Humans run using two legs, a form of locomotion known as bipedalism, which contrasts sharply with the four-limbed movement (quadrupedalism) seen in most mammals. Humans are obligate bipeds, meaning two legs are the primary means of movement. Exploring a quadrupedal gait reveals profound differences in human physical structure and probes the limits of our unique evolutionary path. This analysis explores the feasibility, efficiency, and performance of human quadrupedal running.
Anatomical Limitations to Quadrupedal Running
The human skeletal system is fundamentally reconfigured for an upright stance, creating several limitations for movement on four limbs. Our spinal column exhibits a vertical S-curve, including lumbar lordosis, which functions as a shock absorber for bipedal walking. This differs from the horizontal, arched spine of a true quadruped. A quadruped’s spine is designed to distribute weight evenly between the forelimbs and hindlimbs and contribute to powerful propulsion.
The human pelvis is broad and bowl-shaped, designed to support internal organs and transmit weight directly downward to the legs. In contrast, a quadruped’s narrower pelvis is oriented to allow the hindlimbs to drive the body forward horizontally. Humans also have a high, far-back center of gravity, optimized for the narrow base of support provided by two feet. Quadrupedal animals typically have a lower, more forward center of gravity, often bearing a greater share of body weight on the front limbs for better stability and acceleration.
The structure of the human wrist and hand presents a considerable challenge to sustained quadrupedal running. Our wrists are highly mobile condyloid joints, possessing eight small carpal bones arranged for flexibility and manipulation, such as grasping and tool use. This arrangement is not structurally suited for bearing the entire force of the upper body during a running gait. Running on all fours places immense, non-axial stress on these delicate carpal bones and ligaments, which are designed for mobility, not longitudinal load-bearing.
Human limb proportions are optimized for bipedalism, with legs significantly longer than arms. This ratio is quantified by the intermembral index, which is much lower in humans than in primates that regularly move on all fours. This length discrepancy makes an efficient, rhythmic four-limbed gait difficult. The runner is forced to awkwardly compensate for the short reach of the arms with the long stride of the legs.
Gait Mechanics and Energy Cost
The transition from bipedal to quadrupedal locomotion results in a drastic reduction in mechanical efficiency. When moving on all fours, humans typically adopt a four-beat gait, where each limb touches the ground sequentially, or a bounding gallop at higher speeds. This movement pattern requires the upper body muscles to function as both support and propulsion, a role for which they are poorly suited.
Metabolic efficiency suffers severely in this unnatural gait, as the body expends energy to counteract structural disadvantages. Studies comparing quadrupedal walking to bipedal walking at the same speed show a significant increase in energy consumption. Quadrupedal locomotion can increase total energy expenditure by over 250% compared to upright walking. This increase is due to the higher metabolic demands, potentially increasing expenditure by more than four kilocalories per minute.
This increase in the cost of transport is due to inefficient muscle recruitment. It particularly forces the muscles of the upper limbs and core to work harder than they would in a natural quadruped. In bipedal running, the arms swing to counterbalance the angular momentum created by the legs, which is a highly energy-efficient process. When running on four limbs, the arms must serve as a primary contact point, sacrificing their role in momentum compensation for a high-impact, weight-bearing function.
The overall speed achieved is severely limited by this inefficiency. While a human can achieve a top bipedal sprint speed of over 25 miles per hour, the fastest recorded speed on all fours is drastically slower. The human body cannot generate the same powerful, elastic recoil and long stride length with its arms that it can with its specialized legs.
The World of Specialized Quadrupedal Athletes
Despite the profound anatomical hurdles, a few specialized individuals have demonstrated that high-performance quadrupedal running is physically possible through intense training. These athletes manage to overcome the inherent inefficiencies of the human body to achieve remarkable speeds over short distances. The Japanese athlete Kenichi Ito is a prominent example, having dedicated years to perfecting his technique by studying the movements of primates.
Ito held the Guinness World Record for the fastest 100-meter dash on all fours, achieving a time of 15.71 seconds in 2015. To reach this performance level, he engaged in rigorous training focused on conditioning the joints and musculature. This training involves strengthening the wrists, shoulders, and core to withstand the repetitive, high-force impacts of landing and propulsion.
Although specialized training allows for speeds far beyond what a typical person could achieve, it does not negate the fundamental anatomical and metabolic disadvantages. The speed achieved by these athletes is still only a fraction of the world record for a bipedal 100-meter dash, which sits under ten seconds. This specialized adaptation demonstrates that while the human body is not naturally suited for this locomotion, it possesses a plasticity that can be molded for extreme, short-burst performance.