Electromyographic and cinematographic analysis of movement from a kneeling to a standing position in healthy 5- to 7-year-old children.

Electromyographic and Cinematographic Analysis of Movement from a Kneeling to a Standing Position in Healthy 5- to 7-Year-Old Children The most important locomotor milestone in humans--walking--requires that a bipedal position be assumed. Healthy infants attain this bipedal posture as early as 7 months of age when they begin to pull to a standing posture, using half-kneeling as a transitional posture. [1-4] The half-kneel transitional posture continues to be used by some adolescent children when rising to a standing position from a supine position. [5]

Although the half-kneel transitional posture is used easily and effectively by healthy children to attain a standing posture, many children with cerebral palsy (CP) and other developmental disabilities have difficulty with this movement transition. As a result, the attainment of an upright posture in children with CP is often an important goal of physical therapy. The half-kneeling posture is used to facilitate normal muscle tone as well as to facilitate normal movement from a kneeling to a standing position in children with CP. [6]

Although physical therapists attempt to facilitate more normal movement as a child with CP rises to a standing from a kneeling position, the patterns of muscle activation and joint movement necessary to accomplish this transitional task in healthy children have not been reported. How, then, can physical therapists suggest more normal movements are being facilitated in children with CP, when a normal movement template from which to pattern "facilitated movement" has not been described? A quantitative analysis of the movements physical therapists facilitate, including the kneel-to-stand movement, will allow physical therapists to be more accurate and specific in the techniques used to facilitate movement. In addition, quantitative data on movement in healthy children will enable physical therapists to identify objective goals related to the movement facilitated (eg, joint range of motion [ROM], speed of movement, trajectory of movement) and to objectively analyze the effectiveness of their treatment. The purpose of this study, therefore, was to describe quantitatively the kinematics and muscle activation patterns of the lower extremity initiating the movement from a kneeling to a standing position in healthy children. The quantitative description of this movement may be helpful to therapists to use to guide their facilitated movements when working with children with movement dysfunction.

Background

Cinematography (high-speed photography), which allows for frame-by-frame analysis of movement, has been used to study gait in healthy adults and children [7-13] and in children with abnormal or pathological movement. [14-18] By using these methods of movement analysis, normative data for temporal and distance factors as well as for kinematic and electromyographic (EMG) factors have been established for normal gait in children and adults. [10,19-21]

Functional human movement that developmentally precedes independent walking has been studied much less extensively than gait. Bayley [1] and Illingworth [22] relied on visual observation to record the motor development of healthy infants. Other early investigators [23-26] supplemented direct observation with filming when documenting the gross motor abilities of the developing infant. Although early child development researchers such as Gesell [23,24] and McGraw [25,26] realized the value of photography and cinematography in describing movement, they were interested primarily in documenting attainment of motor milestones in the healthy child and did not attempt to quantify specific components of movement.

Electromyography has been used sparingly in the study of movements that precede skilled ambulation. In very young infants (5 hours to 6 weeks of age), biphasic patterns of activity were reported for hamstring, quadriceps femoris, gastrocnemius, and tibialis anterior muscles during reflexive walking on a treadmill. [27] Although the pattern was biphasic, a high degree of coactivation was reported between agonists and antagonists during the weight-bearing phase of the walking activity. Other researchers [28] have reported reciprocal activity of the tibialis anterior and gastrocnemius muscles during creeping in infants.

In an EMG study of static postural control, Shambes [29] reported that 4-year-old children in a maintained posture of half-kneeling exhibited sustained activity in the tibialis anterior and erector spinae muscles with burst activity in the semitendinosus, rectus femoris, and soleus muscles. Eight-year-old children exhibited steady activity in the erector spinae muscles, but only slight burst activity in the soleus muscles and no activity in the semitendinosus or rectus femoris muscles.

Coactivation of lower extremity antagonist muscles has been reported during a sit-to-stand task as well as during stair climbing. [30,31] In both of these tasks, the center of mass of the body is elevated against gravity by the lower extremities.

Using motion analysis and EMG, previous investigators have examined the components of movement that are necessary to walk, climb stairs, and move from a sitting to a standing position. No previous reports describing the kinematics and muscle activation patterns of rising from a kneeling to a standing posture, using half-kneeling as a transitional posture, could be found.

Method

Subjects

Ten healthy children, 6 girls and 4 boys, ranging in age from 5.2 to 7.9 years (X=6.3, SD=0.76), were recruited for this study. Their weight ranged from 15.6 to 25.0 kg (X=21.1, SD=1.03), and their height ranged from 108.0 to 127.0 cm (X=119.3, SD=14.91). Subjects were limited to children who had no history of developmental delay, prolonged illness, or orthopedic problems. In addition, all subjects were able to rise to a standing from a kneeling position with hands resting on a horizontal platform for balance only.

After the procedure was explained to the child and his or her parent(s), each subject's parent(s) provided written, witnessed informed consent. One parent stayed with the child throughout the entire data-collection procedure.

Equipment and Materials

Silver-silver chloride bipolar surface electrodes (*1) with a recessed 4-mm-diameter cup were used to record the muscle unit action potentials of four muscles of one lower extremity. The EMG signals were amplified by a differential preamplifier (*2) and further amplified (differential input impedance of 3,000 M[Omega], frequency bandwidth of 10 Hz-30 kHz, common mode rejection ratio of 110 db). The amplified signal from each of the four muscles was recorded on a light oscillograph (*3) at a paper speed of 25 mm/sec.

A motor-driven 16-mm Bolex motion picture camera (*4) was used to record the movement activity for kinematic analysis. Tungsten ASA 400 color film with a film speed of 64 frames/sec was used to film the subjects. A variable shutter with a shutter factor of 2.7 or 3.3 was used, with corresponding appropriate f-stop calculated and recorded for light conditions at the time of filming. The camera was mounted on a tripod and positioned 68 cm above the floor and 5.5 m from the subject, perpendicular to the sagittal plane of the subject's movement. Additional lighting was provided by tungsten lamps. After being processed, the film was analyzed on a Vanguard Motion Analyzer. (*5) Pressure-sensitive switches were used to record the beginning and the midpoint of the movement. The switches simultaneously sent a signal to the light oscillograph and illuminated a small light in the field of the camera. This switch system synchronized the recording of muscle activity on the light oscillograph and the movement of the subject on film for later analysis. One pressure-sensitive switch, placed under the knee of the leg initiating movement, recorded the beginning of the movement. The other pressure-sensitive switch recorded foot-down of the leg initiating movement.

A small adjustable table was placed in front of the subject at a height that corresponded to the subject's sternum when the subject assumed a kneeling position. The subjects used the table only as an assist in maintaining balance while they performed the movement. A pressure-sensitive switch was mounted on the table and illuminated a small light if the child placed more than 1 kg of force on the table during the movement. The floor on which the subjects performed the movement was lightly padded to avoid discomfort when they were in the kneeling position.

A dark curtain was used as a background for filming, and a plumb bob was used to identify the vertical plane. A standard of known length was filmed in the plane in which the movement occurred to calibrate horizontal and vertical displacement. A ball drop was recorded on the film to calibrate the film speed.

Procedure

The preferred leg for initiating movement for each subject was determined by asking the subject to stand up from a kneeling position. The leg the child used to initiate the movement in two of three trials was the leg studied. Each subject practiced moving from a kneeling position to an erect standing position, using half-kneeling as a transitional posture, three to five times. Subjects were instructed to rise to a standing position at a rate that was comfortable to them.

Subjects were dressed in bathing suits or gym trunks to allow for proper identification and marking of the anatomical landmarks. After the leg initiating movement had been determined and the subject had practiced the movement three to five times, anatomical landmarks were palpated on the support leg, trunk, ipsilateral acromion, and head and then marked with a small white circle on a black background for accurate identification during film analysis. The following anatomical landmarks were labeled for film analysis: (1) greater trochanter, (2) point 5 cm distal to the greater trochanter along the femur, (3) axis of rotation at the knee, (4) lateral malleolus, (5) lateral weight-bearing surface of the foot from the heel to the base of the fifth metatarsal, (6) line between the anterior superior iliac spine (ASIS) and the posterior superior iliac spine (PSIS), (7) crest of the ilium, (8) point midway between the axilla and the crest of the ilium, (9) acromion of the shoulder ipsilateral to the leg initiating movement, and (10) point anterior to the ear in the Frankfort plane (Fig. 1). The Frankfort plane is established by the points identified by the left and right tragi and the left orbitale. The point anterior to the ear in the Frankfort plane has been suggested to represent the approximate center of mass of the head for analysis of movement in the sagittal plane. [32]

The two active EMG electrodes were placed 2 cm apart, parallel to the muscle fibers, and over the middle one third of the muscle belly of the biceps femoris, rectus femoris, medial gastrocnemius, and tibialis anterior muscles of the support leg. A third, ground electrode was placed 2 cm perpendicular to and equidistant from the active electrodes. [21] The muscle bellies were identified by palpation during resisted voluntary contraction of each muscle. The skin at the electrode sites was prepared by rubbing the skin with alcohol and lightly shaving the area to remove hair, oil, and the superficial layer of dead skin. Interelectrode resistance was 150 k[Omega] or less, as measured between the electrode leads.

After the anatomical markers and surface electrodes were attached, the subject practiced the movement two additional times to ensure that foot placement was recorded by the pressure-sensitive switch and to ensure that the subject was familiar and comfortable with the movement. The subject was instructed to kneel on both knees with the trunk erect so that the shoulders, hips, and knees were vertically aligned, as determined by visual examination by the principal investigator (DVL). Initial position of the ankles was plantar flexion. The subject was also instructed to place his or her hands lightly on the table only for the purpose of maintaining balance during the movement (Fig. 2A). The investigator then gave the signal "Ready. Stand up, please." to the subject. The subject rose from a kneeling position to an erect standing position using half-kneeling as a transitional posture at the command of "Stand up, please." (Figs. 2A-F).

Five trials of the movement were recorded with at least 30 seconds of rest in the kneeling position between trials to avoid fatigue. The light attached to the pressure-sensitive switch was monitored to ensure that the subject did not put more than 1 kg of force on the table during the movement. If the switch was closed, the subject was reminded to use the table only for balance and the trial was repeated. Preparation of the subject and data collection were completed in approximately 1 1/4 hours.

Data Reduction

The 16-mm film was analyzed to determine the duration of the movement of each trial for each subject. An average duration in seconds for each subject was calculated from the five trials. The one trial with a duration that most closely approximated the average duration was designated the representative trial for that subject and was used for EMG and kinematic analysis. [10]

A visual inspection of the EMG recording on light-sensitive paper from each muscle was made to determine the phasic pattern and the duration of muscle activity. For the representative trial, a muscle was considered to show significant activity and determined to be "on" if the EMG tracing was above the baseline by at least 15% of the maximum tracing measured for that trial for more than 3% of the trial. A muscle was determined to be "off" if the EMG tracing fell below a level of 15% of the maximum tracing for that trial for more than 3% of the trial. [30]

All representative trials were normalized over time by designating knee-off of the leg initiating movement as the beginning, or 0% of the movement, and maximum extension of the knee of the leg initiating movement as the end, or 100% of the movement (Fig. 2F).

To report group data for the onset and offset of muscle activity, the following normalization procedure was used:

1. Knee-off of the leg initiating movement was defined as the beginning, or 0%, of the movement, whereas maximum extension of the knee of the leg initiating movement was defined as the end, or 100%, of the movement.

2. Movement time (in seconds) from knee-off to stand was calculated from the film (no. of frames/[60 frames/sec]=movement time).

3. Distance (in millimeters) the oscillograph paper moved from knee-off to stand was calculated (25 mm/secXno. of seconds=distance moved).

4. Distance (in millimeters) the oscillograph paper moved from knee-off to onset or offset of muscle activity was divided by total distance moved (in millimeters) and multiplied by 100 to calculate the percentage of movement from knee-off (mm/mm=%).

Example: 72 frames for movement/ (60 frames/sec) =1.2 seconds

25 mm/secX1.2 seconds =30mm

Medial gastrocnemius muscle onset (3 mm from knee-off on oscilloscope paper)=3 mm/30 mm =0.1=10%

The 16-mm film was projected frame by frame onto a screen onto which vertical and horizontal cross hairs were manually adjusted to intersect the middle of each anatomical marker. Vertical and horizontal coordinates of each landmark were then sent to a personal computer. A computer program was used to calculate joint angles from the vertical and horizontal coordinates. Joint angles were calculated from the appropriate coordinates for the h ip, knee, and ankle (Fig. 1). Hip angle was calculated as the angle between the femur and a line perpendicular to a line connecting the ASIS and PSIS of the pelvis. The marker 5 cm distal to the greater trochanter and the marker over the axis of the knee joint were used to determine the position of the femur. Joint angles were calculated for every fourth frame of 16-mm film (0.0625 second) from the anatomical landmarks. Joint angles were also calculated for those frames that contained the beginning position (kneeling), knee-off, and foot-down of the leg initiating movement; toe-off and footdown of the opposite leg; and when an erect posture had been attained (Fig. 2). For the purposes of this study, swing phase was defined as the period from knee-off to foot-down and stance was defined as the period of the movement from foot-down to erect standing (Fig. 2).

The principal investigator digitized all of the cinematographic data. Intrarater reliability was calculated on 24 frames of film for one subject, on two separate days. Interrater reliability was calculated on 10 frames for the same subject. A physical therapy graduate student who had previous experience extracting cinematographic data was the other rater. Intrarater reliability and interrater reliability for angle calculations were .99 or better for all joints, as determined by using Pearson product-moment correlation. [33] The largest measurement difference between raters was 5 degrees, found for the hip angle near maximum flexion. Most differences, however, were 2 degrees or less for both intrarater and interrater measurements.

The x and y coordinates of the crest of the ilium, acromion, and the point anterior to the ear in the Frankfort plane were digitized from the film and then multiplied by a scalar, the value of which was calculated from the known length of a standard and the length of that same standard on the film. For knee-off, foot-down, toe-off, and foot-down of the opposite leg, x and y values were normalized as a percentage of the total x and y displacement for each of the three landmarks. The normalized values were then multiplied by the mean of the total x and y displacement for the 10 subjects for each landmark. By this calculation, the mean displacement of the crest of the ilium, acromion, and head for the 10 subjects could be presented.

Data Analysis

Descriptive statistics (means and standard deviations) were used to present group data on mean joint position of the ankle, knee, and hip at specific points in the movement, as well as EMG data regarding onset and offset of muscle activity. [33] Angle-angle diagrams were plotted using a cubic spline function that connects the points on the plot with a smoothed line. [34] Displacement of the crest of the ilium, acromion, and head was also plotted using a cubic spline function. [34] An estimate of the ratio of vertical to horizontal movement of the head in the sagittal plane was made for the group data. This ratio was estimated by drawing a perpendicular line from the outermost point of the trajectory to a point that lies on a line drawn between the initiation of movement to the end of the movement. [32] The ratio of maximum vertical velocity to maximum horizontal velocity for the head was calculated by finding the maximum horizontal and vertical velocity of the head during the movement and then dividing the maximum vertical velocity by the maximum horizontal velocity. This calculation was made for each subject.

Results

The duration of the kneel-to-stand movement of the 10 subjects ranged from 1.17 to 1.98 seconds (X=1.55, SD=0.31).

Electromyographic Data

Data from EMG recording of the tibialis anterior, medial gastrocnemius, rectus femoris, and biceps femoris muscles are presented in Tables 1 through 4, respectively, and in Figure 3. Subjects exhibited both monophasic and biphasic muscle activity during the movement from a kneeling to a standing posture. Initial onset of activity in the tibialis anterior and biceps femoris muscles exhibited somewhat less variability than onset of activity in the gastrocnemius and rectus femoris muscles (Tabs 1-4). Offset of activity and the second onset during biphasic contractions exhibited more variability, as expressed by a percentage of movement (Fig. 3). No subjects exhibited only monophasic activity for all muscles, and only two subjects (subjects 8 and 10) had biphasic activity of all four of the muscles during the kneel-to-stand movement.

Kinematic Data

Group data for maximum and minimum joint ROM during the kneel-to-stand movement are presented in Table 5. The mean total joint ROM at the hip, knee, and ankle was 76.3, 130.5, and 60.3 degrees, respectively. The mean maximum ankle dorsiflexion was 29.2 degrees (range=23.2[degrees]- 36.6[degrees]).

Mean joint angles of the hip, knee, and ankle at six selected points during the movement are presented in Table 6. Representative hip-knee and knee-ankle angle-angle diagrams for subject 7 are shown in Figures 4 and 5. Plots of hip-knee and knee-ankle angle-angle diagrams for all 10 subjects are shown in Figures 6 and 7.

Representative tajectories of the iliac crest, acromion, and head are shown in Figure 8 for subject 7. Mean trajectories of the iliac crest, acromion, and head for six points in the movement for all subjects are plotted in Figure 9. For purposes of plotting the displacement of each landmark, the following beginning horizontal and vertical coordinates were chosen to represent an "average" subject in height and posture: crest of the ilium, 0 cm/35 cm; acromion, 0 cm/70 cm; head, 2 cm/80 cm. The estimated ratio of the mean vertical-to-horizontal movement of the head was 3.2 (AB/CD) (Fig. 9). The ratio of maximum vertical velocity to maximum horizontal velocity of the head ranged from 1.21 to 4.06 (X=2.57, SD=0.90).

Discussion

Limitations

The data from this study provide quantitative information about a functional movement that is antecedent to independent ambulation in healthy children. There are several limitations, however, to study. The study of 10 children is inadequate to establish norms for movement variables during the kneel-to-stand movement. The purpose of the study, however, was not to establish norms for the kneel-to-stand movement, but to quantitatively describe some components of the movement in healthy children. Slaton [13] has reported characteristics of gait with a similar sample size. The gait data reported by Sutherland and colleagues [10] are widely used as normative data, although as few as 9 subjects were studied in one age group. Although data from 10 subjects are not sufficient to establish norms for the kneel-to-stand movement, physical therapists can gain an appreciation for the general patterns of muscle activity and active ROM necessary to accomplish this task even if the data are obtained from a small sample of healthy children.

Because only one camera was used, kinematic analysis was limited to motion in the sagittal plane. Little rotation of the leg initiating stance occurred in the healthy children studied. In children with developmental disabilities such as CP, however, in which medial or lateral rotation of the lower extremities may be more likely to occur during movement, a one-camera system may not be adequate to study the kneel-to-stand movement.

Only four muscles of one lower extremity were studied because of limitations of laboratory equipment. The muscles studied were selected to minimize cross-talk between antagonists (ie, rectus femoris versus biceps femoris muscles, medial gastrocnemius versus tibialis anterior muscles) and because all of thesemuscles may exhibit chronic length changes in children with CP that may in turn affect the children's ability to attain an upright posture. The muscles studied have synergists that were likely active during this movement, which may explain some of the variability observed in the EMG patterns.

An additional noteworthy limitation is that one representative trial from each subject was selected to examine EMG and kinematic data, rather than averaging several trials. A number of trials could have been averaged, but fine points regarding the kinematics and muscle activity may have been lost by this averaging method. Selecting a representative trial that most closely matches the average duration of the movement has been found to be a satisfactory method of studying human motion by previous investigators. [10]

Electromyographic Results

Data from Figure 3 and Tables 1 through 4 indicate that the muscles of the leg initiating movement were active in either a monophasic or biphasic pattern during the kneel-to-stand movement. The tibialis anterior muscle was active throughout most of the swing phase as the ankle was dorsiflexed from an initial plantar-flexed position in the preparation for foot-down (Fig. 3). The tibialis anterior muscle continued to be active after the swing phase or became active again after foot-down. This activity most likely represented an isometric and then an eccentric contraction that helped stabilize the tibia against the movement created by the center of mass of the body, which remained posterior to the ankle during the first part of stance as the hip and knee were extending.

Activity of the medical gastrocnemius muscle that exceeded the criterion threshold occurred most consistently during the swing phase in which the gastrocnemius muscle was coactive with the tibialis anterior muscle (Fig. 3). Coactivation was most likely the result of an eccentric contraction acting to modulate the movement of the ankle into dorsiflexion or the result of a concentric contraction that assisted in flexing the knee.

Burst activity of the gastrocnemius muscle during the stance phase was associated with hyperextension of the knee in the standing position or with anterior-to-posterior sway when the movement was nearly complete. The lack of consistent gastrocnemius muscle activity during the stance phase indicated that, in most cases, the gastrocnemius muscle was not responsible for propelling the body upward against gravity. Because the center of mass was posterior to the ankle during early stance, the gastrocnemius muscle's activity would have had a destabilizing postural effect as the child attempted to assume the standing position. This was substantiated by inspection of the 16-mm film, which revealed that the estimated center of mass usually did not move anterior to the ankle until near the end of the movement.

The tibialis anterior muscle was active in both the swing and stance phases during the kneel-to-stand movement, whereas Joseph and Watson [30] found that the tibialis anterior muscle was active only during the swing phase in stair climbing. They also found that the soleus muscle was active only during the stance phase of stair climbing. The pattern of muscle activity of the tibialis anterior and soleus muscles in stair climbing would indicate that the center of mass of the body was anterior to the ankle joint during the stance phase of stair climbing, which was not the case during the kneel-to-stand movement.

Of the more proximal muscles, the rectus femoris muscle was active in most subjects for at least a short duration during the swing phase and most likely assisted in hip flexion. In two subjects, whose rectus femoris muscle activity did not exceed the threshold level during the swing phase, knee flexion may have inhibited the rectus femoris muscle activity or hip flexion may have been accomplished by using other hip flexors. During the stance phase, the rectus femoris muscle was active in all subjects for at least a portion of the phase, presumably to assist in extending the knee and propelling the subject against gravity as an upright posture was assumed. In those subjects with relatively short-duration rectus femoris muscle activity, other knee extensor muscles may have been primarily responsible for knee extension. Six subjects exhibited no rectus femoris muscle activity during the last 30% of the movement (Tab. 3). Based on the study of Fujiwara and Basmajian, [35] activity of the hamstring muscles during hip extension is not likely to have inhibited the rectus femoris muscle during knee extension. Rather, muscle activity from foot-down to 70% of the movement may have generated enough torque that little knee extensor activity was necessary after that time, as momentum of the center of mass may have propelled the body upward toward standing.

The primary biceps femoris muscle activity pattern was biphasic, and the variability for onset and cessation of activity was rather large (Fig. 3). Inspection of individual EMG tracings revealed that in most subjects the biceps femoris muscle was active in the swing phase to flex the knee. In most cases, the biceps femoris muscle also exhibited activity in the stance phase. During the first half of the stance phase, the biceps femoris muscle was most likely acting to extend the hip. Activity of the biceps femoris muscle late in stance may have been an eccentric contraction that helped modulate knee extension. Late biceps femoris muscle activity may also have been present to counteract anterior sway in stance. The large variability of biceps femoris muscle activity during stance may be explained by different levels of synergistic hip extensor activity across subjects. A forceful contraction of the quadriceps femoris muscle also may have caused reciprocal inhibition of the hamstring muscles to allow the knee to extend. [35]

In summary, the tibialis anterior muscle exhibited a rather consistent pattern of activity during the swing phase, whereas the rectus femoris muscle was generally active from the beginning of stance until opposite toe-off. During the kneel-to-stand movement, the tibialis anterior and rectus femoris muscles contribute to ankle dorsiflexion and knee extension, respectively. The biceps femoris muscle acts to flex the knee during the initial part of the swing phase and assists in hip extension during the stance phase. The medial gastrocnemius muscle contracts eccentrically during the swing phase, apparently to modulate ankle dorsiflexion. Both the biceps femoris and the gastrocnemius muscles appear to work in synergy to counteract the forward movement of the body as erect stance is attained.

Kinematic Results

Flexion and extension of the hip, knee, and ankle must occur in a controlled and coordinated manner for a child to efficiently use the half-kneeling transitional posture when assuming a standing position. The mean maximum hip flexion, knee flexion, and ankle dorsiflexion of 89.0, 130.7, and 29.2 degrees, respectively, (Tab. 5) were substantially larger than the values obtained during normal gait in 7-year-old children. [10] This finding is functionally significant in that greater joint ROM and a greater ability to control movement in that larger ROM are needed during a movement to attain an upright posture than during walking. In the child with CP or other neuromuscular disorders, the attainment of standing using a half-kneeling transitional posture may be more difficult than ambulation, based on joint flexibility alone.

The relationship between hip and knee joint angles for all subjects is shown in Figure 6. In general, hip flexion and knee flexion occurred simultaneously during the initial portion of the swing phase. Just prior to foot-down, the knee extended as the hip angle changed little. During the stance phase, knee extension and hip extension were coupled rather tightly, as demonstrated by the nearly linear characteristic of the plots from foot-down to assuming a standing position. The representative hip-knee diagram for subject 7 exhibits a similar relationship between hip and knee motion (Fig. 4). The relative hip and knee motion from foot-down to assuming a standing position during the kneel-to-stand movement is similar to the relative motion of the hip and knee observed during the sit-to-stand movement. [31]

Figure 7 shows the relationship between knee and ankle joint angles for all subjects during the kneel-to-stand movement. Knee and ankle motion are not as tightly coupled during the swing phase (kneeling to foot-down) as during the stance phase (foot-down to standing). All subjects demonstrated a relatively linear relationship between knee extension and ankle plantar flexion from foot-down to assuming a standing position as the ankle moved from a dorsiflexed to a neutral position. A representative trial for subject 7 is shown in Figure 5.

Mean horizontal and vertical displacement of the crest of the ilium, acromion, and head in the 10 subjects show that the total vertical displacement was about 1-1/2 times as great as the horizontal displacement for each landmark (Fig. 9). Jones and associates [32] suggested that the trajectory of the head during the sit-to-stand movement may be useful in assessing the efficiency of the movement. By drawing a perpendicular from the outermost point of the trajectory to a point on a line between the initiation of movement to the end of the movement, Jones and colleagues calculated a ratio that estimates vertical to horizontal movement. They found this ratio of vertical to horizontal movement of the head in healthy adults to be 2.52 during the sit-to-stand movement. In the 10 children who performed the kneel-to-stand movement in our study, the mean of the vertical-to-horizontal displacement ratio was 3.2. Jones and associates suggested that a shallow curve (ratio of vertical to horizontal movement greater than 1.0) reflected an efficiency of movement. A ratio of 3.2 in the 10 children in our study would suggest little forward momentum was necessary to gain an upright posture and that they also exhibited an "efficiency of movement" when coming to stand from a kneeling position. The ratio of total vertical displacement of the mean head trajectory to total horizontal displacement was 1.67, which is another measure that may be used to quantify the movement of the head during the kneel-to-stand movement.

A mean ratio of maximum vertical velocity to maximum horizontal velocity of the head during the kneel-to-stand movement was 2.57 (SD=0.90). This ratio compares to a ratio of 1.40 that Jones and colleagues [32] found for healthy adults during the sit-to-stand movement. The displacement and velocity ratios found in our study would suggest that the 5- to 7-year-old subjects were able to produce a coordinated contraction of their antigravity, functional extensor muscles to attain an upright posture.

Variability of Results

The substantial variability in EMG patterns among the subjects may have been exhibited for several reasons. Children with normal movement may have a wide variety of muscle activation patterns available to accomplish a functional task such as rising from a kneeling to a standing posture. Because the kneel-to-stand movement is not a cyclical movement, or as automatic as gait, the control of the movement is also less automatic and therefore more likely to be variable. Although the EMG activity of the lower extremity muscles was quite variable, less variability was observed in the relationship of lower extremity joints as the movement proceeded (Figs. 6, 7). Specifically, hip and knee motion appeared to be coupled throughout the movement to a substantial extent (Fig. 6). Knee and ankle motion was tightly coupled from footdown to assuming a standing position. Hence, the relative motions of three limb segments may be similar from one subject to another, whereas the EMG patterns used to move the same limb segments are widely variable. Given the number of synergists and two-joint muscles in the lower extremity, the variability is not surprising. For the physical therapist attempting to train a movement, the kinematics of the movement (eg, joint movement, trajectory of the head or limb segments, velocity of movement) may be more useful to use as a template than the EMG activity of the muscles producing the movement.

Implications for

Physical Therapy

Although this study attempted to quantify the kinematics and muscle activity required to attain an upright posture in healthy children, the results may be useful to physical therapists working with developmentally delayed children who have difficulty in transitional movements against gravity. The ROM of the knee in flexion, the hip in flexion, and the ankle in dorsiflexion exceeded the ROM that is necessary during ambulation. Maintaining adequate muscle length in a child with CP would then be important when attempting to train such a child to rise from a kneeling to a standing posture using half-kneeling as a transitional posture. Too often, physical therapists may emphasize inhibitory of facilitatory techniques during treatment, whereas the maintenance of adequate muscle length is not given enough attention.

If children with CP have a decreased ability to activate teh tibialis anterior muscles, stabilization of the tibia would be very difficult when rising from a kneeling to a standing position. During the kneel-to-stand, sit-to-stand, and squat-to-stand movements, the center of mass is posterior to the ankles for a portion of the movement. The tibialis anterior muscle must be able to counteract the moement created by the center of mass, or rising to a standing position cannot be accomplished. Increasing tibialis anterior muscle activity would then appear to be an essential component of a therapy program to improve function during movement to a standing position.

Although pediatric physical therapists often attempt to improve knee extensor muscle activity and control to improve antigravity movement, the hamstring muscles are active as well during the stance phase in healthy children. Therapeutic intervention to improve activity of not only the knee extensors, but also the hip extensors, including the hamstrings muscles and probably the gluteus maximus muscle, may also be useful to improve the functional ability of a child with CP to attain an upright posture from a kneeling or a sitting position.

Finally, the trajectory of the head during movement from a sitting to a standing position has been shown to be substantially different in healthy adults as compared with adults with neurological disorders.[32] Plotting the trajectory of the head and calculating the vertical and the horizontal velocity of the head during an antigravity movement can be accomplished with relatively inexpensive motion analysis equipment. Because many hospitals and clinics now have such equipment, quantifying the movement of the head before and after physical therapy may be a very useful, objective, and reliable way to document the effectiveness of the therapy program. If physical therapy is having a positive effect, changes in the vertical velocity would suggest an improvement in the strength or coordination of antigravity muscles in a child with CP. Being able to quantify such changes in movement variables will allow physical therapists to more objectively evaluate therapeutic intervention techniques.

Future studies are needed to determine whether the movement variables investigated in this study exhibit differences in children with movement dysfunction. In addition, intervention studies will be required to determine which of these or other movement variables are amenable to change and which correlate to a positive change in function in children with movement disorders.

Conclusions

Assuming an upright posture during the kneel-to-stand movement, using half-kneeling as a transitional posture, was accomplished in a controlled, efficient manner in children aged 5 to 7 years. Greater ROM of the hip, knee, and ankle joints was necessary to attain an upright position from a kneeling position than has been reported for subject during ambulation. Variability in EMG activity patterns of the lower extremity muscles studied suggest children use a variety of different patterns of muscle activity and synergies to attain an upright posture. The results of this study may be useful when studying the same or similar movements in children with disability

(*1) Beckman Instruments Inc, 110 Technology Park, Norcross, GA 30071.

(*2) Biomedical Engineering Department, University of North Carolina at Chapel Hill, Chapel Hill, NC 27514.

(*3) Honewell 906C, Honewell Industrial Products Group, 4800 E Dry Creek Rd, Denver, CO 80201.

(*4) Model HR 16, Bolex International SA, Yverdon Switzerland.

(*5) Vanguard Instruments Corp, 1860 Walt Whitman Rd, Melville, NY 11746.

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D Vander Linden, PHd, PT, is Assistant Proessor, Physical Therapy Department, University of Florida, Gainesville, FL 32607 (USA). He was a graduate student, Division of Physical Therapy, Department of Medical Allied Health Professions, School of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, NC, when this study was completed in partial fulfillment of the requirements for his master of science degree.

I Wilhelm, MS, is Research Associate Professor, Division of Physical Therapy, Department of Medical Allied Health Professions, School of Medicine, University of North Carolina at Chapel Hill.

This study was supported in part by Maternal and Child Health Training Grant No. MCJ0000149 from the Bureau of Health Care Delivery and Assistance, Health Services Administration, US Department of Health and Education, Public Health Service.

This study was approved by the Committee on the Protection of the Rights of Human Subjects, School of Medicine, University of North Carolina at Chapel Hill.

This article was submitted January 2, 1990, and was accepted September 4, 1990.