4 of 8 iew article increasing stride length rather than rate (Fig. 4). Stride le

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4 of 8 iew article increasing stride length rather than rate (Fig. 4). Stride lemgths in reliance on more slowly contracting,oxidative and fatigue-resistan humans during ER are typically more than 2m, and can esceed sle fibres, which are relatively more abundant in the legs of 3.5m in elite runners", approximately a metre longer than the competitive distance nunners than in sprinters". The high percen strides predicted for a 65-kg quadruped or measured in chimpantage of slow-twitch muscle fibees necessary for endurance running zees" at the same speeds, men when gallopung (Fig. 4a)Long may have onpated in hurmaas from a nove mall mutation of the absolute (rather than relative) stride lengths in humans are made ACTNS gone possible by a combination of effective leg springs (see above) and relatively long legs. Long legs benefit walking by increasing opti Skeletal strength mum walking speed, but they also increase ground contact time in Amother factior to comsider when evaluating the evolution of ER in both walking and running. Relatively long contact times may be humans is skeletall strength. Running exposes the skeletal system to advantageous for ER because the inverse of contact time has beem ch higher stresses than walking, especially when the foot collides found to corelate across species with the energetic cost of rumning with the ground peoducing a shock wave that passes up the body (running is priced by the step)". Long legs relative to body mass from the heel through the spine to the head. Peak vertical ground typical of most specialized cursons, first appear unequivocally in reaction foroes (GRFs) at heel strike are approximately twice as high hominids 1.8 Myr ago with H. erectss, whose relative leg length during running than during walking and may approach 3-4 times (assessed from the femur) is possibly up to 50% greater than in body wight at higher ER speeds. Human runners roduce these Australopithecus afarensis, Leg length in H. habilis (estimated from stresses to some extent through limb compliance and mid-foot the OH 62 skeletom) and other specimens as carly as 2.5 Myr agoi striking (thereby also storing elastic strain emergy in the kg and currently the subject of debate foot), but must otherwise dissipate impact forces within their Oscillating long legs, however, increases the energy cost of and jins One stratiegy to lower joint stress is to expand joint running in proportion to the limb's mass moment of inertia surfaces spreading forces over larger areas Many studies have Reductions in distal limb mass have litle effect on the energetics found that compared to both Pan and Australiopithecnes, Fiome has of walking but produce substantial metabolic savings during ER substantially larger articular surface areas relative to body mass in roughly proportional to the square of the distance of the mass from most joints of the lower body, including the femoral head and the hip. Redistributing 36kg from the ankles to the hip, for knee, the sacrolliac join,and the lumbar centra. Enlarge- example, decreases the metabolic cost of human running at slow ment of these joints, which is not matched in the upper limb of peeds (26 m s-, by l 5% (ref. 41 ). Although we do not know the Him. lowers the streses that impact forces generate at heel strike relative mass of the distal linsb in fossil hominids. humans differ during wak but would contribute more critically to dissapate from australoecies and resemble many specialined curors the moch higher impact loads generated in running. Another in having more compact feet and relatively short toes the haman possiblemodification of the pelvis for resisting the stresses associ- foot is only 9% of total leg mass, compared to 14% in chimpan- ated with running is enlargement of the iliac pillar in early zees , Humans also have relatively low stride rates at ER speeds H.Humans may also have a larger cross-sectional area even lower than are predicted for a 500-kg quadruped (Fig, 4b)o the clcamcal tuber selative to body mass than australopithe- Low stride rates that increase little in the ER range roduce the fonce cines" required to oscillate the heavy legs 00% of body mass in humans. Both waking and running also cause diaplyal loading, which is compared to 18% in chimpanzee") and may favour greater higher.n ang and ineases relative to body mass as afunction Long Achilles tendion (me 02004 Nature Pubilishing Group

Explanation / Answer

3 anatomical features unique to humans which may be an adaptation for endurance running compared to other primates are

1. Larger stride length that is the distance between heels of same foot when walked across a distance. This helps in taking longer steps and covering more distance than other primates while walking or running.

2. Long legs may help in increasing walking speed and ground contact time, hence humans may consume less energy in moving faster, convering longer distance.

Compact feet and relatively shorter toes which may help in maintaining good balance of body while moving and causes low stride rate which may reduce force required to take steps while walking or running.

3. Stronger skeletal muscles which may have less impact and force on legs due to weight of the body while running due to limb compliance and mid foot striking.

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