Caracterització de l’efecte funcional del producte IDI3A

Various rocker soles designs have previously been prescribed for different pathologies and rehabilitation programs in an attempt to alter muscle working patterns, limit motion in the ankle during stance phase, or reduce pressure distribution over specific areas (Hutchins et al., 2012, Richardson, 1991, Romkes et al., 2006, Shakoor et al., 2008, Viswanathan et al., 2004, Yung-Hui and Wei-Hsien, 2005).

The actual shape of the sole unit varies in the literature according to the biomechanical effects required to be achieved, and the pathology presented. Some examples where rocker soled shoes have the potential to be used or have already been used as an orthotic intervention are:

 To improve gait performance in sport and medicine;

 To redistribute plantar foot pressures for subjects with diabetic peripheral neuropathy;

 To improve the rehabilitation of the Achilles tendon injuries;

 To improve the efficacy of rehabilitation programs in reducing pain in the ankle, knee, hip and back;

 To provide compensation for weak muscles or altered gait patterns, and to train or offload muscles to influence the work done by them;

79

 To reduce energy expenditure, and improve comfort during walking.

Improved performance relies on efficient transformation of mechanical power output produced by the musculoskeletal system through footwear design. Understanding the biomechanical implications of different rocker-soled shoes can help ensure that the appropriate footwear selection/prescription for people with different pathologies can be made and it may also assist in the understanding of causes of lower limb injury. The following section will describe the basic biomechanical effects on specific lower limb muscles caused by different footwear features.

4.6.3 Alteration to heel height

High-heeled shoes are extensively worn by women and are influenced by fashion trends. This type of footwear has been associated with a range of foot deformities. High heel shoes can increase forefoot plantar pressures, increase loading to the toes, (especially the first metatarsal head), increase the activity of leg muscles to maintain balance and increase the activity of muscles of the lower spine. They also alter posture and gait, cause muscles to fatigue and cause pain, induce kinematic and kinetic changes, increase the risk of ankle sprains, increase anterior pelvic tilt and reduce walking speed (Ravindra, 2012, Mika et al., 2012, Stefanyshyn et al., 2000, Gefen et al., 2002, Gehlsen et al., 1986, Esenyel et al., 2003). However, high heeled shoes also have a ‘benefit’ in so far that they produce a positive cosmetic effect by making the legs and look slimmer and longer, and make the feet appear smaller.

Conversely, negatively-heeled shoes increase the internal PF moment, cause premature activation of the calf muscles, and place the ankle in a more dorsiflexed position during walking. They therefore stretch the triceps surae muscles and are therefore not suitable for people with Achilles tendon injuries (Li and Hong, 2007, An and Lee, 2007).

4.6.3.1The effect of heel height on EMG activity

The effect of different heel heights on muscle activity by using EMG analysis has been investigated in several studies. The results of these studies are presented in table 4.5.

80

The existing evidence demonstrates variation in results with regard to EMG activity by increasing heel height, and does not clearly justify why muscle activity is increased or decreased. This is because it is unclear what causes EMG activity to be higher or lower in magnitude by altering the heel height. Gastrocnemius activity was shown by Lee et al (1990) to be reduced once the heel height exceeded 5cm, but tibialis anterior was increased once the heel height was less than 5cm. A separate study, however, showed that a 10cm heel increased EMG activity in the GM muscle compared to a 4cm heel height. Soleus increased activity with a 4cm heel compared to a 3.7cm heel. However, negatively pitched heels have been shown to increase gastrocnemius activity.

The alterations to EMG values reported in the literature may have been due to the following:

 The ankle moments and powers may have been altered or the velocity of specific muscle contractions changed due to the different footwear test conditions;

 Alteration to walking speed could have caused these consequential changes in the velocity of specific muscle contractions (Arnold et al., 2013), as velocity of contraction is related to force generation in muscles (Winter, 2009);

 Muscle-tendon lengths may not have been close to their optimal length for certain footwear test conditions or the muscle moment arms were adversely altered by the change in heel height also.

Table 4.5: The effect of heel height on lower limb muscle EMG activity. Author/s (Date) Subjects, Age Footwear features

Task Muscles (EMG) tested Main findings HEEL HEIGHTS (Lee et al., 1990) 6 Women Age range: 20-31 years Barefoot, 2.5 cm heel, 5.0 cm heel, 7.5 cm heel Walking Med. Gastroc. Tib. Anterior

Med.gastroc - significantly lower mean peak EMG values with a 2.5 cm and 5 cm heel height when compared to a 7.5 cm heel.

Tib. anterior - significantly greater mean peak EMG with a 2.5 cm heel compared to both 5.0 cm and 7.5 cm heel heights. Both muscles showed

significantly lower mean peak EMG for all heel heights versus barefoot.

81 (Stefanysh yn et al., 2000) 13 female subjects Age: 40.6 (±8.3) years Flat shoe 1.4 cm heel, Low-heeled shoe 3.7cm, 5.4cm heel and 8.5cm heel Walking at a speed of 1.4 m/s Gastroc. Soleus

Soleus showed a significantly greater RMS EMG amplitude with 1.4 cm heel versus a 3.7 cm heel.

Soleus - significantly greater RMS EMG amplitude with an 8.5 cm heel versus all other shoes tested. EMG using a 5.4 cm heel was greater than 1.4 and 3.7 cm heel heights (Lee et al., 2001) 5 female subjects Age: in their 20s 0 cm heel, 4.5 cm heel and 8 cm heel Walking at a speed of 1.1 m/s

Tib. anterior Peak tib. anterior EMG values were significantly increased with increased heel heights

(Li and Hong, 2007) 13 female subjects Age: 23.1 (±3.9) Normal shoes plus negative- heeled shoes

Walking Tib. anterior Lat. gastroc.

Lat. Gastroc and Tib. anterior – significantly greater EMG amplitude in negative-heeled shoes and duration of EMG activity was significantly longer for both muscles when walking in negative-heeled shoes versus normal. (An and Lee, 2007) 15 male subjects Flat 15° heel 20° heel Walking at a speed of 1.33m/s Tib. anterior Med. gastroc.

Both negative heel shoes showed significant increases in Tib. anterior EMG activity. Both negative-heel shoes showed an increase in peak EMG activity during stance phase but not significantly (p=0.08) (Mika et al., 2012) 31 female subjects age: 20-25 15 female subjects Age: 45-55 Barefoot 4 cm heel 10 cm heel Walking (self-selected speed) Med. gastroc. Tib. anterior

Tib. Anterior- significantly greater EMG amplitude using a 10 cm heel compared to both barefoot and a 4 cm heel. Med. Gastrocnemius significantly greater mean peak EMG in 10 cm heel versus barefoot and 4 cm heel. (Simonsen et al., 2012) 14 female subjects Age range: 21-38 Barefoot 9 cm heel Walking at 4km/h Med. gastroc. Soleus Tib. anterior

Tib. anterior - significantly greater EMG amplitude in 9 cm heel versus barefoot.

Soleus – significantly greater EMG amplitude in 9 cm heel and duration of EMG activity was significantly longer.

Med. Gastroc – premature activation and greater EMG amplitude during mid-stance for 9 cm heel versus barefoot

82

The answers to these assumptions are as yet unclear. Muscles produce movement at the joints. The forces which cause the joint to rotate produce the joint moment. External moments are produced by gravitational forces and internal moments are generated by muscles and usually oppose external forces (Kirtley, 2006, Trew and Everett, 2005, Whittle, 2003). Muscle moments or moments about the ankle are calculated as the force (muscle or GRF) multiplied by its moment arm. Therefore, both the length of the moment arm and the magnitude of the force can change the joint moment or moment produced by the muscles. The triceps surae muscle group generate internal plantarflexion (PF) moments (muscle force x internal moment arm), which is opposed by an external dorsiflexion (DF) moment (ground reaction force x external moment arm) as shown in figure 4.25 (Sobhani et al., 2013).

Figure 4.25: An example during stance phase of gait of how the external dorsiflexion moment is opposed by internal plantar flexion moment generated by calf muscles. From figure 4.25 above, it is clear that changing the muscle moment arm (denoted d2) and its projected point with the ground would also potentially change muscle force generation. High-heeled shoes place the ankle into a more plantarflexion position than flatter shoes (Simonsen et al., 2012, Esenyel et al., 2003). That would result in an increased Achilles muscle moment arm relative to the ankle joint centre (Nagano and Komura, 2003, Maganaris et al., 1998a). High-heeled shoes have also been shown to decrease the internal ankle moment (Esenyel et al., 2003). Consequently, muscle force magnitude can be increased as the Achilles tendon moment arm is increased until the muscle-tendon length reaches the length at which it loses force generation (Delp, 1990). There is therefore the potential to

83

influence these two parameters by footwear features so that specific muscles produce more or less muscle force generation depending on the application required.

Correlation between internal ankle moments and the related level of muscle activity at the ankle joint (i.e. the intensity of contraction and rate of motor unit recruitment) is not yet fully understood. For instance, skeletal muscles develop only 50% of its maximum available force when its length is shortened to 85% of the resting length (Panjabi and White, 2001). Figure 4.26 demonstrates that as the moment arm of the point of application of the GRF to ankle joint centre is shortened when the ankle is in plantarflexion, the calf muscles potentially work harder to generate force as their muscle-tendon length is shortened.

Figure 4.26: A visual example of EMG activity for the calf muscle during standing on the toes [adapted from (Kirtley, 2006)].

4.6.3.2Heel Height (HH) hypothesis

A feasibility pilot study investigating the effect of walking with shoes with different heel heights was performed for this thesis prior to the main biomechanical testing. It showed that the point of application of the GRF during initial contact can be significantly altered (figure 4.27). These initial results resulted in the following analysis and the hypotheses being formed with regards to the effects of wearing footwear with different heel heights.

84

When walking with high heeled footwear, the ankle is more plantarflexed and therefore tibialis anterior (TA) muscle is stretched compare to when walking with the flat shoe. This would result in changes to TA muscle work and function.

Figure 4.27: GRF point of application data during initial contact (ICt) obtained in a pilot study using visual3D software (C-Motion) for 3.5 and 5.5 cm heel heights.

High heeled shoes when compared to low or negative heels may also change the velocity of TA contraction. For instance, a negative heel would keep the ankle in a dorsiflexed position relative to the ground, but the shank would still rotate forwards. The inclination velocity of the shank relative to the foot would be different in high heel shoes compared to a negative heel. It would consequently increase the eccentric contraction velocity of TA, and EMG activity may be higher in magnitude. Figure 4.28 demonstrates visual differences for negative-heeled and high-heeled shoes during ICt.

Figure 4.28: Visual example of the position of the foot and ankle at ICt for negative heeled and high heeled shoes (where d1 and d2 are the muscle moment arms).

85

Different heel heights can therefore potentially offload or load the TA muscle and may also cause premature activation of the calf muscles.

The sole thickness directly underneath ankle joint centre can also increase the ankle moment. The moment about ankle in the sagittal plane is calculated as summary of vertical GRF magnitude multiplied by the moment arm, and horizontal force multiplied by the moment arm (Richards, 2008). If the sole is thicker, it increases the vertical moment arm as shown in figure 4.29.

Figure 4.29: An ankle moment calculation example. The red coloured section illustrates the extra sole thickness.

However, a thicker sole may not have a significant effect on the total moment when the point of application of the GRF is moving away from the joint centre.

At mid-stance and terminal stance phases of gait, the triceps surae muscles (attached to the Achilles tendon) may produce varying force when walking with negatively-heeled, flat or high-heeled shoes if participants are walking with the same speed. There are several reasons for this:

 It can increase/decrease the external ankle moment during stance phase;

 It can change the velocity of muscle contractions along with changes to ankle angle;

 It can change passive/active muscle force generation;

86

 It can alter muscle-tendon lengths during stance phase;

 The calf muscle moment arm can be increased or reduced;

 It can change walking patterns and adaptation mechanisms by other part of the body.

Figure 4.30: demonstrates a simple visual geometrical example of how different heel heights change the ankle position during terminal stance and therefore muscle biomechanics.

From the example above, the results of pilot/feasibility testing show that for a negative-heel shoe, the triceps surae requires around 2,918 N to perform the task, for a flat shoe 2,236 N and for a high heeled shoe 1,692 N. However, for the positively heeled shoe it requires less muscle force, but the muscles can also be shortened by 85% and consequently lose 50% of their force generation and therefore theoretically may require in excess of 3,384 N (1,692*2) of muscle force to oppose the external DF moment. This example does not include body adaptation such as knee flexion to the shoe. Gastrocnemius muscle length also depends on the knee flexion angle, and the total length can vary with force generation.

87

The following footwear design parameters were analysed in this study to understand their effect on muscle function:

 Rocker sole apex angle and the apex position (i.e. the position along the shoe at which the outsole begins to curve or angle upwards);

 The effect of altering the stiffness of the outsole;

 Variation in heel height (and therefore also the overall pitch of the shoe);

 Addition of a heel curve (otherwise known as a rolled, negatively-curved or chamfered heel) (figure 4.31).

Only one footwear feature was changed at a time during the gait laboratory testing to demonstrate ensure it was clear which footwear features were responsible for alterations to muscle function.

Figure 4.31: An illustration of the footwear features analysed in this thesis.

The methodology section describes in more detail the footwear rocker sole profile selection criteria for this research and illustrates the rocker sole designs, which were tested in this thesis.

88

4.6.3.3Possible effects of varying heel height and resulting hypotheses

1. There is a certain heel height level at which the muscle moment arm has an optimal position in relation to the muscle-tendon length during push off phase of gait. A heel height of between 3.0 and 4.5 cm is the estimated heel height at which triceps surae can produce maximum efficient force. This heel height along with other adaptations to the sole could be beneficial in offloading the calf muscles for patients with IC.

2. High heeled shoes may increase triceps surae EMG activity even though the external ankle DF moment is reduced (this should be equal to the internal plantarflexion moment), but the muscle is too short to generate the force required compared when it is at optimal length, therefore the resulting energy cost may be increased with high- heeled shoes.

3. A negatively-heeled shoe may increase triceps surae muscle work, because it may increase the external DF ankle moment, reduce the muscle moment arm and keep the muscles stretched and therefore they may lose force generation. However, the muscles will work harder to compensate for the force required.

4. A negatively-heeled shoe may prematurely activate the calf muscles which will result in increased external ankle moment during loading response and mid stance phase and therefore an increase in EMG muscle activity will result.

5. A negatively-heeled shoe may reduce ankle ROM during loading response as the foot and shoe may not move relative to the ground, but the shank will still incline.

6. A negatively-heeled shoe may reduce the time taken during loading response and also activate mid stance phase earlier due to a more dorsiflexed foot position.

7. A high-heeled shoe may increase ankle plantarflexion during loading response because it may change the point of application of the GRF (more ankle movement could also result, for example a more inclined shank may result in order to achieve a more comfortable ankle position when compared to a low heeled shoe) and this will increase the external plantarflexion moment.

8. A negatively-heeled shoe will stretch the calf muscles and therefore change their velocity of contraction, EMG activity, and keep the ankle more dorsiflexed through the entire gait cycle and also affect the knee and hip.

9. A positive (high) heel will shift the ankle into a more plantarflexion position during the entire gait cycle and therefore change kinematic, kinetic, EMG and muscle function.

89