Question: How do humans achieve locomotion?
About the article "Human swimming", section "Abstract".

Candidate passages:
The regulatory mode theory depicts how people approach situations to achieve the goal.
This theory is part of E. Tory Higgins research in motivation theories and goal pursuit theories.
People can either use the locomotion or the assessment method for goal-pursuit.
E. Tory Higgins states, “When people self-regulate they decide what they want that they don’t currently have.
They then figure out what they need to do to get what they want, and then they do it”.
People who are geared towards the locomotion mode are focused on moving and getting things done.
In contrast, those that are strong in assessment will compare different goals and analyze different options. 
The following adaptations are for arboreal and knuckle walking locomotion and are not found in humans: 
An increase in leg length since the evolution of bipedalism changed how leg muscles functioned in upright gait.
In humans the "push" for walking comes from the leg muscles acting at the ankle.
A longer leg allows the use of the natural swing of the limb so that, when walking, humans do not need to use muscle to swing the other leg forward for the next step.
As a consequence, since the human forelimbs are not needed for locomotion, they are instead optimized for carrying, holding, and manipulating objects with great precision.
Having long hindlimbs and short forelimbs allows humans to walk upright, while orangutans and gibbons had the adaptation of longer arms to swing on branches.
Apes can stand on their hindlimbs, but they cannot do so for long periods of time without getting tired.
This is because they haven’t adapted their femur for bipedalism.
Apes have vertical femurs, while humans have femurs that are slightly angled medially from the hip to the knee.
This adaptation allows our knees to be closer together and under the body’s center of gravity.
This permits humans to lock their knees and stand up straight for long periods of time without much effort from the muscles. 
Researchers have begun implementing controllers in into robots to control for stiffness.
One such model adjusts for stiffness during robotic locomotion by virtually cocontracting antagonistic muscles about the robot's joints to modulate stiffness while a central pattern generator (CPG) controls the robot's locomotion.
Other models of the neural modulation of stiffness include a feedforward model of stiffness adjustment.
These models of neural control support the idea that humans use a feedforward mechanism of stiffness selection in anticipation of the required stiffness needed to accomplish a given task.
Most models of the neural control of stiffness promote the idea that humans choose an optimal limb stiffness based on their environment or the task at hand.
Studies postulate the humans do this in order to stabilize unstable dynamics of the environment and also to maximize the energy efficiency of a given movement.
The exact method by which humans accomplish this is unknown, but impedance control has been used to give insight into how humans may choose an appropriate stiffness in different environments and as they perform different tasks.
Impedance control has served as the basis for much of the work done in the area of determining how humans interact with their environment.
The work of Neville Hogan has been particularly useful in this area as much of the work being done today in this area is based on his previous work. 
There appears to be three main methods of movement: free swimming, inch-worming along a substrate or sessility.
Inch-worming (or crawling) involves taking alternate steps with the head and tail, as do certain leeches, which gives the group their name (Greek "βδελλα" or "bdella", meaning leech).
This video demonstrates how bdelloids move in three different situations: locomotion and feeding of bdelloid rotifers.