1. A. B.
In collaboration with Dr Ramesh Balasubramanium and a current graduate student, we are trying to understand how infants who are unable to stand independently can regulate the necessary forces and motions of the legs to produce very exquisite control that leads to bouncing. These infants can not only bounce but if their mom or dad crosses in front of them, they can turn their body so they follow the movement of their parent. They can bounce sideways, forward and backward in what appears to be very controlled manners – yet they can’t stand!

2. = = = Mechanical System Bouncing Behavior Infant Contribution
Sinusoidal
(S2) =
Elongated
Sinusoidal
(S3) =
A previous graduate student who is now on Faculty at Guelph, did the original study a number of years ago. She found that babies used a couple of different patterns when they bounced – the patterns seemed to be differentiated by the way they regulated the stiffness of their legs and she reported that they either made their legs match the spring they were bouncing on or that they used a pattern that could be represented by a different mechanical system.
Variable
(LS2) =

3. Elongated Sinusoidal (S3)
Ankle
Knee
Hip
Sinusoidal (S2)
Elongated Sinusoidal (S3)
Variable (LS2)
She used a video camera to record the data – we now have Vicon systems scattered through out the city and so have the ability to do a more comprehensive analysis since the data sets can be much larger and more representative of the movements the babies perform. We are currently redoing some of the original work and will next look at experimental paradigms where we will destabilize or perturb the patterns to see if and how infants respond to environmental changes.
Joint angular velocity (º/sec)
Flexion
Extension

4. Continuously oscillating platform
Safe
Efficient
Unpredictable
postural challenge
Feedback postural
adjustments
Feedforward
postural activity
Predictable
If a person loses their balance unexpectedly, regardless of whether the loss of balance was due to something they did or due to an external perturbation, a feedback mechanism will be used. So, if the floor you are standing on moves unexpectedly, it is the feedback mechanism that is elicited. There is little redundancy in the system and the postural activity must be efficient in order to prevent a fall.
On the other hand, if a person expects to lose their balance, they will use a feedforward mechanism to prepare so they decrease the risk.
Finally, if you stand on a platform that is constantly moving in a consistent manner, over time the movement is anticipated and postural adjustments are used in anticipation of potential perturbations. For example, if a platform is moving backward and forward in a consistent fashion, a person may activate postural muscles in order to minimize the destabilizing effect of the change in direction. There is an added degree of safety since if the feedforward postural adjustment is inappropriate or ineffective, a feedback adjustment can be used for additional control.
These two mechanisms of postural control are not exclusive. Rather, a person using feedforward postural is also able to use feedback postural responses thus taking advantage of possible redundancy in the manner of their preparation and response to a perturbation. However, if a person’s feedforward postural adjustment is inefficient or inappropriate, there is no redundancy and a person must rely on feedback control. When the feedback pattern has deteriorated or is insufficient, the risk of a loss of balance is much greater.
J. Frank, M. Earl, Phy. Ther, vol. 70(12): , 1990
Continuously oscillating platform

5. Methodology: Experiment 1: Experimenter-induced perturbation
Young adults (YA) (22.25 ± 2.12 years old, n=8)
Older adults (OA) (70 ± 4.17 years old, n=8)
Experiment 2: Self-induced perturbation
YA (22.12 ± 2.29 years old, n=8)
OA (70.12 ± 4.61 years old, n=8)
Platform oscillation frequencies:
0.1 Hz (10 cycles), 0.25 Hz (20 cycles),
0.5 Hz (40 cycles), 0.61 Hz (50 cycles).
We use an experimental paradigm where we can study both a person’s feedback and feedforward balance mechanisms. The platform moves forward and backward and we can change how fast it goes.
We record how the body moves, the forces at the ground under the feet and how the person activates their muscles.
10 cm
10 cm

6. Experimenter-induced change in
platform frequency
-80
-60
-40
-20 20 40 60
Percentage of half cycle TA G H BE Q
0.61 Hz
0.5 Hz
0.1 Hz
0.25 Hz
Forward perturbation
Backward perturbation
This is an example of how young and old adults respond when you change the speed. If you look at just the left side and from top to bottom.
The four boxes represent the different platform speeds (slow at the top and fastest at the bottom). The black line at time 0 is when the platform changes direction from moving backward to forward. If a person is able to predict and use a feedforward control mechanism, the muscles should be activated at around -50.
So – look at the blue circles only. The first few cycle at a new speed are always controlled using a feedback mechanism since the subject doesn’t know when the speed will change – in the young adult, this is the open blue circle – and you can see that the muscles are turned on at around the same time as when the platform changes direction. However, after the first few cycles, the young adults quickly adapt and use a feedforward mechanism where they “predict” when the platform might change direction – this is the filled in flue circle and the muscles are turned on quite early. Now compare this to the data in red – from the older adult. The older adults don’t change to a feedforward pattern – even after 50 or 60 cycles at the same frequency.
We have done a number of studies to try to understand what happens with aging and will continue to explore this question.

7. Experimental Setup 2 AMTI force plates (240 Hz)
Participants: 13 healthy toddlers autonomously walking for less than a year participated in this study
Procedures:
2 AMTI force plates (240 Hz)
3 Cameras (60 Hz)
8 Surface markers
Body segment parameters recorded according to Schneider and Zernicke (1992)
Between 6 and 14 steps were analyzed for each toddler
With Dr Robertson, we have done two studies on toddler locomotion. We are interested in how toddlers develop locomotor skills, obstacle crossing and how they will deal with perturbations of their environment.

8. Moment / Body Mass (N.m/kg)
Results
Support Moments
Moment / Body Mass (N.m/kg)
Toddler support moments are approximately half or less than that of the adults in relative amplitude.
The first 20% of stance for both the one and two month walker show negative support moments in approximately 50% of the trials.
The number of steps with negative initial support moments then diminish for the four and five month walkers
the nine and eleven month walkers’ support moments show no negative values during the first 20% of stance.
In the latest study, we are interested in how toddlers get over obstacles. It appears that when they first start walking, they don’t register that the obstacle is even present and they step on it – they then make exaggerated steps over the obstacle until finally they start to look somewhat like a more mature walker and integrate the step over into the locomotor pattern.
We will continue to explore this question to find out how visual information and other environmental challenges are resolved.
% Stance