Trunk muscle endurance, strength and flexibility in rural subsistence farmers and urban industrialized adults in western Kenya.

OBJECTIVES: High trunk muscle endurance, strength, and moderate flexibility reportedly help maintain musculoskeletal health, but there is evidence for tradeoffs among these variables as well as sex differences in trunk muscle endurance and strength. To test if these observations extend similarly to both men and women in nonindustrial and industrial environments, we investigated intra-individual associations and group and sex differences in trunk muscle endurance, strength, and flexibility among 74 (35 F, 39 M; age range: 18-61 years) adults from the same Kalenjin-speaking population in western Kenya. We specifically compared men and women from an urban community with professions that do not involve manual labor with rural subsistence farmers, including women who frequently carry heavy loads. METHODS: Trunk muscle endurance, strength, and flexibility were measured with exercise tests and electromyography (EMG). RESULTS: We found a positive correlation between trunk extensor strength and endurance (R = .271, p ≤ .05) and no associations between strength or endurance and flexibility. Rural women had higher trunk extensor and flexor endurance, EMG-determined longissimus lumborum endurance, and trunk extensor strength than urban women (all p ≤ .05). Rural women had higher trunk extensor and flexor endurance than rural men (both p ≤ .05). Urban women had lower trunk flexor and extensor endurance than urban men (both p ≤ .01). CONCLUSIONS: High levels of physical activity among nonindustrial subsistence farmers, particularly head carrying among women, appear to be associated with high trunk muscle endurance and strength, which may have important benefits for helping maintain musculoskeletal health.

Foot callus thickness does not trade off protection for tactile sensitivity during walking.

Until relatively recently, humans, similar to other animals, were habitually barefoot. Therefore, the soles of our feet were the only direct contact between the body and the ground when walking. There is indirect evidence that footwear such as sandals and moccasins were first invented within the past 40 thousand years1, the oldest recovered footwear dates to eight thousand years ago2 and inexpensive shoes with cushioned heels were not developed until the Industrial Revolution3. Because calluses-thickened and hardened areas of the epidermal layer of the skin-are the evolutionary solution to protecting the foot, we wondered whether they differ from shoes in maintaining tactile sensitivity during walking, especially at initial foot contact, to improve safety on surfaces that can be slippery, abrasive or otherwise injurious or uncomfortable. Here we show that, as expected, people from Kenya and the United States who frequently walk barefoot have thicker and harder calluses than those who typically use footwear. However, in contrast to shoes, callus thickness does not trade-off protection, measured as hardness and stiffness, for the ability to perceive tactile stimuli at frequencies experienced during walking. Additionally, unlike cushioned footwear, callus thickness does not affect how hard the feet strike the ground during walking, as indicated by impact forces. Along with providing protection and comfort at the cost of tactile sensitivity, cushioned footwear also lowers rates of loading at impact but increases force impulses, with unknown effects on the skeleton that merit future study.

Physical fitness differences between rural and urban children from western Kenya.

OBJECTIVES: To study the effects of urbanization on physical fitness (PF), we compare PF between urban and rural children from western Kenya. We hypothesize that active rural children are stronger, more flexible, and have greater endurance, and that PF differences are predictive of endurance running performance. METHODS: We recruited an age-matched, cross-sectional sample of participants (55 males, 60 females; 6-17 years) from schools near Eldoret, Kenya. PF and anthropometrics were assessed using the FITNESSGRAM®. General linear mixed models (GLMM) and path analyses tested for age, sex, and activity group differences in PF, as well as the effects of PF variables on mile run time. RESULTS: On average, urban participants had greater body mass (36.8 ± 15.9 vs. 31.9 ± 10.9 kg) but were not taller than rural participants (1.4 ± 0.2 vs. 1.4 ± 0.2 cm). Greater urban body mass appears driven by higher body fat (28.2 ± 9.4 vs. 16.8 ± 4.4%), which increased with age in urban but not rural participants. GLMM analyses showed age effects on strength variables (P<0.05) and sex differences in hip flexibility, sit-ups, and mile run (P<0.05). There were few differences in PF between groups except rural participants had stronger back muscles (18.2 ± 4.5 vs. 14.18 ± 4.3 cm) and faster mile times (6.3 ± 0.7 vs. 7.9 ± 2.0 min). Body composition and abdominal strength were predictive of mile time (P < 0.06), but the path analysis revealed a network of interacting direct and indirect effects that influenced endurance performance. CONCLUSIONS: Although differences in endurance and body composition are marked between urban and rural groups, strength and flexibility are not always correlated with overall activity levels. Am. J. Hum. Biol. 28:514-523, 2016. © 2015 Wiley Periodicals, Inc.

Variation in Foot Strike Patterns among Habitually Barefoot and Shod Runners in Kenya.

Runners are often categorized as forefoot, midfoot or rearfoot strikers, but how much and why do individuals vary in foot strike patterns when running on level terrain? This study used general linear mixed-effects models to explore both intra- and inter-individual variations in foot strike pattern among 48 Kalenjin-speaking participants from Kenya who varied in age, sex, body mass, height, running history, and habitual use of footwear. High speed video was used to measure lower extremity kinematics at ground contact in the sagittal plane while participants ran down 13 meter-long tracks with three variables independently controlled: speed, track stiffness, and step frequency. 72% of the habitually barefoot and 32% of the habitually shod participants used multiple strike types, with significantly higher levels of foot strike variation among individuals who ran less frequently and who used lower step frequencies. There was no effect of sex, age, height or weight on foot strike angle, but individuals were more likely to midfoot or forefoot strike when they ran on a stiff surface, had a high preferred stride frequency, were habitually barefoot, and had more experience running. It is hypothesized that strike type variation during running, including a more frequent use of forefoot and midfoot strikes, used to be greater before the introduction of cushioned shoes and paved surfaces.