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IBENEME, Sam C.

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Title

The three Neural Speed and Associated Postural Control Strategies Adopted in Human Walking as Revealed by the

Velocity Field Diagram

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The three Neural Speed and Associated Postural Control Strategies Adopted in Human Walking as Revealed by the

Velocity Field Diagram

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Health Sciences and Technology

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The three Neural Speed and associated Postural Control Strategies Adopted in Human Walking as Revealed by the

Velocity Field Diagram

Ibeneme Sam C, BMRPT, MSc (Nig.), Phd, Misn, PT. Lecturer

Dept of medical rehabilitation, faculty of Health Sciences and Technology, College of Medicine, University of

Nigeria, Enugu Campus. E-mail address: samibeneme@,,vahoo.com

I Abstract

Purpose: This study is to determine the postural control strategies adopted across the whole spectrum of human walking using the velocity field diagram (VFD). Background: In different environment contexts, the neuromuscular system is expected to employ several relevant, sensitive and efficient postural control strategies in other to adapt. Adopting different patterns of limb co-ordination, while walking, may reveal these strategies through their operational application. Method: Two gait conditions, namely, normal walking (reciprocal limb movements) and pace (ipsilateral limb movements) walking (PW) were examined. Twenty subjects (10 males and 10 females) demonstrated their gait twice for each speed along a 10 metre walkway at five speeds varying from very slow to very fast. The mean of time and steps taken to cover the distance was recorded. The regression of velocity (V), stride length (L), stride frequency (F), double support (DS), swing (SW) and stance (ST) durations were adopted to form a VFD. The energy profile was determined and plotted in relation to speed. Result: Despite the five speeds of walking only three values were recorded for DS duration (an indicator of postural control) which decreased as speed was increased during pace walking. In addition, only three velocity zones were highlighted by the VFD and corresponded to the three values observed for DS duration. Conclusion: The three velocity zones of the VFD and corresponding DS values suggest the possibility that the CNS appreciates only three speeds and three postural control strategies across the whole spectrum of walking.

4

Keywords: Posture, walking, velocity field diagram.

Gait is the pattern of human walking, and represents the general translatory

motion of the body, emanating from the angular displacement of the comprising

parts or segments. Thus, human walking pattern is a product of a very finely

interwoven action of the neuromuscular, musculoskeletal and other subsystems

(Kamm, et al, 1 989), including the environmental factors; together they form

and modify the motion apparatus.

In humans, bipedalism is the pattern of locomotion adopted from infancy

(Stolov, 1980) as a functional adaptation for fblfilling obvious biological roles

and necessities. Since humans are bipeds with about two-third of the body

weigh from the ground (Winter et al, 1997), and the supporting area are

relatively small compared to quadrupeds (Kummer, 1962), balance and posture

become relevant to locomotion. In man, maintaining the centre of gravity stable

I represent the crucial variable for maintaining the posture in upright stance

(Diezt, 1993). . ,, . . wl. \?. , '

To achieve this goal, the neuromuscular system usually adopts many

context driven, sensitive and efficient strategies to ensure functional adaptation

to task demands. Previous researchers (Nashner, et al, 1979; Gahery, 1987) had

identified several patterns of postural control using electromyographic studies.

Postural control system is important, because it entrances an individual's

capacity to adjust the locomotor patterns effectively by applying several

feedback strategies to elicit automatic adjustments necessary to avoid obstacles,

and precipitate changes in the pattern of gait necessary to transverse any given

topography (Bate, 1997). This suggests that during walking, posture and

stability must require the integration of the sensory, mechanical and motor

processing strategies that support the upright stance. Meanwhile, posture had

been defined as the rotational and translational position of adjoining body

segments and their orientation relative to gravity (Dannis et al, 1996), while

dynamic stability refers to the capacity to control the amplitude and velocity of

displacement of the body's center of gravity while walking. Control in this

context relates to a process of scaling the neuromuscular components up or

down on some variables such position, stiffness, force or speed (Frank and Earl,

1990). In essence, gait is a neurally driven process, and the stability of its

patterns and control is expected to be neurally programmed.

This suggests that changes in the values of the parameters which

characterize the gait process such as velocity which define progression

(Ibeneme, 2002), and double support duration which is a measure of postural

control (Gabel and Nayak, 1984), are also neurally determined. This view is ,, . , * ( .I' .'i

supported, by the results of a previous study (Eke-Okoro, 1999).

However, the efficiency of movement with each pattern change is related

to the metabolic cost of locomotion at that instance. Hence, movement I '

efficiency is a function of the energy expenditure encountered in the gait

process (Frank and Earl, 1990), and is enhanced when the energy cost of

locomotion is at a minimum and vice versa (Ibeneme, 2002). Thus, the energy

needs, goals and the mechanics of human gait should define the postural control

strategies adopted with changing patterns o r movement. This possibility was

examined in this study; and hence an attempt was made to identity the postural

control strategies adopted in human walking.

To achieve this, the whole spectrum of human gait, from very slow to

very fast, was explored to explain the events that occur during walking. In

fulfilling this objective, a velocity field diagram (VFD) was described (Eke-

okoro, 1989; Eke-okoro, 1999). The VFD is a regressional interaction of basal

gait parameters namely, stride length (L) stride frequency (F) and velocity (V),

in response to the neural drive. Two gait conditions which required different

locomotors patterns were examined in healthy adults, namely ordinary walking

(reciprocal limb movements) and pace (ipsilateral limb movements) walking

(P.W). It is believed that adopting different patterns of limb co-orientation while

walking may reveal the underlying postural control strategies adopted through

their operational application.Also, the energy profiles of these subjects were

described and considered in relation to the VFD, to further appreciate the

< relevance of movement efficiency to postural control.

Material and Method

Subjects Selection

Twenty four (20) subjects (10 males and10 females) who gave their written

informed consent participated in the study; after an ethical approval had been

obtained from the hospital medical advisory committee. They had no history of

orthopaedic or neurological disorders confirmed by the orthopaedic

examinations and radiological reports. Their mean age was 24.9 k 1.60 years,

whereas their mean height was 1.79 A 0.08 m; with a mean weight of about

59.10 A 9.90 Kg. Their mean leg length measured ikon1 the line of the knee

joint to the lower border of the lateral malleolus was 0.42* 0.03m

Quantitative Gait Assessment

. The subjects walked along a 10 metre distance measured out on the quiet

corridors of Faculty of Health Sciences and Technology building, within the

University of Nigeria, Enqp~Gampus; after the purpose of the study was

explained to them.

Each subject walked the distance at five different speeds: ordinary, very slow,

slow, fast and very fast, in that order."The subject walked twice the distance, for

each speed, during which the steps were counted and time taken to complete the

distance obtained (Using a Stop-Watch-Hanbart Germany); and the mean

recorded. These speeds were completed under two test conditions to which the

limbs swing patterns were subjected namely;

(b) Normal Walking (control) :The subjects walked swinging the arms in

opposite direction to the lower limbs (alternately), as employed in

normal walking pattern. This was used as the control (C) experiment.

Velocity Field Diagram (VFD).

The means of these values were used to calculate the mean values of stride

length (L), stride frequency (F) and velocity (V) for each subject. Their

regression lines were known as L-line, F-line and V-line respectively; which

were then used to describe the VFD. The speeds varying from very slow to very

fast, were serially numbered 1-5, in the VFD. The numbers were used for the X-

axis, while the numerical values of velocity, stride length and stride frequency

were used on the Y-axis. These lines make up the primary features of the VFD.

With the regressions it was possible to calculate the stride phase duration for I

each subject at each velotype. These values were plotted into the VFD at the

corresponding velotype.,,Tbe phases of stride studied were, stance (ST), Swing

(SW) and double-support (DS)The equality point of the numerical values of

velocity and stride frequency (El) marked the upper limit of very slow speed

and a speed transition to the path &minimal energy trajectory (Ibeneme, 2002).

Results

The results (table 1 & 2) revealed a decrease in maximum gait output at

Velotype 5, with a significant decrease (p<0.05) in velocity, despite significant

increases (P<0.05) in stride frequency during pace walking. Also, during normal

i

walking the stride length was greater at velotypes 4 and 5 compared to what

obtains during pace walking and vice verse at velotypes 1-3. Nevertheless, these

differences were not statistically significant (p>0.05). Meanwhile, despite the

five speeds of walking adopted by the subject during pace walking, only three

different values recorded for double-support duration (table 2). In contrast,

during normal walking a progressive decline was obtained in the DS duration in

five different values, corresponding to each velotype. In addition, the double-

support durations were greater, (though not significant p>0.05) during pace

walking, except at velotype 1 : where both conditions recorded equal values.

The VFD (figs 1 & 2) revealed three functional (lower, intermediate and

upper) speed zones geometrically demarcated by lines NIPl and E2N2 which

were precipitated through the equality points of the regression lines of velocity

and stride frequency (El); and velocity and stride length (E2) respectively.

Corroborating all the results (table 1-3, figs 1-3), it is revealed that during

normal walking, and within the lower zone of the VFD, the double-support

duration recorded its gree8test"valbe likewise energy expenditure. I-Iowever, in

the intermediate zone (beyond Elj, the energy expenditure dropped to a

minimum and the double-support duration recorded moderate values. The I /

double-support duration, however, tends to zero in the upper zone, while energy

expenditure increased. In contrast, during pace walking, though the same trend

was observed, the minimum energy expenditure occurred in the upper zone

where double-support duration attained a critical value beyond which it could

no longer be decreased.

The energy profile described in relation to speed (fig 3) revealed that the

energy expenditure is lesser (though not significant p>0.05) during normal

walking, except at velotype 5. Moreover, though both curves have similar

characteristics, normal walking presents a slightly steeper curve.

Analysis of the phases of stride in pace walking showed that within the

upper zone, the double-support duration was constant while the swing duration

decreased, and the stance period progressively increased. Thus, the regression

lines of swing and stance phases never met to define the equality point El. In

contrast, during normal walking, the double-support phase tend to zero while

the swing phase increased to equalize the stance phase ; and hrther projected

above it to define the equality point E3. Meanwhile, increased differences

between the values of stance and double-support duration at the lower and upper

zones were observed. These differences though not significant (p>0.05), were

greater during pace walking.

Discussion . ,* . *<. ... .,>

The results suggest that with gait alterations maximum speed decreases

due to a decrease also in stride length though stride frequency is increased in a

compensatory response. However, this increase could not compensate for the

decreased stride length otherwise the values of velocity in both conditions will

have approximated each other. Thus, the equation V=SLxSF experiences a

physiological limitation probably due to a restrain imposed on the optimum

functioning of the central pattern generators (Eke-Okoro, 1985) due to gait

alteration. The stride length was greater in normal walking at velotypes 4-5

compared to pace walking, suggesting a greater stability due to increased base

area of support which is necessary to optimize stability and thus facilitate speed

transitions.

However, the trend was reversed at velotypes 1-3, because the direction of

' / momentum of the arms is the same as that of the ipsitateral leg in pace walking.

As such, the summation of the momentum of the arm and leg that impacted on

trunk translates to an exaggerated pelvic rotation which effectively lowers the

centre of mass (CM) resulting in increased stride length apart from the expected

i postural compensation. increased gravitational work (which translates to greater

energy expenditure) and increased stride length (base area of support) which is a

tendency to stability. It appears, however that this trend was reversed at

velotypes 4-5, because of an increase in stride frequency in a attempt to increase

the velocity of movement to equal that of normal walking. Nevertheless, despite

the five speeds of walking the VFD had revealed three speeds, (Eke-Okoro, <

1989, Eke-Okoro, 1999' Ibeneme 2002) which had been defined in its context as

low, intermediate (ordinary) 1 ., ,and high speeds. Interestingly, the three sets of

values recorded for double support duration were obtained at these three

corresponding neural speed zones of the VFD. This pattern became evident

during pace walking, because the system had been further challenged to make

adaptation to handle the miscorrelation in the pattern of the limb trajectory

unlike in normal walking.

These three values of the double-support duration within the three neural

speed zones could suggest that the neurones of the CNS probably discharge in

three patterns for the determination of speed and postural control. This could

translate to three neuromuscular configurations or postural control strategies to

maintain stability in the whole spectrum of human walking. It further highlights

the possibility of a connection between the neurones of speed and postural

control in the CNS.

That these patterns were not easily reflected on the VFD (fig 1) during

normal walking does not indicate the absence of such neuronal activities. Kather

it could suggest that these neuronal activities are intensified with gait

alterations, especially when the stability of the upright stance is threatened.

However, the fact that this pattern of balance control correlates with the

three neural speed zones could indicate the possibility that both processes are

not only neurally driven, but reinforces the earlier speculation that there could

be a connection between the neurones of speed and balance control. Also, the

decrease in double-support duration with increase in speed could suggest the

existence of a negative feedback mechanism between the neurones of speed and

1 balance control in the CNS. This mechanism is thus expected to be evoked

when stability of the upright posture is threatened with increase in speed, and . ,, 4 - 7 . .?' , . I >

vice versa, depending on the perceived need of the system. This was in fact

obtained (table 1-2) and explains why during pace walking the double-support

duration could not decrease toyards.'zero with increase in speed, but remained

constant at a critical point between velotypes 4-5. It further explains why the

equality point Ej (whereby the process of walking stops and running begins) is

not reflected in the VFD (fig 2). Thus, the increase in double-support duration

during pace walking could suggest an increased tendency to greater stability

probably to compensate for instability in the system. The increased difference

between stance and double-support durations could suggest an increase in single

-support duration, since the stance duration is the sum of double-support and

single-support durations. When considered in relation to the upright stance, the

increased single-support phase translates to reduced stability. For these increase

i / in single-support durations to be evident at both lower and upper zones of the

VFD suggest that stability and hence gait is optimised at the intermediate zone

which corresponds to slow and ordinary walking speeds. It further makes it

reasonable to speculate that the postural control strategy adopted at ordinary

walking speed represents the optimum configuration of the neuromuscular

system necessary to bring about efficient and safe locomotion.

These understanding makes it easier to appreciate the earlier observations

made from the VFD (fig 1). It was observed that at the lower zone, the double-

support duration increased likewise energy expenditure. This reflects the pattern

of postural control strategy adopted at slow speed such that it emphasises

increased stability or safety of the upright stance achieved through an inefficient

melding of the body parts nqcessarpto achieve the goal of the movement task.

In fact, the increased energy expenditure could in addition to other factors be

related to the greater active use of the muscles to deliberately retard the leg

swing below the natural pendulu'm rate (Eke Okoro, 1985). Thus, for

emphasizing safety at the expense of movement efficiency it is termed "the

sufe postural control strategy". At the intermediate zone (ie slow and ordinary

speed) the double-support duration recorded an optimal value while the energy

expenditure is at the minimum. It suggests that this strategy emphasises both

stability and movement efficiency equally. The minimum energy cost of

locomotion recorded in this zone, further suggests that the natural pendulum

rate of leg swing which relates to the path of minimum energy trajectory in

i walking (Eke-okoro, 1985) may have been attained. For these reasons this

strategy is termed "the optimum postural control strategytt. At the upper zone

(fast and very fast walking), the double-support duration tends to zero while

energy expenditure increased slightly again. 'This suggests that this strategy is

inefficient and unsafe. This inefficiency could relate to the increased active use

of muscles to increase the rate of leg swing beyond its natural pendulum rate in

an attempt to increase the velocity of walking.

It is possible that in response to this drive, the double-support duration

tends to zero to facilitate speed transition from walking to running at Ej. This

implies that the double -support duration, which is a measure of balance control

(Gabel and Nayak, 1984), is also an important indicator of speed change,

especially of a switch from walking to running (Eke-Okoro, 1996, Ibeneme,

2002). It further suggests that there is a possibility that speed and balance

control is probably programmed . ., . r l t o g ~ ~ h e r r% in the CNS. However, since neither

efficiency nor stability is emphasized by this strategy it is tenned "the deficient

postural control strategytt. The values of double-support duration at equality

points El and E2 could thus signify critical values beyond which a systems

postural control strategy begins to change according to the perceived need for

safe regulation of the body's cenlre of mass and movement efficiency. Thus, the

stability of any adopted walking pattern is a function of the co-operation and

competition anlong the systems components as represented by movement

erficiency and balance control. As such, when the safe regulation of the body's

centre of mass is threatened, the system adopts a motor pattern that not only

compensates for the instability, but represents an inefficient melding of the body

parts necessary to achieve the goal of movement. This explains the observed

trends in pace walking. Also, the minimum energy recorded during pace

walking at maximum speed suggests that the path of minimum energy trajectory

was attained at this point. In essence, the attainment of this path is delayed with

gait alterations. It is possible also, that all the gait pathologies which alter the

neuromuscular configurations involved in the gait process in the same direction

as pace walking, will likewise present similar features.

In essence, a three zone layout of speed (Eke-Okoro, 1989; Eke-Okoro,

1999) may not only represent a velotypic organization of speed in the CNS, but

also the associated postural control strategies. This view is further supported by

the earlier indications that there could be an interaction between the neurons of

balance control and speed in the CNS. Attempts at theoretical analysis of the

postural control function of the parts of the CNS was made and projects the

basal ganglia and the cerebellum~as possible sites for the velotypic mapping of

these strategies.

The roles of the deep cerebellar nuclear complex in the determination of

speed had earlier been highlighted (~ke-okoro, 1999). It was suggested (Eke-

Okoro, 1999) that the neurons of the dentate nucleus of the cerebelluni may be

more active at slow movement since its cooling reduces low threshold response

neurons. Cooling the interpositus (globose and the emboliform nuclei) in cats

reduces high threshold response neurons (Murphy et al, 1975) thus the neurons

of the interpositus may be more active at high speed; especially the emboliform

nucleus (Eke-Okoro, 1999).

As regards the basal ganglia, clinical evidences has shown that certain

neurons within it are active during specific types of movement. A large

percentage of neurons in the putamen (45%) and a smaller percentage (17%) in

the globus pallidus fire preferentially under slow -ramps movements (Delong

and Strick, 1974). Also, recent studies has shown putamen and globus pallidus

activity during fast movements (Mink and Thach, 1987). This could suggest that

both structures have specialised neurons which are selectively active at slow and

fast speed movements.

Furthermore, in conformity with the principles of organisation observed

in other motor systems (Strick, 1985), neurons of the primate putamen and

globus pallidus appear to be organized in multiple functional clusters (lansek

and Porter, 1980). These small functional neurons clusters correspond to

somatopcially organized microexcitable zones, which could represent a more

finely graded breakdown of function within the basal ganglia. It is suggested . ,, . 4 *, 3,' ,>

that these neuronal clusters code specific movement types of particular body

parts (Alexander et al, 1986) and thus "represent the basic functional units of the

striatum" (Crutcher and Delong 1984);

These selective neuronal responses observed within the cerebellum and

the basal ganglia in relation to the speed of movement do not appear to occur

haphazardly. Rather, some degree of organized neuronal feedback and

feedforward interactions between their structures are suggested such that high

and low threshold response neurons in both structures discharge in synchronous

phases during high and low speeds respectively. This could be possible through

their neuronal connections. For instance, the basal ganglia structures through

pallidonigral efferent fibres project to the thalamus, specifically the

ventrolateral pars medialis, the ventrolateral nucleus pars oralis and then to the

supplementary motor area in the cortex (Schell and Strick, 1984). Meanwhile, a

pathway from the caudal portions of the deep cerebellar nuclei projects to area x

in the thalamus and then to the arcuate premotor area. However, another

pathway from rostra1 portions of the deep cerebellar nuclei projects to the

ventral - posterior lateral nucleus pars oralis and then to the motor cortex.

Interestingly, these circuits are linked through a reciprocal connection between

the motor cortex, the arcuate premotor area and the supplementary motor area

(Schell and Strick 1984). In essence, the basal ganglia can influence the

cerebellum and the lower motor neurones through this corticofugal system.

Functionally, this output is involved in limb movements (Ilinsky and

< Ilinsky, 1995) and may also be involved in the determination of speed and

postural control strategies a&@ed,jn movements. In fact, evidence has shown

that single neurons in monkeys putamen respond differentially to information

about an expected movement (Alexander, 1987). Thus, the basal ganglia activity

appears to be involved in preparatory states (postural adjustments) prior to

nlovements otherwise known as motor set. This is of functional relevance as the

inability of patients with Parkinson's disease to initiate movements could be

linked to diminished or impaired motor preparedness (Chan, 1986). In fact,

temporary cooling of the globus pallidus structures had resulted along with

periods of agoinst-antagonist muscle co-contraction (Hore et al, 1977) thus

phases during high and low speeds respectively. This could be possible

their neuronal connections. For instance, the basal ganglia structures

through

through

pallidonigral efferent fibres project to the thalamus, specifically the

ventrolateral pars medialis, the ventrolateral nucleus pars oralis and then to the

supplementary motor area in the cortex (Schell and Strick, 1984). Meanwhile, a

pathway from the caudal portions of the deep cerebellar nuclei projects to area x

in the thalamus and then to the arcuate premotor area. However, another

pathway from rostra1 portions of the deep cerebellar nuclei projects to the

ventral - posterior lateral nucleus pars oralis and then to the motor cortex.

Interestingly, these circuits are linked through a reciprocal connection between

the motor cortex, the arcuate premotor area and the supplementary motor area

(Schell and Strick 1984). In essence, the basal ganglia can influence the

cerebellum and the lower motor neurones through this corticofugal system.

Functionally, this output is involved in limb movements (Ilinsky and

G

Ilinsky, 1995) and may also be involved in the determination of speed and

postural control strategies ad,~,p~ed,jn movements. In fact, evidence has shown

that single neurons in monkeys putamen respond differentially to information

about an expected movement (Alexander, 1987). Thus, the basal ganglia activity

appears to be involved in preparatory. states (postural adjustments) prior to

movements otherwise known as motor set. This is of functional relevance as the

inability of patients with Parkinson's disease to initiate movements could be

linked to diminished or impaired motor preparedness (Chan, 1986). In fact,

temporary cooling of the globus pallidus structures had resulted along with

periods of agoinst-antagonist muscle co-contraction (Hore et al, 1977) thus

leading to the disabling of learned arm movements similar to what is observed

in patients with Parkinsonism. This has a profound clir~ical implication for

posture and equilibrium control in man, since the arm movement in walking

serves to stabilizc thc trunk by counteracting its rotation about the inidline

(lilftman, 1939, Stolov, 1980, Ibeneme, 2002). This may explain why other

clinical evidence (Babinski, 1989;

Rogcrs et al, 1987 and Kanoeke et al, 1989) suggest that the basal ganglia and

the cereblluin are remotely involved in the translation of motor plans into

movement parameters (with emphasis on balance coordination) by a way of a

model of body dynamics. For the cerebellum already associated with speed

determination also to be linked with balance co-ordination, suggests the

possibility that speed and balance control could be programmed at the sanie

sites in the CNS during walking, probably to optimise their functions.

Meanwhile, the involvement of the basal ganglia and the supplementary

motor area in motor programnling had earlier been highlighted (Evarts and

Wise, 1984, Frank and Earl, 1990$. .This .'hypothesis is further supported by

evidence of supplementary motor area activation as seen with regional cerebral

blood flow prior the onset of movement and during tasks requiring con~plex

sequential movement of foot, hand and orofia&l structures (Orgogoro and

l,arscn, 1979). However, most neurons in the putamen and globus pallidus fire

well after the onset of movement and later than those in the supplementary and

primary motor cortex (Kunzle, 1977). Such tinling patterns may not be

unexpected considering the large reciprocal inputs Gom these cortical areas to

the basal ganglia (Delong et al, 1984). Though the late striatal activation do not

conform to the classical definition of motor prograinining (Kelle, 1968), such

disruptio~is are reasonably thougl~t to contribute to I-'arkinsonian motor

ilnpairi~lent (Marsden, 1982).

Ilowever, other investigators (Jacgcr ct al, 1989) have discovered a

significant degree of neuronal firing which preccdcs moveinent onset thus

making the aforementioned issue unresolved. This highlights the need lor

further neurophysiological i~lvestigations. 111 summary, this study usii~g the

VFD had demonstrated the possibility that only three postural control strategies

are programmed in the CNS across the spectrum or human walking. It has

ri~rther demonstrated that the VFD has the potential of further elucidating the

neurophysiology of human walking.

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