Paradigms of Lower Extremity Electrical Stimulation Training After Spinal Cord Injury.

Skeletal muscle atrophy, increased adiposity and reduced physical activity are key changes observed after spinal cord injury (SCI) and are associated with numerous cardiometabolic health consequences. These changes are likely to increase the risk of developing chronic secondary conditions and impact the quality of life in persons with SCI. Surface neuromuscular electrical stimulation evoked resistance training (NMES-RT) was developed as a strategy to attenuate the process of skeletal muscle atrophy, decrease ectopic adiposity, improve insulin sensitivity and enhance mitochondrial capacity. However, NMES-RT is limited to only a single muscle group. Involving multiple muscle groups of the lower extremities may maximize the health benefits of training. Functional electrical stimulation-lower extremity cycling (FES-LEC) allows for the activation of 6 muscle groups, which is likely to evoke greater metabolic and cardiovascular adaptation. Appropriate knowledge of the stimulation parameters is key to maximizing the outcomes of electrical stimulation training in persons with SCI. Adopting strategies for long-term use of NMES-RT and FES-LEC during rehabilitation may maintain the integrity of the musculoskeletal system, a pre-requisite for clinical trials aiming to restore walking after injury. The current manuscript presents a combined protocol using NMES-RT prior to FES-LEC. We hypothesize that muscles conditioned for 12 weeks prior to cycling will be capable of generating greater power, cycle against higher resistance and result in greater adaptation in persons with SCI.

Effects of electrical stimulation parameters on fatigue in skeletal muscle.

STUDY DESIGN: Experimental laboratory study. OBJECTIVES: The primary purpose was to investigate the independent effects of current amplitude, pulse duration, and current frequency on muscle fatigue during neuromuscular electrical stimulation (NMES). A second purpose was to determine if the ratio of the evoked torque to the activated area could explain muscle fatigue. BACKGROUND: Parameters of NMES have been shown to differently affect the evoked torque and the activated area. The efficacy of NMES is limited by the rapid onset of muscle fatigue. METHODS AND MEASURES: Seven healthy participants underwent 4 NMES protocols that were randomly applied to the knee extensor muscle group. The NMES protocols were as follows: standard protocol (Std), defined as 100-Hz, 450-micros pulses and amplitude set to evoke 75% of maximal voluntary isometric torque (MVIT); short pulse duration protocol (SP), defined as 100-Hz, 150-micros pulses and amplitude set to evoke 75% of MVIT; low-frequency protocol (LF), defined as 25-Hz, 450-micros pulses and amplitude set to evoke 75% of MVIT; and low-amplitude protocol (LA), defined as 100-Hz, 450-micros pulses and amplitude set to evoke 45% of MVIT. The peak torque was measured at the start and at the end of the 4 protocols, and percent fatigue was calculated. The outcomes of the 4 NMES protocols on the initial peak torque and activated cross-sectional area were recalculated from a companion study to measure torque per active area. RESULTS: Decreasing frequency from 100 to 25 Hz decreased fatigue from 76% to 39%. Decreasing the amplitude and pulse duration resulted in no change of muscle fatigue. Torque per active area accounted for 57% of the variability in percent fatigue between Std and LF protocols. CONCLUSIONS: Altering the amplitude of the current and pulse duration does not appear to influence the percent fatigue in NMES. Lowering the stimulation frequency results in less fatigue, by possibly reducing the evoked torque relative to the activated muscle area.

The role of pulse duration and stimulation duration in maximizing the normalized torque during neuromuscular electrical stimulation.

STUDY DESIGN: Controlled laboratory study OBJECTIVES: To determine the effects of pulse duration and stimulation duration on the evoked torque after controlling for the activated area by using magnetic resonance imaging (MRI). BACKGROUND: Neuromuscular electrical stimulation (NMES) is commonly used in the clinic without considering the physiological implications of its parameters. METHODS AND MEASURES: Seven able-bodied, college students (mean +/- SD age, 28 +/- 4 years) participated in this study. Two NMES protocols were applied to the knee extensor muscle group in a random order. Protocol A applied 100-Hz, 450-microsecond pulses for 5 minutes in a 3-seconds-on 3-seconds-off duty cycle. Protocol B applied 60-Hz, 250-microsecond pulses for 5 minutes in a 10-seconds-on 20-seconds-off duty cycle. The amplitude of the current was similar in both protocols. Torque, torque time integral, and normalized torque for the knee extensors were measured for both protocols. MRI scans were taken prior to, and immediately after, each protocol to measure the cross-sectional area of the stimulated muscle. RESULTS: The skeletal muscle cross-sectional areas activated after both protocols were similar. The longer pulse duration in protocol A elicited 22% greater torque output than that of protocol B (P<.05). After considering the activated area in both protocols, the normalized torque with protocol A was 38% greater than that with protocol B (P<.05). Torque time integral was 21% greater with protocol A (P = .029). Protocol B failed to maintain torque at the start and the end of the 10-second activation. CONCLUSIONS: Longer pulse duration, but not stimulation duration, resulted in a greater evoked and normalized torque compared to the shorter pulse duration, even after controlling for the activated muscular cross-sectional areas with both protocols. LEVEL OF EVIDENCE: Therapy, level 5.

High specific torque is related to lengthening contraction-induced skeletal muscle injury.

Animal models implicate multiple mechanical factors in the initiation of exercise-induced muscle injury. Muscle injury has been widely studied in humans, but few data exist regarding the underlying cause of muscle injury. This study sought to examine the role of torque per active muscle volume in muscle injury. Eight subjects performed 80 electrically stimulated [via electromyostimulation (EMS)] eccentric contractions of the right and left quadriceps femoris (QF) through an 80 degrees arc at 120 degrees /s. Specific torque was varied by applying 25-Hz EMS to one thigh and 100-Hz EMS to the contralateral thigh. Transverse relaxation time (T2) magnetic resonance images of the QF were collected before and 3 days after the eccentric exercise bouts. Injury was assessed via changes in isometric force and ratings of soreness over the course of 14 days after exercise and by determining changes in T2 and muscle volume 3 days after exercise. The 100-Hz EMS induced greater force loss (P < 0. 05), soreness (P < 0.05), change in muscle volume (P = 0.03), and volume of muscle demonstrating increased T2 (P = 0.005) than the 25-Hz EMS. In addition, injury was found to be similar across the QF in all but the most proximal regions of the QF. Our findings suggest that, in humans, high torque per active volume during lengthening muscle contractions is related to muscle injury.

Low-frequency fatigue in individuals with spinal cord injury.

BACKGROUND/OBJECTIVE: This study examined magnitude and recovery of low-frequency fatigue (LFF) in the quadriceps after electrically stimulated contractions in spinal cord-injured (SCI) and able-bodied subjects. SUBJECTS: Nine SCI (ASIA A-C, levels C5-T9, injured 13.6 +/- 12.2 years) and 9 sedentary able-bodied subjects completed this study. METHODS: Fatigue was evoked in 1 thigh, and the nonfatigued leg served as a control. The fatigue test for able-bodied subjects lasted 15 minutes. For SCI, stimulation was adjusted so that the relative drop in force was matched to the able-bodied group. Force was assessed at 20 (P20) and 100 Hz (P100), and the ratio of P20/P100 was used to evaluate LFF in thighs immediately after, at 10, 20, and 60 minutes, and at 2, 4, 6, and 24 hours after a fatigue test. RESULTS: The magnitude of LFF (up to 1 hour after fatigue) was not different between able-bodied and patients with SCI. However, recovery of LFF over 24 hours was greater in able-bodied compared with patients with SCI in both the experimental (P < 0.001) and control legs (P < 0.001). The able-bodied group showed a gradual recovery of LFF over time in the experimental leg, whereas the SCI group did not. CONCLUSIONS: These results show that individuals with SCI are more susceptible to LFF than able-bodied subjects. In SCI, simply assessing LFF produced considerable LFF and accounted for a substantial portion of the response. We propose that muscle injury is causing the dramatic LFF in SCI, and future studies are needed to test whether "fatigue" in SCI is actually confounded by the effects of muscle injury.

Caffeine attenuates delayed-onset muscle pain and force loss following eccentric exercise.

UNLABELLED: This double-blind, placebo-controlled, repeated-measures experiment examined the effects of a 5 mg . kg(-1) body weight dose of caffeine on delayed-onset muscle pain intensity and force loss in response to 64 eccentric actions of the dominant quadriceps induced by electrical stimulation. Low caffeine-consuming college-aged females (n = 9) ingested caffeine or placebo 24 and 48 hours following electrically stimulated eccentric exercise of the quadriceps. One hour after ingestion, maximal voluntary isometric contractions (MVIC) and submaximal voluntary eccentric actions were used to determine force loss during activation of damaged quadriceps and whether caffeine attenuates muscle pain intensity. Pain intensity was measured using a 0 to 100 visual analog scale. Caffeine produced a large (12.7 raw visual analog scale [VAS] units; -48%; Cohen's d effect size = -0.88), statistically significant hypoalgesia during the MVIC (t = -2.52; df = 8; P = .036). The reduction in pain scores during submaximal voluntary eccentric movements was smaller (7.8 raw VAS units; -26%, d = -0.34), as was the increase in MVIC force (4.4%; d = 0.13). PERSPECTIVE: Eccentric exercise occurs when skeletal muscles produce force while being lengthened. For example, the biceps brachii muscles act eccentrically when a cup of coffee is lowered from the mouth to a tabletop. This experiment found that caffeine (equal to approximately 2 cups of brewed coffee) could produce a large reduction in pain resulting from eccentric exercise-induced, delayed-onset muscle injury. This finding may improve the quality of life of individuals who experience skeletal muscle pain after engaging in unaccustomed, eccentrically biased exercise.

Oxygen cost of dynamic or isometric exercise relative to recruited muscle mass.

BACKGROUND: Oxygen cost of different muscle actions may be influenced by different recruitment and rate coding strategies. The purpose of this study was to account for these strategies by comparing the oxygen cost of dynamic and isometric muscle actions relative to the muscle mass recruited via surface electrical stimulation of the knee extensors. METHODS: Comparisons of whole body pulmonary delta VO2 were made in seven young healthy adults (1 female) during 3 minutes of dynamic or isometric knee extensions, both induced by surface electrical stimulation. Recruited mass was quantified in T2 weighted spin echo magnetic resonance images. RESULTS: The delta VO2 for dynamic muscle actions, 242 +/- 128 ml x min(-1) (mean +/- SD) was greater (p = 0.003) than that for isometric actions, 143 +/- 99 ml x min(-1). Recruited muscle mass was also greater (p = 0.004) for dynamic exercise, 0.716 +/- 282 versus 0.483 +/- 0.139 kg. The rate of oxygen consumption per unit of recruited muscle (VO2(RM)) was similar in dynamic and isometric exercise (346 +/- 162 versus 307 +/- 198 ml x kg(-1) x min(-1); p = 0.352), but the VO2(RM) calculated relative to initial knee extensor torque was significantly greater during dynamic exercise 5.1 +/- 1.5 versus 3.6 +/- 1.6 ml x kg(-1) x Nm(-1) x min(-1) (p = 0.019). CONCLUSION: These results are consistent with the view that oxygen cost of dynamic and isometric actions is determined by different circumstances of mechanical interaction between actin and myosin in the sarcomere, and that muscle recruitment has only a minor role.

Determining the extent of neural activation during maximal effort.

PURPOSE: The purpose of this study was to compare the extent of neural activation assessed by the central activation ratio (CAR) versus activation estimated from T2 magnetic resonance imaging (MRI) and neuromuscular electrical stimulation (NMES). METHODS: Seven college-age individuals volunteered for this study. CAR was determined by manually superimposing a train of NMES (50 Hz, 450-mus biphasic pulses) for 1 s during a maximal voluntary effort. The MRI-NMES method assessed activation by stimulating the knee extensors for 3 min in a 2 s on, 2 s off cycle. T2 MR images were taken at rest and after NMES was administered. Theoretical maximal torque (TMT) of the knee extensors was calculated based on the MRI-NMES activation data. The TMT was then divided by the maximal voluntary isometric contraction (MVIC) of each subject to determine the extent of neural activation during a MVIC. RESULTS: The results for CAR reveal the percent activation (mean +/- SD) of the quadriceps femoris during a MVIC was 92 +/- 7% for the right thigh and 96 +/- 4% for the left thigh. The MRI-NMES method estimated that MVIC could be achieved if 75 +/- 14% of the knee extensors on the right thigh and 74 +/- 14% on the left thigh were activated. These results are similar to findings that showed MVIC could be achieved by stimulating 71% of the knee extensors. CONCLUSIONS: We conclude that CAR overestimates the extent of neural activation during an MVIC because the 3D shape of the thigh is altered. This will change electric current flow to the axonal motor neuron branches and limit the artificially evoked torque, thereby resulting in an overestimation of CAR.

Effects of neuromuscular electrical stimulation parameters on specific tension.

This study examined the effects of altering surface neuromuscular electrical stimulation (SNMES) parameters on the specific tension of the quadriceps femoris muscle. Seven able-bodied subjects had magnetic resonance images taken of both thighs prior to and immediately after four SNMES protocols to determine the activated muscle cross-sectional area (CSA). The four protocols were: (1) research (RES, 100 Hz, 450 micros, and amplitude set to evoke 75% of maximal voluntary isometric torque, MVIT); (2) pulse duration (PD, 100 Hz, 150 micros, same current as in RES); (3) frequency (FREQ, 25 Hz, 450 micros, and same current as in RES); (4) amplitude (AMP, 100 Hz, 450 mus, and current set to evoke the average of the initial torques of PD and FREQ, 45 +/- 9% of MVIT). Reducing the amplitude of the current from 75 to 45% of MVIT did not alter specific tension, 25 +/- 8 N/cm2, suggesting that the amplitude probably affects torque and the area of activated muscle proportionally. Shortening the pulse duration from 450 to 150 micros caused specific tension to drop from 25 +/- 6 to 20 +/- 6 N/cm2 (P < 0.05), indicating that pulse duration increased torque and the activated CSA disproportionally. Alternatively, reducing the frequency from 100 to 25 Hz decreased specific tension from 25 +/- 6 to 17 +/- 4 N/cm2 (P < 0.05), suggesting that the frequency increased torque without affecting the activated CSA. Clinicians who administer SNMES should be aware of the magnitude of adaptations to a given amplitude, pulse duration, and frequency.

Exercise responses and adaptations in rowers and spinal cord injury individuals.

Elite rowers (ROWERS) and those who have had a spinal cord injury (SCI) are different physically in many realms. Both have physical activity histories that affect their lower-extremity extensor muscles in a dramatically different fashion. ROWERS can sustain a 500-W power output during their 5- to 6-min race. After a complete SCI, a 75-W power output might be achieved during a VO(2peak) test. Elite SCI wheelchair racers can achieve a higher value that is similar to that of a sedentary able-bodied person. ROWERS can attain a VO(2 max) of more than 7.5 L.min(-1) and can tolerate a blood lactate of 30 mmol.L(-1). After a complete SCI in which muscles become markedly atrophied, a peak VO(2) of 2 L.min(-1) and a blood lactate of 10 mmol.L(-1) might be achieved. ROWERS rely on the 75% slow-twitch fiber composition of their trained thigh muscles to train and race. Such activity modestly increases fiber size and markedly increases mitochondrial content. After a complete SCI, affected muscle fibers markedly atrophy, maintain most of their mitochondrial content, and become fast-twitch. These data suggest remarkable plasticity of physical function to the extreme that a marked increase in energy demanding, rather continuous physical activity can make a muscle more "slow-twitch"; so it will demand less energy when contracted. In contrast, SCI eventually causes muscle to be composed of more fast-twitch fibers. Molecular biologists may explain why fast-twitch fibers, which appear ideal for some athletes because of their high power output, are abundant in muscles that are seldom recruited. Until then, our results indicate that the fiber type composition of muscle in humans is stable unless extreme alterations in physical activity are endured.

Lower-extremity muscle cross-sectional area after incomplete spinal cord injury.

OBJECTIVES: (1) To quantify skeletal muscle size in lower-extremity muscles of people after incomplete spinal cord injury (SCI), (2) to assess differences in muscle size between involved lower limbs, (3) to determine the impact of ambulatory status (using wheelchair for community mobility vs not using a wheelchair for community mobility) on muscle size after incomplete SCI, and (4) to determine if differential atrophy occurs among individual muscles after incomplete SCI. DESIGN: Case-control study. SETTING: University research setting. PARTICIPANTS: Seventeen people with incomplete SCI and 17 age-, sex-, weight-, and height-matched noninjured controls. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Maximum cross-sectional area (CSA) of individual lower-extremity muscles (soleus, medial gastrocnemius, lateral gastrocnemius, tibialis anterior, quadriceps femoris, hamstrings) as assessed by magnetic resonance imaging. RESULTS: Overall, subjects with incomplete SCI had significantly smaller (24%-31%) average muscle CSA in affected lower-extremity muscles as compared with control subjects (P<.05). Mean differences were highest in the thigh muscles ( approximately 31%) compared with the lower-leg muscles ( approximately 25%). No differences were noted between the self-reported more- and less-involved limbs within the incomplete SCI group. Dichotomizing the incomplete SCI group showed significantly lower muscle CSA values in both the wheelchair (range, 21%-39%) and nonwheelchair groups (range, 24%-38%). In addition, the wheelchair group exhibited significantly greater plantarflexor muscle atrophy compared with the dorsiflexors, with maximum atrophy in the medial gastrocnemius muscle (39%). CONCLUSIONS: Our results suggest marked and differential atrophic response of the affected lower-extremity muscles that is seemingly affected by ambulatory status in people with incomplete SCI.

Metabolic and phenotypic characteristics of human skeletal muscle fibers as predictors of glycogen utilization during electrical stimulation.

Characteristics of skeletal muscle such as fiber type composition and activities of key metabolic enzymes have been purported to affect glycogen utilization. However, the relative importance individual factors may have in predicting glycogen utilization of individual muscle fibers has not been addressed. Thus, we sought to determine the relative importance that metabolic characteristics and phenotypic expression of individual fibers have in predicting fiber specific glycogen utilization during neuromuscular electrical stimulation (NMES) exercise. Biopsies were taken from the m, vastus lateralis (VL) of eight recreationally active males before and immediately after 30 min of non-fatiguing NMES and analyzed for type (I, IIa and IIx), succinate dehydrogenase activity (SDH), glycerol-phosphate dehydrogenase activity (GPDH), quantitative-actomyosin adenosine triphosphatase activity (qATPase), and glycogen content. Our results demonstrate that a ratio of enzyme activities representing pathways for energy supply and energy demand (SDH: qATPase) accounted for more of the variance in glycogen utilization (y=0.2091 e(-0.0329x ), R2=0.622, P< or = 0.0001) than SDH (R2=0.321) or qATPase (R2=0.365) alone. Fiber phenotype was also a significant predictor of glycogen utilization, but to a lesser extent than the other variables studied (R2=0.201). A ratio of the activities of enzymes representing pathways of energy supply and energy demand, represented by SDH:qATPase, is a better predictor of glycogen utilization than either of its components independently while fiber phenotype, although a statistically significant predictor of glycogen utilization, may not be the most appropriate determinate of the functional characteristics of an individual fiber.

Changes in skeletal muscle size and glucose tolerance with electrically stimulated resistance training in subjects with chronic spinal cord injury.

OBJECTIVE: To determine the effect of residence-based, resistance exercise training (RET) on affected skeletal muscle size and glucose tolerance after long-standing, complete spinal cord injury (SCI). DESIGN: Before-after trial. SETTING: University laboratory trial. PARTICIPANTS: Five men with chronic, complete SCI (C5-T9). INTERVENTION: Magnetic resonance images of the thighs and an oral glucose tolerance test were performed before and after RET. Subjects performed RET with both thighs, 2 d/wk for 4 sets of 10 unilateral, dynamic knee extensions for 12 weeks. Neuromuscular electric stimulation induced RET by activating the knee extensors. MAIN OUTCOME MEASURES: Quadriceps femoris muscle cross-sectional area (CSA), plasma glucose, and insulin concentrations were measured before and after RET. Results Skeletal muscle CSA increased by 35% in the right quadriceps femoris (from 32.6 cm2 to 44.0 cm2) and by 39% in the left quadriceps femoris (from 34.6 cm2 to 47.9 cm2) as a result of training (P < .05). There were no significant changes in blood glucose or insulin after training. However, a trend for a reduction in plasma glucose levels was observed (P = .074). Conclusions Affected skeletal muscle can achieve substantial hypertrophy years after SCI with resistance exercise. Furthermore, our results suggest that this type of training may enhance glucose disposal.

Trabecular bone is more deteriorated in spinal cord injured versus estrogen-free postmenopausal women.

The prevalence of osteoporosis is high among postmenopausal women and individuals sustaining a spinal cord injury (SCI). We assessed the effects of estrogen loss and unloading on the trabecular bone of the knee in women. Pre- and postmenopausal ambulatory women (n=17) were compared to pre- and postmenopausal women with SCI (n=20). High-resolution magnetic resonance imaging was used to compare groups on apparent measures of trabecular bone volume, trabecular number, trabecular spacing, and trabecular thickness in the distal femur and proximal tibia, regions with a high proportion of trabecular bone and the most common fracture site for SCI patients. Trabecular bone was deteriorated in women with SCI compared to ambulatory women. SCI groups had fewer, (-19 and -26% less) and thinner trabeculae (-6%) that were spaced further apart (40% and 62% more space between structures) resulting in less trabecular bone volume (-22% and -33%) compared to the ambulatory groups (tibia and femur, respectively). Postmenopausal women with SCI also had 34% greater trabecular spacing in the tibia compared to the 40-year-old premenopausal women with SCI, showing an interaction between unloading and estrogen loss. Middle-aged postmenopausal, ambulatory women, not taking estrogen or medications that affect bone, did not show the deteriorated trabeculae that were evident in women with SCI, nor did they show differences in distal femur and proximal tibia trabeculae compared to a premenopausal group. We conclude that the effect of unloading on bone architecture is greater than that of estrogen loss in middle-aged women.

Time course of molecular responses of human skeletal muscle to acute bouts of resistance exercise.

Resistance exercise (RE) training, designed to induce hypertrophy, strives for optimal activation of anabolic and myogenic mechanisms to increase myofiber size. Clearly, activation of these mechanisms must precede skeletal muscle growth. Most mechanistic studies of RE have involved analysis of outcome variables after many training sessions. This study measured molecular level responses to RE on a scale of hours to establish a time course for the activation of myogenic mechanisms. Muscle biopsy samples were collected from nine subjects before and after acute bouts of RE. The response to a single bout was assessed at 12 and 24 h postexercise. Further samples were obtained 24 and 72 h after a second exercise bout. RE was induced by neuromuscular electrical stimulation to generate maximal isometric contractions in the muscle of interest. A single RE bout resulted in increased levels of mRNA for IGF binding protein-4 (84%), MyoD (83%), myogenin (approximately 3-fold), cyclin D1 (50%), and p21-Waf1 (16-fold), and a transient decrease in IGF-I mRNA (46%). A temporally conserved, significant correlation between myogenin and p21 mRNA was observed (r = 0.70, P < or = 0.02). The mRNAs for mechano-growth factor, IGF binding protein-5, and the IGF-I receptor were unchanged by RE. Total skeletal muscle RNA was increased 72 h after the second serial bout of RE. These results indicate that molecular adaptations of skeletal muscle to loading respond in a very short time. This approach should provide insights on the mechanisms that modulate adaptation to RE and may be useful in evaluating RE training protocol variables with high temporal resolution.

Variable-frequency-train stimulation of skeletal muscle after spinal cord injury.

Skeletal muscle, after spinal cord injury (SCI), becomes highly susceptible to fatigue. Variable-frequency trains (VFTs) enhance force in fatigued human skeletal muscle of able-bodied (AB) individuals. VFTs do this by taking advantage of the "catch-like" property of skeletal muscle. However, mechanisms responsible for fatigue in AB and SCI subjects may not be the same, and the efficacy of VFT stimulation after SCI is unknown. Accordingly, we tested the hypothesis that VFT stimulation would augment torque-time integral in SCI subjects. The quadriceps femoris muscle was stimulated with constant frequency trains (CFTs) (six 200 s square wave pulses separated by 70 ms) or VFTs (a train identical to the CFT, except that the first two pulses were separated by 5 ms) in SCI and AB subjects. After 180 contractions (50% duty cycle), isometric peak torque decreased 44, 56, and 67 percent, in the AB (n = 10), acute SCI (n = 10), and chronic SCI (n = 12) groups, respectively. In fatigued muscle, VFTs enhanced the torque-time integral by 18 percent in AB subjects and 6 percent in chronic SCI patients, and had no effect in acute SCI patients when compared to the corresponding CFT. The much faster rise times in SCI subjects (approximately 80 ms vs. 120 ms in AB subjects) probably contributed to the inability of VFTs to enhance torque-time integrals in SCI patients. The results suggest that the use of VFT stimulation in patients with SCI may not be as efficacious as it is in AB persons.

Trabecular bone microarchitecture is deteriorated in men with spinal cord injury.

UNLABELLED: Using magnetic resonance imaging, men with spinal cord injury (n = 10) were found to have fewer trabeculae that were spaced further apart in the knee than able-bodied controls of similar age, height, and weight (n = 8). The deteriorated trabecular bone microarchitecture may contribute to the increased fracture incidence after injury. INTRODUCTION: Spinal cord injury results in a dramatic decline in areal bone mineral density (aBMD) and a marked increase in lower extremity fracture; however, its effect on trabecular bone microarchitecture is unknown. The purpose of this study was to determine if trabecular bone microarchitecture is deteriorated in the knee of men with long-term, complete spinal cord injury. MATERIALS AND METHODS: Apparent bone volume to total volume (appBV/TV), trabecular number, (appTb.N), trabecular thickness (appTb.Th), and trabecular separation (appTb.Sp), measures of trabecular bone microarchitecture, were assessed in the distal femur and proximal tibia of men with long-term (>2 years) complete spinal cord injury (SCI; n = 10) and able-bodied controls (CON; n = 8) using high-resolution magnetic resonance imaging. Proximal tibia and arm aBMD were determined using DXA. Independent t-tests were used to assess group differences in anthropometrics and bone parameters. Pearson correlation analysis was used to assess the relationships among trabecular bone microarchitecture, aBMD, and time since injury. RESULTS: There were no group differences in age, height, or weight; however, the distal femur and proximal tibia of SCI had 27% and 20% lower appBV/TV, 21% and 20% lower appTb.N, and 44% and 33% higher appTb.Sp, respectively (p < 0.05). The distal femur of SCI also had 8% lower appTb.Th (p < 0.05). Whereas arm aBMD was not different in the two groups, proximal tibia aBMD was 43% lower in SCI. In SCI and CON combined, aBMD was correlated with appBV/TV (r = 0.62), appTb.N (r = 0.78), and appTb.Sp (r = -0.82) in the proximal tibia (p < 0.05). Time since injury was more strongly correlated with appTb.N (r = -0.54) and appTb.Sp (r = 0.56) than aBMD (r = -0.36) in the distal tibia, although none of the relationships were statistically significant (p > 0.05). CONCLUSION: Men with complete spinal cord injury have markedly deteriorated trabecular bone microarchitecture in the knee, which may contribute to their increased fracture incidence.

Assessment of skeletal muscle mass in men with spinal cord injury using dual-energy X-ray absorptiometry and magnetic resonance imaging.

The purpose of this study was to determine whether the proportion of skeletal muscle in the fat-free soft tissue mass (FFST) is the same in men with spinal cord injury (SCI) and able-bodied controls. Skeletal muscle mass and FFST of the midthigh were determined by using magnetic resonance imaging and dual-energy X-ray absorptiometry, respectively, in men with long-term (>2 yr) complete SCI (n = 8) and able-bodied controls of similar age, height, and weight (n = 8). Muscle mass (1.36 +/- 0.77 vs. 2.44 +/- 0.47 kg) and FFST (1.70 +/- 0.94 vs. 2.73 +/- 0.80 kg) were lower in the SCI group than in the controls (P < 0.05), but the lower ratio of muscle to FFST in the SCI group (0.80 +/- 0.09 vs. 0.91 +/- 0.10, P < 0.05) suggested that they had a lower proportion of muscle in the FFST than in controls. This notion was supported by analysis of covariance, in that the mean muscle adjusted to the mean FFST of the groups combined was lower in the SCI group. Despite the lower proportion of muscle in the FFST of the SCI group, the relation between muscle and FFST was strong in the SCI group (r = 0.99) and controls (r = 0.96). The findings suggest a disproportionate loss of muscle in the paralyzed thighs after SCI relative to other nonfat constituents, which may be accurately estimated in men with long-term SCI by dual-energy X-ray absorptiometry if the lower proportion of muscle in the FFST (approximately 15%) is taken into account.

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) cm(2) versus 44 (6) cm(2), 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.

Increasing blood flow before exercise in spinal cord-injured individuals does not alter muscle fatigue.

Previous studies have shown increased fatigue in paralyzed muscle of spinal cord-injured (SCI) patients (Castro M, Apple D Jr, Hillegass E, and Dudley GA. Eur J Appl Physiol 80: 373-378, 1999; Gerrits H, Hopman MTE, Sargeant A, and de Haan A. Clin Physiol 21: 105-113, 2001). Our purpose was to determine whether the increased muscle fatigue could be due to a delayed rise in blood flow at the onset of exercise in SCI individuals. Isometric electrical stimulation was used to induce fatigue in the quadriceps femoris muscle of seven male, chronic (>1 yr postinjury), complete (American Spinal Injury Association, category A) SCI subjects. Cuff occlusion was used to elevate blood flow before electrical stimulation, and the magnitude of fatigue was compared with a control condition of electrical stimulation without prior cuff occlusion. Blood flow was measured in the femoral artery by Doppler ultrasound. Prior cuff occlusion increased blood flow in the first 30 s of stimulation compared with the No-Cuff condition (1,350 vs. 680 ml/min, respectively; P < 0.001), although blood flow at the end of stimulation was the same between conditions (1,260 +/- 140 vs. 1,160 +/- 370 ml/min, Cuff and No-Cuff condition, respectively; P = 0.511). Muscle fatigue was not significantly different between prior cuff occlusion and the control condition (32 +/- 13 vs. 35 +/- 10%; P = 0.670). In conclusion, increased muscle fatigue in SCI individuals is not associated with the prolonged time for blood flow to increase at the onset of exercise.