Overuse injury is a common and frequent problem in the running population (6). Runners sustain overuse injury symptoms of the lower extremity more often compared with other sports like soccer where running is also an integral part (35).
The contributions of multifaceted factors to overuse injury development impede the identification of clear causative relationships (41). Static lower extremity alignment is often stated to be relevant, but prospective studies show, depending on the study cohorts and alignment measures, inconclusive results (20,41,42). Static subtalar joint pronation seems to be associated with increased injury risk, but dynamic factors promoting overuse injury development are considered to be of greater importance (29,45). The concept of increased foot pronation is therefore extensively discussed in this context (12,30). Experimental studies on injured cohorts show an increased subtalar joint eversion displacement during midstance for Achilles tendinopathy patients. This contributes to a pronated position of the foot (37). This concept is not accompanied by studies on neuromuscular control, which certainly affects subtalar and ankle joint motion (18). Neuromuscular impairments due to overuse injury are considered to be evident, but reported data are sparse (45).
Treatment of overuse injury is initially undertaken by nonoperative treatment strategies, whereas foot orthoses are one option for therapy (13,21). The therapeutic effectiveness of orthoses is strongly challenged despite their widespread use in clinical practice because evidence-based data from randomized controlled trials are rare (4,17). Two recent randomized controlled trials show an earlier and more pronounced therapy effect in patients experiencing patellofemoral pain and a distinct pain reduction and reduction of functional impairment in runners with overuse complaints (5,13).
Mechanisms concerning how orthoses lead to or contribute biomechanically to a therapeutic benefit are inconclusive. Mechanical effects are thought to lead by "aligning the skeleton" to an optimized movement path of the segmental bone configuration of the lower extremity (31). Mills et al. (24) report in a recent systematic review on biomechanical effects of foot orthoses that small reductions of peak rear foot eversion as one aspect of foot pronation and tibial internal rotation in noninjured subjects. Studies analyzing injured cohorts are rare in this context. The kinematic approach is partially confuted by bone-pin studies showing only small and unsystematic influences of insoles on bony structures during dynamic movements (39). Alternative approaches assume that orthoses might change afferent information at the plantar surface leading to changes in muscle activation and therefore to "muscle tuning" (32,39). This sensorimotor approach is currently not backed up with sufficient data (24,26,27). Descriptive results report "some evidence" that foot orthoses increase activation of tibialis anterior and peroneus longus muscle, but no causative modes of action (short or long term) are discussed (25,26,28). A recent review on the effect of foot orthoses on lower limb muscle activation suggests more research of rigorous methodological quality to allow better evaluation of EMG effects of foot orthoses (26). Furthermore, research on long-term neuromotor adaptations in patients experiencing overuse injury is requested (24).
The musculus peroneus longus is crucial in actively stabilizing the ankle joint in the mediolateral direction (15). Malfunction of the neuromuscular control of the peroneal muscle due to immoderate stress is thought to contribute not only to acute injury risk but also to overuse injury (15,36). Foot orthoses may affect the motor control of the musculus peroneus longus leading to alternations of afferent input (sensorimotor approach) or using optimized mechanical alignment (biomechanical approach). The purpose of this study was therefore to analyze the neuromuscular activation of the musculus peroneus longus before and after an 8-wk foot orthoses intervention in runners with common overuse injury symptoms.
MATERIALS AND METHODS
The study was conducted according to Good Clinical Practice (EC-GCP-Note for Guidance) and followed CONSORT guidelines for randomized clinical trials. The study was approved by the ethics committee of the local university. Patients were recruited from the institutional outpatient clinic, the local running community (running clubs, local running events) by publicity measures. All persons potentially interested were checked for inclusion criteria (see below). All included subjects agreed to take part in the study voluntarily and signed an informed consent form.
A total of 125 runners (63 males and 62 females) were initially evaluated at the university outpatient clinic by the study physician (F.M.). Inclusion criteria were the presence of running-related overuse symptoms (tendinopathies at the lower extremity, patellofemoral pain syndrome, iliotibial band syndrome, traction periostitis, and plantar fasciitis) with a duration of >3 months, age range between 18 and 60 yr, a running distance of >20 miles·wk−1 (32 km·wk−1), and the absence of other illnesses or complaints. The rationale for the defined age range was to include a representative cohort of the running population. According to a recent review, there is conflicting evidence whether greater age is a single risk factor for overall lower extremity injury (41). Therefore, a wide age range was chosen.
Exclusion criteria were signs and symptoms of acute injury, parallel or previous therapies including insole or orthoses use, nonsteroidal anti-inflammatory drugs, corticosteroid injections or physiotherapy during the 6 months before study inclusion, and any history of surgery to the lower extremity. From those 125 runners, 50 males and 49 females passed the inclusion criteria. They were randomized to the intervention group "orthoses" (OR, n = 51) or to the control group (CO, n = 48).
The randomization was performed by permuted block randomization (blocks of four), separated by gender. A research assistant not involved in the study or any accompanying analysis revealed group allocation.
The study cohort had a mean age of 37.2 ± 8.3 yr (range = 19-52 yr). Subjects could be classified as experienced runners with a weekly mileage of 44 km. CO and OR did not differ in any of the baseline characteristics shown in Table 1 (P > 0.05).
Owing to the random allocation of patients, both groups had an equal distribution of diagnoses. Most often patients experienced Achilles tendinopathy (n = 26; CO = 14, OR = 12), patellar tendinopathy (n = 18; CO = 8, OR = 10), patellofemoral pain syndrome (n = 14; CO = 7, OR = 7), iliotibial band syndrome (n = 13; CO = 7, OR = 6), plantar fasciitis (n = 7; CO = 3, OR = 4), periostitis tibiae (n = 7; CO = 3; OR = 4), and other diagnoses (n = 14; CO = 6, OR = 8).
Sports orthoses fabrication.
The initial screening and final study inclusion were followed by a 2-wk period where orthoses for the OR group were fabricated. The orthoses were individually fitted on the basis of the patient's dynamic plantar pressure distribution. Individual customization was conducted by the same orthopedic technician (H.T.). The sports orthoses used in this study were fabricated out of compression-molded semirigid polyurethane foam (MoveControl; IETEC, Fulda, Germany). They all had a medial longitudinal arch support (25 mm), a detorsion wedge in the forefoot (lateral post, 3 mm), and a bowl-shaped heel (13). Runners were advised to use neutral running shoes with the orthoses. The orthoses showed promising results in daily clinical routine and biomechanical testing in healthy runners (2). Proper fit between shoe and foot orthoses was secured by the orthopedic technician to rule out discomfort. The study coordinator had to be contacted immediately in the case of any new injury symptoms, blisters, or fitting problems.
Patients were instructed to document wear time of orthoses in prepared training diaries. It was required to wear the orthoses for all physical activities during the intervention period (>80% of training sessions). All subjects were advised to follow regular training habits without any modifications. This was also ensured by documenting each training unit (duration, distance, and average running velocity) in the training diary. None of the subjects had to be excluded from the study because of the above-mentioned criteria.
A clinical reevaluation was performed by the study physician at the end of the intervention (after the study was finished). If pain persisted in control patients, orthoses were individually fabricated for them as well, and regardless of group assignment, further therapeutic measures (physiotherapy, ice, ultrasound, load management, and eccentric exercises) were prescribed.
All patients underwent a biomechanical measurement before and after the 8-wk intervention period. The musculus peroneus longus of all subjects was prepared on both tests for surface EMG measurements by carefully localizing the placement of bipolar surface electrodes (type P-00-S; Ambu, Medicotest, Ballerup, Denmark; center-to-center distance = 25 mm) according to Winter and Yack (44). The longitudinal axes of the electrodes were in line with the presumed direction of the underlying peroneal muscle fibers (Fig. 1). A ground reference electrode was placed on the tibial tuberosity.
The interelectrode resistance was kept below 5 kΩ. This was achieved by shaving, light abrasion, degreasing, and disinfecting the skin with alcohol. The EMG electrodes were directly connected to custom-built differential preamplifiers (gain = 500, input impedance = 4000 MΩ, common mode rejection = 90 dB at 60 Hz) and taped to the skin. The preamplified signals were transmitted via shielded cables (fixed to the leg with Velcro straps lattice bracing) to the main amplifier (band-pass filter [10 Hz to 1 kHz], gain 5.0 [resulting in an overall gain of 2500]) and sampled with 1000 Hz.
All subjects ran barefoot and with a standardized neutral running shoe without stabilizing elements or dual-density midsole on a motorized treadmill (HP Cosmos Quasar®; Nussdorf-Traunstein, Germany) at a speed of 3.3 m·s−1 for an interval of 5 min. At the end of each 5-min interval (barefoot and shoe) 20 complete stride cycles were collected. To trigger muscle activation signals to initial ground contact, touchdown was detected by simultaneous plantar pressure measurements (Novel Pedar®, Munich, Germany). A threshold load of 30 kPa was defined for the initiation of touchdown (toe off = pressure below 30 kPa, respectively).
Postprocessing of EMG signals included full-wave rectification and the calculation of one average time-normalized stride cycle of 10 consecutive stride cycles. The on-off pattern of muscle activity was defined above a threshold of the resting signal plus 2 SD (14). Above this threshold, the muscle was considered "on." "Off" was defined after decline of the EMG signal below this threshold.
"On" and "off" were identified automatically by customized software (LabView®-based; National Instruments, Austin, TX) and visually controlled by a person experienced in EMG data processing. The person was blinded to group assignment of the edited individual trial. This is considered to be a reasonable procedure securing both high standardization and validity (14).
Total time of activation (Ttot) was calculated as a percentage of total stride time (1.0). The initial onset of activation (Tini) and the time of maximum activation (Tmax) were expressed in relation to touchdown (=0.0) in percent of stride (1). Values below 0.0 for the onset indicate an onset of activity before touchdown (before activity; Fig. 2). Amplitudes in preactivation (Apre), weight acceptance (Awa), and push-off (Apo) phase were calculated according to the gait cycle phase definition by Winter (43). The mean amplitude value (MAV) per gait cycle phase was extracted, and values of the shoe condition were normalized to the MAV of the entire stride cycle of the barefoot reference condition. Therefore, Apre, Awa, and Apo were determined relative to the average stride cycle activity and were expressed as a percentage of the average amplitude (=1.0) (Fig. 2).
The described procedure was repeated after 8 wk of intervention. After the first measurement, electrode placements were marked with a water-resistant pen, and subjects were instructed to renew the markers throughout the intervention for identical placement of electrodes at the time of retest. Training diaries, pain rating, and functional impairment scales were completed by all subjects in parallel. These were not the main outcome measure in the presented protocol, but information can be obtained elsewhere (13).
Effect sizes in percent can later be appraised on the basis of the test-retest variability (TRV) of the described quantities. This was previously checked in our laboratory using a test-retest design with 17 healthy male participants measured 1 wk apart with the same protocol. TRV for the quantities in the time domain (Tini, Tmax, Ttot) ranged from 8% to 12%. The amplitude domain (Apre, Awa, Apo) revealed TRV ranging from 12% to 14%.
Data are presented as group means and SD and upper and lower 95% confidence intervals (CI). Possible baseline differences between groups were tested with independent t-tests (α = 0.05). To test for differences between interventions, two-way ANOVA with repeated measures (factors: group and test) were used. The α level was 0.05. The interaction effect group × test was calculated, and in case of a significant result, pre-post differences within groups were analyzed using Tukey post hoc tests. All analyses were executed using JMP® statistical software package 5.0.1 (SAS Institute, Cary, NC).
The analysis of muscle activation of the musculus peroneus longus in the time and amplitude domain at baseline did not reveal any group differences (Tini: P = 0.97, Tmax: P = 0.08, Ttot: P = 0.45, Apre: P = 0.77, Awa: P = 0.74, Apo: P = 0.43).
Timing of peroneal activity also showed no interaction effect (Tini: P = 0.59, Tmax: P = 0.14, Ttot: P = 0.57). The onset of activation was before touchdown (values below 0.0) in both groups indicating a distinct preactivation in anticipation of initial contact. The time of maximum activation was slightly after touchdown in both groups. CO and OR also did not differ in the total time of activation (Table 2).
The 8-wk orthoses therapy led to a statistically significant interaction effect in preactivation amplitudes (Apre; P = 0.006). Post hoc analysis showed a statistically significant (P = 0.001) increase in EMG activity of 22% ± 48% (95% CI = 9%-32%) in OR compared with CO (Fig. 3).
During stance, no interaction effect was found in EMG amplitudes after intervention. Both groups showed the same activity levels in weight acceptance (Awa: P = 0.24) and push-off (Apo: P = 0.84) (Table 3).
The purpose of this study was to analyze the muscle activation of the musculus peroneus longus before and after an 8-wk foot orthoses intervention in runners with overuse injury symptoms. EMG activity in the time domain showed no baseline difference between groups and also no intervention effects. Timing of muscle activity was therefore not influenced by wearing the orthoses for 8 wk. Reduced activity in terms of overall time of activation throughout the stride cycle has been shown to be associated with reduced ankle stability in patients with chronic ankle instability. Unfortunately, this was not differentiated in preheel and postheel strike fractions (38). Because no comparison with healthy controls was performed in the present study, it remains open if overuse injury symptoms diminish timing of muscle activity in a similar way. Both groups start of activation was well before touchdown indicating preactivity before the foot hits the ground. This was already observed in several movements like walking, running, or jumping in healthy subjects (23). The overuse symptoms did apparently not lead to an absence of preactivity.
There was a statistically significant difference after intervention in preactivation. The group wearing foot orthoses showed a 22% increase in preactivity in the time frame from onset of activity to initial touchdown. This implies a different preprogrammed muscle activation strategy in the orthoses-wearing group. Muscle activity is preprogrammed before touchdown in situations where considerable load is anticipated because the sole reliance on reflex mechanisms triggered by foot-ground contact is not adequate to successfully perform the desired task of stabilizing the foot-ankle complex (7,18,22). Moreover, peroneal preactivity is adjusted according to the anticipated level of external load. The higher preactivation amplitudes in the orthoses-wearing group therefore show a change in feed-forward motor control (8). This is suggested to contribute to dynamic control of ankle stability (19). The feed-forward motor control implies the planning of motor programs on the basis of past experiences (9,11). The functional significance of this preprogrammed activity lies in the control of subsequent stiffness regulations of the ankle joint when touching the ground. This contributes to ankle stability and even to the subsequent performance of the ankle, for example, in a drop jump task (16). In pathological conditions like functional ankle instability, a diminished feed-forward motor control is discussed as a contributing factor leading to the development of recurrent inversion sprains by negatively influencing joint stability (3).
Moreover, the increased preactivity of the musculus peroneous longus could be an expression of proper foot placement at touchdown and an improved alignment of the foot. The additional stability provided by the orthoses could have decreased mobility of the midtarsal joints by limiting pronation and thus leading to an advanced movement path of the lower extremity. The stability provided by the foot orthoses could serve as an "optimized" end point around which the neuromuscular system can reorganize itself to achieve the desired movement outcome. This might lead over time and training to adjustments in neuromuscular activation and further highlights the importance of orthotics in the management of selected lower limb impairments where joint stability and/or alleviating pressure on an injured area are needed.
Higher preactivation is also associated with a gain in reflex response after touchdown (22). This aspect cannot be answered by the descriptive quantities used in the present study. Muscle activity during stance (in weight acceptance as well as in push-off) did not differ after intervention. Analysis of short, medium, and late latency responses of the peroneal muscle would give further insight into this aspect. However, this is only reasonable in movement tasks where considerable reflex responses can be assumed like perturbation tasks or jumps (8,18). The observed running movement might not elicit selective responses to be differentiated.
It can be hypothesized that the higher peroneal preactivity in the orthoses group is an indicator for a feed-forward motor control mechanism leading to increased dynamic ankle stability. Studies working on preparatory muscle activity at the knee joint also link the amount of preactivity to joint stabilizing mechanisms (10,33).
The subsequent question arises concerning how the 8-wk wear of foot orthoses is linked to an adaptation of peroneal preactivity. If the assumption of sensorimotor mechanisms involved is pursued, a modulation of afferent input at the plantar surface can be discussed. The use of foot orthoses with a medial post (as used in the current study) leads to an increase in local pressure at the longitudinal arch and the medial midfoot area (2,34). These changes in plantar pressure are detected by cutaneous receptors, peroneal muscle spindles, or Golgi apparatuses modulating afferent information transferred to the interneuron pool. Spinal and supraspinal adaptations may then lead to a change in the underlying motor program. Reductions of inhibition at the presynaptic level lead successively to an increase in efferent drive to the peroneal muscle. The exact pathways and spinal and supraspinal modulations cannot be tracked down at this point. Nevertheless, it might be worth developing new paradigms investigating sensorimotor adaptation mechanisms to gain a greater insight into physiological modes of action of foot orthoses. This is currently already used to evaluate the effects and training adaptations of sensorimotor and balance training regimens (40).
Limitations of the study have to be considered. First, no blinding of the subjects was performed and no sham treatment was used in the control group. This aspect might have been a confounding factor. Although it has to be considered that experienced runners feel right away if an orthoses is to be destined to alter the running condition or not. Furthermore, it has to be mentioned that the foot orthoses intervention and the hypothesized mode of action can only work for subjects who are able to continue a considerable amount of running despite their pain experience. If running can still be performed in patients with overuse symptoms, foot orthoses seem to work as a single-measure approach (13). If the experienced pain level prevents patients from running, the foot orthoses cannot be used, and therefore, no alterations of neuromuscular control can take place. In this case, different therapeutic approaches are essential (21).
In addition, the focus in this study was solely on the musculus peroneus longus. Other muscle of the lower leg may also adapt to the use of foot orthoses. Therefore, other muscular adaptation mechanisms than the described ones cannot be ruled out. Another limitation refers to the test-retest variability of the measurement quantities. Because not the same protocol (8-wk intervention) was used in a prestudy (1 wk between measurements) to assess variability, full appraisement of detected effects is impeded. Higher variability due to the longer intervention period has to be assumed.
Adaptations of muscle activation to foot orthoses are far from being conclusive (26). This might be due to the acute approach most laboratory studies use. Mostly study subjects put on different orthotic or control conditions and immediate measurements are performed. The presented results imply a certain wear time necessary to see full adaptations to a change of plantar surface condition. Therefore, a shift to the further development of interventional approaches is recommended (24).
An intervention with individually accustomed foot orthoses for use in running shoes alters muscle activation of the peroneal muscle (higher preactivation) in runners with overuse injury symptoms. This might subsequently lead to enhanced ankle stability and argues for a "sensorimotor" mode of action of foot orthoses. Added mechanical stability by wearing foot orthoses may additionally contribute to changes in neuromuscular control suggesting interplay between sensorimotor and mechanical effects.
The development of functional paradigms and accompanying methodological approaches integrating spinal and supraspinal adaptation mechanisms might be promising to enhance knowledge on the effects of foot orthoses on motor control.
This study was supported by a grant from the Federal Institute for Sport Science (BISP), Bonn, Germany. Project: VF 0407/01/18/2001-2002 and VF 0407/01/49/2003-2005.
The authors acknowledge study coordinator Mrs. Katrin Renner and the doctoral thesis students involved in data collection. Our appreciation is also extended to the IETEC GmbH, Fulda, Germany, and Herbert Türk (HT), Fuß-Vital-Center Türk, Freudenstadt, Germany, for the supply of the orthoses and their customization.
The authors have no conflict of interest.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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