Caracterització de la interacció in vitro dels pèptids C18-RCAN1 i C18-RCAN3 amb CnA

tibialis anterior (TA)

Tibialis anterior (TA) contributes over 50% of the total dorsiflexion moment over the entire range of ankle motion (Delp, 1990). The gastrocnemius, the most superficial calf muscle, comprises of two portions, or heads, and forms the greater bulk of the calf. The soleus is a postural muscle, and is a flat muscle situated immediately deep, or anteriorly, to the gastrocnemius. Together they form a muscle group called the triceps surae, which contributes 90% of the total plantar flexion force of the posterior calf muscles (Alter, 2004). Since gastrocnemius and soleus have short fibres relative to their moment arms, the fibres change length (force) significantly as the ankle is moved (Delp, 1990). The triceps surae connects to the Achilles tendon, which is the largest and strongest tendon in the body. The

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distal end is attached to the posterior surface of the calcaneus. Although it is the strongest tendon, it can be injured. The most common injury to the Achilles tendon is tendinitis, mainly caused by overuse. Footwear features could feasibly be designed to effect walking patterns and therefore to potentially aid in the recovery of an Achilles tendon injury by offloading it during rehabilitation. Gastrocnemius and soleus muscles for the average adult (i.e. not athletes) comprise of different fibre types. Slow twitch fibres (type I) which are mainly used for postural control is primarily used in soleus muscle. It also contains a high amount of oxidative fibres (Alter, 2004, Abernethy et al., 2013).

It is known that patients with IC experience pain in the calf region due to vascular complications, which results in an inadequate oxygen supply to the calf muscles. Theoretically, if footwear could be designed to offload the soleus muscle (type I muscle), it should increase pain-free walking distance, and also improve (train) the cardiovascular system. There could be benefits related to aerobic exercise and muscle performance that improve overall oxygen consumption, its delivery and muscle tissue as well as nutrition storage.

The gastrocnemius muscle provides forceful contraction and contains a high proportion of fast twitch muscle (Type II). Regular strength training of the gastrocnemius muscle could theoretically build up more muscle fibres and store more nutrition for muscle performance provided the blood vessels grow with it. Thus, footwear, which makes muscle work harder, may be a good way to strengthen gastrocnemius and could be used for short periods of walking as an adjunct therapy to strengthen the ankle plantarflexors.

Knowledge of muscle fibre types is therefore important for several reasons:

 IC patients suffer from oxygen supply deficiency to the triceps surae muscle group. It may therefore help to understand what level of oxygen usage each muscle needs and which one can be trained or offloaded with a rocker shoe and/or training programs which could be targeted for subjects with vascular complications;

 It may also help to understand what kind of footwear could be developed for IC patients.

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The triceps surae’s critical phase of walking cycle is the push off phase. At this time, the gastrocnemius and soleus muscles are heavily activated in order to push the centre of mass of the body forward and upward just before the heel strike of the contra-lateral leg. Recent analysis has shown that gastrocnemius and soleus both develop their peak force at the same time during the push off phase (Abernethy et al., 2013). The peak force that can be generated by GAS is around 900N, whereas the peak force in SOL is around 2000N. However, the maximum peak isometric force is around 3500N generated by SOL and around 1350N generated by GAS (Arnold et al., 2010).

4.5.1 The Achilles tendon moment arm

The range of joint angles over which a muscle can develop active force depends on its fibre length and moment arm. The change in muscle-tendon length with joint angle depends on the moment arm. For a given range of a specific joint, muscle-tendon excursion increases with the moment arm. Thus, the ratio of a muscle’s fibre length to its moment arm determines the range of joint angles over which the muscle can develop active force (Delp, 1990, Hoy et al., 1990b).

The Achilles tendon moment arm length increases when the ankle moves from a dorsiflexed into a plantarflexed position (Nagano and Komura, 2003, Maganaris et al., 1998a). A study investigating alteration to the Achilles tendon moment arm with respect to the ankle joint centre using MRI scanning has shown that its length changes (i.e. increases) from 4.4 cm to 7 cm between -15° to +30° of plantarflexion (Maganaris et al., 1998a). This is one of the crucial factors for power generation and alteration to MTU moments about the ankle. Even if the triceps surae muscle force is high, if the moment arm is small, then this will cause a reduction in the internal ankle moment generated by the calf musculature. Therefore, it is crucial that MTU-joint centre moment arms be optimised and considered as one of the factors when deciding on the position the ankle joint needs to be in to reduce the work done by the calf muscles.

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4.5.2 Optimal pennation angle for gastrocnemius and soleus during dynamic movement

Muscle fibre pennation angle is an important parameter with regards to musculoskeletal function (force generation). Recent developments have used ultrasound to measure muscle pennation angle and simultaneous EMG during isometric contraction of the gastrocnemius muscle. The pennation angle for the gastrocnemius muscle varies between 14° to 20° during isometric plantarflexion (Zhou et al., 2012). Maximum torque is produced at a pennation angle of around 18°. It was also noticeable from the graphs that with higher values of pennation angle, EMG was significantly higher too. With low values pennation angle was significantly lower, however tendon force is equal to fibre force multiplied by cosine of muscle pennation angle. If the pennation angle is less it consequently should increase force transferred to the tendon. However, the results suggest that the muscle produce more active force when it is shortening. Further research with MRI scanning has shown that by lengthening muscles fibres, pennation angle is reduced (Narici et al., 1996). This would help to transmit elastic force to the joint; however the muscle moment arm may be short and it may reduce the effect. It is therefore interesting to test different heel heights in footwear designs in order to understand relationships between alteration to muscle-tendon lengths, and muscle moment arms in relation to EMG and ankle moments in the lower limb.

Another study using ultrasound has calculated the pennation angle values for the gastrocnemius, soleus and tibialis anterior muscles (see table below) at rest and at maximum voluntary contraction for male and female adult subjects (Manal et al., 2006).

Table 4.4: Pennation angle measured at rest and maximum voluntary contraction for the right (R) and left (L) legs of male and female subjects. Pennation angle is reported as degrees with standard deviations in parentheses (Manal et al., 2006).

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Optimal pennation angle angles of 8 males and 8 females for the GAS, SOL, and TA muscles were calculated. Joint angles were chosen to control muscle tendon lengths so that the muscles were near their optimal length within the length-tension relationship (figure 4.24).

Figure 4.24: Mean values of optimal pennation angle with standard deviation for tiabialis anterior (TA), lateral and medial gastrocnemius, and soleus muscles (Manal et al., 2006).

Further ultrasound research has produced detailed measurement of the triceps surae complex architecture in six males across the muscle belly at rest and during maximum voluntary contraction trials at angles of -15° (dorsiflexion position), 0° (neutral position), +15°and +30° (plantarflexion position) (Maganaris et al., 1998b). The results demonstrated that pennation angle increases when the ankle is plantarflexing and GAS and SOL muscle fibres are shortening. Therefore, it is unclear which is the best ankle position to produce an optimal ankle moment force that would recruit less motor units to generate the internal moment by the ankle and therefore theoretically produce less oxygen consumption. This thesis was therefore designed to investigate this question using ambulatory activities with regards to footwear design by testing different footwear features, collecting kinematic, kinetic and by analysing muscle-tendon property data (lengths, velocities, Achilles moment arm) and EMG signals.

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