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As women age, changes in hormones affect muscle strength, joint laxity and bone density making them more susceptible to conditions such as osteoporosis, tendon and ligament injuries (Cauley, 2015 ; Chidi-Ogbolu and Baar, 2019) and hip and vertebral fractures (Cauley, 2015). It has been reported women lose more of their strength in their 5th decade as a result of menopause (Cauley, 2015; Leblanc et al., 2017).
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Power has been reported to be significantly more valuable than loss of muscle strength (Maltais, Desroches and Dionne, 2009). Power enables us in old age to continue with normal daily living activities and supports the loss of balance and reduces risk and occurrences of falls (Maltais, Desroches and Dionne, 2009). This report will investigate the factors influencing loss of strength and power on the musculoskeletal system from the effects of menopause.
This review paper will focus on age related changes to musculoskeletal health of females from a clinical biomechanical perspective, with a specific focus on the impact of hormonal changes due to menopause. The review paper will discuss the effects of estrogen and other steroid hormones on bone mineral density (BMD) and the mechanism of muscle tendon unit (MTU) stiffness and their influence on increase risk to injury.
Changes in BMD and MTU composition during and after menopause
Estrogen regulates bone metabolism and turnover, during menopause the loss of ovarian estrogen is strongly connected to the decline in BMD (Cauley, 2015). The loss of estrogen increases the risk of hip and vertebral fractures in older women (Cauley, 2015). With the decline in BMD occurrences of falls, joint pain, stiffness and reduced power negatively influence daily living activities (Faria et al., 2010).
Muscle tendon unit (MTU) is associated with human motion performance and injury (Butler et al., 2003). Symptoms of menopause include muscle and joint pain that are related to physical fragility and reduced mobility affecting the fitness of postmenopausal women (Faria et al., 2010).
MTU elasticity and muscle tendon stiffness in perimenopause and post-menopause
Faria et al (2010) makes a statement that menopause has no statistical influence on stiffness (Faria et al., 2010). The statement is based on Faria et al’s (2010) study of 93 Caucasian menopausal women whereby data was collected measuring MTU stiffness using vertical ground reaction force, sampling rate of 12000Hz, to calculate maximum voluntary isometric contraction (Faria et al., 2010). The data compared postmenopausal women taking hormone replacement therapy to those who did not and a group with induced menopause and found there was no significant difference between the groups in MTU stiffness (Faria et al., 2010). This study suggests that menopause had no direct influence on muscle tendons (Faria et al., 2010).
However, a later study conducted by Faria et al (2011) concluded contrasting results. The study compared the biomechanical properties of triceps surae MTU between 39 young women and 37 menopausal women. A force plate was used to obtain vertical ground reaction force, at a sampling rate of 12000Hz, to calculate Maximal voluntary isometric contraction (Faria et al., 2011). The results from this study indicated a significant increase in MTU stiffness in the postmenopausal group. This is due to the declining levels of estrogen during menopause that will contribute to reduced bone mass density and higher risk of musculoskeletal related ailments (Faria et al., 2011; Leblanc et al., 2017).
Another factor contributing to the altering MTU stiffness is the decline in lean muscle mass and increased fat mass during menopause, which may have an effect on viscoelastic properties of muscles and tendons (Faria et al., 2009). This reduction of muscle mass, known as sarcopenia, occurs with aging especially during menopause (Faria et al., 2009; (Messier et al., 2011). Changes and a decline in hormone levels during menopause impact muscle protein synthesis (Faria et al., 2009). Increased degeneration of the sarcopenia and bone mineral density is more significant in females during perimenopause, this is when the greatest change in estrogen levels occurs (Cauley, 2015; Leblanc et al., 2017)
Menopause adversely affects muscles and tendons which influences the behaviour of contractile components and series elastic component, that may have an impact on MTU stiffness (Faria et al 2011). The contractile component turns nerves signal into force and affects the response of the series elastic component (Faria et al 2011). Other contributing factors of the viscoelasticity of the series elastic components are tendon and other collagenous structures within muscle fibres (Faria et al 2011). The changes in length and speed of the contractile component can therefore lead to differences in the way the MTU functions (Faria et al 2011). The interaction between contractile component and series elastic component has an effect on the MTU stiffness (Faria et al 2011).
Having insufficient or excessive stiffness can lead to injuries (Faria et al 2011). The reduced stiffness of MTU increases joint motion with eccentric muscle activity, which raises the risk of soft tissue injuries (Butler et al 2003; (Faria et al 2010). However, decreased compliance due to too much stiffness increases the load further up the kinetic chain, thereby contributing to a greater risk of musculoskeletal injuries such as osteoarthritis and stress fractures (Butler et al 2003; (Faria et al., 2009; (Faria et al., 2010).
Changes in muscle power and strength due to menopause
With age there is a loss of fast motor units thereby negatively affecting muscle strength and power required for physical movement (Messier et al., 2011). The specific fibre tension reduces, which is a factor in triggering a decline in intrinsic muscle force in older individuals (Faria et al., 2011). Muscle power has been reported to be more significant than loss of isometric strength for carrying out daily living activities (Maltais, Desroches and Dionne, 2009). The loss of power declines more than that of isometric strength in old age (Maltais, Desroches and Dionne, 2009).
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A early study by Bassey et al (1996) of 89 premenopausal, 92 menopausal and 33 peri-menopausal women indicated estrogen status had no influence in retaining strength in leg extensor power between these groups, Bassey at al’s (1996) concluded that there is no protective mechanism of estrogen on muscle strength and power (Bassey, Mockett and Fentem, 1996; (Maltais, Desroches and Dionne, 2009). In contrast to this statement a later study conducted by Carville et al’s (2005) supports the statement that estrogen influences muscle power (Carville, Rutherford and Newham, 2005) Carville et al’s (2005) study reported postmenopausal women taking hormone replacement therapy (HRT) with higher concentration of estrogen, were protected from loss of power but not isometric strength (Carville, Rutherford and Newham, 2005).
Ronkainen et al’s (2009) study of 15 pairs of monozygotic twins, one of each pair of twins was taking HRT for a long period whilst the other pairs was not (Ronkainen et al., 2009). The twins in the study using HRT showed to have far more muscle power and higher leans muscle mass than the twins not taking HRT (Ronkainen et al., 2009). The results conclude that changes to muscle mass and viscoelastic properties to muscle tendon are influenced by the levels of estrogen (Ronkainen et al., 2009). The study of identical twins is more convincing as it is comparing like for like, the genetic and physiological variations are minimal between the twin group study of those taking HRT and the non-users of HRT (Ronkainen et al., 2009; Tiidus, 2011)
Stiffness affecting muscle strength and power and risk to injury
Menopause contributes to the decline in muscle mass and strength (Narici, and Maganaris, 2006). Faria et al’s (2011) study concludes that this promotes changes in the triceps surae MTU stiffness (Faria et al., 2011). The results from Faria et al’s (2011) study of the triceps surae reported postmenopausal women had higher MTU stiffness compared to young women (Faria et al., 2011). The excessive MTU stiffness in postmenopausal women, proposes a higher risk of musculoskeletal injuries (Faria et al., 2011).
Viscoelastic properties are influenced by menopausal symptoms which then in turn affects the muscle power generated (Faria et al., 2010). The Ronkainen et al’s (2009) study of the monozygotic twin also confirms this statement. Viscoelastic properties affected the level of power generated and enhanced the muscle performance of the twin group taking HRT to reduce symptoms of menopause on the musculoskeletal system (Ronkainen et al., 2009).
Estrogen levels in muscle fibres affecting muscle strength and power
The decreased production of adrenal steroid hormone androstenedion, dehydroepian- drostenedion (DHEA) also occurs with menopause. DHEA is converted into estradiol, progesterone and testosterone in ovarian tissue. These deceased levels have been associated with reduced muscle strength and force (Sipilä and Poutamo, 2003).
Sarcopenia is the loss of muscle mass and the increase in muscle weakness that is related with aging (Messier et al., 2011), during menopause sarcopenia is accelerated due to the decline in estrogen levels (Messier et al., 2011). Estrogen increases collagen content in connective tissue and improves muscle mass and strength. During menopause, the decline in estrogen has shown to negatively influence muscle force resulting in increased risk of injury (Cauley, 2015).
Higher levels of estrogen increase laxity in soft tissue structures resulting in increased risk of ligament and tendon injury (Chidi-Ogbolu and Baar, 2019). The low levels of estrogen that occur during menopause, however, have shown to increase tendon stiffness, which reduces the risk of soft tissue injury (Cauley, 2015; Chidi-Ogbolu and Baar, 2019). Conversely, more frequent periods of ovulation during the perimenopause phase means that women have a greater risk of tendon injuries than postmenopausal women, due to the higher estrogen levels being released into the muscle tissue (Leblanc et al., 2017).
Conclusion 200 words
The effect of estrogen on bone health is well established; more recent studies on the effects of other soft tissue structures such as tendons, ligaments and collagen synthesis have shown contrasting results (Cauley, 2015).
Studies of estrogen protecting strength and power during and postmenopausal women have conflicting results. Bassey et al (1996) study suggest estrogen has no protecting influence on muscle strength and power in contrast Carville et al’s (2005) supports the statement that estrogen influences muscle power based on a study of menopausal women taking HRT. Results from Carville et al’s (2005) is supported by the Ronkainen et al’s (2009) study of the monozygotic twins.
Due to the complexity of the female sex hormones further research needs to be conducted to understand the effects hormones have on MTU stiffness, muscle strength and power, thus, influencing ligament and tendon laxity and other soft tissue structures in menopausal women (Sipilä and Poutamo, 2003).
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