Direct muscle electrical stimulation as a method for the in vivo assessment of force production in m. abductor hallucis

© 2020 Elsevier Ltd In vivo assessment of the force-generating capacity of m. abductor hallucis (AbH) is problematic due to its combined abduction-flexion action and the inability of some individuals to voluntarily activate the muscle. This study investigated direct muscle electrical stimulation as a method to assess isometric force production in AbH about the 1st metatarsal phalangeal joint (1MPJ) at different muscle-tendon lengths, with the aim of identifying an optimal angle for force production. A 7 s stimulation train was delivered at 20 Hz pulse frequency and sub-maximal (150% motor threshold) intensity to the AbH of the left foot in 16 participants whilst seated, and with the Hallux suspended from a force transducer in 0°,5°,10°,15° and 20° 1MPJ dorsal flexion. Reflective markers positioned on the foot and force transducer were tracked with 5 optical cameras to continuously record the force profile and calculate the external 1MPJ joint flexion moment at each joint configuration. A parabolic relationship was found between AbH force production and 1MPJ configuration. The highest 1MPJ joint moments induced by electrical stimulation were found between 10° and 15° of Hallux dorsal flexion. However, the joint angle (p < 0.001; η2 = 0.86) changed significantly across all but one 1MPJ configurations tested during the stimulation-evoked contraction, resulting in a significant change in the corresponding external moment arm (p < 0.001; η2 = 0.83). Therefore, the changes in joint geometry during contraction should be accounted for to prevent an underestimation of the resulting joint moment. We conclude that direct muscle electrical stimulation combined with dynamometry offers a robust method for standardised assessment of AbH sub-maximal isometric force production

Muscle architecture and passive lengthening properties of the gastrocnemius medialis and Achilles tendon in children who idiopathically toe-walk

Children who idiopathically toe-walk (ITW) habitually operate at greater plantarflexion angles and thus, at shorter muscle-tendon unit (MTU) lengths than typically developing (TD) children. Therefore, it is often assumed that habitual use of the gastrocnemius muscle in this way will cause remodelling of the muscle-tendon architecture compared to TD children. However, the gastrocnemius muscle architecture of children who ITW has never been measured. It is essential that we gain a better understanding of these muscle-tendon properties, to ensure that appropriate clinical interventions can be provided for these children. Five children who ITW (age 8 ± 2 years) and 14 TD children (age 10 ± 2 years) participated in this study. Ultrasound was combined with isokinetic dynamometry and surface electromyography, to measure muscle architecture at common positions and passive lengthening properties of the gastrocnemius muscle and tendon across full range of motion. Regardless of which common condition groups were compared under, both the absolute and normalised to MTU muscle belly and fascicle lengths were always longer, and the Achilles tendon length was always shorter in children who ITW than TD children (p 0.05); however, passive joint stiffness was greater in children who ITW at maximum dorsiflexion (p = 0.001) and at a joint moment common to all participants (p = 0.029). Consequently, the findings of this pilot study indicate a remodelling of the relative MTU that does not support the concept that children who ITW commonly experience muscle shortening. Therefore, greater consideration of the muscle and tendon properties are required when prescribing clinical interventions that aim to lengthen the MTU, and treatments may be better targeted at the Achilles tendon in children who ITW

Wide-pulse, high-frequency, low-intensity neuromuscular electrical stimulation has potential for targeted strengthening of an intrinsic foot muscle: A feasibility study

Background: Strengthening the intrinsic foot muscles is a poorly understood and largely overlooked area. In this study, we explore the feasibility of strengthening m. abductor hallucis (AH) with a specific paradigm of neuromuscular electrical stimulation; one which is low-intensity in nature and designed to interleave physiologically-relevant low frequency stimulation with high-frequencies to enhance effective current delivery to spinal motoneurones, and enable a proportion of force produced by the target muscle to be generated from a central origin. We use standard neurophysiological measurements to evaluate the acute (~ 30 min) peripheral and central adaptations in healthy individuals. Methods: The AH in the dominant foot of nine healthy participants was stimulated with 24 × 15 s trains of square wave (1 ms), constant current (150% of motor threshold), alternating (20 Hz-100 Hz) neuromuscular electrical stimulation interspersed with 45 s rest. Prior to the intervention, peripheral variables were evoked from the AH compound muscle action potential (Mwave) and corresponding twitch force in response to supramaximal (130%) medial plantar nerve stimulation. Central variables were evoked from the motor evoked potential (MEP) in response to suprathreshold (150%) transcranial magnetic stimulation of the motor cortex corresponding to the AH pathway. Follow-up testing occurred immediately, and 30 min after the intervention. In addition, the force-time-integrals (FTI) from the 1st and 24th WPHF trains were analysed as an index of muscle fatigue. All variables except FTI (T-test) were entered for statistical analysis using a single factor repeated measures ANOVA with alpha set at 0.05. Results: FTI was significantly lower at the end of the electrical intervention compared to that evoked by the first train (p < 0.01). Only significant peripheral nervous system adaptations were observed, consistent with the onset of low-frequency fatigue in the muscle. In most of these variables, the effects persisted for 30 min after the intervention. Conclusions: An acute session of wide-pulse, high-frequency, low-intensity electrical stimulation delivered directly to abductor hallucis in healthy feet induces muscle fatigue via adaptations at the peripheral level of the neuromuscular system. Our findings would appear to represent the first step in muscle adaptation to training; therefore, there is potential for using WPHF for intrinsic foot muscle strengthening

A voluntary activation deficit in m. abductor hallucis exists in asymptomatic feet.

M. abductor hallucis (AbH) is the strongest intrinsic foot muscle and its dysfunction underlies various foot disorders. Attempts to strengthen the muscle by voluntary exercises are constrained by its complex morphology and oblique mechanical action, which leads to an inability even in asymptomatic individuals to fully activate AbH. This study investigated the extent and magnitude of this inability whilst also providing preliminary evidence for the virtue of targeted sub-maximum neuromuscular electrical stimulation (NMES) as a countermeasure for an AbH activation deficit. The voluntary activation ratio (VAR) was assessed via the twitch interpolation technique in the left AbH of 13 healthy participants during maximum voluntary 1st metatarsophalangeal joint flexion-abduction contractions (MVC). Participants were grouped ("able" or "unable") based on their ability to fully activate AbH (VAR ≥ 0.9). 7 s-NMES trains (20 Hz) were then delivered to AbH with current intensity increasing from 150% to 300% motor threshold (MT) in 25% increments. Perceived comfort was recorded (10 cm-visual analogue scale; VAS). Only 3 participants were able to activate AbH to its full capacity (able, mean (range) VAR: 0.93 (0.91-0.95), n = 3; unable: 0.69 (0.36-0.83), n = 10). However, the maximum absolute forces produced during the graded sub-maximum direct-muscle NMES protocol were comparable between groups implying that the peripheral contractility of AbH is intact irrespective of the inability of individuals to voluntary activate AbH to its full capacity. These findings demonstrate that direct-muscle NMES overcomes the prevailing inability for high voluntary AbH activation and therefore offers the potential to strengthen the healthy foot and restore function in the pathological foot. [Abstract copyright: Copyright © 2021 Elsevier Ltd. All rights reserved.

Consistent patterns and the idiosyncstatic effects of biodiversity in marine ecosysytems

Revealing the consequences of species extinctions for ecosystem function has been a chief research goal 1, 2, 3, 4, 5, 6, 7 and has been accompanied by enthusiastic debate 8, 9, 10, 11. Studies carried out predominantly in terrestrial grassland and soil ecosystems have demonstrated that as the number of species in assembled communities increases, so too do certain ecosystem processes, such as productivity, whereas others such as decomposition can remain unaffected12. Diversity can influence aspects of ecosystem function, but questions remain as to how generic the patterns observed are, and whether they are the product of diversity, as such, or of the functional roles and traits that characterize species in ecological systems. Here we demonstrate variable diversity effects for species representative of marine coastal systems at both global and regional scales. We provide evidence for an increase in complementary resource use as diversity increases and show strong evidence for diversity effects in naturally assembled communities at a regional scale. The variability among individual species responses is consistent with a positive but idiosyncratic pattern of ecosystem function with increased diversity