Muscle tuning during running: implications of an un-tuned landing

BACKGROUND: The impact force in heel-toe running is an input signal into the body that initiates vibrations of the soft tissue compartments of the leg. These vibrations are heavily damped and the paradigm of muscle tuning suggests the body adapts to different input signals to minimize these vibrations. The objectives of the present study were to investigate the implications of not tuning a muscle properly for a landing with a frequency close to the resonance frequency of a soft tissue compartment and to look at the effect of an unexpected surface change on the subsequent step of running. METHOD: Thirteen male runners were recruited and performed heel-toe running over two surface conditions. The peak accelerations and biodynamic responses of the soft tissue compartments of the leg along with the EMG activity of related muscles were determined for expected soft, unexpected hard and expected hard landings. RESULTS AND CONCLUSIONS: For the unexpected hard landing there was a change in the input frequency of the impact force, shifting it closer to the resonance frequency of the soft tissue compartments. For the unexpected landing there was no muscle adaptation, as subjects did not know the running surface was going to change. In support of the muscle-tuning concept an increase in the soft tissue acceleration did occur. This increase was greater when the proximity of the input signal frequency was closer to the resonance frequency of the soft tissue compartment. Following the unexpected change in the input signal a change in pre-contact muscle activity to minimize soft tissue compartment vibrations was not found. This suggests if muscle tuning does occur it is not a continuous feedback response that occurs with every small change in the landing surface properties. In previous studies with significant adaptation periods to new input signals significant correlations between the changes in the input signal frequency and the EMG intensity have been shown, however, changes in soft tissue accelerations have not been found. The results of the present study showed that changes in these soft tissue accelerations can occur in response to a resonance frequency input signal when a muscle reaction has not happened.     

Quantification of the input signal for soft tissue vibration during running

Soft tissue compartment vibrations are initiated at heel-strike in heel-toe running. The concept of muscle tuning suggests that the body tries to minimize these vibrations with a muscle adaptation that changes the mechanical properties of the soft tissue compartment. A muscle tuning adaptation can be quantified by determining the biodynamic response, of the soft tissue compartment for different experimental conditions. To determine the biodynamic response a measure of both the input signal and the soft tissue compartment vibrations are required. The input signal for the vibrations is the rapid deceleration of the leg after initial ground contact. The aim of this study was to evaluate three non-invasive methods to quantify the input signal for the triceps surae soft tissue vibrations. Data from a force platform, a shoe mounted accelerometer and a video analysis of a reflective skin marker were used to quantify leg deceleration. Both the shoe mounted accelerometer and skin marker method provided a satisfactory evaluation of the input signal and could be used to determine the biodynamic response of the soft tissue compartment. The impact portion of the ground reaction force is primarily due to the deceleration of the leg at landing. However, due to the influence of the effective body mass on the impact magnitude, the force plate data was not appropriate for quantifying a muscle tuning response.     

Soft tissue vibrations within one soft tissue compartment

The concept of muscle tuning suggests that vibrations of the soft tissue compartments of the leg initiated by impacts are minimized by muscular activity prior to heel-strike of heel-toe running. For the quantification of muscle tuning it has been assumed (1) that the soft tissue compartment acts as one lumped mass and (2) that vibration energy dissipation does occur within one muscle. The purpose of this study was to test these two assumptions. It was hypothesized that (H1) the movement of the soft tissue compartment is not homogeneous, (H2) the vibration frequencies for different muscles within one soft tissue compartment are different and (3) attenuation of vibration movement within one muscle does occur. Soft tissue vibrations were measured using accelerometers on four locations on the quadriceps soft tissue compartment during heel-toe running. There were differences in the peak soft tissue acceleration and time of peak acceleration between accelerometer locations. The dominant frequency was similar throughout the soft tissue compartment, however; there was an attenuation of high-frequency vibration energy between distal and proximal points overlying one muscle. This evidence suggests that accelerometer placement is important when quantifying the acceleration magnitude and timing of peak soft tissue compartment but not when estimating the resonant vibration characteristics of a soft tissue compartment. It also provides initial evidence to support the idea that vibration control through muscle tuning may be achieved through changes in energy dissipating properties within the soft tissue compartment.     

Muscle activity in the leg is tuned in response to impact force characteristics

Based on results from quasi-static experiments, it has been suggested that the lower extremity muscle activity is adjusted in reaction to impact forces with the goal of minimizing soft-tissue vibrations. It is not known whether a similar muscle tuning occurs during dynamic activities. Thus, the purpose of this study was to determine the effect of changes in the input signal on (a) vibrations of lower extremity soft-tissue packages and (b) EMG activity of related muscles during heel-toe running. Subjects performed heel-toe running in five different shoe conditions. Ground reaction forces were measured with a KISTLER force platform, soft-tissue vibrations were measured with tri-axial accelerometers and muscle activity was measured using surface EMG from the quadriceps, hamstrings, tibialis anterior and triceps surae groups from 10 subjects. By changing both the speed of running and the shoe midsole material the impact force characteristics were changed. There was no effect of changes in the input signal on the soft-tissue peak acceleration following impact. A significant correlation (R2=0.819) between the EMG pre-activation intensity and the impact loading rate changes was found for the quadriceps. In addition, the input frequency was shown to approach the vibration frequency of the quadriceps. This evidence supports the proposed paradigm that muscle activity is tuned to impact force characteristics to control the soft-tissue vibrations.     

Modification of soft tissue vibrations in the leg by muscular activity.

Vibration characteristics were recorded for the soft tissues of the triceps surae, tibialis anterior, and quadriceps muscles. The frequency and damping of free vibrations in these tissues were measured while isometric and isotonic contractions of the leg were performed. Soft tissue vibration frequency and damping increased with both the force produced by and the shortening velocity of the underlying muscle. Both frequency and damping were greater in a direction normal to the skin surface than in a direction parallel to the major axis of each leg segment. Vibration characteristics further changed with the muscle length and between the individuals tested. The range of the measured vibration frequencies coincided with typical frequencies of impact forces during running. However, observations suggest that soft tissue vibrations are minimal during running. These results support the strategy that increases in muscular activity may be used by some individuals to move the frequency and damping characteristics of the soft tissues away from those of the impact force and thus minimize vibrations during walking and running.     

Tissue vibration in prolonged running

The impact force in heel–toe running initiates vibrations of soft-tissue compartments of the leg that are heavily dampened by muscle activity. This study investigated if the damping and frequency of these soft-tissue vibrations are affected by fatigue, which was categorized by the time into an exhaustive exercise. The hypotheses were tested that (H1) the vibration intensity of the triceps surae increases with increasing fatigue and (H2) the vibration frequency of the triceps surae decreases with increasing fatigue. Tissue vibrations of the triceps surae were measured with tri-axial accelerometers in 10 subjects during a run towards exhaustion. The frequency content was quantified with power spectra and wavelet analysis. Maxima of local vibration intensities were compared between the non-fatigued and fatigued states of all subjects. In axial (i.e. parallel to the tibia) and medio-lateral direction, most local maxima increased with fatigue (supporting the first hypothesis). In anterior–posterior direction no systematic changes were found. Vibration frequency was minimally affected by fatigue and frequency changes did not occur systematically, which requires the rejection of the second hypothesis. Relative to heel-strike, the maximum vibration intensity occurred significantly later in the fatigued condition in all three directions. With fatigue, the soft tissue of the triceps surae oscillated for an extended duration at increased vibration magnitudes, possibly due to the effects of fatigue on type II muscle fibers. Thus, the protective mechanism of muscle tuning seems to be reduced in a fatigued muscle and the risk of potential harm to the tissue may increase.     

Soft-tissue vibrations in the quadriceps measured with skin mounted transducers

The purpose of this study was to develop a method to characterize the frequency and damping of vibrations in the soft tissues of the leg. Vibrations were measured from a surface-mounted accelerometer attached to the skin overlying the quadriceps muscles. The free vibrations in this soft tissue were recorded after impact whilst the muscle was performing isometric contractions at 0, 50, and 100% maximum voluntary force and with the knee held at 20, 40, and 60 degrees angles of flexion. The acceleration signals indicated that the soft tissue oscillated as under-damped vibrations. The frequency and damping coefficients for these vibrations were estimated from a model of sinusoidal oscillations with an exponential decay. This technique resolved the vibration coefficients to 2 and 7% of the mean values for frequency and damping, respectively.     

Muscle activity damps the soft tissue resonance that occurs in response to pulsed and continuous vibrations.

This study tested the hypotheses that when the excitation frequency of mechanical stimuli to the foot was close to the natural frequency of the soft tissues of the lower extremity, the muscle activity increases 1) the natural frequency and 2) the damping to minimize resonance. Soft tissue vibrations were measured with triaxial accelerometers, and muscle activity was measured by using surface electromyography from the quadriceps, hamstrings, tibialis anterior, and triceps surae groups from 20 subjects. Subjects were presented vibrations while standing on a vibrating platform. Both continuous vibrations and pulsed bursts of vibrations were presented, across the frequency range of 10-65 Hz. Elevated muscle activity and increased damping of vibration power occurred when the frequency of the input was close to the natural frequency of each soft tissue. However, the natural frequency of the soft tissues did not change in a manner that correlated with the frequency of the input. It is suggested that soft tissue damping may be the mechanism by which resonance is minimized at heel strike during running.     

Vibration transmission to lower extremity soft tissues during whole-body vibration

In order to evaluate potential risks of whole-body vibration (WBV) training, it is important to understand the transfer of vibrations from the WBV platform to the muscles. Therefore, the purpose of this study was to quantify the transmissibility of vibrations from the WBV platform to the triceps surae and quadriceps soft tissue compartments.     

Modeling muscle activity to study the effects of footwear on the impact forces and vibrations of the human body during running

A previously developed mass-spring-damper model of the human body is improved in this paper, taking muscle activity into account. In the improved model, a nonlinear controller mimics the functionality of the Central Nervous System (CNS) in tuning the mechanical properties of the soft-tissue package. Two physiological hypotheses are used to determine the control strategies that are used by the controller. The first hypothesis (constant-force hypothesis) postulates that the CNS uses muscle tuning to keep the ground reaction force (GRF) constant regardless of shoe hardness, wherever possible. It is shown that the constant-force hypothesis can explain the existing contradiction about the effects of shoe hardness on the GRF during running. This contradiction is emerged from the different trends observed in the experiments on actual runners, and experiments in which the leg was fixed and exposed to impact. While the GRF is found to be dependent on shoe hardness in the former set of experiments, no such dependency was observed in the latter. According to the second hypothesis, the CNS keeps the level of the vibrations of the human body constant using muscle tuning. The results of the study show that this second control strategy improves the model such that it can correctly simulate the effects of shoe hardness on the vibrations of the human body during running.     

Analysis of EMG measurements during bicycle pedalling

Activity of eight leg muscles has been monitored for six test subjects while pedalling a bicycle on rollers in the laboratory. Each electromyogram (EMG) data channel was digitized at a sampling rate of 2 kHz by a minicomputer. Data analysis entailed generating plots of both EMG activity regions and integrated EMG (IEMG). For each test subject, data were recorded for five cases of pedalling conditions. The different pedalling conditions were defined to explore a variety of research hypotheses. This exploration has led to the following conclusions: Muscular activity levels of the quadriceps are influenced by the type of shoes worn and activity levels increase with soft sole shoes as opposed to cycling shoes with cleats and toeclips. EMG activity patterns are not strongly related to pedalling conditions (i.e. load, seat height and shoe type). The level of muscle activity, however, is significantly affected by pedalling conditions. Muscular activity bears a complex relationship with seat height and quadriceps activity level decreases with greater seat height. Agonist (i.e. hamstrings) and antagonist (i.e. quadriceps) muscles of the hip/knee are active simultaneously during leg extension. Regions of peak activity levels, however, do not overlap. The lack of significant cocontraction of agonist/antagonist muscles enables muscle forces during pedalling action to be computed by solving a series of equilibrium problems over different regions of the crank cycle. Regions are defined and a solution procedure is outlined.     

On the reflex coactivation of ankle flexor and extensor muscles induced by a sudden drop of support surface during walking in humans.

Recent studies have revealed that the stretch reflex responses of both ankle flexor and extensor muscles are coaugmented in the early stance phase of human walking, suggesting that these coaugmented reflex responses contribute to secure foot stabilization around the heel strike. To test whether the reflex responses mediated by the stretch reflex pathway are actually induced in both the ankle flexor and extensor muscles when the supportive surface is suddenly destabilized, we investigated the electromyographic (EMG) responses induced after a sudden drop of the supportive surface at the early stance phase of human walking. While subjects walked on a walkway, the specially designed movable supportive surface was unexpectedly dropped 10 mm during the early stance phase. The results showed that short-latency reflex EMG responses after the impact of the drop (<50 ms) were consistently observed in both the ankle flexor and extensor muscles in the perturbed leg. Of particular interest was that a distinct response appeared in the tibialis anterior muscle, although this muscle showed little background EMG activity during the stance phase. These results indicated that the reflex activities in the ankle muscles certainly acted when the supportive surface was unexpectedly destabilized just after the heel strike during walking. These reflex responses were most probably mediated by the facilitated stretch reflex pathways of the ankle muscles at the early stance phase and were suggested to be relevant to secure stabilization around the ankle joint during human walking.     

The effect of material characteristics of shoe soles on muscle activation and energy aspects during running

The purposes of this study were (a) to determine group and individual differences in oxygen consumption during heel-toe running and (b) to quantify the differences in EMG activity for selected muscle groups of the lower extremities when running in shoes with different mechanical heel characteristics. Twenty male runners performed heel-toe running using two shoe conditions, one with a mainly elastic and a visco-elastic heel. Oxygen consumption was quantified during steady state runs of 6 min duration, running slightly above the aerobic threshold providing four pairs of oxygen consumption results for comparison. Muscle activity was quantified using bipolar surface EMG measurements from the tibialis anterior, medial gastrocnemius, vastus medialis and the hamstrings muscle groups. EMG data were sampled for 5 s every minute for the 6 min providing 30 trials. EMG data were compared for the different conditions using an ANOVA (alpha=0.05). The findings of this study showed that changes in the heel material characteristics of running shoes were associated with (a) subject specific changes in oxygen consumption and (b) subject and muscle specific changes in the intensities of muscle activation before heel strike in the lower extremities. It is suggested that further study of these phenomena will help understand many aspects of human locomotion, including work, performance, fatigue and possible injuries.     

Foot center of pressure manipulation and gait therapy influence lower limb muscle activation in patients with osteoarthritis of the knee

Background

Foot center of pressure (COP) manipulation has been associated with improved gait patterns. The purpose of this study was to determine lower limb muscle activation changes in knee osteoarthritis patients, both immediately after COP manipulation and when COP manipulation was combined with continuous gait therapy (AposTherapy).

Methods

Fourteen females with medial compartment knee osteoarthritis underwent EMG analyzes of key muscles of the leg. In the initial stage, trials were carried out at four COP positions. Following this, gait therapy was initiated for 3 months. The barefoot EMG was compared before and after therapy.

Results

The average EMG varied significantly with COP in at least one phase of stance in all examined muscles of the less symptomatic leg and in three muscles of the more symptomatic leg. After training, a significant increase in average EMG was observed in most muscles. Most muscles of the less symptomatic leg showed significantly increased peak EMG. Activity duration was shorter for all muscles of the less symptomatic leg (significant in the lateral gastrocnemius) and three muscles of the more symptomatic leg (significant in the biceps femoris). These results were associated with reduced pain, increased function and improved spatiotemporal parameters.

Conclusions

COP manipulation influences the muscle activation patterns of the leg in patients with knee osteoarthritis. When combined with a therapy program, muscle activity increases and activity duration decreases.     

Constrained tibial vibration in mice: a method for studying the effects of vibrational loading of bone

Vibrational loading can stimulate the formation of new trabecular bone or maintain bone mass. Studies investigating vibrational loading have often used whole-body vibration (WBV) as their loading method. However, WBV has limitations in small animal studies because transmissibility of vibration is dependent on posture. In this study, we propose constrained tibial vibration (CTV) as an experimental method for vibrational loading of mice under controlled conditions. In CTV, the lower leg of an anesthetized mouse is subjected to vertical vibrational loading while supporting a mass. The setup approximates a one degree-of-freedom vibrational system. Accelerometers were used to measure transmissibility of vibration through the lower leg in CTV at frequencies from 20 Hz to 150 Hz. First, the frequency response of transmissibility was quantified in vivo, and dissections were performed to remove one component of the mouse leg (the knee joint, foot, or soft tissue) to investigate the contribution of each component to the frequency response of the intact leg. Next, a finite element (FE) model of a mouse tibia-fibula was used to estimate the deformation of the bone during CTV. Finally, strain gages were used to determine the dependence of bone strain on loading frequency. The in vivo mouse leg in the CTV system had a resonant frequency of 60 Hz for +/-0.5 G vibration (1.0 G peak to peak). Removing the foot caused the natural frequency of the system to shift from 60 Hz to 70 Hz, removing the soft tissue caused no change in natural frequency, and removing the knee changed the natural frequency from 60 Hz to 90 Hz. By using the FE model, maximum tensile and compressive strains during CTV were estimated to be on the cranial-medial and caudolateral surfaces of the tibia, respectively, and the peak transmissibility and peak cortical strain occurred at the same frequency. Strain gage data confirmed the relationship between peak transmissibility and peak bone strain indicated by the FE model, and showed that the maximum cyclic tibial strain during CTV of the intact leg was 330+/-82microepsilon and occurred at 60-70 Hz. This study presents a comprehensive mechanical analysis of CTV, a loading method for studying vibrational loading under controlled conditions. This model will be used in future in vivo studies and will potentially become an important tool for understanding the response of bone to vibrational loading.     

The computer-vibromyography as a biometric progress in studying muscle function

Muscular vibrations were recorded from different relaxed and contracted skeletal muscles in human subjects, with the use of a piezo-electric device. Simultaneous wire-EMG recordings were performed. Spectral analysis of the acceleration curves (vibromyograms) disclosed muscle and function dependent compound frequency patterns. We suggest that the activity of motor units including the action of central reflex loops and oscillatory driving is mainly responsible for the muscular vibrations. Other sources are discussed. Computer-Vibromyography as a mechanical ensemble measurement supplements bioelectric EMG techniques and classical tremor analysis and provides further insights into the function of muscle and motor-system.     

The influence of increasing steady-state walking speed on muscle activity in below-knee amputees

The goal of this study was to identify changes in muscle activity in below-knee amputees in response to increasing steady-state walking speeds. Bilateral electromyographic (EMG) data were collected from 14 amputee and 10 non-amputee subjects during four overground walking speeds from eight intact leg and five residual leg muscles. Using integrated EMG measures, we tested three hypotheses for each muscle: (1) there would be no difference in muscle activity between the residual and intact legs, (2) there would be no difference in muscle activity between the intact leg and non-amputee legs, and (3) muscle activity in the residual and intact legs would increase with speed. Most amputee EMG patterns were similar between legs and increased in magnitude with speed. Differences occurred in the residual leg biceps femoris long head, vastus lateralis and rectus femoris, which increased in magnitude during braking compared to the intact leg. These adaptations were consistent with the need for additional body support and forward propulsion in the absence of the plantar flexors. With the exception of the intact leg gluteus medius, all intact leg muscles exhibited similar EMG patterns compared to the control leg. Finally, the residual, intact and control leg EMG all had a significant speed effect that increased with speed with the exception of the gluteus medius.     

Human postural responses to different frequency vibrations of lower leg muscles

We analyzed human postural responses to muscle vibration applied at four different frequencies to lower leg muscles, the lateral gastrocnemius (GA) or tibialis anterior (TA) muscles. The muscle vibrations induced changes in postural orientation characterized by the center of pressure (CoP) on the force platform surface on which the subjects were standing. Unilateral vibratory stimulation of TA induced body leaning forward and in the direction of the stimulated leg. Unilateral vibration of GA muscles induced body tilting backwards and in the opposite direction of the stimulated leg. The time course of postural responses was similar and started within 1 s after the onset of vibration by a gradual body tilt. When a new slope of the body position was reached, oscillations of body alignment occurred. When the vibrations were discontinued, this was followed by rapid recovery of the initial body position. The relationship between the magnitude of the postural response and frequency of vibration differed between TA and GA. While the magnitude of postural responses to TA vibration increased approximately linearly in the 60-100 Hz range of vibration frequency, the magnitude of response to GA vibration increased linearly only at lower frequencies of 40-60 Hz. The direction of body tilt induced by muscle vibration did not depend on the vibration frequency.     

Explicit finite element modelling of heel pad mechanics in running: inclusion of body dynamics and application of physiological impact loads

Many heel pathologies including plantar heel pain may result from micro tears/trauma in the subcutaneous tissues, in which internal tissue deformation/stresses within the heel pad play an important role. Previously, many finite element models have been proposed to evaluate stresses inside the heel pad, but the majority of these models only focus on static loading boundary conditions. This study explored a dynamics modelling approach to the heel pad subjected to realistic impact loads during running. In this model, the inertial property and action of the body are described by a lumped parameter model, while the heel/shoe interactions are modelled using a viscoelastic heel pad model with contact properties. The impact force pattern, dynamic heel pad deformation and stress states predicted by the model were compared with published experimental data. Further parametrical studies revealed the model responses, in terms of internal stresses in the skin and fatty tissue, change nonlinearly when body dynamics changes. A reduction in foot's touchdown velocity resulted in a less severe impact landing and stress relief inside the heel pad, for example peak von-Mises stress in fatty tissue, was reduced by 11.3%. Applications of the model may be extendable to perform iterative analyses to further understand the complex relationships between body dynamics and stress distributions in the soft tissue of heel pad during running. This may open new opportunities to study the mechanical aetiology of plantar heel pain in runners.