Contributions to the Understanding of Gait Control

1. What they did

This is the summary of a doctoral thesis (Doctor of Medical Science, University of Copenhagen) built on ten of the author’s published studies of how people walk and run. It had two aims: to explain where the forces (net joint moments) at the hip, knee and ankle come from during walking, and to find out what role sensory signals from the muscles play in controlling walking and running.

The thesis opens with the evolution of upright walking. It describes the 3.6-million-year-old footprints at Laetoli, Tanzania, left by at least two early hominids, probably an adult and a child walking side by side. It cites studies that compared 3D scans of these footprints with footprints people made in wet sand, and a computer model of “Lucy” (Australopithecus afarensis), to argue that these hominids already walked on straight legs.

In the gait laboratory:

• Movement and forces. People walked over force platforms set in the floor while five video cameras (or a six-camera infrared system) recorded reflective markers on their bodies. Combining the two gave the forces acting at each joint.
• Muscle activity. Electromyography (EMG) with surface electrodes, and in one study fine wires inserted into the deep hip flexor (iliacus) muscle.
• Reflexes. Electrical stimulation of the nerve behind the knee to measure the Hoffmann reflex (H-reflex) in the calf muscles during walking and running.
• Changing the gait on purpose. Walking in 9 cm high-heeled shoes, walking with the upper body leaning back or forward, and comparing patients with drop-foot (who cannot lift the front of the foot) or joint hypermobility with healthy people.

2. What they found

People walk in different ways

• Healthy people walking at the same speed and barefoot (or in identical shoes) showed large individual differences, especially at the knee. Some put the most force through the ankle, others through the knee.
• A cluster analysis split 15 healthy people into two groups. One group walked with more bent knees, more thigh muscle activity and higher knee forces, with higher bone-on-bone forces in the knee (64 versus 55 newtons per kilogram of body mass). The author suggests this could help explain why some people seem predisposed to knee arthritis.
• Each person’s pattern appears unique. The thesis cites an earlier study by the same group that identified 21 people walking on different days from just three joint movements.

Disturbing the gait

• High heels: the ankle produced less push-off force, while the knee extensor force almost doubled in the first half of the stance phase, with more bent knees and more muscle activity.
• Drop-foot: the knee extensor force rose in both legs, the affected and the healthy one, most likely because patients try to keep their gait symmetrical. The healthy leg also pushed off harder at the ankle.
• Upper body posture: the hip flexor force late in each step did not balance the upper body. Instead it came from the strong hip ligaments being stretched, which store elastic energy and then swing the leg forward with almost no muscle work.

Reflexes

• The calf (soleus) H-reflex was strongly adjusted through each stride: switched up while the foot is on the ground, and always switched off at toe-off.
• About half of people had the reflex almost fully suppressed while the leg swings forward; the other half had it rising before heel strike. Each person’s pattern was highly repeatable from day to day.
• In running, the reflex rose sharply about 200 milliseconds before heel strike in every subject, and both calf reflexes increased with running speed.

3. What they concluded

• Normal human walking is done with different strategies, and the forces at the joints can be shared out and redistributed differently because of disability, footwear or pain.
• The knee shows the largest differences between people, which leads to differences in the forces on the joint.
• During walking the trailing leg is brought forward without muscle force, using elastic energy stored in the hip ligaments, an important energy-saving feature of human walking.
• The upper body is balanced by speeding up and slowing down the pelvis together with the abdominal and back muscles.
• Reflex adjustment is an integral part of the human gait pattern, and it differs between people while staying consistent within each person.

6. Other technologies, skills or innovations

• Three-dimensional video analysis (Ariel Performance Analysis System, five cameras at 50 frames per second) and a Vicon infrared system (six cameras at 100 frames per second), with force platforms measuring ground reaction forces and centre of pressure.
• A wireless EMG system using Bluetooth (Marq Medical MQ16 and MQ8).
• A double stimulus method for measuring the H-reflex, and a “double-differential” electrode setup to reduce cross-talk between neighbouring calf muscles.