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A wider pelvis does not increase locomotor cost in humans, with implications for the evolution of childbirth.

Warrener AG, Lewton KL, Pontzer H, Lieberman DE - PLoS ONE (2015)

Bottom Line: This trade-off, known as the obstetrical dilemma, is invoked to explain the relative difficulty of human childbirth and differences in locomotor performance between men and women.The results show that pelvic width does not predict hip abductor mechanics or locomotor cost in either women or men, and that women and men are equally efficient at both walking and running.Since a wider birth canal does not increase a woman's locomotor cost, and because selection for successful birthing must be strong, other factors affecting maternal pelvic and fetal size should be investigated in order to help explain the prevalence of birth complications caused by a neonate too large to fit through the birth canal.

View Article: PubMed Central - PubMed

Affiliation: Department of Human Evolutionary Biology, Harvard University, 11 Divinity Avenue, Cambridge, Massachusetts, 02138, United States of America.

ABSTRACT
The shape of the human female pelvis is thought to reflect an evolutionary trade-off between two competing demands: a pelvis wide enough to permit the birth of large-brained infants, and narrow enough for efficient bipedal locomotion. This trade-off, known as the obstetrical dilemma, is invoked to explain the relative difficulty of human childbirth and differences in locomotor performance between men and women. The basis for the obstetrical dilemma is a standard static biomechanical model that predicts wider pelves in females increase the metabolic cost of locomotion by decreasing the effective mechanical advantage of the hip abductor muscles for pelvic stabilization during the single-leg support phase of walking and running, requiring these muscles to produce more force. Here we experimentally test this model against a more accurate dynamic model of hip abductor mechanics in men and women. The results show that pelvic width does not predict hip abductor mechanics or locomotor cost in either women or men, and that women and men are equally efficient at both walking and running. Since a wider birth canal does not increase a woman's locomotor cost, and because selection for successful birthing must be strong, other factors affecting maternal pelvic and fetal size should be investigated in order to help explain the prevalence of birth complications caused by a neonate too large to fit through the birth canal.

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Metabolic cost of hip abductor activation and net locomotor cost in men and women.a. Hip abductor mass-specific cost is higher in women at both a walk (P = 0.006) and a run (P = 0.006) due to lower EMA from shorter average abductor moment arm length (see text, Table 2) b. Net mass-specific locomotor cost does not differ between men and women at a walk or a run. Black bar indicates mean, box indicates mean±1SD and whiskers are mean±2SD.
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pone.0118903.g003: Metabolic cost of hip abductor activation and net locomotor cost in men and women.a. Hip abductor mass-specific cost is higher in women at both a walk (P = 0.006) and a run (P = 0.006) due to lower EMA from shorter average abductor moment arm length (see text, Table 2) b. Net mass-specific locomotor cost does not differ between men and women at a walk or a run. Black bar indicates mean, box indicates mean±1SD and whiskers are mean±2SD.

Mentions: Hip abductor cost estimated from the active muscle volume cost regression was 35% higher in women than men during both walking (P = 0.006) and running (P = 0.006) due to lower hip abductor EMA in women (Fig. 3a, Table 2). However, because the hip abductors accounted for a relatively small proportion of the total cost of walking (6.3±1.8%) and running (9.7±2.3%) (also see [41,42]), net mass-specific locomotor economy did not differ significantly between men and women (walk, P = 0.63; run, P = 0.94; Fig. 3b, Table 2). Therefore, the differences between men and women in hip abductor EMA and cost attributed to smaller hip abductor r were not great enough to influence overall locomotor economy.


A wider pelvis does not increase locomotor cost in humans, with implications for the evolution of childbirth.

Warrener AG, Lewton KL, Pontzer H, Lieberman DE - PLoS ONE (2015)

Metabolic cost of hip abductor activation and net locomotor cost in men and women.a. Hip abductor mass-specific cost is higher in women at both a walk (P = 0.006) and a run (P = 0.006) due to lower EMA from shorter average abductor moment arm length (see text, Table 2) b. Net mass-specific locomotor cost does not differ between men and women at a walk or a run. Black bar indicates mean, box indicates mean±1SD and whiskers are mean±2SD.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4356512&req=5

pone.0118903.g003: Metabolic cost of hip abductor activation and net locomotor cost in men and women.a. Hip abductor mass-specific cost is higher in women at both a walk (P = 0.006) and a run (P = 0.006) due to lower EMA from shorter average abductor moment arm length (see text, Table 2) b. Net mass-specific locomotor cost does not differ between men and women at a walk or a run. Black bar indicates mean, box indicates mean±1SD and whiskers are mean±2SD.
Mentions: Hip abductor cost estimated from the active muscle volume cost regression was 35% higher in women than men during both walking (P = 0.006) and running (P = 0.006) due to lower hip abductor EMA in women (Fig. 3a, Table 2). However, because the hip abductors accounted for a relatively small proportion of the total cost of walking (6.3±1.8%) and running (9.7±2.3%) (also see [41,42]), net mass-specific locomotor economy did not differ significantly between men and women (walk, P = 0.63; run, P = 0.94; Fig. 3b, Table 2). Therefore, the differences between men and women in hip abductor EMA and cost attributed to smaller hip abductor r were not great enough to influence overall locomotor economy.

Bottom Line: This trade-off, known as the obstetrical dilemma, is invoked to explain the relative difficulty of human childbirth and differences in locomotor performance between men and women.The results show that pelvic width does not predict hip abductor mechanics or locomotor cost in either women or men, and that women and men are equally efficient at both walking and running.Since a wider birth canal does not increase a woman's locomotor cost, and because selection for successful birthing must be strong, other factors affecting maternal pelvic and fetal size should be investigated in order to help explain the prevalence of birth complications caused by a neonate too large to fit through the birth canal.

View Article: PubMed Central - PubMed

Affiliation: Department of Human Evolutionary Biology, Harvard University, 11 Divinity Avenue, Cambridge, Massachusetts, 02138, United States of America.

ABSTRACT
The shape of the human female pelvis is thought to reflect an evolutionary trade-off between two competing demands: a pelvis wide enough to permit the birth of large-brained infants, and narrow enough for efficient bipedal locomotion. This trade-off, known as the obstetrical dilemma, is invoked to explain the relative difficulty of human childbirth and differences in locomotor performance between men and women. The basis for the obstetrical dilemma is a standard static biomechanical model that predicts wider pelves in females increase the metabolic cost of locomotion by decreasing the effective mechanical advantage of the hip abductor muscles for pelvic stabilization during the single-leg support phase of walking and running, requiring these muscles to produce more force. Here we experimentally test this model against a more accurate dynamic model of hip abductor mechanics in men and women. The results show that pelvic width does not predict hip abductor mechanics or locomotor cost in either women or men, and that women and men are equally efficient at both walking and running. Since a wider birth canal does not increase a woman's locomotor cost, and because selection for successful birthing must be strong, other factors affecting maternal pelvic and fetal size should be investigated in order to help explain the prevalence of birth complications caused by a neonate too large to fit through the birth canal.

Show MeSH