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Variability in a dynamic postural task attests ample flexibility in balance control mechanisms
No full textWhen humans stand upright on a platform that sinusoidally translates in the
anterior-posterior direction, the movements of upper and lower body segments are
appropriately coordinated, in order to keep the body within its limits of
stability. A significant fluctuation in this behaviour is evident across subjects
and perturbation conditions. The inter- and intrasubject variability in the body
segment kinematics, as occurs during repeated trials across different conditions,
is quantitatively described here. Twenty normal subjects stood upright with eyes
open (EO) or eyes closed (EC) on a platform moving to-and-fro in the horizontal
plane for 30 s, at a frequency of 0.2 and 0.6 Hz, with a peak-to-peak amplitude
of 6 cm. Each subject made two trial repetitions for each visual and frequency
condition. The last 20 s of each trial was acquired. The displacement of markers
fixed on the lateral malleolus, hip and head was sampled at a frequency of 50 Hz.
An index of the 'average' displacement of each marker during the trial was the
standard deviation (SD) of its anterior-posterior displacements, calculated
across the acquired trial cycles. The cross-correlation (CC) between pairs of
marker displacement traces gave an indication of the degree of coupling of the
body segments. All subjects showed two basic modes of coping with the
perturbation, depending on the availability of the visual input: with EO, they
tended to stabilize the head in space; with EC, the head oscillated in the
anterior-posterior direction more than hip and platform. Within this general
behaviour, the values of the SD of horizontal displacement of head and hip marker
traces varied within an ample range during different trials of the same
perturbation condition. Even within a single trial there was an ample variability
of the body segments' position. In spite of this, neither head nor hip ever
bypassed the anterior or posterior limits of stability. Remarkably, the range of
variability of the whole population of normal subjects, both across and within
trials, was analogous to the range of variability of single subjects across
numerous trials. This large variability notwithstanding, it appeared that the
relationship of head to hip SD across trials was almost constant, independent of
visual and perturbation frequency condition. The results show that there exist a
large variety of dynamic postures, rather than one particular configuration,
which assure stability. The findings also suggest a neural or biomechanical
constraint underlying the operations of the equilibrium control strategy
Free and supported stance in Parkinson's disease. The effect of posture and 'postural set' on leg muscle responses to perturbation, and its relation to the severity of the disease
No full textUpright stance and its reflex control were studied in parkinsonian patients and
in age-matched normal subjects. They stood unperturbed on a force-measuring
surface (static conditions), or were displaced by movement of a supporting
platform (dynamic conditions). During quiet stance the following variables were
analysed, with eyes open or closed: position of the centre of foot pressure
(CFP), average sway area, length of sway path, amplitude and distribution of
tonic leg muscle EMG activity. Perturbations of stance were induced by toe-up or
toe-down rotations, and by backward or forward translations of the platform.
Amplitude of short, medium and long-latency EMG responses to displacement were
measured in the tibialis anterior (TA) and in the three heads of the triceps
surae (TS) muscle. The perturbations were produced during both free and supported
stance (holding onto a stable structure), under which condition normal subjects
suppress medium and long-latency responses. Under static conditions, the only
significant finding in parkinsonians was represented by a shift in the position
of the CFP. This was correlated with the severity of the disease (Webster scale),
the less affected patients being shifted backwards and the more affected patients
forwards, with respect to normals. Under dynamic conditions, the reflex responses
to perturbations of free stance were similar in both groups. Only the
medium-latency burst of gastrocnemius lateralis and the long-latency burst TA
evoked by TS stretch were larger in parkinsonians. The amplitude of these
responses, as well as of all the others, was not related to the Webster score.
Within the patients' group, a relationship between position of CFP and area of
EMG burst was found for both medium and long-latency TA responses evoked by
forward translation and toe-up rotation, respectively. Under supported
conditions, the capability to suppress all medium and long-latency muscle
responses to any perturbation was lost or impaired in the parkinsonians. The
degree of impairment was unrelated to the position of the CFP, but was
significantly related to the severity of the disease. The suppression to 40%
(supported/nonsupported), of TA response to toe-down rotation is proposed as the
point of separation between normals and parkinsonians. The forward projection of
the CFP, occurring in the severe stages of the disease, and the increase in
amplitude of some responses to perturbations of free stance might be a
compensatory adaptation to the anomalous upright posture
Fatigue effects on body balance
No full textBody sway variables (sway area and sway path) were recorded by a dynamometric
platform in 13 young subjects, standing quiet with feet together, with eyes open
(EO) and eyes closed (EC), prior to and following two types of physical exercise
(treadmill walking and cycle ergometer pedalling). Each exercise was performed
under both fatiguing (above anaerobic threshold) and non-fatiguing conditions.
Following fatiguing treadmill exercise, we observed a significant increase in
body sway with respect to pre-exercise values. The increase was present under
both visual conditions, affected both sway area and sway path and lasted until
about 15 min from the end of the exercise. The Romberg quotient (the ratio of
EC/EO of sway area, or sway path) significantly increased after the fatiguing
exercise with respect to the non-fatiguing exercise. The mean position of the
centre of foot pressure (CFP) was unchanged after the exercise. Fatigue induced
an increase in the median frequency of oscillation of the centre of foot
pressure, independent of the amplitude of sway. Non-fatiguing treadmill exercise
induced no significant changes in sway or in its frequency content. Following
fatiguing cycle ergometer exercise, a negligible increase or a decrease (under
eyes closed condition) in body sway were observed. Non-fatiguing cycling exercise
induced no significant changes or a decrease in sway. Control experiments showed
that simple repetition of successive stance trials (without intercalated
exercise) was able by itself to induce a decrease in sway. By taking this effect
into account, both types of cycling exercises revealed a mild capacity to
increase sway. We concluded that body sway increased after strenuous physical
exercise, but was little affected by exercise performed below the estimated
anaerobic threshold. The effects of fatigue on sway were short-lasting and of
moderate extent, and therefore were not liable to seriously threaten body
equilibrium
Shift of activity from slow to fast muscle during voluntary lengthening contractions of the triceps surae muscles in humans
No full text1. Raw or rectified and integrated electromyograms (integrated EMGs) of the leg muscles were recorded during (a) isotonic ramp shortening or lengthening contractions consisting of foot plantar flexions against a constant load, or dorsal flexions accomplished by braking the load and yielding to it, respectively, and (b) isometric increasing or decreasing plantar torques accomplished by graded contractions or relaxations of the triceps muscles. 2. During plantar flexions or increasing torques, the EMG of soleus, gastrocnemius lateralis, medialis, and peroneus increased in parallel. During decreasing torques, motor unit derecruitment took place gradually and simultaneously. The tibialis anterior was silent. During dorsal flexions, one of two characteristic patterns was observed in different subjects: (a) soleus was abruptly derecruited at the beginning of the task, while gastrocnemius lateralis (or medialis) exhibited a large recruitment lasting throughout the lengthening contraction; (b) soleus remained active during the task, showing large motor unit potentials, while the gastrocnemius lateralis recruitment was of a lesser extent than in (a). Peroneus derecruitment was gradual and tibialis anterior activity was absent in both cases. 3. The EMG patterns observed during plantar flexions or in increasing and decreasing torques, and the two patterns observed during shortening or lengthening contractions, were closely reproduced during sinusoidal oscillations of the foot or in isometric contractions and relaxations. 4. When recruitment of the gastrocnemius lateralis was present during dorsal flexion, the slope of its integrated EMG envelope was steeper, the higher the velocity of lengthening contraction. The most rapid and the slowest tasks, however, did not require its activation. Gastrocnemius lateralis integrated EMGs of an amplitude similar to those occurring during lengthening contractions were observed only during ballistic plantar flexions. 5. The two patterns of triceps activation occurring during lengthening contraction could be traced to different mechanical characteristics of the soleus muscles, the gastrocnemius lateralis being activated preferentially in subjects with long soleus half-relaxation times, and the soleus in subjects with short soleus half-relaxation times. 6. The soleus and gastrocnemius lateralis H reflexes were tested during shortening and lengthening contractions
Effects of stimulus intensity, cervical cord tractotomies and cerebellectomy on somatosensory evoked potentials from skin and muscle afferents of cat hindlimb.
No full textThe somatosensory evoked potentials (SEPs) recorded from the sensory cortex were investigated by using graded stimulation of skin and muscle nerves from contralateral hind limb in the cat. Sections were made of the middle cervical cord to assess the pathways involved in mediating SEPs evoked by large and small diameter fibers. Dorsal column (DC) section caused a decrease of SEPs from skin group I afferents, and a small increase in those from group I muscle afferents. A subsequent section of dorso-lateral fasciculus (DLF) further decreased SEPs from skin and eliminated SEPs from muscle, evoked at low stimulus intensity. When the stimulus recruited group III fibres, SEPs were still present after DC and DLF section, both from skin and muscle nerves. Section of ALT in addition to DC confirmed a major role played by DLF (mainly spino-cervical tract of Morin) in transmitting impulses from muscle afferents; the role of DLF in mediating potentials evoked from skin is less remarkable than that of DC. Cerebellectomy did not change any SEP, however evoked. Previous results in the literature are discussed, taking into account the methodologies employed by various authors, and the possible interactions among pathways mediating SEPs
Spinal pathways mediating SEPs from cutaneous and muscle nerves in the cat.
No full texthe Authors give evidence on the function of a pathway mediating somatosensory evoked potentials (SEPs) from muscle nerves, other than the dorsal columns: physiological and anatomical data prove its location to be in the spinothalamic tract. Previous contrasting results on the topic are discussed
Short latency cortical potentials evoked by air-jet tactile stimulation of body and face in man.
No full textWhy rare diseases?
No full textPatients with rare diseases are awaiting an answer to their needs. Traditionally, however, research on rare diseases has been limited by the idea that it was too difficult to do and too little rewarding in terms of return of profit. This attitude has actually changed during the last decade, because it was realized that research on rare diseases may help finding solutions valid also for common conditions. Indeed, while we all invoke translational research as the way to adapt results of laboratory studies into therapeutic interventions for patients, rare diseases often need the opposite path: we observe rare patients in the clinical practice, then we find out that they have a genetic defect, and finally we reproduce the defect in an animal model to extend the observation further beyond the clinic. In the process we also learn a lot about the physiology and the pathology and have insight into the mechanisms of common diseases. In other words, studying a rare condition may enlighten the path to other discoveries and to break the boundaries between disciplines and specialities to provide solutions for the sake of the patients
Medium-latency stretch reflexes of foot and leg muscles analysed by cooling the lower limb in standing humans
No full text1. In standing subjects, an ankle-dorsiflexing perturbation of the supporting
surface evokes a short-latency response (SLR) and a medium-latency response (MLR)
to stretch in both soleus (Sol) and flexor digitorum brevis (FDB) muscles. The
SLR is the counterpart of the monosynaptic reflex, whilst the MLR might be either
mediated by Ia fibres, the delay being due to a long-loop central circuit, or by
fibres of slower conduction velocity. Since small afferents are slowed more than
large ones by low temperature, a greater latency increment for the MLR than the
SLR induced by cooling of the limb would point to a peripheral origin of the MLR.
2. In nine subjects, one limb was cooled by circulating water in a tube wrapped
around it for about 120 min. Perturbations were delivered to the same limb prior
to and during cooling, and after rewarming. EMG was recorded by surface
electrodes from the Sol and FDB muscles. 3. The mean increase in latency of MLRs
was significantly greater than that of SLRs in both muscles. On average, the Sol
SLR increased from 42.4 to 47.0 ms and the Sol MLR from 72.0 to 82.3 ms. The FDB
SLR increased from 58.1 to 66.5 ms and the FDB MLR from 94.9 to 110.5 ms. The
mean difference (MLR minus SLR) increased from 29.6 to 35.2 ms for Sol, and from
36.8 to 43.9 ms for FDB at the end of cooling. After 30 min of rewarming, the
responses of both muscles recovered towards control values. 4. The greater
latency increment of the MLRs than of the SLRs favours the hypothesis of a slower
conduction velocity of the responsible afferent fibres. The most likely candidate
fibres are the spindle group II afferents
Reflex contribution of spindle group Ia and II afferent input to leg muscle spasticity as revealed by tendon vibration in hemiparesis
No full textOBJECTIVE: Foot dorsiflexion evokes a short- (SLR) and a medium-latency EMG
response (MLR) in the soleus of standing subjects. SLR is mediated by spindle
group Ia, while group II fibres contribute to MLR through an oligosynaptic
circuit. We studied the effects of Achilles' tendon vibration on both responses
in spastic patients to disclose any abnormal excitability of these pathways.
METHODS: SLR and MLR were evoked in 11 hemiparetics and 11 normals. The
vibration-induced changes in both responses were correlated to the Ashworth score
of the affected leg.
RESULTS: There were no differences between normals and patients in the size of
control SLR or MLR. Vibration decreased SLR to 70% in normal subjects, but
increased it to 110% in patients, in both affected and unaffected leg. Vibration
did not affect MLR in normals, but increased it to 165% on the affected and 120%
on the unaffected side of patients. Ashworth score was solely correlated with the
degree of vibration-induced increase of MLR.
CONCLUSIONS: While the lack of inhibitory effect of vibration on SLR confirms a
reduced inhibitibility of the monosynaptic reflex, the increased MLR indicates a
disinhibition of group II pathway in patients, connected to the loss of
descending control on group II interneurones. Spastic hypertonia depends on
release of group II rather than group Ia reflex pathways.
SIGNIFICANCE: These findings give a neurophysiological support for the
pharmacological treatment of spastic hypertonia and suggest a method for the
assessment of its effects
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