Walk/Run Transition Mechanics

Study Notes

The preferred transition speed (PTS) from walking to running occurs at around 2 m/s or 7.2 km/h for most people.
The PTS does not correspond to the speed where the energetic cost of walking becomes greater than running.
Increasing walking speed leads to increased ground reaction force (GRF) during the loading phase and decreased GRF during the pre-swing/propulsion phase.
Increased walking speed also leads to increased posterior GRF during the loading phase (up to a point) and increased anterior GRF during the propulsion phase (up to a point).
Walking requires higher plantarflexion torque compared to running at various speeds.
At speeds above the PTS, walking requires greater ankle dorsiflexion, knee flexion, and hip extension during the swing phase.
The effort required to increase muscle activation around the ankle, hip, and knee could be a factor in triggering the transition to running.

Transitioning to running slightly increases plantarflexor and dorsiflexor activation, with a slight increase in dorsiflexor force output and a more than doubled plantarflexor force output.
Muscle mechanics make plantarflexor force production more difficult at high walking speeds.
Running improves the mechanical situation for plantarflexors.

The running cycle consists of a stance phase and an airborne phase.
Stride length in running includes take-off distance, flight distance, and landing distance.
Stride rate in running is determined by the time spent in contact with the ground and the time spent in the air.
Running involves an airborne phase, increased joint flexion, higher energy expenditure, and increased stride length and stride rate compared to walking.
Foot contact in front of the center of mass (CoM) produces a braking force opposing the direction of motion.
Removing this braking force through foot placement and active hip extension increases CoM velocity.
Running can be modeled as a spring-mass system, where leg stiffness dictates the duration of the stance phase, vertical displacement of the CoM, and the rate of force application during the stance phase.
Leg stiffness is calculated as force divided by displacement.
High leg stiffness helps oppose ground reaction forces quickly, minimize force absorption, and shorten the stance phase. However, it also increases vertical displacement of the CoM.

Leg stiffness is influenced by joint torques, muscle forces (muscle activity and motor unit recruitment), and the stiffness of elastic soft tissues (muscle, tendon, connective tissue).
Stiffness in both vertical and anterior-posterior directions is important because it affects the magnitude of GRF moments (torques) at each joint and the muscle force required to counteract them.
Higher GRF moments at each joint require greater muscle force to counteract.

Sprinting aims to cover a distance in the shortest possible time by increasing stride frequency, reducing ground contact time, and increasing leg stiffness.
Sprinting also results in increased stride length, flight distance, speed of release (take-off), decreased angle of release, reduced air resistance, and minimization of landing distance and braking force.

Leg swing during sprinting creates a torque around the longitudinal body axis.
Movement of the opposite arm counteracts this torque, preventing trunk rotation and medio-lateral force generation.

Limiting braking force through foot strike and muscle activation is crucial for running economy.
High, but not maximal, leg stiffness is essential for running economy, influenced by elastic element stiffness, muscle activation, and joint configuration.
Sprint and distance running differ in their reliance on passive forces, with distance running relying more heavily on passive forces.

Dynamical systems theory states that the motor system simplifies its degrees of freedom by developing coordinated structures or temporary assemblages of muscle complexes.
Dynamical systems theory posits that the reduced complexity of the motor system leads to functionally preferred coordination patterns or "attractor" states that promote goal-directed actions.
Within each attractor region (the "neighborhood" of an attractor), system dynamics remain stable and ordered, resulting in consistent movement patterns.
Transitions between multiple attractor regions provide flexibility and adaptation in motor system behavior.
The brain may also have stable and unstable patterns of activity that interact with limb dynamics to produce stable movement.

Hierarchical models emphasize the role of the brain in specifying muscle and movement commands.
Dynamical system models focus on the mechanics of the human musculoskeletal system and suggest that coordinated movement can occur without descending control.
Both ideas are being integrated into more recent models of movement control.