Patricia Ann Kramer

The obstetrical dilemma (OD) posits that hominin pelvic morphology reflects a trade-off between obstetric demands and efficient bipedal locomotion– wider pelves facilitate childbirth but increase mechanical demands on stabilizing muscles, potentially raising the energetic cost of walking. Several studies challenge this view, however, demonstrating no strong associations between pelvic width and increased energy expenditure, suggesting that musculoskeletal compensation minimizes locomotor costs via muscle coordination and posture dynamics. Because assessment of evolutionary constraints requires examination of forms outside observed species variation, we used musculoskeletal simulations to evaluate how extreme deviations in functional hip width affect gait parameters. Motion capture and ground reaction data from 10 adults (45 trials) drove subject-specific musculoskeletal models where functional hip widths were altered by ±20% and ±40%. Simulations used identical gait kinematics to isolate the effect of pelvic breadth. Wider pelves increased hip abductor force and metabolic power during mid-stance, while narrower pelves showed inverse trends. Compensatory changes in antagonistic muscle groups (e.g., adductors, flexors) limited net effects. Across all pelvic conditions, the dominant contributors to stride and muscle metabolic cost remained consistent, and total muscle metabolic power stayed within observed inter-individual variation. These results support hypotheses suggesting human musculoskeletal systems can adapt to extreme shifts in pelvic width without substantial muscle metabolic penalties. Contrary to core assumptions of the OD, pelvic breadth alone does not constrain walking effectiveness, suggesting that pelvic evolution was shaped primarily by reproductive, developmental, and structural pressures than by locomotor energetics alone.

Objective: Although pathological variations of the first pedal ray have been well‐studied, non‐pathologic variations, including those potentially linked to hormonal changes during pregnancy, have yet to be examined. Here we quantify the angle between the longitudinal axes of the first and third metatarsals and investigate variation in this angle between females and males and nulliparous and parous individuals. Materials and Methods: Foot CT scans were accessed from Harborview Medical Center and the New Mexico Decedent Image Database. A 3D model was constructed in Avizo Lite 9.0.1 and landmarks were placed on the proximal and distal ends of the first and third metatarsal. The vectors between the landmark sets were used to represent the first and third metatarsal axes. The angle between the two axes was calculated in MATLAB. Wilcoxon Rank Sum tests were used to test for significant differences in this angle between (1) females and males, (2) nulliparous and parous females, and (3) nulliparous females and males. Results: Females have a statistically larger angle compared to males (average 10.16° vs. 8.64°, p = 0.03). Parous females have a statistically larger angle than nulliparous females (average 10.44° vs. 8.74°, p = 0.04). Nulliparous females do not significantly differ from males ( p = 0.70). Discussion: We propose that the relatively larger angle in females is likely due to hormonal exposure, specifically relaxin, throughout pregnancy given that parous females have a relatively larger angle than both males and nulliparous females by 1.5°–1.7°, while nulliparous females share a similar average angle with males.

Objectives: Bone mineral density (BMD), a critical measurement of overall bone health, is sensitive to nutritional, hormonal, immunological, lifestyle, and genetic factors through their impact on bone remodeling and age‐related BMD loss. Further, bone responds to repetitive mechanical loading by increasing bone mineral deposition, suggesting that differences in bone loading could moderate associations between age and BMD by buffering weight‐bearing regions against age‐related resorption. We examine whether age‐BMD relationships differ across skeletal sites and between weight‐bearing (WBR) and nonweight‐bearing (non‐WBR) regions.Methods: Data from adult participants (n = 12,403) of the National Health and Nutrition Examination Survey (NHANES; 2007–2018) were analyzed using multivariate linear regression, adjusting for body mass index, gender, racialized group, socioeconomic status, smoking, alcohol consumption, and sedentary behavior. BMD was evaluated across 12 anatomical regions.Results: WBR and non‐WBRs showed similar age‐related declines in BMD overall, though this pattern varied by gender. Women showed a steeper age‐related BMD decline in non‐WBRs; men showed comparable negative associations between regions. Non‐Hispanic Black participants had significantly higher BMD in every region than non‐Hispanic White participants. Socioeconomic status and heavy smoking were strong predictors of BMD. Notably, increased sedentary time was linked exclusively to lower BMD in non‐WBRs (all p < 0.001).Discussion: These results indicate habitual loading alone may be insufficient to buffer bones against age‐related loss. Rather, social and lifestyle factors significantly moderate the age‐BMD relationship. Further research employing longitudinal data, direct mechanical loading measurements, and bone microarchitecture assessments could help clarify the interplay of biomechanical and social factors influencing BMD.

Objectives: We recently reported that functional pelvic width is not a statistically significant predictor of the metabolic energy expenditure of the hip abductor muscles during walking when mass, stature, and progression velocity were controlled. Others revised this analysis, suggesting that a different statistical approach would yield statistical significance. In order to address this suggestion, herein we refine our original analysis and increase our sample size.Materials and Methods: Using a musculoskeletal model of the human body that estimates the metabolic energy expenditure of muscle activation for 66 people (33 females), 1093 trials were evaluated. Metabolic energy consumption for the abductor muscles was summed over a stride to create total abductor energy expenditure. A mixed effects model allowing the coefficient and slope of velocity to be a random effect was fit to the data.Results: We confirm our original analysis: functional pelvic width is not a statistically significant predictor of muscle metabolic energy expenditure of the hip abductors during walking.Discussion: We take as an initial assumption that a constraint on pelvic width exists and seek to establish if evidence for an energetic constraint is present in estimates of hip abductor muscle metabolic energy expenditure in humans. We find no such evidence in a sample of 66 people. Our results do not address whether or not a constraint on pelvic width exists, and we suggest that simulation of pelvic widths substantially wider or narrower than those observed in laboratory settings is necessary to assess this idea.

Hip stabilization through muscular activation of the gluteals is a key feature of hominin walking, but the role of pelvic shape on muscular activation remains uncertain. Coupled with this is the uncertainty regarding whether the kinematics and kinetics of modern humans are appropriate in extinct hominins. We apply modern human kinematics and kinetics to musculoskeletal models with modern human-like and australopithecine-like hips. We test the prediction that the hip functional complex that includes biacetabular breadth, femoral neck length, and iliac blade flare, produces hip abductor muscle activations that are similar in the modern human- and australopithecine-like forms. Using previously developed musculoskeletal models, we calculated muscle forces using inverse dynamics analyses and a muscle redundancy algorithm for ten individuals who walked at their normal velocity. We found that the shape of the australopithecine-like pelvis produces absolutely higher muscle activations in gluteus medius and gluteus minimus, but lower muscle activations across a long period of stance in gluteus maximus compared to the modern human-like pelvis when kinematics and size are held constant. These results suggest that, while the australopithecine-like pelvis is compatible with human walking patterns, influences on pelvic shape other than accommodating muscle and joint reaction forces during walking are present.

Turning is a ubiquitous feature of human locomotion and like straight path walking, requires muscular force both to propel the individual forward and to stabilize the trunk over the stance limb. The purpose of this study is to identify muscle force patterns while making a turn and compare them to those of straight path walking. Kinematic and kinetic data were collected from 10 adults who walked unshod at their self-selected normal velocity in four conditions: straight line, 45° and 90° turns with a sidestep, and a 45° crossover event. A musculoskeletal model was used to calculate muscle forces in the pelvis and lower limb. Statistical parameter mapping (SPM) was used to determine whether the muscle force patterns of the three turning conditions were different from walking in a straight path. We find that, overall, the muscles that stabilize the hip and ankle during walking demonstrate differences in timing and magnitude of their force patterns across all turning conditions.

Musculoskeletal modeling can be used to estimate forces during locomotion. These models, however, are dependent on underlying assumptions about the model inputs, such as muscle volumes and fiber lengths, to calculate muscle forces. Triceps surae (gastrocnemius medialis, gastrocnemius lateralis, soleus) muscle volume distributions vary among humans. Here we quantify how this muscle volume variation impacts maximum estimated lower limb muscle forces during the braking and propulsive phases of the stance phase of walking. Three triceps surae muscle volume distributions (AnyBody Modeling System standard cadaver [MS], average of 21 cadavers [C], average of 21 young, healthy adults [YHA]) were evaluated in a standard musculoskeletal model using the kinetic and kinematic data of 10 healthy individuals at three walking velocities. Maximum muscle forces were calculated using inverse dynamics and an algorithm to solve the muscle redundancy problem in the AnyBody Modeling System. Repeated measure ANOVAs were used to test for significant differences among the three muscle distribution configurations for each muscle/muscle group at each velocity. Triceps surae muscle volume distribution significantly affects gastrocnemius lateralis and soleus maximum muscle forces for both braking and propulsion at all three velocities (p < 0.001), with relatively larger muscle volumes typically producing relatively larger muscle forces. There was no significant difference in gastrocnemius medialis maximum force among configurations (p > 0.124) except at the self-selected spontaneous velocity during braking. Significant differences exist at some velocities for the hamstrings and gluteus maximus during braking (p < 0.046) and the other plantarflexors, dorsiflexors, evertors, hamstrings, quadriceps, sartorius, and gluteus maximus during propulsion (p < 0.042). Muscle volumes used in musculoskeletal models impact estimated muscle forces of both the muscles of interest and other muscles in the biomechanical chain. This is consistent with recent analyses demonstrating that input values can substantially impact results and suggests individualized muscle parameters may be needed depending on the research question.

Hip stabilization through muscular activation of the gluteals is a key feature of hominin walking, but the role of pelvic shape on muscular activation remains uncertain. Coupled with this is the uncertainty regarding whether or not the kinematics and kinetics of modern humans are appropriate proxies for the patterns in extinct hominins. In this work we treat modern human kinematics and kinetics as a null hypothesis by applying them to musculoskeletal models with modern human-like and australopithecine-like hips and examine the resulting joint reaction and muscle forces. We test the prediction that the hip functional complex that includes biacetabular breadth, femoral neck length, and iliac blade flare produces hip abductor muscle activations and hip joint reactions that are similar in the modern human- and australopithecine-like forms. Using previously developed musculoskeletal models that approximate gluteus maximus with twelve muscle elements, gluteus medius with twelve, and gluteus minimus with three, we calculated joint reaction and muscle forces using inverse dynamics analyses and a muscle redundancy algorithm. We used data from ten individuals who walked at their normal velocity. We found that the shape of the australopithecine-like pelvis produces absolutely higher muscle activations in gluteus medius and gluteus minimus, but lower muscle activations across a long period of stance in gluteus maximus compared to the modern human-like pelvis when kinematics and size are held constant. Hip joint reaction forces are similarly heterogeneous: the australopithecine-like hip exhibits lower forces in the direction of travel but higher in the vertical. These results suggest that, while the australopithecine-like pelvis is compatible with human walking patterns, influences on pelvic shape other than accommodating muscle and joint reaction forces during walking are present.

Musculoskeletal modeling is emerging as a powerful approach to investigate the locomotor biomechanics of extinct taxa. These models rely on bony morphology to define joint locations and muscle geometry. Using different motion profiles to drive models of extinct and extant taxa complicates comparisons because results reflect both morphological and kinematic differences. Here we report on an approach that permits changes in shape while maintaining model kinematics. Using a human musculoskeletal model, we carried out walking simulations of ten humans. Next, we morphed the model pelvis and proximal femur to match a reconstructed australopithecine pelvis and proximal femur. Using the kinematic results from the human walking simulations, we created new motion files based on the positions of joint centers and tracked anatomical locations. These files were used to drive simulations of the same walking trials with the australopithecine model. Joint centers and axes of the human and australopithecine models were compared for each walking trial simulation. We found that joint centers in the australopithecine simulations were typically within ~1µm of their locations in human simulations, and joint axes differed by less than 0.005 degrees. Such small differences have negligible effects on external joint moments calculated during inverse dynamics analyses. This conservative comparison will serve as a baseline for more complex simulations. Although this work focuses on one taxon, the approach outlined is applicable to a wide variety of extinct animals.

Despite a paucity of physiological evidence, simplistic biomechanical analyses have led researchers to assume that humans who have wider hips use more energy to walk. Pitting biomechanical first principles against physiological data has led to little deepening of our understanding of bipedalism and its evolution. Both approaches, however, use proxies for the energy used by muscles. We decided to approach the question directly. Using a musculoskeletal model of the human body that estimates the metabolic energy expenditure of muscle activation for 48 people (23 women), 752 trials were evaluated. Metabolic energy consumption for the abductor muscles was summed over a stride to create total abductor energy expenditure. We calculated the maximum hip joint moment acting in the coronal plane and the functional distance between the hip joint centers. We hypothesize that wider hips would be correlated with both maximum coronal plane hip moment and increased total abductor energy expenditure when mass and velocity were controlled. Linear regressions with multiple independent variables, clustered by participant to control for the non-independence of the data points, were performed in Stata. We found that hip width does not predict total abductor energy expenditure, although mass and velocity combine to predict 61% of the variation (both p<0.001). Maximum hip joint coronal plane moment is predicted by pelvic width (p<0.001) and, in combination with mass and velocity (both p<0.001), explains 79% of the variation. Our results indicate that people use their morphology in ways that limit differences in energy expenditure. Consistent with recent discussion, intraspecific variation might not be useful to understand differences among species.