Luke A. Kelly

Recruitment of the plantar intrinsic foot muscles with increasing postural demand

BACKGROUND The aim of this study was to determine the difference in activation patterns of the plantar intrinsic foot muscles during two quiet standing tasks with increasing postural difficulty. We hypothesised that activation of these muscles would increase with increasing postural demand and be correlated with postural sway. METHODS Intra-muscular electromyographic (EMG) activity was recorded from abductor hallucis, flexor digitorum brevis and quadratus plantae in 10 healthy participants while performing two balance tasks of graded difficulty (double leg stance and single leg stance). These two standing postures were used to appraise any relationship between postural sway and intrinsic foot muscle activity. FINDINGS Single leg stance compared to double leg stance resulted in greater mean centre of pressure speed (0.24 m s(-1) versus 0.06 m s(-1), respectively, P ≤ 0.05) and greater mean EMG amplitude for abductor hallucis (P ≥ 0.001, ES=0.83), flexor digitorum brevis (P ≤ 0.001, ES=0.79) and quadratus plantae (P ≤ 0.05, ES=0.4). EMG amplitude waveforms for all muscles were moderate to strongly correlated to centre of pressure (CoP) medio-lateral waveforms (all r ≥ 0.4), with muscle activity amplitude increasing with medial deviations of the CoP. Intra-muscular EMG waveforms were all strongly correlated with each other (all r ≥ 0.85). INTERPRETATIONS Activation of the plantar intrinsic foot muscles increases with increasing postural demand. These muscles are clearly important in postural control and are recruited in a highly co-ordinated manner to stabilise the foot and maintain balance in the medio-lateral direction, particularly during single leg stance.

Intrinsic foot muscles have the capacity to control deformation of the longitudinal arch

The human foot is characterized by a pronounced longitudinal arch (LA) that compresses and recoils in response to external load during locomotion, allowing for storage and return of elastic energy within the passive structures of the arch and contributing to metabolic energy savings. Here, we examine the potential for active muscular contribution to the biomechanics of arch deformation and recoil. We test the hypotheses that activation of the three largest plantar intrinsic foot muscles, abductor hallucis, flexor digitorum and quadratus plantae is associated with muscle stretch in response to external load on the foot and that activation of these muscles (via electrical stimulation) will generate sufficient force to counter the deformation of LA caused by the external load. We found that recruitment of the intrinsic foot muscles increased with increasing load, beyond specific load thresholds. Interestingly, LA deformation and muscle stretch plateaued towards the maximum load of 150% body weight, when muscle activity was greatest. Electrical stimulation of the plantar intrinsic muscles countered the deformation that occurred owing to the application of external load by reducing the length and increasing the height of the LA. These findings demonstrate that these muscles have the capacity to control foot posture and LA stiffness and may provide a buttressing effect during foot loading. This active arch stiffening mechanism may have important implications for how forces are transmitted during locomotion and postural activities as well as consequences for metabolic energy saving.

Active regulation of longitudinal arch compression and recoil during walking and running

The longitudinal arch (LA) of the human foot compresses and recoils in response to being cyclically loaded. This has typically been considered a passive process, however, it has recently been shown that the plantar intrinsic foot muscles have the capacity to actively assist in controlling LA motion. Here we tested the hypothesis that intrinsic foot muscles, abductor hallucis (AH), flexor digitorum brevis (FDB) and quadratus plantae (QP), actively lengthen and shorten during the stance phase of gait in response to loading of the foot. Nine participants walked at 1.25 m s−1 and ran at 2.78 and 3.89 m s−1 on a force-instrumented treadmill while foot and ankle kinematics were recorded according to a multisegment foot model. Muscle–tendon unit (MTU) lengths, determined from the foot kinematics, and intramuscular electromyography (EMG) signals were recorded from AH, FDB and QP. Peak EMG amplitude was determined during the stance phase for each participant at each gait velocity. All muscles underwent a process of slow active lengthening during LA compression, followed by a rapid shortening as the arch recoiled during the propulsive phase. Changes in MTU length and peak EMG increased significantly with increasing gait velocity for all muscles. This is the first in vivo evidence that the plantar intrinsic foot muscles function in parallel to the plantar aponeurosis, actively regulating the stiffness of the foot in response to the magnitude of forces encountered during locomotion. These muscles may therefore contribute to power absorption and generation at the foot, limit strain on the plantar aponeurosis and facilitate efficient foot ground force transmission.

Comparison of plantar pressure distribution in adolescent runners at low vs. high running velocity.

This study aimed to compare foot plantar pressure distribution while jogging and running in highly trained adolescent runners. Eleven participants performed two constant-velocity running trials either at jogging (11.2 ± 0.9 km/h) or running (17.8 ± 1.4 km/h) pace on a treadmill. Contact area (CA in cm(2)), maximum force (F(max) in N), peak pressure (PP in kPa), contact time (CT in ms), and relative load (force time integral in each individual region divided by the force time integral for the total plantar foot surface, in %) were measured in nine regions of the right foot using an in-shoe plantar pressure device. Under the whole foot, CA, F(max) and PP were lower in jogging than in running (-1.2% [p<0.05], -12.3% [p<0.001] and -15.1% [p<0.01] respectively) whereas CT was higher (+20.1%; p<0.001). Interestingly, we found an increase in relative load under the medial and central forefoot regions while jogging (+6.7% and +3.7%, respectively; [p<0.05]), while the relative load under the lesser toes (-8.4%; p<0.05) was reduced. In order to prevent overloading of the metatarsals in adolescent runners, excessive mileage at jogging pace should be avoided.

Discharge properties of abductor hallucis before, during, and after an isometric fatigue task

Abductor hallucis is the largest muscle in the arch of the human foot and comprises few motor units relative to its physiological cross-sectional area. It has been described as a postural muscle, aiding in the stabilization of the longitudinal arch during stance and gait. The purpose of this study was to describe the discharge properties of abductor hallucis motor units during ramp and hold isometric contractions, as well as its discharge characteristics during fatigue. Intramuscular electromyographic recordings from abductor hallucis were made in 5 subjects; from those recordings, 42 single motor units were decomposed. Data were recorded during isometric ramp contractions at 60% maximum voluntary contraction (MVC), performed before and after a submaximal isometric contraction to failure (mean force 41.3 ± 15.3% MVC, mean duration 233 ± 116 s). Motor unit recruitment thresholds ranged from 10.3 to 54.2% MVC. No significant difference was observed between recruitment and derecruitment thresholds or their respective discharge rates for both the initial and postfatigue ramp contractions (all P > 0.25). Recruitment threshold was positively correlated with recruitment discharge rate (r = 0.47, P < 0.03). All motor units attained similar peak discharge rates (14.0 ± 0.25 pulses/s) and were not correlated with recruitment threshold. Thirteen motor units could be followed during the isometric fatigue task, with a decline in discharge rate and increase in discharge rate variability occurring in the final 25% of the task (both P < 0.05). We have shown that abductor hallucis motor units discharge relatively slowly and are considerably resistant to fatigue. These characteristics may be effective for generating and sustaining the substantial level of force that is required to stabilize the longitudinal arch during weight bearing.

Effect of Orthoses on Changes in Neuromuscular Control and Aerobic Cost of a 1-h Run

PURPOSE The studys purpose was to determine the effect of foot orthoses on neuromuscular control and the aerobic cost of running. METHODS Twelve recreational athletes ran for 1 h on a treadmill at a constant velocity (i.e., 10% higher than their first ventilatory threshold) with and without custom-molded foot orthoses, in a counterbalanced order. Surface EMG activity of five lower limb muscles, together with oxygen consumption and HR, was recorded at 8-min intervals, starting after 2 min, during the run. A series of neuromuscular tests including voluntary and electrically evoked contractions of the ankle plantar flexors was performed before and after running. RESULTS Peroneus longus root mean square amplitude decreased with time, independently of the condition (-18.9%, P < 0.01). Lower root mean square signal amplitude for vastus medialis (-13.3%, P < 0.02) and gastrocnemius medialis (-10.7%, P < 0.05), combined with increased peroneus longus burst duration (+14.7%, P < 0.05), occurred when running with orthoses. There was no main effect of the condition for oxygen consumption (P > 0.05), whereas HR was significantly lowered while wearing foot orthoses (-3%, P < 0.02). Maximal strength capacity (-9%, P < 0.01), normalized EMG activity (-17%, P < 0.001), and peak twitch torque (-14%, P < 0.01) declined from before to after exercise, independently of the condition. Smaller fatigue-induced decrements in the rate of torque development within the first 200 ms (-6% vs -33%, P < 0.01) were reported after running with foot orthoses. CONCLUSIONS Wearing foot orthoses alters neuromuscular control during a submaximal 1-h treadmill run and partly protects from the resulting fatigue-induced reductions in rapid force development characteristics of the plantar flexors. However, these changes may be too small to alter the aerobic cost of running.

Augmented Low Dye Taping Changes Muscle Activation Patterns and Plantar Pressure During Treadmill Running

STUDY DESIGN Randomized, crossover study. OBJECTIVE To examine changes in muscle activity and plantar pressure during running with the application of augmented low Dye (ALD) taping. BACKGROUND ALD taping is used clinically as part of management for lower limb injury. As of yet, no studies have examined the effect of this taping method on muscle activity and plantar pressure during running, simultaneously. METHODS Thirteen healthy recreational runners(mean ± SD age, 31.7 ± 4.9 years; height, 181.7 ± 4.6 cm; body mass, 81.6 ± 5.9 kg) completed a 6-minute run on a treadmill at a speed of 10 km·h⁻¹, with 3 different taping conditions (ALD, control tape, no tape), applied in randomized order. Peak and average EMG signal amplitude, onset time, and burst duration were calculated for the vastus medialis, vastus lateralis, and the gluteus medius. In-shoe plantar pressures were also recorded. All data were calculated based on an average of 20 steps collected after 5 minutes of treadmill running. RESULTS ALD taping significantly altered muscle activity and plantar pressure during treadmill running by (1) delaying the onset of the EMG signal of the gluteus medius, vastus medialis, and vastus lateralis, and (2) increasing lateral midfoot plantar pressure. CONCLUSION ALD taping significantly alters plantar pressure and muscle activation patterns during treadmill running. These findings give insight into the neuromuscular effect of a taping procedure that is used commonly in a clinical setting.

Shoes alter the spring-like function of the human foot during running

The capacity to store and return energy in legs and feet that behave like springs is crucial to human running economy. Recent comparisons of shod and barefoot running have led to suggestions that modern running shoes may actually impede leg and foot-spring function by reducing the contributions from the leg and foot musculature. Here we examined the effect of running shoes on foot longitudinal arch (LA) motion and activation of the intrinsic foot muscles. Participants ran on a force-instrumented treadmill with and without running shoes. We recorded foot kinematics and muscle activation of the intrinsic foot muscles using intramuscular electromyography. In contrast to previous assertions, we observed an increase in both the peak (flexor digitorum brevis +60%) and total stance muscle activation (flexor digitorum brevis +70% and abductor hallucis +53%) of the intrinsic foot muscles when running with shoes. Increased intrinsic muscle activation corresponded with a reduction in LA compression (−25%). We confirm that running shoes do indeed influence the mechanical function of the foot. However, our findings suggest that these mechanical adjustments are likely to have occurred as a result of increased neuromuscular output, rather than impaired control as previously speculated. We propose a theoretical model for foot–shoe interaction to explain these novel findings.

High-Intensity Running and Plantar-Flexor Fatigability and Plantar-Pressure Distribution in Adolescent Runners

CONTEXT Fatigue-induced alterations in foot mechanics may lead to structural overload and injury. OBJECTIVES To investigate how a high-intensity running exercise to exhaustion modifies ankle plantar-flexor and dorsiflexor strength and fatigability, as well as plantar-pressure distribution in adolescent runners. DESIGN Controlled laboratory study. SETTING Academy research laboratory. PATIENTS OR OTHER PARTICIPANTS Eleven male adolescent distance runners (age = 16.9 ± 2.0 years, height = 170.6 ± 10.9 cm, mass = 54.6 ± 8.6 kg) were tested. INTERVENTION(S) All participants performed an exhausting run on a treadmill. An isokinetic plantar-flexor and dorsiflexor maximal-strength test and a fatigue test were performed before and after the exhausting run. Plantar-pressure distribution was assessed at the beginning and end of the exhausting run. MAIN OUTCOME MEASURE(S) We recorded plantar-flexor and dorsiflexor peak torques and calculated the fatigue index. Plantar-pressure measurements were recorded 1 minute after the start of the run and before exhaustion. Plantar variables (ie, mean area, contact time, mean pressure, relative load) were determined for 9 selected regions. RESULTS Isokinetic peak torques were similar before and after the run in both muscle groups, whereas the fatigue index increased in plantar flexion (28.1%; P = .01) but not in dorsiflexion. For the whole foot, mean pressure decreased from 1 minute to the end (-3.4%; P = .003); however, mean area (9.5%; P = .005) and relative load (7.2%; P = .009) increased under the medial midfoot, and contact time increased under the central forefoot (8.3%; P = .01) and the lesser toes (8.9%; P = .008). CONCLUSIONS Fatigue resistance in the plantar flexors declined after a high-intensity running bout performed by adolescent male distance runners. This phenomenon was associated with increased loading under the medial arch in the fatigued state but without any excessive pronation.

Dynamic midfoot kinematics in subjects with medial tibial stress syndrome.

BACKGROUND Medial tibial stress syndrome (MTSS) is a common diagnosis. Several studies have demonstrated that excessive static navicular drop (ND) is related to the diagnosis. However, no studies have yet investigated ND and the velocity of ND during dynamic conditions. The aim of this study was to evaluate ND characteristics in patients with MTSS in dynamic and static conditions. METHODS In a case-control study, 14 patients diagnosed as having MTSS were included from an orthopedic outpatient clinic. A control group consisting of 14 healthy participants was matched regarding age, sex, and typical sporting activity. Navicular drop was evaluated during treadmill walking by a two-dimensional video analysis. Static foot posture, static ND, dynamic ND (dND), and velocity of dND were compared. RESULTS The two groups were comparable in relation to age, sex, height, weight, and foot size. No significant difference was found in static foot posture. Static ND showed a mean difference of 1.7 mm between the groups (P = .08). During treadmill walking, patients with MTSS had, on average, a 1.5-mm-larger dND (P =.004) and a 2.4-mm/sec-larger mean velocity of dND (P = .03). CONCLUSIONS Patients with MTSS display a larger ND and a higher ND velocity during treadmill walking. Increased ND velocity may be important to this condition. Future studies should include velocity of dND to investigate the mechanisms of dND in relation to overuse injuries.