Influence of complete spinal cord injury on skeletal muscle cross-sectional area within the first 6 months of injury

In this study we examined the influence of complete spinal cord injury (SCI) on affected skeletal muscle morphology within 6 months of SCI. Magnetic resonance (MR) images of the leg and thigh were taken as soon as patients were clinically stable, on average 6 weeks post injury, and 11 and 24 weeks after SCI to assess average muscle cross-sectional area (CSA). MR images were also taken from nine able-bodied controls at two time points separated from one another by 18 weeks. The controls showed no change in any variable over time. The patients showed differential atrophy (P?=?0.0001) of the ankle plantar or dorsi flexor muscles. The average CSA of m. gastrocnemius and m. soleus decreased by 24% and 12%, respectively (P?=?0.0001). The m. tibialis anterior CSA showed no change (P?=?0.3644). As a result of this muscle-specific atrophy, the ratio of average CSA of m. gastrocnemius to m. soleus, m. gastrocnemius to m. tibialis anterior and m. soleus to m. tibialis anterior declined (P?=?0.0001). The average CSA of m, quadriceps femoris, the hamstring muscle group and the adductor muscle group decreased by 16%, 14% and 16%, respectively (P?≤?0.0045). No differential atrophy was observed among these thigh muscle groups, thus the ratio of their CSAs did not change (P?=?0.6210). The average CSA of atrophied skeletal muscle in the patients was 45–80% of that of age- and weight-matched able-bodied controls 24 weeks after injury. In conclusion, the results of this study suggest that there is marked loss of contractile protein early after SCI which differs among affected skeletal muscles. While the mechanism(s) responsible for loss of muscle size are not clear, it is suggested that the development of muscular imbalance as well as diminution of muscle mass would compromise force potential early after SCI.     

Influence of complete spinal cord injury on skeletal muscle mechanics within the first 6 months of injury

In this study we examined the influence of complete spinal cord injury (SCI) on the mechanical characteristics of skeletal muscle in vivo within 6 months of the injury. Surface electrical stimulation (ES) was applied to the left m. quadriceps femoris of patients at 6, 11 and 24 weeks after injury. Surface ES was also applied to seven able-bodied controls (AB) at two time points 18 weeks apart. ES consisted of 2 bouts of 20, 1-s isometric contractions with 2 s and 2 min of rest between contractions and bouts, respectively. The time from 20–80% of peak torque (rise time) and the half relaxation time (1/2 RT) were determined for the first and for the last few contractions. Force loss over repeat contractions was greater in SCI than AB (27% vs 95%; P = 0.0001), and did not change over the 18-week period. Rise time did not change over repeat contractions, was not different between groups, and nor did it change over the 18-week period (range: 150–172 ms). 1/2 RT showed several group differences. Overall, 1/2 RT was longer at the beginning of ES in SCI than AB [mean (SE) 133 (15) ms vs 90 (6) ms, P = 0.037]. Slowing of relaxation time with force loss over repeat contractions was found in SCI at 24 weeks after injury [167 (18) ms, P = 0.016], but not at 6 [128 (14) ms] or 11 [145 (12) ms] weeks after injury. AB, in contrast, showed prolonged relaxation times, with force loss at both time points [115 (10) ms and 113 (11) ms; P = 0.0001]. The results indicate that SCI alters the relaxation but not contractile properties of mixed skeletal muscle within the first 24 weeks of injury. Altered calcium handling and contraction-induced fiber injury are suggested to explain the slower relaxation time per se, and the prolonged relaxation with force loss observed after SCI. Accepted: 19 July 1999     

Time-course response in serum markers of bone turnover to a single-bout of electrical stimulation in patients with recent spinal cord injury

The objective of the present repeat-measures study was to determine whether plasma serum levels of testosterone, cortisol, osteocalcin or type I collagen C-telopeptide (CT) are acutely affected following an electro-myostimulation (EMS) bout, and their relation to bone mineral density and muscle mass. Ten men with recent (8 weeks) thoracic spinal cord injury (SCI) (ASIA A) and 10 age-matched able-bodied (AB) men performed one EMS bout on the quadriceps femoris muscle. Blood samples were drawn at basal condition, immediately after EMS, and 15 min, 30 min, 24 h and 48 h post-EMS. Muscle cross-sectional area was measured by magnetic resonance imaging. Bone mineral density (BMD) was determined by dual-energy X-ray absorptiometry. In the SCI group, a significant decrease in testosterone, cortisol and CT together with a significant increase in testosterone/cortisol ratio and osteocalcin/CT ratio was observed after EMS. For the AB subjects, only testosterone and CT decreased significantly following EMS. Muscle size was only related to testosterone/cortisol ratio in the SCI sample (R = 0.659, p < 0.05), whereas BMD did not show any relation to any biomarker. Acute EMS in recent spinal cord injured men seems to induce positive effects on bone turnover biomarkers, and anabolic and catabolic hormones.     

Long-term spinal cord injury increases susceptibility to isometric contraction-induced muscle injury

Complete spinal cord injury (SCI) results in inactivation and unloading of affected skeletal muscles. Unloading causes an increased susceptibility of muscle to contraction-induced injury. This study used magnetic resonance imaging (MRI) to test the hypothesis that isometric contractions would evoke greater muscle damage to the quadriceps femoris muscle (mQF) of SCI subjects than that of able-bodied (AB) controls. MR images were taken of the mQF prior to, immediately post, and 3 days post electromyostimulation (EMS). EMS consisted of five sets of ten isometric contractions (2 s on/6 s off, 1 min between sets) followed by another three sets of ten isometric contractions (1 s on/1 s off, 30 s between sets). Average muscle cross-sectional area (CSA) and the relative areas of stimulated and injured muscle were obtained from MR images by quantifying the number of pixels with an elevated T2 signal. SCI subjects had significantly greater relative area [90 (2)% versus 66 (4)%, P<0.05; mean (SE)] but a lesser absolute area [16 (3) cm² versus 44 (6) cm², P<0.05] of mQF stimulated than AB controls. During EMS, peak torque was reduced by 66% and 37% for SCI and control subjects, respectively. Three days post EMS, there was a greater relative area of stimulated mQF injured for the SCI subjects [25 (6)% versus 2 (1)%, P<0.05]. Peak torque remained decreased by 22% on day 3 in the SCI group only. These results indicate that affected muscle years after SCI is more susceptible to contraction-induced muscle damage, as determined by MRI, compared to AB controls. They also support the contention that electrically elicited isometric contractions are sufficient to cause muscle damage after a prolonged period of inactivity.     

Biarticular and monoarticular muscle activation and injury in human quadriceps muscle

We hypothesized that activation of the quadriceps femoris muscle group during eccentric exercise is related to the increase in magnitude of several markers of muscle injury that developed during the next week. Fourteen male subjects performed six to eight sets of five to ten repetitions of single-leg eccentric-only seated knee extension exercise. Magnetic resonance (MR) images were collected before and immediately after exercise and on days 2–4 and 6 after eccentric exercise. Changes in maximal voluntary contraction (MVC), perceived soreness, muscle volume and muscle transverse relaxation of water protons (T₂) were determined for the quadriceps femoris muscle group each day. Changes in muscle volume and T₂ were determined every day for each muscle [vastus lateralis (VL), vastus medialis (VM), vastus intermedius (VI), rectus femoris (RF)] of the quadriceps femoris group. Post-exercise T₂ was greater than pre-exercise T₂ (P<0.05) for all muscles. The acute ΔT₂ (Post-Pre) was similar (P>0.05) among VL, VM, VI, and RF [5.5 (0.3) ms], suggesting that the four muscles were equally activated during eccentric exercise. In the week after eccentric exercise, subjects experienced delayed-onset muscle soreness (DOMS) and all muscles demonstrated a delayed increase in T₂ above pre-exercise values (P<0.05), suggesting that muscle injury had occurred. For the quadriceps femoris muscle group, there was no correlation between acute ΔT₂ and delayed (peak T₂ during days 2, 3, 4, 6 minus pre-exercise T₂) ΔT₂ (r=0.04, P>0.05). Similar results were obtained when VL, VM, VI and RF were examined separately. Of the four muscles in quadriceps femoris, the biarticular RF experienced greater muscle injury [delayed ΔT₂=15.2 (2.0) ms] compared to the three monoarticular vasti muscles [delayed ΔT₂=7.7 (1.3) ms; P<0.05]. We propose that the disproportionate muscle injury to RF resulted from an ineffective transfer of torque from the knee to hip joint during seated eccentric knee extension exercise, thus causing RF to dissipate greater energy than normal. We conclude that in humans, muscle activation is not a unique determinant of muscle injury. Electronic Publication     

The effect of a repeat bout of exercise on muscle injury in persons with spinal cord injury

Following an initial bout of damaging exercise, a successive bout of similar exercise typically results in less injury, known as the protective effect. Unloading due to spinal cord injury (SCI) increases the susceptibility to contraction-induced muscle injury. We tested the hypothesis that two bouts of isometric actions would evoke the same damage in the quadriceps femoris (QF) of patients with SCI. Six male subjects [32 (5) years old, 182 (9) cm, 81 (21) kg, injury level C6-T7, 6 (2) years post-injury, mean (SD)] were tested at two time points (Time1, Time2), separated by 8 weeks. Magnetic resonance images were taken of the QF prior to, immediately after, and 3 days after electromyostimulation (EMS) that evoked isometric knee extension. EMS (50 Hz) consisted of five sets of ten contractions (2 s on/6 s off, 1 min b/t sets) followed by three sets of ten contractions (1 s on/1 s off, 30 s b/t sets). Relative cross-sectional area of stimulated and injured skeletal muscle was obtained by quantifying pixels with an elevated T2. Relative area of stimulated QF was the same for both time points [92 (6)% and 89 (7)%] as was torque loss (~55%). Three days post-EMS, the relative area of stimulated QF injured was not different between time points [30 (14)% vs 29 (17)%, P>0.05]. These results indicate an absence of a protective effective for repeat exercise bouts separated by 8 weeks in SCI patients using EMS.     

The effect of early therapeutic electrical stimulation on bone mineral density in the paralyzed limbs of the rabbit

The purpose of this animal experiment was to evaluate the changes of bone mineral density in paralyzed limbs, and to assess the effects of electrically stimulating muscle contraction upon bone mineral density (BMD) in paralyzed limbs during the four week period immediately following spinal cord injury (SCI). Ten rabbits were used for the study, spinal cords were totally transected at the T11 spine level. The paralyzed quadriceps femoris of one limb was contracted by electrical stimulation for 60-minutes daily, while the other side was not stimulated as a control. The BMD of each lower limb was measured by Dual Photon Absorptiometry before and four weeks after acute SCI. BMD of both limbs decreased in all rabbits four weeks after SCI. The decrease in BMD for stimulated and non-stimulated limbs was 6.130 +/- 3.212% and 9.098 +/- 3.831%, respectively during the four-week period after SCI. The BMD of stimulated limbs decreased significantly less than that of the non-stimulated limbs. Electrically induced muscular contraction reduced bone mineral loss in the paralyzed limb during the early stage of SCI in the rabbit.     

Variability of the fatigue response of paralyzed skeletal muscle in relation to the time after spinal cord injury: mechanical and electrophysiological characteristics

The aim of this study was to determine the effect of the time after spinal cord injury (less than and greater than 10 months) on the mechanical and electrophysiological characteristics of muscle fatigue of the paralyzed electrically stimulated quadriceps muscle. Morphologically and histochemically, a relationship was observed between muscle fatigue and the delay from injury, revealing a critical period of enzymatic turning and a maximum peak of atrophy around the 10th month after the injury, followed by a long-term stabilization. Knee-torque output and M-wave variables (amplitude, latency, duration, and root mean square, RMS) of two muscular heads of the quadriceps were recorded in 19 paraplegic patients during a 120-s isometric contraction. The fatiguing muscle contraction was elicited by supramaximal continuous 20-Hz electrical stimulation. Compared to the chronic group, the acutely paralyzed group showed a greater resistance to fatigue (amount and rate of force decline, P?≤?0.01), smaller alterations of the M-wave amplitude and RMS, and a limited decrease of the muscle fiber conduction velocity (P?相似文献   

A low-cost thermoelectrically cooled tissue clamp for in vitro cyclic loading and load-to-failure testing of muscles and tendons

In vitro cyclic loading and load-to-failure testing of muscles and tendons require a reliable linking device between tissues and the actuator that can transmit high loads without slippage or tissue damage. This article describes a simple custom-made thermoelectrically cooled freeze clamp. The effectiveness of the clamp to transmit loads without tissue slippage was evaluated on 10 canine quadriceps femoris myo-tendinous junctions in both load-to-failure and cyclic loading settings. Dynamic cyclic loading during an extensive period of time was successfully achieved. Loads up to 4.84 kN were applied in quasi-static conditions without evidence of clamp slippage or failure.     

Low-frequency H-reflex depression in trained human soleus after spinal cord injury

After spinal cord injury (SCI), widespread reorganization occurs within spinal reflex systems. Regular muscle activity may influence reorganization of spinal circuitry after SCI. The purpose of this study is to investigate the effects of long-term soleus training on H-reflex depression in humans after SCI. Seven subjects with acute (<7 weeks) SCI (AC group) underwent testing of H-reflex depression at several frequencies of repetitive stimulation. Eight subjects (including 3 from AC) stimulated one soleus muscle daily, leaving the other leg as an untrained within-subject control. Trained limb H-reflexes were assessed during year 1 (TR1) and year 2 (TR2) of training. Untrained limbs were tested during year 2 (UN). H-reflex amplitude was lower at 1, 2 and 5 Hz than at 0.1 or 0.2 Hz (p < 0.05). The pattern of depression differed between AC and UN (p < 0.05), but not between TR2 and UN (p > 0.05) despite significant adaptations in torque and fatigue resistance (p < 0.05). Three subjects who began training very early after SCI retained H-reflex post activation depression, suggesting that early intervention of daily muscular activity may be important.     

Inhomogeneous architectural changes of the quadriceps femoris induced by resistance training

Purpose

This study aimed to clarify whether resistance training-induced changes in muscle architecture are homogeneous among the quadriceps and over different regions within each muscle.

Methods

Eleven recreationally active men (27 ± 2 years) completed a 12-week resistance training program for knee extensors. Before and after the intervention, muscle thicknesses, fascicle lengths, and pennation angles of the four muscles (vastus lateralis, vastus medialis, vastus intermedius, rectus femoris) in several regions (2–4 regions per each muscle) were measured using ultrasonography. Anatomical cross-sectional areas (ACSAs) at the same positions as the ultrasound measurements were determined from magnetic resonance images.

Results

Relative increases in the ACSA, muscle thickness, and pennation angle of the rectus femoris were significantly greater than those of the vasti. Relative increases in the ACSAs of the vastus lateralis and rectus femoris were significantly greater in the distal than in the proximal region, and those in the muscle thickness and pennation angle of the vastus intermedius were significantly greater in the medial than in the lateral region. Fascicle lengths did not change in any muscles. The interrelations between muscle thickness and pennation angle remained unchanged after the intervention, with a significant association between the relative changes of the two variables.

Conclusion

The current results indicate that (1) hypertrophy of the quadriceps femoris is associated with a proportional increase in pennation angle but not necessarily in fascicle length, and (2) training-induced changes in muscle size and pennation do not evenly occur among the quadriceps, along or across a muscle.     

Force enhancement during and following muscle stretch of maximal voluntarily activated human quadriceps femoris

Force enhancement during and following muscle stretch has been observed for electrically and voluntarily activated human muscle. However, especially for voluntary contractions, the latter observation has only been made for adductor pollicis and the ankle joint muscles, but not for large muscles like quadriceps femoris. Therefore, the aim of this study was to investigate the effects of active muscle stretch on force production for maximal voluntary contractions of in vivo human quadriceps femoris (n = 15). Peak torques during and torques at the end of stretch, torques following stretch, and passive torques following muscle deactivation were compared to the isometric torques at corresponding muscle length. In addition, muscle activation of rectus femoris, vastus medialis and vastus lateralis was obtained using surface EMG. Stretches with different amplitudes (15, 25 and 35° at a velocity of 60° s⁻¹) were performed on the plateau region and the descending limb of the force–length relation in a random order. Data analysis showed four main results: (1) peak torques did not occur at the end of the stretch, but torques at the end of the stretch exceeded the corresponding isometric torque; (2) there was no significant force enhancement following muscle stretch, but a small significant passive force enhancement persisted for all stretch conditions; (3) forces during and following stretch were independent of stretch amplitude; (4) muscle activation during and following muscle stretch was significantly reduced. In conclusion, although our results showed passive force enhancement, we could not provide direct evidence that there is active force enhancement in voluntarily activated human quadriceps femoris.     

Degenerative and regenerative features of myofibers differ among skeletal muscles in a murine model of muscular dystrophy

Skeletal muscle myofibers constantly undergo degeneration and regeneration. Histopathological features of 6 skeletal muscles (cranial tibial [CT], gastrocnemius, quadriceps femoris, triceps brachii [TB], lumbar longissimus muscles, and costal part of the diaphragm [CPD]) were compared using C57BL/10ScSn-Dmd ^mdx (mdx) mice, a model for muscular dystrophy versus control, C57BL/10 mice. Body weight and skeletal muscle mass were lower in mdx mice than the control at 4 weeks of age; these results were similar at 6–30 weeks. Additionally, muscular lesions were observed in all examined skeletal muscles in mdx mice after 4 weeks, but none were noted in the controls. Immunohistochemical staining revealed numerous paired box 7-positive satellite cells surrounding the embryonic myosin heavy chain-positive regenerating myofibers, while the number of the former and staining intensity of the latter decreased as myofiber regeneration progressed. Persistent muscular lesions were observed in skeletal muscles of mdx mice between 4 and 14 weeks of age, and normal myofibers decreased with age. Number of muscular lesions was lowest in CPD at all ages examined, while the ratio of normal myofibers was lowest in TB at 6 weeks. In CT, TB, and CPD, Iba1-positive macrophages, the main inflammatory cells in skeletal muscle lesions, showed a significant positive correlation with the appearance of regenerating myofibers. Additionally, B220-positive B-cells showed positive and negative correlation with regenerating and regenerated myofibers, respectively. Our data suggest that degenerative and regenerative features of myofibers differ among skeletal muscles and that inflammatory cells are strongly associated with regenerative features of myofibers in mdx mice.     

A new paradigm of neuromuscular electrical stimulation for the quadriceps femoris muscle

Purpose

Neuromuscular electrical stimulation (NMES) with large electrodes and multiple current pathways (m-NMES) has recently been proposed as a valid alternative to conventional NMES (c-NMES) for quadriceps muscle (re)training. The main aim of this study was to compare discomfort, evoked force and fatigue between m-NMES and c-NMES of the quadriceps femoris muscle in healthy subjects.

Methods

Ten healthy subjects completed two experimental sessions (c-NMES and m-NMES), that were randomly presented in a cross-over design. Maximal electrically evoked force at pain threshold, self-reported discomfort at different levels of evoked force, and fatigue-induced force declines during and following a series of 20 NMES contractions were compared between c-NMES and m-NMES.

Results

m-NMES resulted in greater evoked force (P < 0.05) and lower discomfort in comparison to c-NMES (P < 0.05–0.001), but fatigue time course and magnitude did not differ between the two conditions.

Conclusions

The use of quadriceps m-NMES appears legitimate for (re)training purposes because it generated stronger contractions and was less discomfortable than c-NMES (due to multiple current pathways and/or lower current density with larger electrodes).     

Recruitment of fibre types and quadriceps muscle portions during repeated, intense knee-extensor exercise in humans

To investigate recruitment of slow-twitch (ST) and fast-twitch (FT) muscle fibres, as well as the involvement of the various quadriceps femoris muscle portions during repeated, intense, one-legged knee-extensor exercise, 12 healthy male subjects performed two 3-min exercise bouts at ~110% maximum thigh O₂ consumption (EX1 and EX2) separated by 6 min rest. Single-fibre metabolites were determined in successive muscle biopsies obtained from the vastus lateralis muscle (n=6) and intra-muscular temperatures were continuously measured at six quadriceps muscle sites (n=6). Creatine phosphate (CP) had decreased (P<0.05) by 27, 73 and 88% in ST fibres and 25, 71 and 89% in FT fibres after 15 and 180 s of EX1 and after 180 s of EX2, respectively. CP was below resting mean–1 SD in 15, 46, 84 and 100% of the ST fibres and 9, 48, 85 and 100% of the FT fibres at rest, after 15 and 180 s of EX1 and after 180 s of EX2, respectively. A significant muscle temperature increase (T_m) occurred within 2–4 s at all quadriceps muscle sites. T_m varied less than 10% between sites during EX1, but was 23% higher (P<0.05) in the vastus lateralis than in the rectus femoris muscle during EX2. T_m in the vastus lateralis was 101 and 109% of the mean quadriceps value during EX1 and EX2, respectively. We conclude that both fibre types and all quadriceps muscle portions are recruited at the onset of intense knee-extensor exercise, that essentially all quadriceps muscle fibres are activated during repeated intense exercise and that metabolic measurements in the vastus lateralis muscle provide a good indication of the whole-quadriceps muscle metabolism during repeated, intense, one-legged knee-extensor exercise.     

Architectural anatomy of the quadriceps and the relationship with muscle strength: An observational study utilising real-time ultrasound in healthy adults

Quadriceps atrophy and morphological change is a known phenomenon that can impact significantly on strength and functional performance in patients with acute or chronic presentations conditions. Real-time ultrasound (RTUS) imaging is a noninvasive valid and reliable method of quantifying quadriceps muscle anatomy and architecture. To date, there is a paucity of normative data on the architectural properties of superficial and deep components of the quadriceps muscle group to inform assessment and evaluation of intervention programs. The aims of this study were to (1) quantify the anatomical architectural properties of the quadriceps muscle group (rectus femoris, vastus intermedius, and vastus lateralis) using RTUS in healthy older adults and (2) to determine the relationship between RTUS muscle parameters and measures of quadriceps muscle strength. Thirty middle aged to older males and females (age range 55–79 years; mean age =59.9 ± 7.08 years) were recruited. Quadriceps muscle thickness, cross-sectional area, pennation angle, and echogenicity were measured using RTUS. Quadriceps strength was measured using hand-held dynamometry. For the RTUS-derived quadriceps morphological data, rectus femoris mean results; circumference 9.3 cm; CSA 4.6 cm²; thickness 1.5 cm; echogenicity 100.2 pixels. Vastus intermedius mean results; thickness 1.8 cm; echogenicity 99.1 pixels. Vastus lateralis thickness 1.9 cm; pennation angle 17.3 ° ; fascicle length 7.0 cm. Quadriceps force was significantly correlated only with rectus femoris circumference (r = 0.48, p = 0.007), RF echogenicity (r = 0.38, p = 0.037), VI echogenicity (r = 0.43, p = 0.018), and VL fascicle length (r = 0.43, p = 0.019). Quadriceps force was best predicted by a three-variable model (adjusted R² = 0.46, p < 0.001) which included rectus femoris echogenicity (B = 0.43, p = 0.005), vastus lateralis fascicle length (B = 0.33, p = 0.025) and rectus femoris circumference (B = 0.31, p = 0.041). Thus respectively, rectus femoris echogenicity explains 43%, vastus lateralis fascicle length explains 33% and rectus femoris circumference explains 31% of the variance of quadriceps force. The study findings suggest that RTUS measures were reliable and further research is warranted to establish whether these could be used as surrogate measures for quadriceps strength in adults to inform exercise and rehabilitation programs.     

Torque and mechanomyogram relationships during electrically-evoked isometric quadriceps contractions in persons with spinal cord injury

The interaction between muscle contractions and joint loading produces torques necessary for movements during activities of daily living. However, during neuromuscular electrical stimulation (NMES)-evoked contractions in persons with spinal cord injury (SCI), a simple and reliable proxy of torque at the muscle level has been minimally investigated. Thus, the purpose of this study was to investigate the relationships between muscle mechanomyographic (MMG) characteristics and NMES-evoked isometric quadriceps torques in persons with motor complete SCI. Six SCI participants with lesion levels below C4 [(mean (SD) age, 39.2 (7.9) year; stature, 1.71 (0.05) m; and body mass, 69.3 (12.9) kg)] performed randomly ordered NMES-evoked isometric leg muscle contractions at 30°, 60° and 90° knee flexion angles on an isokinetic dynamometer. MMG signals were detected by an accelerometer-based vibromyographic sensor placed over the belly of rectus femoris muscle. The relationship between MMG root mean square (MMG-RMS) and NMES-evoked torque revealed a very high association (R² = 0.91 at 30°; R² = 0.98 at 60°; and R² = 0.97 at 90° knee angles; P < 0.001). MMG peak-to-peak (MMG-PTP) and stimulation intensity were less well related (R² = 0.63 at 30°; R² = 0.67 at 60°; and R² = 0.45 at 90° knee angles), although were still significantly associated (P ≤ 0.006). Test-retest interclass correlation coefficients (ICC) for the dependent variables ranged from 0.82 to 0.97 for NMES-evoked torque, between 0.65 and 0.79 for MMG-RMS, and from 0.67 to 0.73 for MMG-PTP. Their standard error of measurements (SEM) ranged between 10.1% and 31.6% (of mean values) for torque, MMG-RMS and MMG-PTP. The MMG peak frequency (MMG-PF) of 30 Hz approximated the stimulation frequency, indicating NMES-evoked motor unit firing rate. The results demonstrated knee angle differences in the MMG-RMS versus NMES-isometric torque relationship, but a similar torque related pattern for MMG-PF. These findings suggested that MMG was well associated with torque production, reliably tracking the motor unit recruitment pattern during NMES-evoked muscle contractions. The strong positive relationship between MMG signal and NMES-evoked torque production suggested that the MMG might be deployed as a direct proxy for muscle torque or fatigue measurement during leg exercise and functional movements in the SCI population.     

Near-infrared assessments of skeletal muscle oxidative capacity in persons with spinal cord injury

After spinal cord injury (SCI) skeletal muscle decreases in size, increases in intramuscular fat, and has potential declines in mitochondrial function. Reduced mitochondrial function has been linked to the development of metabolic disease. The aim of this study was to measure mitochondrial function in persons with SCI using near-infrared spectroscopy (NIRS). Oxygen consumption of the vastus lateralis muscle was measured with NIRS during repeated short-duration arterial occlusions in nine able-bodied (AB) and nine persons with motor complete SCI. Skeletal muscle oxidative capacity (V _max) was evaluated with two approaches: (1) rate constant of the recovery of oxygen consumption after exercise and (2) extrapolated maximum oxygen consumption from a progressive work test. V _max as indicated by the rate constant (k) from the recovery kinetics test was lower in SCI compared with AB participants (k: SCI 0.7 ± 0.3 vs. AB 1.9 ± 0.4 min^?1; p < 0.001). Time constants were SCI 91.9 ± 37.8 vs. AB 33.6 ± 8.3 s. V _max from the progressive work test approached a significant difference between groups (SCI 5.1 ± 2.9 vs. AB 9.8 ± 5.5 % Hb-Mb/s; p = 0.06). NIRS measurements of V _max suggest a deficit of 50–60 % in participants with SCI compared with AB controls, consistent with previous studies using ³¹P-MRS and muscle biopsies. NIRS measurements can assess mitochondrial capacity in people with SCI and potentially other injured/diseased populations.     

Acid hydrolase activities in mouse cardiac and skeletal muscle following exhaustive exercise

Summary Acid hydrolase activities in skeletal and cardiac muscle were studied 5,10, and 20 days after exhaustive intermittent running by untrained and endurance-trained mice. Exhaustion increased the activities of cathepsin D, -glucuronidase and ribonuclease, but not that of p-nitrophenylphos-phatase in skeletal muscle of untrained mice. Activities were highest on the fifth day after exhaustion and decreased during the following two weeks. More intensive loading produced no changes in acid hydrolytic capacity in skeletal muscle of endurance-trained mice. Acid hydrolase activities in cardiac muscle of both untrained and trained mice were unaffected by exhaustive running. It is suggested that exhaustive running causes both lethal and sublethal hypoxic fiber injuries in the skeletal muscle of untrained mice but not in that of endurance-trained mice or in the cardiac muscle of animals of either group. These injuries manifest themselves as fiber necrosis (lethal) and as increased acid hydrolytic capacity in surviving fibers (sublethal).This study was financially supported by the Academy of Finland and the Finnish Research Council for Physical Education and Sport (Ministry of Education)