Absorption was originally organized globally. In horizontal locomotion, forces entered the body through large contact surfaces and were distributed through the trunk. The abdomen and torso functioned as elastic, deformable interfaces with the ground, transforming impact and friction into continuous, wave-like motion. No single joint was required to bear peak loads.

Body-Global Absorption

In the beginning there was the wave, not the step. The abdomen is an organizer of movement. In early animals it was the central contact surface with the ground. It functioned simultaneously as a distributor of force, a base of stability, and a motor—long before limbs became fully weight-bearing. Snakes, salamanders, or semi-amphibious tetrapods such as Ichthyostega moved (and move) through wave-like motions of the trunk; the belly bore (and bears) the mass, generated (and generates) friction, and transmitted (and transmits) forces.

With the evolution of load-bearing legs, the direct propulsive role shifted to the extremities, yet axial organization remained central. Today, many people understand the abdomen primarily as an aesthetic problem. Those who think biomechanics incorrectly overload joints and use the body inefficiently; those who understand it correctly use the abdomen like a primal sole: continuous, rhythmic, organizing.

Horizontal vs. Vertical

Horizontal movements reveal the original function of the abdomen. The belly distributes forces across a large surface, rhythmically stabilizes the trunk, and relieves the joints. Vertical movements shift the main load to the extremities and joints; the abdomen becomes a stabilizer. Intra-abdominal pressure protects the spine.

Walking is controlled falling. Each step is a forward tipping that is caught by the sole of the foot. Upright gait was an energetically costly update of our horizontal genesis. In the early stages of human evolution, the body had to reconfigure a mode of function practiced for millions of years. When walking, the sole contacts the ground, absorbs force, returns it, and enables forward motion.

In early land vertebrates (tetrapods) such as Ichthyostega, around 360 million years ago, the abdomen was the decisive contact surface. Although these animals already had limbs, they were not yet fully weight-bearing legs but rather supports and paddles (paleontologists describe Ichthyostega as semi-amphibious). Locomotion occurred primarily through wave-like movements of the trunk and spinal musculature—a principle we observe in snakes, salamanders, and seals.

The belly bore the weight, allowed mass to circulate, generated friction, transmitted force—and thus functioned like a large, muscular sole. It was the primary contact and force-transfer point between organism and substrate.

Originally, then, the abdomen was an organ that absorbed forces and transformed them into movement. With the evolution of load-bearing legs, the load increasingly shifted to the extremities. Yet the original function of the abdomen remains inscribed in the body. Even today it is not a passive muscular sheath but the central interface for stability, force transmission, and balance—a primal sole that could carry us again at any time.

Aesthetics Instead of Function – The Lost Art of Natural Movement

In the modern fitness and movement world, the guiding motif is aesthetics instead of function. Visible muscles, defined body parts, smooth surfaces—these are the goals. The actual performance of our muscles—their evolutionarily shaped capacity to move the body efficiently, stably, and enduringly—remains largely unused.

The abdominal muscles are evolutionarily designed for sustained work, stabilization, and coordinated force transmission. They hold the trunk, transfer impulses between arms and legs, and secure balance—especially in horizontal movements such as crawling or quadrupedal gait. These muscles can work for hours without fatigue.

Modern training programs usually isolate them in short, visible movements: crunches, sit-ups, machine exercises. The abdominal muscles are forced into explosive, brief contractions—a form foreign to them that obscures their true capacity. Strength is made “nicely visible,” not functionally useful.

The Lost Natural Movement

Evolutionary movement patterns that optimally use our muscles are innate but not instinctive. Children crawl, creep, balance—they use their trunk and abdominal musculature naturally. Adults lose this practice, and social culture offers few alternatives. Instead, we sit, stand upright, lift uncoordinatedly, and expect muscles and joints to perform optimally anyway.

The consequence: joints are overloaded, muscles underused, degenerative phenomena increase. The abdominal muscles remain largely inactive, the kinetic chain breaks, energy is transmitted inefficiently—a mismatch between evolutionary equipment and everyday life.

Abdominal muscles are evolutionarily designed for continuous stabilization and coordinated force transmission, not for isolated, explosive contractions. Horizontal locomotion is their original function. Over millions of years, abdomen, trunk, and limbs served as an integrated system for efficient movement, minimal energy loss, and long loading times. Only upright walking shifted the load, reduced the abdomen’s direct force role, and increased joint stress.

Modern training doctrine often focuses on aesthetic abdominal exercises that produce isolated, short-term contractions. An approach oriented toward horizontal movements—crawling, quadrupedal gait, moving plank variations—activates the abdominal musculature in its evolutionary endurance function. Here it works as a central force and stabilization element; the kinetic chain is optimally used, and joints are protected. This type of training is particularly effective for older adults, since abdominal muscles respond more readily to sustained work with age than the fast fibers of other muscle groups.

The Abdomen in the Horizontal

During crawling, the abdomen lies at the center of events. It bears a substantial portion of body weight, functions as a sliding surface on the ground, and stabilizes the entire trunk through sustained muscular tension. Arms and legs provide impulses, but the abdomen remains the constant contact point that enables movement. In this position, joints are relieved: they do not act as load-bearing structures but as simple openings or hinges that transmit movement energy. The main load does not rest on knees, hips, or spine, but on the broad support of the trunk.

The Abdomen in the Vertical

With upright posture, this logic changes fundamentally. In standing and walking, the legs provide propulsion and the joints assume the load. The abdomen loses its direct function as a propulsive surface and becomes a stabilizer. It tenses, builds intra-abdominal pressure, and thereby protects the spine. Force transmission between upper and lower body passes through it, but it is no longer the motor—rather, the link. The joints, by contrast, are disproportionately loaded: knees, hips, and spine now act as load-bearing columns that must absorb body weight and impact forces with every step.

Evolutionary Perspective

While most animals use their joints primarily as movement openings, humans shifted the entire system through upright gait. Bipedalism brought enormous advantages—free hands, better overview, new possibilities for tool use and communication—but biomechanically it was a compromise. Joints originally designed for horizontal load distribution now had to bear sustained loads for which they are not optimally suited. Osteoarthritis, disc damage, and knee problems are the modern price of this evolutionary shift.

Developmental History of the Child

A child’s development mirrors, in many ways, the evolutionary history of human locomotion. A child begins by crawling, with the trunk as the central contact surface and movement center. This is followed by quadrupedal gait, and finally upright walking. In this sequence, the trunk gradually relinquishes its central role to the legs, and joints transform from movement conduits into load-bearing structures.

Vertical locomotion enforces biomechanical compromises. While in the horizontal the trunk relieves joints and distributes forces efficiently, in upright gait the legs assume the primary load while the trunk becomes the stabilizing element. Bipedalism enables free hands, better overview, and new possibilities for tool use and communication, but it permanently stresses knees, hips, and spine.

From a biomechanical perspective, walking creates a constant interplay between muscle force and ground reaction: the foot presses against the ground, and the ground exerts an equal and opposite force. In crawling, the trunk assumes a comparable role. Continuous contact with the substrate generates frictional forces that ensure stability, while arms and legs provide push and pull impulses. The trunk bears a significant share of body weight, distributes forces over a larger area, and optimizes both balance and propulsion.

From a motor-control standpoint, the trunk functions as the central stabilizing component that connects upper and lower body, transmits forces, and holds the body in a controlled position. Without an activated trunk, the kinetic chain would be interrupted, movements would become inefficient, and joints would be overloaded. Evolutionarily, this shift began already in early hominins: Australopithecus was primarily a short-distance mover, while Homo erectus hunted as a long-distance runner, requiring adaptations of the pelvis and spine.

In addition, the trunk musculature protects and supports the internal organs—stomach, liver, kidneys—during locomotion. Despite these central functions, the abdomen is often viewed in modern perception as an isolated entity. Visible muscles or fat pads dominate attention.