**1.3. Differences in locomotion due to shoes**

De Wit et al. (2000) describe different angular displacements of the knee and ankle when subjects ran barefoot and when running shod. During running, the body reacts to the external environment which produces the ground reaction force (GRF) that occurs in response to the force action transmitted by the leg in contact with the ground. The GRF reflects the net effect of the muscle action and the accelerations of the segments while the foot is in contact with the ground (Martin & Morgan 1992). All segments contribute to the total acceleration of the body in proportion to the acceleration of its centre of gravity and its relative mass.

The three components of the GRF (vertical, anterior-posterior, and medial-lateral) change their size when using footwear (Nigg 1983). The GRF reflects the acceleration and deceleration of the centre of gravity. The gravity eases the contact of the foot with the ground.

### *1.3.1. Vertical component (GRFv)*

During movement, the GRFv varies above and below the body weight due to the positive and negative accelerations undergone by the body. The difference between the vertical

Computational Intelligence in Electromyography Analysis – 70 A Perspective on Current Applications and Future Challenges

component and the body weight is due to acceleration of the body. The direction of displacement of centre of gravity and acceleration influences also the magnitude of the vertical component.

Comparison by EMG of Running Barefoot and Running Shod 71

suggested that the repetitive impact forces during physical activities are not responsible for possible injury but are the cause of changes in myoelectric activity (activation time and amplitude), and these changes are responsible for the injury. Gollhofer & Komi (1987) found differences in the electrical activity of muscles when subjects ran barefoot first, then shod; Gavilanes & Goiriena-de-Gandarias (2004) found changes in myoelectric activity throughout the gait cycle when the subjects walked barefoot or with two different types of footwear, with no differences due to types of footwear used. Wakeling et al. (2002) found that the muscle activity concomitant to the impact can be altered by changing the hardness of the shoe. Frederick (1986) concluded that footwear induces adjustments in the movement of the

As found in the literature, footwear induces adaptations in the motion of the joints of the lower extremities, changes in the reaction force, and modifications in the myoelectric activity. When designing and making shoes, different types of considerations are taken into account: (1) reduce excessive burden, by absorbing the impact, (2) improve the dynamic stability, (3) increase the performance, and (4) feel comfortable (Ramiro et al. 1988, Segesser & Nigg 1993). The impact absorption is carried out through the midsole of sport shoes that acts as a filter by changing the impact forces (Luethi & Stacoff 1987). The increase in performance with the use of the shoe has been an argument used by athletes and shoe manufacturers. The midsole of athletic shoes is a layer of resilient, deformable material that is interposed between the upper shoe and the outside. The main function is to provide a protective layer between the foot and the ground and soften the shock of impact. During the first stage of the stride of jogging, the midsole is compressed by the pressure of the foot and the forces acting on it do some work on the viscoelastic material of the sole. Part of this work becomes stored as deformation energy in the material (elastically deformed). When the load on the midsole is reduced, the material undergoes an elastic recovery to its original shape

The literature review reveals as well that the ability of sport shoes to mitigate the impact forces between the ground and the body has been examined by different researchers (Denoth et al. 1981, Bates et al. 1983, Nigg et al. 1986, Gollhofer & Komi 1987, Dufek JS et al. 1991, Forner et al. 1995, De Wit et al. 2000). Less studied are the effects of footwear on kinematics (Frederick 1986, Nigg et al. 1986), or muscle activity (Gollhofer & Komi 1987, Wakeling et al. 2001, Nigg & Wakeling 2001, Wakeling et al. 2002, Gavilanes & Goiriena de

The design of sport shoes and the elasticity of the materials used in their sole influence, respectively, the location of the application point of the GRF and its magnitude. These parameters influence the ability to produce an angular movement of the joints. The elasticity of the materials can be characterized based on the concepts of elasticity, rigidity, deformability, hysteresis, resilience, and viscosity. Elasticity: ability of a body to recover its original shape once the force that has deformed it has disappeared. Stiffness: a body resistance to deformation. Deformability: the inverse of the stiffness, requiring little force

legs, which in turn have secondary effects on the kinetics.

(Shorteen 1993).

Gandarias 2004).

The vertical component is biphasic and has a first peak of early impact (at 20ms after the impact), representing between 140% and 160% (and up to 200%) of body weight (BW), in the runners who touch the ground first with the heel, and a second peak in the stance phase, which appears at 80 ms and can almost triple the body weight. The two peaks have different slopes, the first very fast and the second more gradual. Contact time is about 0.25 s. The first peak is associated with heel strike and indicates pronation. It is surprising that this peak is smaller in magnitude than the second peak that is associated with the propulsion.

#### *1.3.2. Antero-posterior component (GRF a-p)*

When the foot contacts the ground, it is pushed forward and suffers a reaction force that slows it down. At the time the body passes over the foot that is resting on the ground, the horizontal component is zero. When the body moves over the foot which is resting, the foot is pushed against the ground and the antero-posterior component becomes positive, facilitating the forward propulsion. Its magnitude represents 50% of the BW during jogging.

#### *1.3.3. Medial-lateral component (GRF m-l)*

The medial-lateral component is the smallest of all components. It has two polarities, the first in reaction to the force transmitted by the foot on the medial direction, and the second in reaction to the force transmitted by the foot in the lateral direction. The polarity of this component of one leg is opposite to the polarity of this component in the contralateral leg. Thus the sign of the lateral component of the right foot would be first positive and then negative, and for the left foot would be reversed. The variation in magnitude of the vertical, antero-posterior, and medial-lateral components means that during the displacement, the speed is not constant, as the body moves faster in one point and slower in another.

The parameters related to the vertical component are the peak impact and the rate of increase of force (obtained with a force plate), and were used to examine the load under which the locomotor system is during locomotion (Nigg 1983). One of the main functions of the footwear is to cushion the strength of the action the subject exerts on the ground and to absorb the reaction force in order to protect the musculoskeletal system. In jogging, running shoes nullifies the impact peak and is involved in delaying the onset of the support vertical force by changing the gradient loading (Nigg 1983, De Wit et al. 2000). The anteroposterior force, which has two phases (braking and propulsion), is influenced by the friction introduced by the shoe sole. The medial-lateral component that guides us on the pronosupination movement can be modified by the shoes as they change the distance between the point of application of the GRF and the subtalar or calcaneo-talar joint.

Wakeling et al. (2001) speculated that the muscle activation levels in the lower extremities are adjusted depending on the loading speed of impact forces. Nigg & Wakeling (2001) suggested that the repetitive impact forces during physical activities are not responsible for possible injury but are the cause of changes in myoelectric activity (activation time and amplitude), and these changes are responsible for the injury. Gollhofer & Komi (1987) found differences in the electrical activity of muscles when subjects ran barefoot first, then shod; Gavilanes & Goiriena-de-Gandarias (2004) found changes in myoelectric activity throughout the gait cycle when the subjects walked barefoot or with two different types of footwear, with no differences due to types of footwear used. Wakeling et al. (2002) found that the muscle activity concomitant to the impact can be altered by changing the hardness of the shoe. Frederick (1986) concluded that footwear induces adjustments in the movement of the legs, which in turn have secondary effects on the kinetics.

Computational Intelligence in Electromyography Analysis – 70 A Perspective on Current Applications and Future Challenges

*1.3.2. Antero-posterior component (GRF a-p)* 

*1.3.3. Medial-lateral component (GRF m-l)* 

vertical component.

component and the body weight is due to acceleration of the body. The direction of displacement of centre of gravity and acceleration influences also the magnitude of the

The vertical component is biphasic and has a first peak of early impact (at 20ms after the impact), representing between 140% and 160% (and up to 200%) of body weight (BW), in the runners who touch the ground first with the heel, and a second peak in the stance phase, which appears at 80 ms and can almost triple the body weight. The two peaks have different slopes, the first very fast and the second more gradual. Contact time is about 0.25 s. The first peak is associated with heel strike and indicates pronation. It is surprising that this peak is

When the foot contacts the ground, it is pushed forward and suffers a reaction force that slows it down. At the time the body passes over the foot that is resting on the ground, the horizontal component is zero. When the body moves over the foot which is resting, the foot is pushed against the ground and the antero-posterior component becomes positive, facilitating the forward propulsion. Its magnitude represents 50% of the BW during jogging.

The medial-lateral component is the smallest of all components. It has two polarities, the first in reaction to the force transmitted by the foot on the medial direction, and the second in reaction to the force transmitted by the foot in the lateral direction. The polarity of this component of one leg is opposite to the polarity of this component in the contralateral leg. Thus the sign of the lateral component of the right foot would be first positive and then negative, and for the left foot would be reversed. The variation in magnitude of the vertical, antero-posterior, and medial-lateral components means that during the displacement, the

The parameters related to the vertical component are the peak impact and the rate of increase of force (obtained with a force plate), and were used to examine the load under which the locomotor system is during locomotion (Nigg 1983). One of the main functions of the footwear is to cushion the strength of the action the subject exerts on the ground and to absorb the reaction force in order to protect the musculoskeletal system. In jogging, running shoes nullifies the impact peak and is involved in delaying the onset of the support vertical force by changing the gradient loading (Nigg 1983, De Wit et al. 2000). The anteroposterior force, which has two phases (braking and propulsion), is influenced by the friction introduced by the shoe sole. The medial-lateral component that guides us on the pronosupination movement can be modified by the shoes as they change the distance between the

Wakeling et al. (2001) speculated that the muscle activation levels in the lower extremities are adjusted depending on the loading speed of impact forces. Nigg & Wakeling (2001)

speed is not constant, as the body moves faster in one point and slower in another.

point of application of the GRF and the subtalar or calcaneo-talar joint.

smaller in magnitude than the second peak that is associated with the propulsion.

As found in the literature, footwear induces adaptations in the motion of the joints of the lower extremities, changes in the reaction force, and modifications in the myoelectric activity. When designing and making shoes, different types of considerations are taken into account: (1) reduce excessive burden, by absorbing the impact, (2) improve the dynamic stability, (3) increase the performance, and (4) feel comfortable (Ramiro et al. 1988, Segesser & Nigg 1993). The impact absorption is carried out through the midsole of sport shoes that acts as a filter by changing the impact forces (Luethi & Stacoff 1987). The increase in performance with the use of the shoe has been an argument used by athletes and shoe manufacturers. The midsole of athletic shoes is a layer of resilient, deformable material that is interposed between the upper shoe and the outside. The main function is to provide a protective layer between the foot and the ground and soften the shock of impact. During the first stage of the stride of jogging, the midsole is compressed by the pressure of the foot and the forces acting on it do some work on the viscoelastic material of the sole. Part of this work becomes stored as deformation energy in the material (elastically deformed). When the load on the midsole is reduced, the material undergoes an elastic recovery to its original shape (Shorteen 1993).

The literature review reveals as well that the ability of sport shoes to mitigate the impact forces between the ground and the body has been examined by different researchers (Denoth et al. 1981, Bates et al. 1983, Nigg et al. 1986, Gollhofer & Komi 1987, Dufek JS et al. 1991, Forner et al. 1995, De Wit et al. 2000). Less studied are the effects of footwear on kinematics (Frederick 1986, Nigg et al. 1986), or muscle activity (Gollhofer & Komi 1987, Wakeling et al. 2001, Nigg & Wakeling 2001, Wakeling et al. 2002, Gavilanes & Goiriena de Gandarias 2004).

The design of sport shoes and the elasticity of the materials used in their sole influence, respectively, the location of the application point of the GRF and its magnitude. These parameters influence the ability to produce an angular movement of the joints. The elasticity of the materials can be characterized based on the concepts of elasticity, rigidity, deformability, hysteresis, resilience, and viscosity. Elasticity: ability of a body to recover its original shape once the force that has deformed it has disappeared. Stiffness: a body resistance to deformation. Deformability: the inverse of the stiffness, requiring little force per unit area to produce large deformation. Hysteresis: represents the energy dissipated between the deformation and recovery of the original shape. Resilience: the amount of energy returned by the deformed material during the discharge phase. Viscosity of a liquid or semiliquid substance is the resistance of a body to deformation in response to a load.

Comparison by EMG of Running Barefoot and Running Shod 73

We have classified the hardness of the sole of each footwear type during jogging on the basis of their subjective hardness: barefoot condition was interpreted as the maximum hardness, as the outer protection and reduction of the shock at the beginning of the stance phase was minimal. The own athletic shoes of each subject (typically used to run) was the condition interpreted as the softest. The standard shoes were harder than the athletic shoes,

Prior to obtaining EMG recordings, subjects got used to carrying the electrodes and contact sensors (foot switches –FS) by walking freely in the laboratory until obtaining a normal gait. For each individual there have been five successive records barefoot at spontaneous speed (no specific speed requested), five with a standard sports shoes and five with his/her own shoes. Subjects ran at ground level at their preferred speed in both the first registration and in the remaining four. When subjects were shod, they were given also some time to get

Each record registered the EMG corresponding to the cycles required to cross a distance of 10 m. From the cycles registered, clearly identified by the FS signal, only the central 2 were further analyzed; thus avoiding the effect of acceleration and deceleration on muscle activity. In order to calculate the average speed (in ms-1), the time taken to cross the 10 m has been timed. Between each of the five records made for each condition, a pause of one minute

The eight FSs (B & L, U.S.A.) facilitated the identification of the phases of the stride for each of the lower extremities. These sensors are flexible disks of two sizes: 18mm and 30mm. They were placed under the heel (30mm FS) and on the heads of the first and fifth metatarsal and toe tip (18mm FS). When subjects were shod, the FSs were placed on the bottom of the shoes at the sites corresponding to the outer edge of the heel, first and fifth metatarsal, and toe tip. A FS is activated when a pressure greater than 150 g is applied on it.

Visual monitoring of the signal from the FS eased the removal of stride records with deficiencies. The simultaneous recording of signals originating in the FS and the EMG has also allowed identifying the cycle phases (support, swing, or flight) in which the muscles were active. Prior to the start of the records, we checked the signals obtained through the

The electrode characteristics are presented in **Table 1**. The surface electrodes used were active, equipped with pre-amplifiers providing a gain of x320. EMG signal thus obtained is better than that achieved with passive electrodes respect to the level of noise. They are composed of three stainless steel electrodes, acting two as active electrodes and one as a common ground.

**2.2. Experimental conditions** 

**2.3. Proceeding** 

was given.

**2.4. EMG** 

electrodes and the FSs.

and therefore feature less cushioning of the impact.

habituated to the shoes, and moved at a freely chosen speed.

The duration of each phase of the jogging stride (support, flight, and swing) depends on the control of the muscles of each leg executed by the Nervous System: suprasegmental centres, spinal networks, and afferent information from the different senses and from the osteoarticular system. If the information from the feet changes due to modifications on the interface between the foot and the floor, it is expected that the muscular activity will be also modified.

The recording of electrical activity obtained during muscle contraction or electromyogram (EMG) reflects the muscle involvement in the movement of the joints and therefore in the kinetic response of the ground or reaction force. The EMG amplitudes are related both to the nervous system and to muscle tension, although the response of the mechanical system is not directly related to the nervous system signal (Bouisset 1973), as the mechanical response depends on more variables than the muscle activity (such as the length of the muscle, the rate of change of length, time of contraction, and the lever arm magnitude). The relationship between muscle activity and force is not straightforward; however, EMG amplitude, duration, and coordination among different muscles can provide information about the neural and mechanical systems. Therefore, the electromyography is a powerful tool in the study of the neuromusuclar control of movement. The EMG signal is not easily recorded, as it is very susceptible to interferences and cable movement, and it is quite small, varying its amplitude between microvolts and millivolts (Kleissen et al. 1998) being its maximum amplitude (peak-to-peak) only 5 mV when using surface electrodes (Winter 1979).

During locomotion, the muscles of the legs are used to meet the following mechanical demands: progression, dynamic stability, and improving the impact and energy conservation (Inman et al. 1981); if any of these tasks is altered, the record of the muscles electrical activity will provide information about its contribution.

The aim of our present study was to evaluate the influence of footwear on the electrical activity of muscles of both legs when running barefoot and running with two different types of sports footwear, in order to assess the effects on: the extent of muscle electrical signal, the profile of muscular electrical activity, the order of muscle involvement, and the coactivation of antagonist muscles.
