, with a return to baseline within 60 seconds after manipulation. Cervical spinal manipulation had no significant effect on lumbar motoneuron activity.

CONCLUSION

These data indicate that the inhibitory effects of spinal manipulation on motoneuronal excitability are regional, rather than global.

Posture-related changes of soleus H-reflex excitability

We investigated whether the modulatory effects of segmental and descending inputs on the soleus H reflex are modified by postural conditions. Fourteen healthy volunteers received a transcranial magnetic stimulus (TMS) or percutaneous electrical stimulation of the posterior tibial nerve (PTN), preceding by 0 to 400 ms the elicitation of the soleus H reflex in supine, sitting, and standing positions. In all positions, TMS induced an early period of facilitation at interstimulus intervals (ISIs) ranging between 5 and 35 ms. In supine and sitting positions, there was a second period of facilitation at ISIs between 60 and 90 ms, which was absent or significantly reduced in the standing position. PTN induced a strong inhibition of the H reflex in all positions up to 125 ms. In supine and sitting positions, inhibition continued up to 400 ms, whereas it was significantly reduced or completely absent beyond 125 ms in the standing position. These results demonstrate posture-related differences in the modulatory effects of descending and segmental inputs on the excitability of the H-reflex circuit.

Postural modulation of the segmental reflex: effect of body tilt and postural sway

This purpose of this study was to clarify the relationship between segmental reflex excitability and posture and to investigate potential mechanisms responsible for modulation of the H-reflex (HR) in unsupported standing. Soleus (S) and lateral gastrocnemius (LG) HRs were recorded from subjects (N=12S, N=11LG) while their static posture was altered from supine to vertical (5 positions). This was compared to an unsupported standing position in which the subjects naturally underwent a small degree of postural sway, a dynamic posture condition. Although individual profiles suggested varied relationships between the S and LGHR and the angle of body tilt, the group data did not reveal significant differences. There was, however, a significant (p < .01) decrease in the S (43% 49%) and LG (34%-46%) HR when subjects stood without support compared to all static postures. This decrease occurred even though the tonic or background activity of the S and LG was present only when subjects were free standing. To determine whether weight-bearing was responsible for the HR depression, 3 additional conditions were compared (N=3), supported standing without weight-bearing (90 degrees NWB), supported standing with weight-bearing (90 degrees WB), and free standing. Again, S and LGHRs were depressed only when subjects were free standing. Presynaptic inhibition presumably accounts for the depression of the HR in unsupported standing. Data from 8 of the subjects were collected under the same 6 conditions using a shorter interstimulus interval (1 Hz stimulus instead of 0.1 Hz) which produced low frequency depression (LFD) of the S and LG HR. LFD reduced the amplitude of the S HR an average of 43% (p < .05) when subjects were in a supported static position but only 21% when subjects were free standing. Although tonic activity of the S was present only when subjects were free standing, in 2 (of 8) individuals the EMG in free standing was not measurably different from static conditions. In these individuals, free standing still depressed the SHR by 35%; however, the shorter stimulus interval now produced the same degree of LFD when subjects were free standing (35%) as when they were standing with support (37%). The data suggest that 2 presynaptic mechanisms, although independent, interact to control spindle afferent feedback when subjects are free standing. Postural sway appears to be necessary to reduce the gain of the HR when subjects are standing, whereas, LFD is influenced by the degree of muscle activation.

Movement-induced modulation of soleus H reflexes with altered length of biarticular muscles

Passive pedaling movements of the leg results in the phasic modulation of the soleus H reflex of that leg. In contrast, the H reflex of the contralateral leg is attenuated tonically. The phasic modulation of the reflex ipsilaterally can be attributed to the afferent discharge associated with the cyclic lengthening of the extensor muscles. We hypothesized that the tonic attenuation of the contralateral reflex could be explained if the afferent feedback arising from the lengthening of the biarticular muscles had an increased importance in regulating the amplitude of the contralateral reflex. To test this, the passive pedaling movements were reduced to those about either the knee or hip alone. Despite the alteration in the pattern of stretching of the biarticular muscles, the contralateral soleus H reflex was tonically attenuated during both forms of single joint movements. We suggest that the same phasic afferent discharge responsible for the modulation of the ipsilateral soleus H reflex initiates the tonic attenuation contralaterally, but that the signal undergoes a complex transformation in crossing the cord. These results do not rule out the possibility that the stretching of the biarticular muscles contributes to the attenuation of the ipsilateral soleus H reflex, which is subsequently masked by a powerful influence from the stretching of the uniarticular extensor muscles. To test this possibility, a second experiment manipulated the lengths of the muscles of the leg by altering the positions of the static joints during isolated rotation of either the knee or hip and measuring the amplitude of the ipsilateral soleus H reflex. From the results, it was clear that stretching the uniarticular extensor muscles produced the most dramatic effects. However, the stretch of the biarticular muscles yielded mild inhibitory influences if these muscles were near their maximal lengths.

Evidence for soleus H-reflex depression induced by ballistic head rotation

In this study, we described a sequence of changes of the electromyographic (EMG) and depression of the soleus (Sol) H-reflex that precede and follow the onset of ballistic head rotation in freely standing human subjects. During head rotation, a sequence of EMG changes occurred simultaneously in Sol and in biceps femoris muscle (BF). Specifically, changes to Sol EMG included deactivations (EMG silent period) and H-reflex depression coincident with BF excitation. Thus, in the case of ballistic head movement, control commands for anticipatory postural adjustment appear to excite BF and inhibit Sol in a similar way to that previously described for the onset of arm movement.

The relationship between the kinematics of passive movement, the stretch of extensor muscles of the leg and the change induced in the gain of the soleus H reflex in humans

The gain of the H reflex attenuates during passive stepping and pedalling movements of the leg. We hypothesized that the kinematics of the movement indirectly reflect the receptor origin of this attenuation. In the first experiment, H reflexes were evoked in soleus at 26 points in the cycle of slow, passive pedalling movement of the leg and at 13 points with the leg static (the ankle was always immobilized). Maximum inhibition occurred as the leg moved through its most flexed position (P < 0.05). Inhibition observed in the static leg was also strongest at this position (P < 0.05). The increase in inhibition was gradual during flexion movement, with rapid reversal of this increase during extension. In the second experiment, the length of stretch of the vasti muscles was modelled. Variable pedal crank lengths and revolutions per minute (rpm) altered leg joint displacements and angular velocities. Equivalent rates of stretch of the vasti, achieved through different combinations of joint displacements and velocities, elicited equivalent attenuations of mean reflex magnitudes in the flexed leg. Reflex gain exponentially related to rate of stretch (R2 = 0.98 P < 0.01). The results imply that gain attenuation of this spinal sensorimotor path arises from spindle discharge in heteronymous extensor muscles of knee and/or hip, concomitant with movement.

Reconsidering the 90 degrees head-rotation paradigm used in neuropsychological research: are there reflexive rather than hemispatial effects?

The 90 degrees head-rotation paradigm has often been used in neuropsychological studies to manipulate external hemispace (circumcorporeal space) relative to the head. Under the 90 degrees head-rotation paradigm, the performance of limb and hand movements carried out within the left or right hemispace as defined by head positions relative to the body is likely to be affected by the reflexive effect due to the neck and vestibular afferent inputs elicited by the head rotations, as well as by the hemispatial effect. Using the H-reflex technique, the present study examined whether the reflexive effect on the spinal motoneuron excitability occurred with head rotations under the 90 degrees head-rotation paradigm. The results showed that the amplitudes of H-reflexes evoked on both the thumb flexor and soleus muscles were not affected by head rotations, indicating no reflexive change in the spinal motoneuron excitability for both the thumb and soleus muscles. This finding suggests that the reflexive effect due to neck and vestibular afferent inputs can be ruled out from possible causal factors influencing the motor performance of limb and hand movements performed within the left or right hemispace as manipulated by the 90 degrees head-rotation paradigm.

Influence of posture and voluntary background contraction upon compound muscle action potentials from anterior tibial and soleus muscle following transcranial magnetic stimulation

Compound muscle action potentials (CMAPs) were recorded from anterior tibial (TA) and soleus (SOL) muscles following transcranial magnetic stimulation of motor cortex in 10 healthy subjects: (1) while standing upright without support, (2) while sitting, and (3) while lying supine. The results of this study demonstrate a significant influence of posture upon the amplitudes of CMAPs. Postural facilitation presented itself, firstly, in terms of a higher incidence of bilateral activation of distal leg muscles during stance and, secondly, as significant enhancement of the amplitude of CMAPs while standing as compared to lying supine. The onset latency, however, did not disclose a significant shortening during stance. To assess the effects of preinnervation subjects voluntarily adjusted the level of TA activity to 5%, 10% and 20% of maximum isometric force respectively before cortex stimulation. Voluntary background contraction resulted in a significant increase of amplitude of CMAPs but, in contrast to postural facilitation, concomitant with a significant decrease in onset latency. These results point to a somewhat different mechanism of facilitation during stance as compared to voluntary preinnervation. But it cannot be decided whether cortical mechanisms, different descending systems, the spinal circuitry or a combination of these factors is responsible for the observed effects.

Orientation-induced artifacts in the measurement of monosynaptic reflexes

Hoffmann and tendon jerk reflexes were tested in human subjects at various orientations between vertical, prone and supine. The data suggest that artifacts due to movement of soft tissue may explain the current disagreement in the literature on otolithic influences on these reflexes.