CARACTERITZACIÓ FUNCIONAL DE LA INTERACCIÓ RCAN1 CALCINEURINA A LIMFÒCITS T HUMANS

For the entire muscle-tendon complex, the amount of force tension generated depends on the number of fibres recruited, their pennation angle, the velocity of contraction and on the relative length of the entire configuration with the respect of their optimal length (Hong and Bartlett, 2008).

The force developed by the muscle during isometric contraction varies with its starting length. For each muscle there is an optimal length (the length it assumes in the body at rest) at which a muscle can generate maximal active contraction (Delp, 1990, Hoy et al., 1990b, Winter, 2009, Panjabi and White, 2001). It has been shown that muscle develops only 50% of its maximum force when its length is shortened to 85% of its resting length (Panjabi and White, 2001). The connective tissues that surround the contractile elements influence the force-length curve (called the parallel elastic components) act much like an elastic band (Winter, 2009). The summation of all the connective tissues in series with the contractile component, including the tendon, are called the series elastic elements. When the muscle- tendon complex is at resting length or less, the parallel elastic component is in a slack state and is not tense. When the muscle-tendon lengthens, the elastic tension begins to build up in the muscle and tendon, slowly at first and then more rapidly. This is known as passive tension (force). The total tension is the sum of the active and passive tensions and depends on the amount of connective tissues (elastic elements) that a specific muscle-tendon complex has. For single joint muscles, the amount of stretch is less efficient with regards to passive tension when compared to two-joint muscles such as soleus and gastrocnemius. Therefore, the total force generated by two-joint muscles (i.e the sum of active and passive tension) may reach the maximum available tension in the stretched muscle.

Figure 4.18 shows the total (passive and active) tension for a muscle-tendon isometric contraction.

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Figure 4.18: Length-tension relationship for the whole muscle during isometric contraction [adapted from (Hong and Bartlett, 2008)].

Figure 4.19: Tendon tension resulting from various levels of muscle activation. Parallel elastic element generates tension independent of the activation of contractile element [adapted

from (Winter, 2009)].

The typical overall force-length characteristics of an MTU as function of the percentage of excitation is shown in figure 4.19.

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Arnold et al (2010) recently published the mechanical properties and architecture of lower limb muscles obtained from 21 cadavers (table 4.3) where PCSA represents their physiological cross-section areas, and where optimal fibre length denotes the resting size of the fibre at which it can generate maximum tension.

Table 4.3: Muscle architecture parameters of 21 cadavers (Arnold et al., 2010).

Muscle PCSA (cm) Peak force (N) Optimal fibre length (cm) Tendon slack length (cm) Pennation Angle (°)

Biceps femoris long head 11.6 705.2 9.8 32.2 11.6

Rectus femoris 13.9 848.8 7.6 34.6 13.9 Soleus 58.8 3585.9 4.4 28.2 28.3 Tibials anterior 11.0 673.7 6.8 24.1 9.6 Gastrocnemius medial head 21.4 1308.0 5.1 40.1 9.9

Tendon slack length is the length of tendon at which force begins to develop when stretched. This data can be used to calculate the maximum force of the entire muscle system and it can also be used to aid the development of gait analysis simulation software such as OpenSim to estimate the effect of gait alterations on muscle-tendon properties. It also means that this muscle-tendon unit position is the best to generate internal muscle force to perform the task with the less muscle work done.

The maximum force (𝐹_𝑚𝑎𝑥) for the whole muscle may be calculated as: 𝐹_𝑚𝑎𝑥 = 𝑃𝐶𝐴 ∗ 𝐾

Where PCA is the physiological cross-section area and K is a constant (20 to 100 N*cm-2_). For pennated muscles, PCA is calculated as:

𝑃𝐶𝐴 =𝑚 ∗ 𝑐𝑜𝑠𝛼 𝜌 ∗ 𝐿

Where m is the mass of the muscle, ρ is its density (1.056 g*cm-2_{), L is the length of the} muscle fibres and α is pennation angle of the muscle (Winter, 2009).

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4.4 Comment

If a footwear test condition were designed to potentially keep the ankle plantarflexors at their optimal length during a powerful isometric contraction, this would be the most efficient tension, which would result in less muscle unit recruitment and therefore less oxygen consumption. It may not be comfortable for older people with IC to stretch their calf muscles by wearing an extremely negatively-pitched shoe in order to produce enough passive elastic force to achieve efficient tension generation in the calf muscle in order to reduce oxygen consumption. This would also alter their typical normal gait resulting in joint angle alterations in the knee, ankle and hip joints. Stretching the calf muscle would produce greater ankle dorsiflexion and would also cause different changes in knee angle. Dorsiflexion tends to be greater with a flexed knee than with it extended because of the influence of the gastrocnemius, which crosses both the ankle and knee joints. When the knee is flexed, the gastrocnemius is slacker at the knee, allowing it to stretch more at the ankle, and if the knee is extended, the gastrocnemius is more stretched proximally, allowing it to stretch less at the ankle [this is known as passive insufficiency - (Alter, 2004)].

A negative heel may therefore not be able to produce significant elastic force as the knee could adapt to the walking pattern by being flexed to facilitate more comfortable walking. Soleus is a postural muscle and stretching it may not improve posture control. The other factor is ageing. As a result of ageing, elastic fibres lose their resiliency and undergo various other alterations (Bick, 1961). When the ankle is dorsiflexed, it has a smaller gastrocnemius muscle moment arm with respect to the ankle joint centre, and it may also reduce internal moment generation about ankle joint even if elastic energy is high.

There is also evidence to suggest that older subjects tend to reduce their walking speed and it consequently reduces their soleus muscle length during the entire stance phase (Panizzolo et al., 2013). In one study, eight healthy subjects (aged 25.8 ± 3.5 years) and eight healthy older adults (66.1 ± 2.3 years) were compared by analysing their natural walking speed and matched walking speed in relation to soleus muscle lengthening using ultrasound and a motion analysis system in the gait laboratory. Natural walking speed for older subjects was reduced by 20% compared to healthy subjects. Also, because of walking speed adaptation, the kinematic data, EMG and soleus length data were very similar between elderly and young subjects (everyone walked with the comfortable their own walking speed). However,

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when older subjects adjusted their walking speed to that of the younger subjects (20% faster), there were significant differences in soleus muscle lengths. For the older population, the soleus muscle was significantly stretched throughout the whole gait cycle as shown on the figure 4.20.

Figure 4.20: Comparison of normalised soleus muscle length between young adults (YA) and old adults (OA) during walking: (A) – preferred walking speed in YA and OA, (B) matched

walking speed between YA and OA [adapted from (Panizzolo et al., 2013)].

These results suggest that older people tend to reduce stretching their soleus muscle by slowing down their walking speed to keep optimal length and sustain a more natural gait pattern similar to younger people (Panizzolo et al., 2013). This could be due to muscle architecture changes related to the muscle aging process (for example, muscles tissues become stiffer) and older people tend to reduce soleus muscle stretching to compensate this effect.

Further research which used the simulation software OpenSim and a newly developed muscle architecture data model, has demonstrated that with increasing walking speed, a reduction in soleus force generation is demonstrated (Arnold et al., 2013). Therefore, stretching muscles and walking faster would affect soleus force generation in elderly subjects and it could therefore be suggested that negative-heeled footwear may not be a good option to offload calf muscle by generating additional passive force (elastic energy) for elderly people and subjects with PAD. It is also unclear how calf muscle stretching adaptation can affect knee extension/flexion in relation to passive force generation by the ankle during walking and also muscle architecture changes for patients with IC in relation to walking pattern adaptation. Delp in his PhD demonstrated how elastic and inelastic Achilles

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tendon tissue can affect the active and passive soleus force curves in relation to ankle angle as shown in figure 4.21.

Figure 4.21: Active plus passive soleus forces versus ankle angle with elastic and inelastic tendon. The solid black line was calculated with nominal tendon elasticity. The dotted black

curve shows the effect of making the tendon inextensible.

Figure 4.21 shows that if tendon and fibres are less stiff, this tends to decrease the slope of the force versus angle curve during stretching of the Achilles tendon for the soleus muscle. From the picture above it is visibly clear that for a stiffer soleus MTU, more force would be generated when it is slightly plantarflexed and for an elastic muscle-tendon complex, it would be higher when it is stretched. It is known that the aging process is related to decreased muscle and tendon elasticity and they become stiffer compared with a young healthy population (Abernethy et al., 2013). These are contributing factors to the loss of joint range of motion. Therefore, ankle force generation for older subjects in relation to the level of ankle plantar/dorsiflexion would be different versus a younger population. Another study has demonstrated that in an older age group of females, peak active and passive torque values occurred at a relatively more plantarflexed joint angle (Gajdosik et al., 1996). This would all suggest that the muscle architecture for older subjects with specific complications or diseases should be studied in more detail to understand this relationship to ensure that an appropriate footwear rocker sole profile can be prescribed.

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