Ultrasonographic quantification of architectural response in tibialis anterior to neuromuscular electrical stimulation

While muscle contraction in voluntary efforts has been widely investigated, little is known about contraction during neuromuscular electrical stimulation (NMES). The aim of this study was to quantify in vivo muscle architecture of agonist and antagonist muscles at the ankle joint during NMES. Muscle fascicle lengths and pennation angles of the tibialis anterior (TA) and lateral gastrocnemius muscles were assessed via ultrasonography in 8 healthy young males. Measures were obtained during maximal NMES and torque-matched voluntary dorsiflexion contractions. In the TA, NMES induced a shorter fascicle length (67.2 ± 8.1 mm vs 74.6 ± 11.4 mm; p = 0.04) and a greater pennation angle (11.0 ± 2.4° vs 9.3 ± 2.5°; p = 0.03) compared with voluntary torque-matched dorsiflexion contractions. Architectural responses in the antagonist lateral gastrocnemius muscle did not significantly differ from rest or between voluntary and electrically induced contractions (p > 0.05). Contraction of the antagonist muscle was not a contributing factor to a greater fascicle shortening and increased pennation angle in the TA during NMES. TA architectural response during NMES likely arose from the contribution of muscle synergists during voluntary contractions coupled with a potentially localized contractile activity under the stimulation electrodes during NMES induced contractions.     

Changes in lower limb muscle activity after walking on a split-belt treadmill in individuals post-stroke

Background: There is growing evidence that stroke survivors can adapt and improve step length symmetry in the context of split-belt treadmill (SBT) walking. However, less knowledge exists about the strategies involved for such adaptations. This study analyzed lower limb muscle activity in individuals post-stroke related to SBT-induced changes in step length. Methods: Step length and surface EMG activity of six lower limb muscles were evaluated in individuals post-stroke (n = 16) during (adaptation) and after (after-effects) walking at unequal belt speeds. Results: During adaptation, significant increases in EMG activity were mainly found in proximal muscles (p ⩽ 0.023), whereas after-effects were observed particularly in the distal muscles. The plantarflexor EMG increased after walking on the slow belt (p ⩽ 0.023) and the dorsiflexors predominantly after walking on the fast belt (p ⩽ 0.017) for both, non-paretic and paretic-fast conditions. Correlation analysis revealed that after-effects in step length were mainly associated with changes in distal paretic muscle activity (0.522 ⩽ r ⩽ 0.663) but not with functional deficits. Based on our results, SBT walking could be relevant for training individuals post-stroke who present shorter paretic step length combined with dorsiflexor weakness, or individuals with shorter nonparetic step length and plantarflexor weakness.     

Can self-report instruments of shoulder function capture functional differences in older adults with and without a rotator cuff tear?

Rotator cuff tears (RCT) are prevalent in older individuals and may compound age-associated functional declines. Our purpose was to determine whether self-report measures of perceived functional ability are valid for older patients with RCT. Twenty five subjects participated (12M/13F; age = 63.9 ± 3.0 years); 13 with RCT and 12 controls (CON). Participants completed self-report measures of shoulder function (SST, ASES, WORC) and health-related quality of life (SF-36). Isometric joint moment and range of motion (ROM) were measured at the shoulder. Relationships among functional self-reports, and between these measures and joint moment and ROM were assessed; group differences for total and subcategory scores were evaluated. There were significant correlations among self-reports (r_s = 0.62–0.71, p ⩽ 0.02). For RCT subjects, ASES was associated with all joint moments except adduction (p ⩽ 0.02); SST, ASES, and WORC were associated with abduction and external rotation ROM (p ⩽ 0.04). For RCT subjects, SST and WORC were associated with SF-36 physical function subcategory scores (p ⩽ 0.05). The RCT group scored worse than CON on all functional self-reports (p < 0.01) and WORC and ASES subcategories (p < 0.01). In conclusion, SST, ASES, and WORC demonstrate utility and discriminant validity for older individuals by distinguishing those with RCT, but this work suggests prioritizing ASES given its stronger association with functional group strength.     

A benchtop biorobotic platform for in vitro observation of muscle-tendon dynamics with parallel mechanical assistance from an elastic exoskeleton

We present a novel biorobotic framework comprised of a biological muscle-tendon unit (MTU) mechanically coupled to a feedback controlled robotic environment simulation that mimics in vivo inertial/gravitational loading and mechanical assistance from a parallel elastic exoskeleton. Using this system, we applied select combinations of biological muscle activation (modulated with rate-coded direct neural stimulation) and parallel elastic assistance (applied via closed-loop mechanical environment simulation) hypothesized to mimic human behavior based on previously published modeling studies. These conditions resulted in constant system-level force-length dynamics (i.e., stiffness), reduced biological loads, increased muscle excursion, and constant muscle average positive power output—all consistent with laboratory experiments on intact humans during exoskeleton assisted hopping. Mechanical assistance led to reduced estimated metabolic cost and MTU apparent efficiency, but increased apparent efficiency for the MTU + Exo system as a whole. Findings from this study suggest that the increased natural resonant frequency of the artificially stiffened MTU + Exo system, along with invariant movement frequencies, may underlie observed limits on the benefits of exoskeleton assistance. Our novel approach demonstrates that it is possible to capture the salient features of human locomotion with exoskeleton assistance in an isolated muscle-tendon preparation, and introduces a powerful new tool for detailed, direct examination of how assistive devices affect muscle-level neuromechanics and energetics.     

Sex differences in jaw muscle duty factors during exercise in two environments: A pilot study

It is unknown if females and males use jaw muscles similarly during exercise. This pilot study assessed jaw elevator muscle duty factors (DFs = time of muscle activity/total recording time) at repeated sessions to test if DFs are reliable and different between sexes during exercises in two environments. Ten female and seven male subjects recruited from university soccer teams provided informed consent. Surface electromyography was recorded from masseter and temporalis muscles during biting and leg-extension laboratory exercises. Average activities to produce 20 N bite-forces for each muscle and subject determined thresholds (5–80%·T20 N) for subject-specific DF calculations during exercises performed in laboratory and natural environments. Subjects self-recorded via portable electromyography equipment during in-field leg-extension and weight-lifting exercises. Effects of variables on DFs were assessed via ANOVA (α = 0.05) and simple effects testing (Bonferroni-adjusted p ⩽ 0.012). All subjects used jaw muscles during exercises in both environments. DFs between laboratory sessions were reliable (R = 0.84). During laboratory exercises, male temporalis DFs were significantly higher than female DFs from both muscles (p ⩽ 0.001). During in-field exercises females had higher DFs during weight-lifting while males had higher DFs during leg-extensions. In-field sex differences were significant at most thresholds and showed larger effect sizes for leg-extension compared to weight-lifting exercises.     

Foot structure is correlated with performance in a single-joint jumping task

Variability in musculoskeletal structure has the potential to influence locomotor function. It has been shown, for example, that sprinters have smaller Achilles tendon moment arms and longer toes than non-sprinters, and toe length has been found to correlate with toe flexor work in running humans. These findings suggest that interindividual variation in human foot structure allows for function that is adapted to various motor tasks. The purpose of this study was to test for correlations between foot anthropometry and single-joint maximal-height jumping performance. Ten male subjects performed static jumps using only their ankles for propulsion. Several anthropometric measures were taken. Bivariate correlation analyses were performed between all anthropometric variables and the average jump height for each subject. Results showed that the best jumpers had longer lateral heel lengths (r = 0.871; p = 0.001) and longer toes (r = 0.712; p = 0.021). None of the other anthropometric variables (stature, mass, lower extremity lengths) measured were found to correlate significantly with jump height. A factor analysis was performed to investigate whether some underlying feature related to body stature could explain jumping performance. Taller subjects did not necessarily jump higher. Specific variations in foot structure, unrelated to other general stature measures, were associated with performance in this single-joint jumping task.     

Ergogenic and metabolic effects of oral glucocorticoid intake during repeated bouts of high-intensity exercise

All systemically administered glucocorticoids (GC) are prohibited in-competition, because of the potential ergogenic effects. Although short-term GC intake has been shown to improve performance during submaximal exercise, literature on its impact during brief intense exercise appears to be very scant. The purpose of this study was to examine the ergogenic and metabolic effects of prednisone during repeated bouts of high-intensity exercise. In a double-blind randomized protocol, ten recreational male athletes followed two 1-week treatments (Cor: prednisone, 60 mg/day or Pla: placebo). At the end of each treatment, they hopped on their dominant leg for 30 s three times consecutively and then hopped until exhaustion, with intervals of 5 min of passive recovery. Blood and saliva samples were collected at rest and 3 min after each exercise bout to determine the lactate, interleukin-6, interleukin-10, TNF-alpha, DHEA and testosterone values. The absolute peak force of the dominant leg was significantly increased by Cor but only during the first 30-s hopping bout (p < 0.05), whereas time to exhaustion was not significantly changed after Cor treatment vs Pla (Pla: 119.9 ± 24.7; Cor: 123.1 ± 29.5 s). Cor intake lowered basal and end-exercise plasma interleukin-6 and saliva DHEA (p < 0.01) and increased interleukin-10 (p < 0.01), whereas no significant change was found in blood lactate and TNF-alpha or saliva testosterone between Pla and Cor. According to these data, short-term glucocorticoid intake did not improve endurance performance during repeated bouts of high-intensity exercise, despite the significant initial increase in absolute peak force and anti-inflammatory effect.     

Performance limitation and the role of core temperature when wearing light-weight workwear under moderate thermal conditions

The objective of this investigation was to achieve an understanding about the relationship between heat stress and performance limitation when wearing a two-layerfire-resistant light-weight workwear (full-clothed ensemble) compared to an one-layer short sports gear (semi-clothed ensemble) in an exhaustive, stressful situation under moderate thermal condition (25 °C). Ten well trained male subjects performed a strenuous walking protocol with both clothing ensembles until exhaustion occurred in a climatic chamber. Wearing workwear reduced the endurance performance by 10% (p=0.007) and the evaporation by 21% (p=0.003), caused a more pronounced rise in core temperature during submaximal walking (0.7±0.3 vs. 1.2±0.4 °C; p≤0.001) and from start till exhaustion (1.4±0.3 vs. 1.8±0.5 °C; p=0.008), accelerated sweat loss (13±2 vs. 15±3 g min⁻¹; p=0.007), and led to a significant higher heart rate at the end of cool down (103±6 vs. 111±7 bpm; p=0.004). Correlation analysis revealed that core temperature development during submaximal walking and evaporation may play important roles for endurance performance. However, a critical core temperature of 40 °C, which is stated to be a crucial factor for central fatigue and performance limitation, was not reached either with the semi-clothed or the full-clothed ensemble (38.3±0.4 vs. 38.4±0.5 °C). Additionally, perceived exertion did not increase to a higher extent parallel with the rising core temperature with workwear which would substantiate the critical core temperature theory. In conclusion, increased heat stress led to cardiovascular exercise limitation rather than central fatigue.     

Vitamin D sufficiency associates with an increase in anti-inflammatory cytokines after intense exercise in humans

The purpose of this study was to identify the influence of vitamin D status (insufficient vs. sufficient) on circulating cytokines and skeletal muscle strength after muscular injury. To induce muscular injury, one randomly selected leg (SSC) performed exercise consisting of repetitive eccentric–concentric contractions. The other leg served as the control. An averaged serum 25(OH)D concentration from two blood samples collected before exercise and on separate occasions was used to establish vitamin D insufficiency (<30 ng/mL, n = 6) and sufficiency (>30 ng/mL, n = 7) in young, adult males. Serum cytokine concentrations, single-leg peak isometric force, and single-leg peak power output were measured before and during the days following the exercise protocol. The serum IL-10 and IL-13 responses to muscular injury were significantly (both p < 0.05) increased in the vitamin D sufficient group. The immediate and persistent (days) peak isometric force (p < 0.05) and peak power output (p < 0.05) deficits in the SSC leg after the exercise protocol were not ameliorated with vitamin D sufficiency. We conclude that vitamin D sufficiency increases the anti-inflammatory cytokine response to muscular injury.     

Acute effect of heel-drop exercise with varying ranges of motion on the gastrocnemius aponeurosis-tendon’s mechanical properties

The objectives of this study was to investigate the acute effects of various magnitudes of tendon strain on the mechanical properties of the human medial gastrocnemius (MG) in vivo during controlled heel-drop exercises. Seven male and seven female volunteers performed two different exercises executed one month apart: one was a heel-drop exercise on a block (HDB), and the other was a heel-drop exercise on level floor (HDL). In each regimen, the subjects completed a session of 150 heel-drop exercises (15 repetitions × 10 sets; with a 30 s rest following each set). Before and immediately after the heel-drop exercise, the ankle plantar flexor torque and elongation of the MG were measured using a combined measurement system of dynamometry and ultrasonography and then the MG tendon strain and stiffness were evaluated in each subject. The tendon stiffness measured prior to the exercises was not significantly different between the two groups 23.7 ± 10.6 N/mm and 24.1 ± 10.0 N/mm for the HDB and HDL, respectively (p > .05). During the heel-drop exercise, it was found that the tendon strain during the heel-drop exercise on a block (8.4 ± 3.7%) was significantly higher than the strain measured on the level floor (5.4 ± 3.8%) (p < .05). In addition, the tendon stiffness following the heel-drop exercise on a block (32.3 ± 12.2 N/mm) was significantly greater than the tendon stiffness measured following the heel-drop exercise on the level floor (25.4 ± 11.4 N/mm) (p < .05). The results of this study suggest that tendon stiffness immediately following a heel-drop exercise depends on the magnitude of tendon strain.     

Neural control of leg stiffness during hopping in boys and men

The purpose of the study was to investigate whether boys and men utilise different control strategies whilst hopping. Eleven boys (11–12 yr old) and ten men completed hopping at 1.5 Hz, 3.0 Hz and at their preferred frequency. A footswitch measured contact and flight times, from which leg stiffness was calculated. Simultaneously, surface electromyograms (EMGs) of selected lower limb muscles were recorded and quantified for each 30 ms period during the first 120 ms post-ground contact. At 1.5 Hz there were no differences between the groups in relative stiffness or muscle activity. At 3.0 Hz men had significantly shorter contact times (P = 0.013), longer flight times (P = 0.002), greater relative stiffness (P = 0.01) and significantly greater soleus (P = 0.012) and vastus lateralis (P < 0.001) activity during the initial 30 ms post-ground contact. At the preferred frequency men hopped significantly faster than the boys (P = 0.007), with greater leg stiffness (P < 0.01) and with more extensor activity in most time periods. Boys and men demonstrated similar control strategies when hopping at a slow frequency, but when hopping frequency increased men were able to better increase feedforward and reflex muscle activity to hop with greater relative stiffness.     

Toward the validation of a new method (MUNIX) for motor unit number assessment

Introduction: This prospectively designed study analyzed the correlation of a new, non-invasive neurophysiological method (Motor Unit Number Index – MUNIX) with two established Motor Unit Number Estimation (MUNE) methods. Methods: MUNIX and incremental stimulation MUNE (IS-MUNE) were done in the abductor digiti minimi muscle (ADM), while MUNIX and spike-triggered averaging MUNE (STA-MUNE) were tested in the trapezius muscle. Twenty healthy subjects and 17 patients with amyotrophic lateral sclerosis (ALS) were examined. Results: MUNIX and MUNE values correlated significantly (ADM: n = 108; Spearman-Rho; r = 0.88; p < 0.01; trapezius muscle: n = 49; Spearman-Rho; r = 0.46; p < 0.01). Discussion: MUNIX indeed reflects the number of motor units in a muscle, and may sensibly be recorded from the trapezius muscle. With MUNIX being both much more patient friendly and much more rapid to assess than MUNE, the results support the use of MUNIX when motor unit number assessment is desired.     

Mechanical properties of the patellar tendon in adults and children

It is not currently known how the mechanical properties of human tendons change with maturation in the two sexes. To address this, the stiffness and Young's modulus of the patellar tendon were measured in men, women, boys and girls (each group, n=10). Patellar tendon force (F_pt) was calculated from the measured joint moment during a ramped voluntary isometric knee extension contraction, the antagonist knee extensor muscle co-activation quantified from its electromyographical activity, and the patellar tendon moment arm measured from magnetic resonance images. Tendon elongation was imaged using the sagittal-plane ultrasound scans throughout the contraction. Tendon cross-sectional area was measured at rest from ultrasound scans in the transverse plane. Maximal F_pt and tendon elongation were (mean±SE) 5453±307 N and 5±0.5 mm for men, 3877±307 N and 4.9±0.6 mm for women, 2017±170 N and 6.2±0.5 mm for boys and 2169±182 N and 5.9±0.7 mm for girls. In all groups, tendon stiffness and Young's modulus were examined at the level that corresponded to the maximal 30% of the weakest participant's F_pt and stress, respectively; these were 925–1321 N and 11.5–16.5 MPa, respectively. Stiffness was 94% greater in men than boys and 84% greater in women than girls (p<0.01), with no differences between men and women, or boys and girls (men 1076±87 N/mm; women 1030±139 N/mm; boys 555±71 N/mm and girls 561.5±57.4 N/mm). Young's modulus was 99% greater in men than boys (p<0.01), and 66% greater in women than girls (p<0.05). There were no differences in modulus between men and women, or boys and girls (men 597±49 MPa; women 549±70 MPa; boys 255±42 MPa and girls 302±33 MPa). These findings indicate that the mechanical stiffness of tendon increases with maturation due to an increased Young's modulus and, in females due to a greater increase in tendon cross-sectional area than tendon length.     

Validity of the twitch interpolation technique for the assessment of quadriceps neuromuscular asymmetries

This study examined the validity of the twitch interpolation technique for evaluating side-to-side asymmetries in quadriceps neuromuscular function. Fifty-six subjects with a wide range of asymmetries (19 healthy, 24 with unilateral and 13 with bilateral anterior cruciate ligament reconstruction) took part in the study. Supramaximal electrical paired stimuli were delivered to the quadriceps muscle during and immediately after a maximal voluntary contraction (MVC) of the knee extensors (twitch interpolation technique). MVC torque, voluntary activation and resting doublet-evoked torque were measured separately for the two sides, and percent side-to-side asymmetries were calculated for each parameter. MVC torque asymmetry was plotted against voluntary activation asymmetry and doublet-evoked torque asymmetry, and a multiple regression analysis was also conducted. Significant positive correlations were observed between MVC torque asymmetry and both voluntary activation asymmetry (r = 0.40; p = 0.002) and doublet-evoked torque asymmetry (r = 0.53; p < 0.001), and their relative contribution to MVC torque asymmetry was comparable (r = 0.64; p < 0.001). These results establish the validity of the twitch interpolation technique for the assessment of neuromuscular asymmetries. This methodology could provide useful insights into the contribution of some neural and muscular mechanisms that underlie quadriceps strength deficits.     

Surface electromyography activity of the rectus abdominis,internal oblique,and external oblique muscles during forced expiration in healthy adults

We aimed to characterize rectus abdominis, internal oblique, and external oblique muscle activity in healthy adults under expiratory resistance using surface electromyography. We randomly assigned 42 healthy adult subjects to 3 groups: 30%, 20%, and 10% maximal expiratory intraoral pressure (PEmax). After measuring 100% PEmax and muscle activity during 100% PEmax, the activity and maximum voluntary contraction of each muscle during the assigned experimental condition were measured. At 100% PEmax, the external oblique (p < 0.01) and internal oblique (p < 0.01) showed significantly elevated activity compared with the rectus abdominis muscle. Furthermore, at 20% and 30% PEmax, the external oblique (p < 0.05 and < 0.01, respectively) and the internal oblique (p < 0.05 and < 0.01, respectively) showed significantly elevated activity compared with the rectus abdominis muscle. At 10% PEmax, no significant differences were observed in muscle activity.Although we observed no significant difference between 10% and 20% PEmax, activity during 30% PEmax was significantly greater than during 20% PEmax (external oblique: p < 0.05; internal oblique: p < 0.01). The abdominal oblique muscles are the most active during forced expiration. Moreover, 30% PEmax is the minimum intensity required to achieve significant, albeit very slight, muscle activity during expiratory resistance.     

Alternative methods of normalising EMG during cycling

We evaluated possible methods of normalisation for EMG measured during cycling. The MVC method, Sprint method and 70% Peak Power Output Method were investigated and their repeatability, reliability and sensitivity to change in workload were compared.Thirteen cyclists performed the same experimental protocol on three separate occasions. Each day, subjects firstly performed MVCs, followed by a 10 s maximal sprint on a cycle ergometer. Subjects then performed a Peak Power Output (PPO) test until exhaustion. After which they cycled at 70% of PPO for 5 min at 90 rpm. Results indicated that normalising EMG data to 70% PPO is more repeatable, the intra-class correlation (ICC) of 70% PPO (0.87) was significantly higher than for MVC (0.66) (p = 0.03) and 10 s sprint (0.65) (p = 0.04). The 70% PPO method also demonstrated the least intra-subject variability for five out of the six muscles. The Sprint and 70% PPO method highlighted greater sensitivity to changes in muscle activity than the MVC method. The MVC method showed the highest intra-subject variability for most muscles except VM.The data suggests that normalising EMG to dynamic methods is the most appropriate for examining muscle activity during cycling over different days and for once-off measurements.     

The reliability and criterion validity of 2D video assessment of single leg squat and hop landing

The objective was to assess the intra-tester, within and between day reliability of measurement of hip adduction (HADD) and frontal plane projection angles (FPPA) during single leg squat (SLS) and single leg landing (SLL) using 2D video and the validity of these measurements against those found during 3D motion capture. 15 healthy subjects had their SLS and SLL assessed using 3D motion capture and video analysis. Inter-tester reliability for both SLS and SLL when measuring FPPA and HADD show excellent correlations (ICC_2,1 0.97–0.99). Within and between day assessment of SLS and SLL showed good to excellent correlations for both variables (ICC_3,1 0.72–91). 2D FPPA measures were found to have good correlation with knee abduction angle in 3-D (r = 0.79, p = 0.008) during SLS, and also to knee abduction moment (r = 0.65, p = 0.009). 2D HADD showed very good correlation with 3D HADD during SLS (r = 0.81, p = 0.001), and a good correlation during SLL (r = 0.62, p = 0.013). All other associations were weak (r < 0.4). This study suggests that 2D video kinematics have a reasonable association to what is being measured with 3D motion capture.     

Biceps femoris and semitendinosus tendon/aponeurosis strain during passive and active (isometric) conditions

The purpose of this study was to quantify strain and elongation of the long head of the biceps femoris (BFlh) and the semitendinosus (ST) tendon/aponeurosis. Forty participants performed passive knee extension trials from 90° of knee flexion to full extension (0°) followed by ramp isometric contractions of the knee flexors at 0°, 45° and 90° of knee flexion. Two ultrasound probes were used to visualize the displacement of BFlh and ST tendon/aponeurosis. Three-way analysis of variance designs indicated that: (a) Tendon/aponeurosis (passive) elongation and strain were higher for the BFlh than the ST as the knee was passively extended (p < 0.05), (b) contraction at each angular position was accompanied by a smaller BFlh tendon/aponeurosis (active) strain and elongation than the ST at higher levels of effort (p < 0.05) and (c) combined (passive and active) strain was significantly higher for the BFlh than ST during ramp contraction at 0° but the opposite was observed for the 45° and 90° flexion angle tests (p < 0.05). Passive elongation of tendon/aponeurosis has an important effect on the tendon/aponeurosis behavior of the hamstrings and may contribute to a different loading of muscle fibers and tendinous tissue between BFlh and ST.