Ewald M. Hennig

Musculoskeletal disorders associated with obesity: a biomechanical perspective

Despite the multifactorial nature of musculoskeletal disease, obesity consistently emerges as a key and potentially modifiable risk factor in the onset and progression of musculoskeletal conditions of the hip, knee, ankle, foot and shoulder. To date, the majority of research has focused on the impact of obesity on bone and joint disorders, such as the risk of fracture and osteoarthritis. However, emerging evidence indicates that obesity may also have a profound effect on soft‐tissue structures, such as tendon, fascia and cartilage. Although the mechanism remains unclear, the functional and structural limitations imposed by the additional loading of the locomotor system in obesity have been almost universally accepted to produce aberrant mechanics during locomotor tasks, thereby unduly raising stress within connective‐tissue structures and the potential for musculoskeletal injury. While such mechanical theories abound, there is surprisingly little scientific evidence directly linking musculoskeletal injury to altered biomechanics in the obese. For the most part, even the biomechanical effects of obesity on the locomotor system remain unknown. Given the global increase in obesity and the rapid rise in musculoskeletal disorders, there is a need to determine the physical consequences of continued repetitive loading of major structures of the locomotor system in the obese and to establish how obesity may interact with other factors to potentially increase the risk of musculoskeletal disease.

Plantar pressure differences between obese and non-obese adults: a biomechanical analysis

OBJECTIVE: To investigate plantar pressure differences between obese and non-obese adults during standing and walking protocols using a pressure distribution platform.SUBJECTS: Thirty-five males (age 42.4±10.8 y; 67–179 kg) and 35 females (age 40.0±12.6 y; 46–150 kg) divided into obese (body mass index (BMI) 38.75±5.97 kg/m2) and non-obese (BMI 24.28±3.00 kg/m2) sub-groups, respectively.MEASUREMENTS: Data collection was performed with a capacitive pressure distribution platform with a resolution of 2 sensors/cm2 (Emed F01, Novel GmbH, München). The measurement protocol included half and full body weight standing on the left, right and both feet, respectively, and walking across the platform, striking with the right foot. Pressures were evaluated for eight anatomical sites under the feet.RESULTS: For both men and women, the mean pressure values of the obese were higher under all anatomical landmarks during half body weight standing. Significant increases in pressure were found under the heel, mid-foot and metatarsal heads II and IV for men and III and IV for women. Foot width during standing was also significantly increased in obese subjects. For walking, significantly higher peak pressures were also found in both obese males and females.CONCLUSION: Compared to a non-obese group, obese subjects showed increased forefoot width and higher plantar pressures during standing and walking. The greatest effect of body weight on higher peak pressures in the obese was found under the longitudinal arch of the foot and under the metatarsal heads. The higher pressures for obese women compared to obese men during static weight bearing (standing) may be the result of reduced strength of the ligaments of the foot.

The influence of stretching and warm‐up exercises on Achilles tendon reflex activity

The aim of this study was to investigate the acute effects of prior exercise (warm-up and stretching) on the electromyographic and force output of mechanically elicited triceps surae reflexes. Fifty male subjects performed eight reflex experiments under each of three successive conditions in one session: (1) no prior exercise, (2) after static stretching of the passive triceps surae (3 min) and (3) after a 10-min warm-up run on a treadmill. Tendon tap reflex force was elicited in the triceps surae of the right leg by means of a standardized reflex hammer and measured in a custom-built fixture. Electromyographic (EMG) signals were recorded with surface electrodes over the medial head of the gastrocnemius (G) and the soleus (S). Low coefficients of variation within subjects contrasted with high between-subject variations, indicating highly individual reflex characteristics. After stretching, reductions in the peak force (-5%; P < 0.05), the force rise rate (-8%; P < 0.01), the half relaxation rate (-5%; N.S.), the EMG amplitudes (G, -16%; S, -17%; P < 0.01) and integrals (G, -15%; S, -18%; P < 0.01), and an increase in EMG latencies (G, +3%; S, +1%; P < 0.01), were found compared with the values obtained without prior exercise. After running, the peak force reached the values obtained without prior exercise (-2%; N.S.), the force rise rate and half relaxation rate increased by 8 and 12%, respectively (P < 0.01), and the impulse (force-time integral; -12%), EMG amplitudes (G, -20%; S, -23%; P < 0.01), integrals (G, -18%; S, -23%; P < 0.01) and latencies (G, -1%; S, -2%; P < 0.01) decreased significantly. The changes in the force characteristics observed after the stretching treatment indicate improved muscle compliance that might reduce the risk of injury. On the other hand, the changes after the additional warm-up run had a more pronounced influence with regard to improved force development and a decreased EMG activity, which can be viewed as a performance-enhancing effect.

The biomechanics of adiposity – structural and functional limitations of obesity and implications for movement

Obesity is a significant health problem and the incidence of the condition is increasing at an alarming rate worldwide. Despite significant advances in the knowledge and understanding of the multifactorial nature of the condition, many questions regarding the specific consequences of the disease remain unanswered. For example, there is a dearth of information pertaining to the structural and functional limitations imposed by overweight and obesity. A limited number of studies to date have considered plantar pressures under the feet of obese vs. non‐obese, the influence of foot structure on performance, gait characteristics of obese children and adults, and relationships between obesity and osteoarthritis. A better appreciation of the implications of increased levels of body weight and/or body fat on movement capabilities of the obese would provide an enhanced opportunity to offer more meaningful support in the prevention, treatment and management of the condition.

The biomechanics of restricted movement in adult obesity

In spite of significant advances in the knowledge and understanding of the multi‐factorial nature of obesity, many questions regarding the specific consequences of the disease remain unanswered. In particular, there is a relative dearth of information pertaining to the functional limitations imposed by overweight and obesity. The limited number of studies to date have mainly focused on the effect of obesity on the temporospatial characteristics of walking, plantar foot pressures, muscular strength and, to a lesser extent, postural balance. Collectively, these studies have implied that the functional limitations imposed by the additional loading of the locomotor system in obesity result in aberrant mechanics and the potential for musculoskeletal injury. Despite the greater prevalence of musculoskeletal disorders in the obese, there has been surprisingly little empirical investigation pertaining to the biomechanics of activities of daily living or into the mechanical and neuromuscular factors that may predispose the obese to injury. A better appreciation of the implications of increased levels of body adiposity on the movement capabilities of the obese would afford a greater opportunity to provide meaningful support in preventing, treating and managing the condition and its sequelae. Moreover, there is an urgent need to establish the physical consequences of continued repetitive loading of major structures of the body, particularly of the lower limbs in the obese, during the diverse range of activities of daily living.

The Pathomechanics of Plantar Fasciitis

Plantar fasciitis is a musculoskeletal disorder primarily affecting the fascial enthesis. Although poorly understood, the development of plantar fasciitis is thought to have a mechanical origin. In particular, pes planus foot types and lower-limb biomechanics that result in a lowered medial longitudinal arch are thought to create excessive tensile strain within the fascia, producing microscopic tears and chronic inflammation. However, contrary to clinical doctrine, histological evidence does not support this concept, with inflammation rarely observed in chronic plantar fasciitis. Similarly, scientific support for the role of arch mechanics in the development of plantar fasciitis is equivocal, despite an abundance of anecdotal evidence indicating a causal link between arch function and heel pain. This may, in part, reflect the difficulty in measuring arch mechanics in vivo. However, it may also indicate that tensile failure is not a predominant feature in the pathomechanics of plantar fasciitis. Alternative mechanisms including ‘stress-shielding’, vascular and metabolic disturbances, the formation of free radicals, hyperthermia and genetic factors have also been linked to degenerative change in connective tissues. Further research is needed to ascertain the importance of such factors in the development of plantar fasciitis.

Plantar Pressure Distribution Patterns of Young School Children in Comparison to Adults

Peak pressures and relative loads were determined under the feet of 125 children between 6 and 10 years of age. These results were compared with previously published data from 111 adults. A capacitive pressure distribution platform with a resolution of 2 sensors/cm 2 was used for data collection during walking. As compared with the group of adults, the school children showed considerably lower peak pressures under all anatomical structures. Larger foot dimensions with respect to body weight result in reduced foot pressures for the children by distributing the ground reaction forces across larger contact areas. With increasing age, a medial load shift in the forefoot could be observed for the older children. Data analysis of the pressures under the midfoot revealed that the longitudinal foot arch development is almost complete before the age of 6. Contrary to the findings in adults, body weight was identified to be of major influence on the magnitude of the pressures under the feet of school children. No differences were found for the foot pressures between boys and girls.

Differential shock transmission response of the human body to impact severity and lower limb posture.

The shocks imparted to the foot during locomotion may lead to joint-degenerative diseases and jeopardize the visual-vestibular functions. The body relies upon several mechanisms and structures that have unique viscoelastic properties for shock attenuation. The purpose of the present study was to determine whether impact severity and initial knee angle (IKA) could alter the shock transmission characteristics of the body. Impacts were administered to the right foot of 38 subjects with a human pendulum device. Combinations of velocities (0.9, 1.05 and 1.2 m s-1) and surfaces (soft and hard foams) served to manipulate impact severity in the first experiment. Three IKA (0, 20 and 40 degrees) were examined in the second experiment. Transmission between shank and head was characterized by measuring the shock at these sites with miniature accelerometers. Velocity and surface had no effect on the frequency profile of shock transmission suggesting a consistent response of the body to impact severity. Shank shock power spectrum features accounted for the lower shock ratio (head/shank) measured under the hard surface condition. IKA flexion caused considerable reduction in effective axial stiffness of the body (EASB), 28.7-7.9 kNm-1, which improved shock attenuation. The high correlation (r = 0.97) between EASB and shock ratio underscored the importance of EASB to shock attenuation. The present findings provide valuable information for the development of strategies aimed at protecting the joints, articular cartilage, spine and head against locomotor shock.

Pressure Distribution Patterns under the Feet of Children in Comparison with Adults

Peak pressures and regional impulses were determined under the feet of 15 children and 111 adults by means of a capacitive pressure distribution platform. The measurements were taken during walking and running and revealed insights into foot function during the process of locomotion skill acquisition. Considerably reduced peak pressures in the infant group could be attributed to a softer foot structure and a lower body-weight to foot-contact area ratio. An almost three times higher relative load under the midfoot of the infant foot shows that the longitudinal foot arch is still a weak structure. Within a few months of gait development remarkable changes toward an adult loading pressure pattern were observed.

The arch index: A measure of flat or fat feet?

Background: Studies using footprint-based estimates of arch height have indicated that obesity results in a lowered medial longitudinal arch in children. However, the potentially confounding effect of body composition on indirect measures of arch height, such as the arch index, has not been investigated. Methods: This study assessed the body composition of 12 male and 12 female adults (mean age: 39.9 ± 8.1 years, height: 1.724 ± 0.101 m; weight: 95.1 ± 13.7 kg, and BMI: 31.9 ± 3.0kg/m 2 ) using bioelectrical impedance analysis to produce a two-component model of fat mass (FM) and fat-free mass (FFM). The dynamic arch index also was determined from electronic footprints captured during gait using a capacitive pressure distribution platform with a resolution of 4 sensors/cm2. Results: While significant correlations were noted between FFM and the area of both the hindfoot (r = .75, p <.05) and forefoot (r = .72, p <.05), the midfoot area was correlated only with FM (r = .54, p <.05). Similarly, the arch index was significantly correlated with the FM percentage (r = .67, p <.05). Conclusions: The findings of this pilot study suggest that body composition influences arch index values in overweight and obese subjects. Consequently, body composition may be a confounding factor in interpreting footprint based estimates of arch height and, as such, these estimates would best be used with supplementary measures of body composition.