Elsevier

Clinical Biomechanics

Volume 24, Issue 10, December 2009, Pages 866-871
Clinical Biomechanics

Triceps-surae musculotendinous stiffness: Relative differences between obese and non-obese postmenopausal women

https://doi.org/10.1016/j.clinbiomech.2009.07.015Get rights and content

Abstract

Background

There is a lack of research into the relationship between obesity and muscle–tendon unit stiffness in postmenopausal women. Muscle–tendon unit stiffness appears to affect human motion performance and excessive and insufficient stiffness can increase the risk of bone and soft tissue injuries, respectively. The aim of this study was to investigate the relationship between muscle–tendon unit stiffness and obesity in postmenopausal women.

Methods

105 postmenopausal women (58 [SD 5.5] years) participated. Four groups (normal weight, pre-obese, obesity class I and obesity class II) were defined according World Health Organization classification of body mass index. The ankle muscle–tendon unit stiffness was assessed in vivo with a free oscillation technique using a load of 30% of maximal voluntary isometric contraction.

Findings

ANOVA shows significant difference in muscle–tendon unit stiffness among the groups defined (P < 0.001). Post hoc analysis reveals significant differences between the following groups: normal weight–pre-obese; normal weight–obesity class I and normal weight–obesity class II. The normal weight group had stiffness of 15789 (SD 2969) N/m, pre-obese of 19971 (SD 3678) N/m, obesity class I of 21435 (SD 4295) N/m, and obesity class II of 23497 (SD 1776) N/m.

Interpretation

Obese subjects may have increased muscle–tendon unit stiffness because of fat infiltration in leg skeletal muscles, range of motion restrictions and stability/posture reasons and might be more predisposed to develop musculoskeletal injuries. Normal weight group had identical stiffness values to those reported in studies where subjects were not yet menopausal, suggesting that stiffness might not be influenced by menopause.

Introduction

There is a scarcity of published research relating muscle–tendon unit (MTU) stiffness with the occurrence of injuries. Nevertheless, some reports suggest that insufficient or excessive stiffness can increase the risk of injuries. Butler et al. found that low levels of stiffness were linked to excessive joint motion leading to soft tissue injuries (Butler et al., 2003). If a system becomes too compliant, an overload of structures associated with force attenuation may occur (Williams et al., 2004). High values of leg stiffness, on the other hand, are typically related to increased peak forces, loading rates and shocks (Butler et al., 2003). If a system becomes too stiff (decreased compliance), there may be an increase in forces up the kinetic chain (Williams et al., 2004). This combination of factors usually increases the likelihood of musculoskeletal injury, such as knee osteoarthritis (OA) and stress fractures (Butler et al., 2003).

An injury, sometimes relatively minor, can lead to joint damage and osteoarthritis (Rao et al., 2008). If trauma occurs, midfoot arthritis may also develop. MTU stiffness may influence motion accuracy and joint position (Magnusson et al., 2008) and this could lead to changes in foot posture and plantar loading. Rao et al. suggested that these factors, singly or in combination, could lead to abnormal articular loads and subsequent damage to the tarsometatarsal joints (Rao et al., 2008).

Other painful conditions, like plantar fasciitis (inflammation of the plantar fascia), may also be related to MTU stiffness of the ankle. The plantar fascia is a structure that runs from the front of calcaneus to the phalanges. Both the plantar fascia and the Achilles’ tendon attach to the calcaneus. When tension on the Achilles tendon increases, so does strain on the plantar fascia (Cheung et al., 2006). Excessive traction forces applied to the calcaneus by the plantar fascia causes pain (Fuller, 2000).

MTU stiffness also plays an important role in the effectiveness and efficiency of human motion performance (Shorten, 1987). It seems that some stiffness is needed for motion performance, but excessive or insufficient stiffness can increase the risk of injuries.

Stiffness in its simplest sense can be defined as the ratio between changes in force and changes in tissue length (Butler et al., 2003). The overall stiffness of the human body depends on several components, such as muscles, tendons, ligaments, cartilage, fáscia and bone (Butler et al., 2003). Ankle MTU stiffness reflects total ankle joint stiffness. However, under active conditions, ankle plantar flexors have been assumed to be the main contributors to stiffness (Shorten, 1987). During plantar flexion, passive stiffness contributes less than 5% of total ankle stiffness (Granata et al., 2004, Riemann et al., 2001) in healthy young non-obese subjects. Throughout the present study, the definition of the MTU stiffness encompasses all the aspects mentioned above but the roles of the muscles and tendons are highlighted.

Muscles and tendons exhibit important viscoelastic time-dependant properties. These properties can influence force transmission, energy storage and recoil, spinal reflex responses and the way that movement accuracy and joint position are controlled (Magnusson et al., 2008, Shorten, 1987). Muscles and tendons do not behave exactly like springs. Even so, they have biomechanical properties that can be described by relatively simple elastic models (Fukashiro et al., 2001).

Obese people have a higher risk of OA (Felson et al., 2000). Downward vertical forces transmitted via the tibia tend to flatten the medial longitudinal arch. As body weight increases, more tension is put on the plantar fascia (Cheung et al., 2006). This may increase the likelihood of obese subjects’ developing plantar fasciitis. Other studies have linked an excess of fat infiltration in leg muscles to low calf muscle strength and power and to functional limitations (Hilton et al., 2008). An increase in obesity, estimated by body mass index (BMI), could also reduce standing stability and increase joint stiffness (Edwards, 2007).

At menopause, low levels of estrogen are associated with decreased bone mineral density, skeletal fractures, joint pain and other musculoskeletal complaints (Sievert and Goode-Null, 2005) and a clear deterioration in muscle performance take place from early menopause (Sipilä et al., 2001). Additionally, muscle power decreases with age and this decrease accelerates after menopause (Russo et al., 2003). Menopause is also related to a decrease in lean mass and an increase in fat mass (Sorensen et al., 2001). These changes could alter the viscoelastic properties of the muscles and tendons, altering MTU stiffness.

Triceps-surae MTU stiffness is particularly important as it is regularly used in daily living activities. Excessive body weight may reduce standing stability and be compensated by joint stiffness. Women undergo diverse musculoskeletal changes during their life time that are enhanced with menopause and may contribute to modify MTU stiffness. Further on, to the authors’ knowledge MTU stiffness was never related with different levels of obesity in postmenopausal women. Therefore, the aim of the present study was to investigate if higher levels of obesity lead to higher values of MTU stiffness. A secondary aim was to evaluate if postmenopausal subjects present different values of MTU stiffness from non-postmenopausal subjects examined in other studies.

Section snippets

Subjects

The sample consisted of 105 postmenopausal women recruited through advertising from the surrounding community. All subjects were healthy and had no injuries. An evaluation of the medical history was performed by a physician before the subjects were included in the study. The evaluation used the Bone Estrogen Strength Training (BEST) Study (Center for Physical Activity and Nutrition, 2004) and the Greene scale (Greene, 2008). Subjects with diabetes and/or signs associated with neuropathy were

Results

After an analysis of outliers, ANOVA tests were performed to identify differences in the values of MTU stiffness, MTU stiffness normalized by mass and MVIC between groups classified according WHO scale. Kruskal–Wallis test was also performed to account damping ratio differences between groups. Significant differences (P < 0.001) were found between groups but only for MTU stiffness (Table 1).

Post hoc Hochberg’s GT2 tests revealed significant difference (P < 0.001) between the following pair groups:

Discussion

The main aim of this study was to investigate if higher levels of obesity lead to higher values of MTU stiffness. We found that as BMI increase MTU stiffness also increases. However our data only show statistical differences for MTU stiffness between normal weight group and all the other obesity groups, but no statistical differences were found for MTU stiffness normalized by mass and damping ratio.

To explain the greater MTU stiffness in obese people, we hypothesized that restrictions in range

Conclusion

As body mass index increases, MTU stiffness also increases. However, statistical differences were only found between the following pair groups: normal weight–pre-obese; normal weight–obesity class I and normal weight–obesity class II. Possible causes for these differences may be fat infiltration in leg skeletal muscles, range of motion restrictions and stability/posture reasons. Our normal weight group had identical stiffness values to those reported in studies where subjects were not yet

Conflict of interest statement

The authors declare that this study was not funded and that they have no competing interests.

Acknowledgments

We acknowledge the comments and suggestions of Professor Darlene A. Kluka, School of Human Performance and Leisure Sciences at Barry University, Florida, USA, President of the International Association of Physical Education and Sport for Girls and Women (IAPESGW), chair of the Editorial Board of the International Council of Sport Science and Physical Education (ICSSPE), and founding member of the USA Volleyball Sports Medicine and Performance Commission.

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