Biomechanical analysis of the role of shoe inserts for over pronated foot during running

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Introduction  
 
1.1 Pronation 
Subtalar joint pronation is a motion which combines dorsiflexion, abduction, and 
eversion of the foot which respect to the leg, providing the primary mechanism by 
which transverse rotations occur between the leg and foot [1]. Excessive and an 
increased velocity of pronation during stance, have  been related with overuse injuries 
in running, although the mechanisms behind this relationship are still unclear [2] [3] [4].  
 
                 
                                    Figura 1.1 Pronated foot Vs Normal foot 
                     (adapted from Philadelphia Runner website: http://www.philadelphiarunner.com) 
 
1.2 Overuse Injuries  
Overuse injuries are those injuries that result from repetitive subtraumatic forces that 
overuse and stress the muscuskeletical tissues [5]. Willems et al. 2006  have identified  
the central heel strike, the excessive eversion and the increased lateral roll off of the 
ankle as risk factors for exercise related lower leg pain. Exercise related leg pain 
includes, shin splints, shin pain, medial tibial stress pain, periostitis, compartment 
syndrome and stress fractures [2]. On a second study, Willlems found that the risk 
factors running with shoes  are the increased pronation excursion and accelerated 
reinversion[3]. These studies were carried on by comparing an injury group of subjects 
that developed exercise related lower leg pain with a group of subjects who had never  
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1.3 Inserts, Orthoses, Shoes 
 
 
had an injury history in the lower limb. The results  from these studies showed that the 
gait pattern of subjects who developed this type of injury  is different from a normal 
population who hadn’t had injuries in the lower limb. The general approach to find the 
causes that lead to overuse injuries from an over-pronated foot is by comparing different 
gait variables between the healthy population and the foot pronated population with 
different type of inserts that artificially change the pronation  [1] [4]. The most common 
types of gait parameter evaluation  used in literature are :  kinematic, kinetics or plantar 
pressure and  electromyography [6][7]. 
 
1.3 Inserts, Orthoses, Shoes  
Inserts, orthoses and special shoes are the common ways of changing the gait pattern  
for rehabilitation or to correct the foot posture in individuals. Murley et al. 2009[7], 
carried out a systematic review  to find the effects of foot posture, foot orthoses and 
footwear on the lower limb  muscle activity during walking and running. The papers 
taken into consideration were included in the study if they satisfied the inclusion criteria 
which were: main outcome measure for muscle were EMG or MRI, independent 
variables include variation in posture, footwear or orthoses used, statistical analysis 
must have been carried out, and more than one subject with no neurological disease 
were tested. Finally  38 papers were chosen: 6 on foot posture, 12 on foot orthoses and 
20 on footwear. A quality assessment was then made based on the way the EMG 
measurement methodology was reported on in each paper . The foot orthoses studies 
compared different types of orthoses of different materials; customized foot 
orthoses[8][9] or fixed medial wedging[9][10], semirigid polypropylene shells [8][11] 
or ethylene vinyl acetate(EVA) made midsoles, with standard control shoes (walking) 
or sandals(running). A general result independent from the type of orthoses was that 
peroneus longus, tibialis anterior EMG amplitude and tibialis anterior duration, were 
greater when wearing foot orthoses. The majority of the papers examined were about 
footwear and showed different styles of footwear. Shoes with varying heel height were 
reported to make changes in both lower back and lower limb EMG muscles activity 
with increased heel height. Destabilizing footwear designed to enhance the ankle 
musculature were used also to treatment injury groups and for prevention[12] [13] [14]. 
Similar results as the orthoses studies were found for the footwear studies but a clear 
conclusion on the effect on muscle function couldn’t be found due to different methods 
and results.  
 
 
 
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1.4 Problem Statement 
 
 
1.3.1 Randomized Controlled Trials (RCTs) 
The previously discussed studies on orthoses, were based on laboratory research and 
aimed at finding the biomechanical effect of footwear and orthoses. An approach to 
establish the effect of footwear to prevent overuse injuries, is the randomized controlled 
trials method (RCTs). The RCTs is a statistical method that compares a  treatmentment 
group with a control group under a defined protocol which follow a generally accepted 
guideline[15]. The aim of this method is to reduce the risk of bias, i.e. when there is an  
error in choosing the population sample and the assumptions on the subjects are not true 
so that no reliable interpretation of the results could be done. A systematic review of 
RCTs studies was conducted to analyze the effect of orthoses or shoes to prevent 
overuse injuries[16]. The quality assessment of the papers reviewed was based on their 
rmethodology and assessed the following features: randomization, concealed allocation, 
baseline similarity of the study group, blinding of participants, blinding of care 
providers, blinding of assessors, co-interventions, compliance, drop out rate, timing of 
outcome measures, and intention-to-treatment analysis.  Many of these studies were 
done on military recruits due to the homogeneity of the population (age, physical 
characteristics, race, gender) and the possibility of controlling the trials in the military 
academy. A subdivision of the papers was done based on the type of study: orthoses or 
inserts, external joint supports, and training programs. The orthoses and insoles studies 
selected were aimed at reducing the shock absorption. The analysis confirmed that the 
use of insoles [17][18][19], could be effective in preventing injuries, but still these 
results are  in contradiction with other studies[20] and a real comparison was difficult 
because of the clinical and methodological differences of these studies.  
 
1.4 Problem Statement 
 
1.4.1 Background 
Solutions to prevent injuries due to  over pronation  are provided by special shoes[21], 
inserts [22] or orthotics [9]. Murley et al. 2009  [7] reviews several papers that have 
found evidences of the feasibility of preventing some lower limb injuries (specially 
femoral stress fractures and shin splints) with foot orthoses. Since the clear mechanism 
that lead to injuries is not identified, the main goal of these solutions is to directly 
reduce the pronation (providing a support to the medial foot) and the shock absorption 
during stance phase to reset the normal condition. Changes have been found in muscle 
activity. According to Mundermann et al. 2006 [9], the tibialis anteriorir, the vastus 
medialis, the vastus lateralis and rectus femoris revealed significant changes in global 
EMG intensity during pre-heel-strike with orthoses compared to control insert.  The  
 
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1.4 Problem Statement 
 
 
tibialis anterioris , the peroneus longus, the gastrocnemius  medialis, the biceps femoris 
showed significant changes in global EMG intensity during post-heel-strike with 
orthoses compared to control insert. Nawoczenski et al. 1999 [8] compared customized 
foot orthoses with running sandals and obtained significant greater RMS EMG 
amplitude values with orthoses in the tibialis anterior, and significantly lower RMS 
EMG amplitude for the biceps femoris. O’Connor et al. 2006[23] found a similar result 
for the tibialis anterior comparing a neutral running shoe with a running shoe with 
medial wedge. Some studies suggest that pronation is associated with greater EMG 
amplitude of invertor muscles such as tibialis anterior and flexor hallus [24,25] and 
lower EMG amplitudes for evertor muscles such as peroneus longus. Nigg et al. 
2001[26] suggested a new paradigm that defines the forces acting on the foot during the 
stance phase, as an input signal. According to this new concept, a muscle adaptation 
followed by these inputs aims at maintaining the preferred joint movement path for a 
given movement task (e.g. running). Any intervention that supports the preferred 
movement path, reduces the muscle activity, if it counteracts the preferred movement 
path, muscle activity must be increased. This concept implies that the “movement 
control” during stance phase is not important to align the skeleton but  attempts to 
control the movement change muscle activity during the stance phase which result for 
the person in an improved comfort.  In the studies cited orthoses and shoe inserts were 
able to influence muscle activity. Shoe inserts, basically provide a support to the foot 
arch and limit its collapsing during foot pronation in the stance phase of gait. This 
solution may alter the ankle joint and the knee joint muscles activity. Foot orthoses and 
shoe inserts aim is also to adsorb the shock transmission to the lower limb during heel 
strike of the foot.   
Several studies on shoe inserts that have given different results were discussed. Some 
confirm the hypothesis that inserts change the gait pattern[9] and others didn’t find any 
differences in the gait pattern compared to  normal insoles[4][27]. The gait pattern can 
be analyzed by studying the dynamic of different parameters during gait. In this project 
we focused on the parameters that in the literature were widely confirmed to change due 
to shoe inserts when positive results were obtained [8][9][10][28]. According to these 
papers shoe inserts, change the muscle activity that stabilize the ankle and knee joint 
kinematic. It’s not clear if the change must be an increased or decreased muscle activity; 
literature showed differences on it, probably because each person has its own favorite 
gait pattern that maximizes comfort[26], therefore there is a subject- specific change in 
the gait pattern of the muscle activity. Several studies were conducted to study the 
shock absorption. Some studies used RCT methods[16][17][18][19] to find out if the 
reduced impact force caused a reduced risk of overuse injuries and other studies were 
carried out in laboratory experiments that use pressure insoles[29] or force 
platforms[30] to compare treatmented shoes with control shoes and found out the 
evidence of  
 
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