General principles

engineering and biomechanics in cycling

Principles – Muscles, joints and activations while pedaling

Influence of the seat position on muscle forces, resulting moments and tangential force contribution at the crank.

During muscular activity while pedaling there is a certain point in time when muscular strength is at its maximum.
The transmission of these forces to the pedal crank depends on a certain geometric position of the limbs in relation to one another.
This geometry is determined by the seat position, depending on the trajectory of the ankle joint.

It makes sense to achieve the maximum force of the biologically strongest muscles (thigh extensors) if the position of the links results in maximum large joint levers*.

This ensures that the power can be optimally transferred to the crank for torque generation. Physically, the greatest torque is generated on the crank when a horizontal position (90°, p/2) is reached.

In order to make predictions about the effect of physiological movements a muscle-mechanical model is required with which the equations of motion can be solved using functional control signals. [Yamaguchi and Zajac]

The effect of the muscles on the skeleton is determined by the geometry of the application of force and the amount of muscle force. [Zajac and Gordon]

The summation of the actively generated moments and the kinetic boundary conditions as well as the inertial forces of the body determine how the body segments are moved.

Basically the following can be determined by assessing the seat position on the road bike:

'A posture on the bike must be examined and found that takes into account the biomechanical physical conditions in such a way that all forces generated during pedaling are physically transferred to the maximum. This is not done as an iterative optimization process but it`s the determination of a relative maximum value.'

Although this does not yet provide a definition. It reduces the criteria to the essential physical connections between the human body and the technical frame of the bicycle.

Jachen Denoth from the ETH in Zurich, Switzerland wrote something very remarkable: 'Real muscular movements are (perhaps) the result of optimization processes; maybe they are just the result of the mechanical properties of the muscles and the given neurological loops. Which parts derive from specific properties of the musculature and which phases of movement sequences are stored in the motor cortex and represent components of the regulation are and remain fascinating questions that have not yet been answered in detail.'

from: Denoth, Jachen Lecture Biomechanics II - Biological and mechanical basics of the musculoskeletal system, chapter 4.1 page 132