Archive for March, 2013

Control of Leg Timing vs Placement during Split-Belt Walking

March 26th, 2013

Student: Ough, I hurt my left knee running this week. And it’s hard not to be limping. It’s sorta like walking on a split-belt treadmill!

Professor: Well, another recent  study from Bastian and colleagues gives us more information on limping. Specifically, Malone and Bastian (2010) studied the effects of “conscious correction versus distraction” upon ‘limping’ during adaptation to split-belt walking. Below is their Fig. 1 showing their methods:

Malone Bastian 2010 Fig 1

FIG. 1. A: diagram of marker location and limb angle convention. B: experimental paradigm showing the periods of split-belt walking and conditions.

Professor: The methods of previous post (Perturbing Infants Locomotor Patterns With A Split Belt Treadmill) from a study of infants by Musselman, Patrick, Vasudevan, Bastian, and Yang (2011) are very similar to this study by Malone and Bastian. However, Malone and Bastian studied adults, and the experiment had two baseline periods each at the different speeds, a fixed adaptation period, and a fixed de-adaptation period. Notice: Malone and Bastian used as 3:1 fast:slow split-belt ratio instead of the 2:1 ratio Musselman et al utilized. Malone and Bastian also studied three groups of subjects each having different instructions. Their detailed description of the groups state that,

  1. “Subjects in the control group were given no instructions (n = 11).
  2. “The conscious correction group was given on-line visual feedback of their steps and instructed to “keep their step lengths equal on both sides” during the entire adaptation block (n = 11). To allow subjects to develop their own error monitoring and correction mechanisms, the experimenter demonstrated what was defined as a step length until the subject had an understanding of the parameter, but allowed the subject to monitor his/her own errors (i.e., the experimenter did not comment on the step lengths) once the experiment began.
  3. “The distraction group (n = 11) watched a television program unrelated to walking and were told to count the number of times a particular word was said using a handheld counter. Additionally, subjects in the distraction group were asked to focus their attention on the television so that they could answer questions about the program’s visual scenes after the adaptation block finished. Therefore the subjects were distracted by audio and visual stimuli.” [Numbering and emphases added.]

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Perturbing Infant’s Locomotor Patterns with a Split-Belt Treadmill

March 21st, 2013

Student: How young can a child be…. Well, how well coordinated can a child be so it can adapt to walking on a split-belt treadmill? Ahh, with a minimum of assistance?

Professor: Musselman et al. (2011) determined that children, older than 8.5 months, were the successful participant in thein study. Below is their Fig. 1 showing and describing their methods:

Musselman etal Fig 1 2011

Fig. 1. Methods. A: experimental protocol. Children walked on a split-belt treadmill with the belts at the same speed (tied), followed by the belts at different speeds (split), and finally again in the tied condition. Time periods of interest are late baseline (open bar), early split (shaded bar), late split (hatched bar), and early postsplit (stippled bar) at 40 steps for each period. B: temporal measures of walking are shown: stride time, stance time, and double support time. Open and shaded horizontal bars indicate the duration of the stance phase, and the space between the bars represents the duration of the swing phase. The duration of a stride includes a stance and a swing phase. Temporal coordination was quantified by double support times (i.e., time when both feet are in contact with the ground), shown for when the slow leg is trailing (slow DS) and when the fast leg is trailing (fast DS). C: center of oscillation is the mean limb angle over a stride. Limb angles of the fast (solid line) and slow (shaded line) legs are plotted for 1 child (35.2 mo) during early split. Dashed horizontal solid and shaded lines represent the mean limb angles for the fast and slow legs, respectively. Limb angle is the angle between the vertical and a vector connecting the hip and ankle markers (shaded line in inset at right). D: step length and stride length are illustrated. Step length, defined as the distance between the ankle markers of the 2 legs in the anteroposterior direction, was measured at the time of foot contact of the leading limb (i.e., instant in time illustrated at middle and right). The step lengths are named according to the leading leg, by convention. Stride length (left) is the distance traveled in the anteroposterior direction by the ankle marker of a single leg through the stance phase (i.e., foot contact to lift off, limb position shown for the 2 instances in time). [Emphases added.]

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Does Our Brainstem select Our Motor Programs?

March 11th, 2013

Eh! What? Motor programs operated by the brainstem?

Okay, my brainstem is the extension of my spinal cord. And I’m guessing this means just my brainstem selects each motor program I use?

Well, the short answer is Yes and No. Let’s  allow the experts to explain!

First, in their paper entitled, ” Is there a brainstem substrate for action selection?”, Humphries et al. (2007) observe,

” Decerebrate animals and altricial (helpless at birth) neonates do not have fully intact basal ganglia but are capable of expressing spontaneous behaviours and coordinated and appropriate responses to stimuli. … Yet, the chronic decerebrate rat can, for example, spontaneously locomote, orient correctly to sounds, groom, perform coordinated feeding actions and discriminate food types …. Such animals clearly have some form of intact system for simple action selection that enables them to both respond to stimuli with appropriate actions (more complex than simple spinal-level reflexes), and sequence behaviours—as demonstrated by the holding, gnawing and chewing required for eating solid food.”

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What Happens When I Switch from Walking to Running?

March 5th, 2013

Before I was retired, I used to take the bus to work.

  • One day, when I am walking toward the bus stop, I saw my bus already pulling up to my bus stop, and I noticed, in a moment later, that I was off and running toward it.
  • Later on, I realized in that brief moment, I had made an abrupt and radical switch from walking to running.

Well what happened? To answer that question, let’s allow Lacquaniti’s group provide the answer from their utterly fascinating study on walking versus running (Cappellini et al 2006). They describe the general characteristics of walking and running in their Fig. 1 shown below (notice that walking is solid black lines and running is fainter grey lines):

Ivanenko etal 2006 Fig 1


FIG. 1. General characteristics of walking and running. A: schematic representation of walking by vaulting (inverted pendulum) and running by a “bouncing” gait (leg spring-loaded behavior during stance). B: ankle, knee, and hip moments of force and vertical ground reaction force (normalized to the subject’s weight) of the right leg during overground walking (5.4 km/h) and running (9.4 km/h) in 1 representative subject. C: relative stance duration and cycle duration (± SD) in walking and running. D: foot (fifth metatarsophalangeal joint, VM) trajectory characteristics (mean ± SD, normalized to the limb length L): horizontal excursion of the VM marker [relative to greater trochanter (GT)] and vertical VM displacements (in the laboratory reference frame).

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