Plantar Flexor Muscle Group Research Articles

Artificial gravity as a countermeasure to microgravity: a pilot study examining the effects on knee extensor and plantar flexor muscle groups

The goal of this project was to examine the effects of artificial gravity (AG) on skeletal muscle strength and key anabolic/catabolic markers known to regulate muscle mass. Two groups of subjects were selected for study: 1) a 21 day-bed rest (BR) group (n = 7) and 2) an AG group (n = 8), which was subjected to 21 days of 6 degrees head-down tilt bed rest plus daily 1-h exposures to AG (2.5 G at the feet). Centrifugation was produced using a short-arm centrifuge with the foot plate approximately 220 cm from the center of rotation. The torque-velocity relationships of the knee extensors and plantar flexors of the ankle were determined pre- and posttreatment. Muscle biopsy samples obtained from the vastus lateralis and soleus muscles were used for a series of gene expression analyses (mRNA abundance) of key factors implicated in the anabolic vs. catabolic state of the muscle. Post/pre torque-velocity determinations revealed greater decrements in knee extensor performance in the BR vs. AG group (P < 0.04). The plantar flexors of the AG subjects actually demonstrated a net gain in the torque-velocity relationship, whereas in the BR group, the responses declined (AG vs. BR, P < 0.001). Muscle fiber cross-sectional area decreased by approximately 20% in the BR group, whereas no losses were evident in the AG group. RT-PCR analyses of muscle biopsy specimens demonstrated that markers of growth and cytoskeletal integrity were higher in the AG group, whereas catabolic markers were elevated in the BR group. Importantly, these patterns were seen in both muscles. We conclude that paradigms of AG have the potential to maintain the functional, biochemical, and structural homeostasis of skeletal muscle in the face of chronic unloading.

Corrigendum

CORRIGENDACorrigendumPublished Online:01 Mar 2009https://doi.org/10.1152/ajpheart.zh4-8713-corr.2009MoreSectionsPDF (25 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmailWeChat Volume 63, May 2008 Wu WC, Wang J, Detre JA, Wehrli FW, Mohler E 3rd, Ratcliffe SJ, Floyd TF. Hyperemic flow heterogeneity within the calf, foot, and forearm measured with continuous arterial spin labeling MRI. Am J Physiol Heart Circ Physiol 294: H2129–H2136, 2008. First published February 29, 2008; doi:10.1152/ajpheart.01399.2007 ( http://ajpheart.physiology.org/cgi/content/full/294/5/H2129).—Due to a spreadsheet layout error, the time-to-peak values shown in Tables 1 and 3 (on p. H2131–H2132) had an offset of one effective repetition time (16 s). The corrected tables are shown below (corrected data displayed in boldface). The correction has no effects on any of the statistical results and text description within the article. Table 1.Calf Muscle GroupnPeak Flow, ml·100 g−1·min−1Mean Hyperemic Flow, ml·100 g−1·min−1Hyperemic Flow Volume, ml/100 gHyperemic Period, sTime To Peak, sDPC19127±4969±2096±3377±2753±11Sol18116±5767±39106±6684±2950±9Gstrc-L1887±3249±2075±4087±2352±13Gstrc-M1985±4349±2575±4880±2952±12AC1986±3353±2272±3376±2550±12LC1775±4743±2564±4979±3250±11Values are means ± SD; n, number of subjects. Flow measurements in the midcalf. Six regions of interest were analyzed: deep posterior compartment (DPC), soleus (Sol), lateral (Gstrc-L) and medial (Gstrc-M) gastrocnemius, and anterior (AC) and lateral (LC) component. Table 3.Muscle GroupnPeak Flow, ml·100 g−1·min−1Mean Hyperemic Flow, ml·100 g−1·min−1Hyperemic Flow Volume, ml/100 gHyperemic Period, sTime To Peak, sFoot D-Ext9117±4466±2394±3985±1546±13 P-Flx9105±46 (0.31)57±20 (0.17)79±49 (0.34)78±23 (0.43)62±5 (0.0085)Forearm F-Flx1572±2246±3356±4175±1033±11 F-Ext15103±37 (0.0013)81±43 (0.0020)91±58 (0.020)66±12 (0.015)33±17 (1.000)Values are means ± SD; n, number of subjects. Values in parentheses are P values from repeated-measures ANOVA. Flow measurements in the foot and forearm. D-Ext, dorsal group; P-Flx, plantar flexor group; F-Flx, forearm flexors; F-Ext, forearm extensors.This article has no references to display. Download PDF Previous Back to Top FiguresReferencesRelatedInformation More from this issue > Volume 296Issue 3March 2009Pages H902-H902 Copyright & PermissionsCopyright © 2009 the American Physiological Societyhttps://doi.org/10.1152/ajpheart.zh4-8713-corr.2009History Published online 1 March 2009 Published in print 1 March 2009 Metrics

Effect of altering neural, muscular and tendinous factors associated with aging on balance recovery using the ankle strategy: A simulation study

Aging is associated with declines in neuromuscular function and reduced ability to recover balance from an imbalance episode. However, little is known about the relations amongst these factors. The purpose of this study was to determine the relative influence of age-related changes in neural, muscular and tendinous properties on the ability to recovery balance from a forward leaning position using the ankle strategy. A computer simulation was developed which consisted of an inverted pendulum with one rotational degree of freedom controlled by two muscles representing the ankle joint plantar flexor (PF) and dorsi flexor (DF) muscle groups. Model parameter values were adjusted so that the isometric torque–angle relation was in agreement with experimental ankle joint torque–angle curves from the literature. Muscle excitation was adjusted to match an experimentally determined maximum recoverable lean angle (MRLA) of 7.2° (baseline condition). The effect of 20% alterations to maximum isometric force, optimum muscle fibre length, maximum shortening velocity, tendon stiffness, reaction time delay (RTD), activation time constant and the maximum excitation of the PF muscles, and maximum excitation of the DF muscles (co-activation) on MRLA was then assessed. The parameters that had the greatest influence on MRLA were maximum isometric force, the maximum excitation of the ankle joint PFs and RTD, which, respectively, resulted in 19.0%, 17.8% and 4.6% reductions in MRLA. Individual changes to other parameters influenced MRLA by less than 1.9%. When selected parameter values were adjusted in accordance with age-related changes reported in the literature, MRLA was reduced to 5.3°, a value in relative agreement with experimental values reported in the literature (4.6±1.8°). In general, these results suggest that MRLA is most sensitive to PF muscle mass and the ability to maximally activate the PFs, and that the combined effect of multiple changes in neural, muscular and tendinous parameters reported to occur with aging can have a profound effect on the ability to recover balance from a forward fall using the ankle strategy.

Locomotor Training and Muscle Function After Incomplete Spinal Cord Injury: Case Series

Background/Objective: To determine whether 9 weeks of locomotor training (LT) results in changes in muscle strength and alterations in muscle size and activation after chronic incomplete spinal cord injury (SCI). Study Design: Longitudinal prospective case series.Methods: Five individuals with chronic incomplete SCI completed 9 weeks of LT. Peak isometric torque, torque developed within the initial 200 milliseconds of contraction (Torque200), average rate of torque development (ARTD), and voluntary activation deficits were determined using isokinetic dynamometry for the knee-extensor (KE) and plantar-flexor (PF) muscle groups before and after LT. Maximum muscle crosssectional area (CSA) was measured prior to and after LT.Results: Locomotor training resulted in improved peak torque production in all participants, with the largest increases in the more-involved PF (43.9% ± 20.0%), followed by the more-involved KE (21.1% ± 12.3%). Even larger improvements were realized in Torque200 and ARTD (indices of explosive torque), after LT. In particular, the largest improvements were realized in the Torque200 measures of the PF muscle group. Improvements in torque production were associated with enhanced voluntary activation in both the KE and ankle PF muscles and an increase in the maximal CSA of the ankle PF muscles.Conclusion: Nine weeks of LT resulted in positive alterations in the KE and PF muscle groups that included an increase in muscle size, improved voluntary activation, and an improved ability to generate both peak and explosive torque about the knee and ankle joints.

Central fatigue affects plantar flexor strength after prolonged running

The primary aim of this study was to examine central fatigue of the plantar flexor muscle group after prolonged running using the twitch interpolation technique. Eight healthy, habitually active male subjects ran on a motorized treadmill for 2 h at a speed corresponding to 75% of peak oxygen uptake (VO(2peak)). Maximal voluntary isometric contraction (MVC) strength as well as the electrically induced twitch produced during MVC [interpolated twitch (IT)] and at rest [resting twitch (RT)] were measured before and after running. The level of activation (LOA) during each MVC was calculated as LOA (%)=100(1-IT/RT). Both MVC and LOA decreased (17+/-16% and 19+/-15%, respectively, P<0.05) after running, whereas RT did not change. The decrease in MVC was correlated with the decrease in LOA (r=0.87, P<0.05). The results demonstrate that after 2 h of treadmill running at an intensity of 75% of VO(2peak), there was a reduction in maximal voluntary plantar flexor muscle strength that was mainly related to central fatigue.

Effect of unloading on muscle volume with and without resistance training

The present study aimed to investigate the effect of resistance training on the volume of four muscle groups and/or 17 individual muscles of the human lower limb during 20 days of 6 ∘ head-down tilt bed rest. Twelve healthy men were divided into two groups: the resistance training group: BR-Tr ( n = 6 ) and the control group: BR-Cont ( n = 6 ) . The volumes of the knee extensor, knee flexor, adductor, plantar flexor, and dorsiflexor muscle groups and their individual muscles were calculated. After the bed rest, the BR-Tr subjects showed no significant change in the volume in almost all tested muscles; in contrast, the volumes of the four muscle groups significantly decreased in the BR-Cont group ( - 12 % to - 8 %). These results suggest that resistance training during bed rest can prevent the deteriorating of thigh muscles and calf muscles.

Effect of aging on the recovery following contraction-induced injury in muscles of female mice

By the age of 80 yr, the skeletal muscles of men and women decrease in mass and maximum force by approximately 30%. Severe contraction-induced injury may contribute to these age-related declines. One to two months after a 225 lengthening contraction protocol (LCP), muscles of young/adult male mice recovered completely, whereas those of old male mice sustained deficits of approximately 15% in mass and approximately 25% in maximum force. Although gender-related differences in the early events of contraction-induced injury have been reported, the recovery phase of muscles in old female animals has not been investigated. The hypothesis tested was that 2 mo after a severe LCP to the plantar flexor muscle group, the magnitude of recovery of mass and force for old female mice is less than that for adult female mice. The LCP was administered to muscles of adult and old, female C57BL/6 mice. At 3 days, 1 mo, and 2 mo following the LCP, maximum isometric force was measured, and muscles were removed and weighed. Two months following the LCP, the muscles of adult female mice recovered mass and force. In contrast, for old female mice, even after 2 mo, muscle masses were decreased by 11% and maximum forces by 38%. We conclude that, as reported previously for old male mice, a severe contraction-induced injury to muscles of old female mice results in prolonged deficits in mass and force.

Reliability of techniques to assess human neuromuscular function in vivo

The purpose of this study was to comprehensively evaluate the reliability of a large number of commonly utilized experimental tests of in vivo human neuromuscular function separated by 4-weeks. Numerous electrophysiological parameters (i.e., voluntary and evoked electromyogram [EMG] signals), contractile properties (i.e., evoked forces and rates of force development and relaxation), muscle morphology (i.e., MRI-derived cross-sectional area [CSA]) and performance tasks (i.e., steadiness and time to task failure) were assessed from the plantarflexor muscle group in 17 subjects before and following 4-weeks where they maintained their normal lifestyle. The reliability of the measured variables had wide-ranging levels of consistency, with coefficient of variations (CV) ranging from ∼2% to 20%, and intraclass correlation coefficients (ICC) between 0.53 and 0.99. Overall, we observed moderate to high-levels of reliability in the vast majority of the variables we assessed (24 out of the 29 had ICC>0.70 and CV<15%). The variables demonstrating the highest reliability were: CSA (ICC=0.93–0.98), strength (ICC=0.97), an index of nerve conduction velocity (ICC=0.95), and H-reflex amplitude (ICC=0.93). Conversely, the variables demonstrating the lowest reliability were: the amplitude of voluntary EMG signal (ICC=0.53–0.88), and the time to task failure of a sustained submaximal contraction (ICC=0.64). Additionally, relatively little systematic bias (calculated through the limits of agreement) was observed in these measures over the repeat sessions. In conclusion, while the reliability differed between the various measures, in general it was rather high even when the testing sessions are separated by a relatively long duration of time.

ANKLE STRENGTHENING TO IMPROVE GAIT AND FUNCTION IN CEREBRAL PALSY—A PILOT STUDY

Purpose/Hypothesis: The purpose of this investigation was to determine if increases in ankle strength can improve gait and function in persons with cerebral palsy (CP). Despite recognizing that muscle weakness is a major impairment in CP, many of the current treatments (e.g. tendon lengthening, Botulinum toxin, orthotics) purposely weaken or result in further weakening of the muscles. Number of Subjects: Twelve subjects (3 males; 9 females, age 10 + 3 years) with spastic diplegia CP and Gross Motor Function Classification System (GMFCS) levels; I n = 5, II n = 5, III n = 2. Materials/Methods: Subjects were randomly assigned to one of four groups: 1) dorsiflexor (DF) group [n = 3], 2) plantarflexor (PF) group [n = 4], 3) dorsi-plantarflexor (DF/PF) group [n = 2], and 4) control group [n = 3] undergoing no strength training program. Subjects in the 3 training groups participated in a 12-week, 3×/week progressive resistance strength training program on an isokinetic dynamometer (concentric/eccentric @ 30 and 90o/s). Pre and post training, the subjects underwent tests for plantarflexor spasticity, end range dorsiflexion, ankle strength, GMFM66 (Gross Motor Function Measure), GMFM-wrj (walk, run, jump) and gait speed. Paired t test and correlations were used to assess changes and relationships (P < 0.05). Results: Results indicated no increase in spasticity or decrease in end range dorsiflexion. There was a significant increase in strength in the trained muscles (P = 0.007). The strength increase was greater for the plantarflexors of PF group (0.46 Nm/kg) than for the dorsiflexors of the DF group (0.20 Nm/kg). For the DF/PF group, the strength increase of the dorsiflexors was not as much as those of the DF group, whereas the strength increase of the plantarflexors was slightly greater than those of the PF group. The muscles that were not trained were unchanged. GMFM66 results showed increases in the 3 training groups and a decrease for the control group (1.0, 1.8, 2.7 and −2.7 for the DF, PF, DF/PF, Control groups, respectively). Results for the GMFM-wrj indicated larger improvements than the GMFM66. There was a significant increase in the GMFM-wrj for the training groups (P = 0.041). Further, the correlation between change in strength and change in GMFM-wrj was r = 0.84 (P = 0.001). Gait speed was increased for the PF group (8.0 cm/s) and for the DF/PF group (2.4 cm/s). The increase in the PF group was primarily due to a 6.4 cm increase in stride length. Speed was unchanged for the DF group (−1.1 cm/s) and for the control group (−1.5 cm/s). Conclusions: Results from this study seem to indicate that increases in ankle strength are possible in individuals with CP. Function was improved for all of the strength training groups. Gait speed improved with strengthening of the plantarflexors but not with isolated dorsiflexor strength training. Clinical Relevance: If the results hold true for a larger investigation, strengthening rather than weakening the plantarflexors, may improve gait and function in persons with CP. Funding: NIH- Ankle Strengthening to Improve Gait and Function in CP (R01 NS046434).

Increasing Ankle Strength to Improve Gait and Function in Children with Cerebral Palsy: A Pilot Study

This pilot study was designed to determine whether increases in ankle strength could improve gait and function in children with spastic diplegia. Data were obtained from 12 children with spastic diplegia who were assigned randomly to a dorsiflexor group, a plantarflexor group, a dorsi- and plantarflexor group, or a control group. Training group subjects participated in a 12-week strength program using an isokinetic dynamometer. The majority of subjects increased their strength in the trained muscles; the strength of untrained muscles was unchanged. The Gross Motor Function Measure (GMFM) walk-run-jump dimension and a quality of life measure (ie, Peds QL) improved significantly for the entire training group. Gait speed was not significantly increased for the entire training group but varied among training groups. Gait kinematics significantly improved or showed trends for improvement for the entire training group. This pilot investigation indicates that ankle strength increases may lead to improved function, gait speed, and quality of life in children with spastic diplegia.

Lower extremity skeletal muscle function in persons with incomplete spinal cord injury

A cross-sectional study design. To characterize and specifically quantify impairments in muscle function after chronic incomplete spinal cord injury (SCI). University of Florida, Gainesville, FL, USA. Voluntary and electrically elicited contractile measurements were performed and voluntary activation deficits were quantitatively determined in the knee extensor and ankle plantar flexor muscle groups in 10 individuals with chronic incomplete SCI (C5-T8, ASIA C or D) and age-, gender-, height- and body weight matched healthy controls. Persons with incomplete-SCI were able to produce only 36 and 24% of the knee extensor torque and 38 and 26% of the plantar flexor torque generated by noninjured controls in the self-reported less-involved and more-involved limbs, respectively (P<0.05). In addition, both indices of explosive or instantaneous muscle strength, torque200 (absolute torque reached at 200 ms) and the average rate of torque development (ARTD) were dramatically reduced in the ankle plantar flexor and knee extensor muscle groups in persons with incomplete-SCI. However, the deficit in instantaneous muscle strength was most pronounced in the ankle plantar flexor muscles, with an 11.7-fold difference between the torque200 measured in the self-reported more involved limb and a 5-fold difference in the less-involved limb compared to control muscles. Voluntary activation deficits ranged between 42 and 66% in both muscle groups. Interestingly, electrically elicited contractile properties did not differ between the groups. The resultant impact of incomplete-SCI is that affected muscles not only become weak, but slow to develop voluntary torque. We speculate that the large deficit in torque200 and ARTD in the ankle plantar flexors muscles of persons with incomplete-SCI may limit locomotor function. The results presented in this study provide a quantitative and sensitive assessment of muscle function upon which future research examining rehabilitation programs aimed at restoring muscle function and promoting functional recovery after incomplete-SCI may be based.

Contractile abilities of normal and “mini” triceps surae muscles from mice (Mus domesticus) selectively bred for high voluntary wheel running

As reported previously, artificial selection of house mice caused a 2.7-fold increase in voluntary wheel running of four replicate selected lines compared with four random-bred control lines. Two of the selected lines developed a high incidence of a small-muscle phenotype ("mini muscles") in the plantar flexor group of the hindlimb, which apparently results from a simple Mendelian recessive allele. At generations 36-38, we measured wheel running and key contractile characteristics of soleus and medial gastrocnemius muscles from normal and mini muscles in mice from these selected lines. Mice with mini muscles ran faster and a greater distance per day than normal individuals but not longer. As expected, in mini-muscle mice the medial and lateral gastrocnemius muscles were approximately 54 and 45% the mass of normal muscles, respectively, but the plantaris muscles were not different in mass and soleus muscles were actually 30% larger. In spite of the increased mass, contractile characteristics of the soleus were unchanged in any notable way between mini and normal mice. However, medial gastrocnemius muscles in mini mice were changed markedly toward a slower phenotype, having slower twitches; demonstrated a more curved force-velocity relationship; produced about half the mass-specific isotonic power, 20-50% of the mass-specific cyclic work and power (only 10-25% the absolute power if the loss in mass is considered); and fatigued at about half the rate of normal muscles. These changes would promote increased, aerobically supported running activity but may compromise activities that require high power, such as sprinting.

In vivo physiological cross-sectional area and specific force are reduced in the gastrocnemius of elderly men

Sarcopenia and muscle weakness are well-known consequences of aging. The aim of the present study was to ascertain whether a decrease in fascicle force (Ff) could be accounted for entirely by muscle atrophy. In vivo physiological cross-sectional area (PCSA) and specific force (Ff/PCSA) of the lateral head of the gastrocnemius (GL) muscle were assessed in a group of elderly men [EM, aged 73.8 yr (SD 3.5), height 173.4 cm (SD 4.4), weight 78.4 kg (SD 8.3); means (SD)] and for comparison in a group of young men [YM, aged 25.3 yr (SD 4.4), height 176.4 cm (SD 7.7), weight 79.1 kg (SD 11.9)]. GL muscle volume (Vol) and Achilles tendon moment arm length were evaluated using magnetic resonance imaging. Pennation angle and fiber fascicle length (Lf) were measured using B-mode ultrasonography during isometric maximum voluntary contraction of the plantar flexors. PCSA was estimated as Vol/Lf. GL Ff was calculated by dividing Achilles tendon force by the cosine of theta, during the interpolation of a supramaximal doublet, and accounting for antagonist activation level (assessed using EMG), Achilles tendon moment arm length, and the relative PCSA of the GL within the plantar flexor group. Voluntary activation of the plantar flexors was lower in the EM than in the YM (86 vs. 98%, respectively, P < 0.05). Compared with the YM, plantar flexor maximal voluntary contraction torque and Ff of the EM were lower by 47 and 40%, respectively (P < 0.01). Both Vol and PCSA were smaller in the EM by 28% (P < 0.01) and 16% (P < 0.05), respectively. Also, pennation angle was 12% smaller in the EM, whereas there was no significant difference in Lf between the YM and EM. After accounting for differences in agonists and antagonists activation, the Ff/PCSA of the EM was 30% lower than that of the YM (P < 0.01). These findings demonstrate that the loss of muscle strength with aging may be explained not only by a reduction in voluntary drive to the muscle, but mostly by a decrease in intrinsic muscle force. This phenomenon may possibly be due to a reduction in single-fiber specific tension.

Voluntary Resistance Exercise Training With Obese and Lean Zucker Rats

PURPOSE Fatty Zucker rats have been used to examine the effects of obesity, and diabetes on the physiological mechanisms associated with exercise-induced musculosketetal adaptation. Because many volitional exercise models use operant conditioning techniques that rely on dietary restrictions to sufficiently motivate voluntary behavior, a study was conducted to determine if obese Zucker rats could be trained to perform a voluntary weight-lifting movement without altering their diet. METHODS Using a custom-designed hind limb loading apparatus, eight 6-weekold male genetically obese Zucker rats (fa/fa) and eight Zucker lean rats (+/fa) were trained with positive reinforcement to enter a vertical tube, insert their head into a neck ring, and repeatedly lift and lower their body weight in a movement that targeted the plantar flexor muscle group. Training sessions were conducted 5 days per week and each lasted until at least 50 lifting and lowering movements were performed or 1.5 hrs. One 45-mg sucrose pellet was given as a reinforcer, after every second lift. Food and water were available in the home cage at all times. RESULTS As expected, mean body weights increased rapidly in the obese rats from 201 g at 6 weeks old to 789 g at 23 weeks old. During the same period mean body weights of the Zucker leans increased from 183 g to 457 g. All rats successfully acquired the weight-lifting movement. However, of the obese Zucker rats only 2 were able to approach the goal of 50 lifts per session with a mean of 47 lifts per session. The mean lifts per session for the remaining 6 obese rats ranged from 5 to 25. 5 of 8 lean Zucker rats were able to approach the 50-lifts/session goal with a mean range of 47–51. The 3 other lean rats recorded means of 28, 30, 35, lifts per session, respectively. CONCLUSIONS Results demonstrate that genetically obese Zucker rats can be trained to perform a voluntary movement for reward. However, food reward was unable to continually motivate the Zucker obese rats towards a goal of 50-body weight lifts per day. Conversely, it was sufficient to elicit continual lifting of Zucker leans. In summary the procedures used in this study were not sufficient to maintain a consistent lifting pattern in obese Zucker rats.

Quantification of level of effort at the plantarflexors and hip extensors and flexor muscles in healthy subjects walking at different cadences

The plantarflexor, hip extensor and hip flexor muscle groups contribute by their concentric action to generate most of the energy during level gait in healthy subjects. The goal of the present study was to determine, during the main energy generation phases, the relative demand of these three groups in 14 healthy subjects walking at four cadences (self-selected, 60, 80 and 120 steps/min). The muscular utilization ratio (MUR), that compares the net joint moment obtained during gait to the maximal potential moment (MPM) at each percentage of the gait cycle, was used to estimate the mechanical relative demand. The MPM values were obtained by regression equations developed from torque data measured with a Biodex dynamometric system. The results showed that the peak MURs increased with gait cadence. The peak values were not significantly different between sides for all cadences despite mean absolute lateral differences ranging from 7% to 10%. The mean peak MURs of both sides ranged from 51.3% to 62.6%, from 20.7% to 49.9% and from 14.9% to 42.5%, for the plantarflexors, hip flexors and hip extensors, respectively. Highly significant associations were found between the MURs and net moments (numerator of the MUR ratio), with Pearson coefficients ( r) superior to 0.80 for all muscles groups. The association between the MURs and the maximal potential moments (denominator of MUR ratio) was lower (0.01 < r < 0.60). The results of this study indicated that the mechanical relative demand during gait increased with gait cadence. The plantarflexor MUR values were always the highest. However, when gait cadence increased, the MURs at the hip reached similar values of those of the ankle, which suggests that these muscles also work significantly at elevated gait cadences. The changes in the MUR values were mainly explained by changes in the net joint moments.

Resistance training improves strength and functional capacity in persons with multiple sclerosis

The purpose of this study was to evaluate the effect of an eight-week progressive resistance training programme on lower extremity strength, ambulatory function, fatigue and self-reported disability in multiple sclerosis (MS) patients (mean disability score 3.7 +/- 0.8). Eight MS subjects volunteered for twice weekly training sessions. During the first two weeks, subjects completed one set of 8-10 reps at 50% of maximal voluntary contraction (MVC) of knee flexion, knee extension and plantarflexion exercises. In subsequent sessions, the subjects completed one set of 10-15 repetitions at 70% of MVC. The resistance was increased by 2-5% when subjects completed 15 repetitions in consecutive sessions. Isometric strength of the quadriceps, hamstring, plantarflexor and dorsiflexor muscle groups was assessed before and after the training programme using an isokinetic dynamometer. Magnetic resonance images of the thigh were acquired before and after the exercise programme as were walking speed (25-ft), number of steps in 3 min, and self-reported fatigue and disability. Knee extension (7.4%), plantarflexion (52%) and stepping performance (8.7%) increased significantly (P < 0.05). Self-reported fatigue decreased (P < 0.05) and disability tended to decrease (P = 0.07) following the training programme. MS patients are capable of making positive adaptations to resistance training that are associated with improved ambulation and decreased fatigue.

Ankle stiffness and tissue compliance in stroke survivors: A validation of Myotonometer measurements

Rydahl SJ, Brouwer BJ. Ankle stiffness and tissue compliance in stroke survivors: a validation of Myotonometer measurements. Arch Phys Med Rehabil 2004;85:1631–7. Objective To determine the construct validity of Myotonometer measurements of tissue compliance as being reflective of ankle plantarflexor tone. Design Cross sectional. Setting Motor performance research laboratory. Participants Twenty-three stroke survivors (67.5±10.9y) and 24 control subjects (71.2±9.0y) recruited from the community. Interventions Not applicable. Main outcome measures Plantarflexor tone was measured using the Modified Ashworth Scale (MAS), ankle stiffness (total, passive, intrinsic and reflex components) was quantified using a torque motor, and tissue compliance during relaxation and activation of the plantarflexors was measured with the Myotonometer. Results MAS scores in the stroke group ranged from 1 to 4, whereas all control subjects had normal tone. Mean total ankle stiffness was higher in the stroke group than in the control group ( P<.02), mainly due to elevated passive stiffness ( P<.03). Compliance did not change as a function of muscle activation in stroke, but it decreased when control subjects contracted their plantarflexors ( P<.04). The difference in Myotonometer measurements acquired during active and relaxed states was associated with total ankle stiffness and, specifically, intrinsic stiffness. The relationships were strongest when only stroke data were considered. Conclusions Stiffness and compliance measures distinguished between control subjects and persons with hypertonia secondary to stroke. Compliance differences in the relaxed and active gastrocnemius muscle reflected intrinsic stiffness associated with the contractile elements of the plantarflexor group of muscles as a whole.

Functional recovery of the plantarflexor muscle group after hindlimb unloading in the rat.

Research into skeletal muscle's response to hindlimb unloading (HU) of the rodent has focused on that of the markedly affected slow-twitch anti-gravity muscles (e.g., soleus). However, the ability of the animal to locomote following HU should be best determined by the in vivo functional properties of the muscle groups involved and, to our knowledge, this has not been investigated. Our objective was to determine how the in vivo functional properties of the rat ankle plantarflexor group change after 28 days of HU and during a subsequent 28-day recovery. Rats ( n=48) were unloaded for 28 days after which they were either tested immediately or allowed to recover for 7, 14, or 28 days before being tested. Control rats ( n=61) were tested at comparable times. In vivo functional properties of the ankle plantarflexors were assessed under anesthesia using an isokinetic dynamometer and included determination of the isometric torque-frequency relationship, the concentric torque-ankle angular velocity relationship, and fatigability. Immediately after HU, plantarflexor muscle weight was reduced by 24% but isometric torque production was reduced by 7-9% only at > or =100 Hz and concentric torque production was not significantly affected. However, after 7 days of recovery, in vivo function was more adversely affected; isometric and concentric torques were reduced by 12-33% and 16-36%, respectively, relative to control levels. In vivo plantarflexor function was recovered by 14 days. In conclusion, 28 days of HU has minor adverse effects on the in vivo function of the rat ankle plantarflexors. During the first week of recovery from HU, injury apparently occurs to the plantarflexors resulting in a transient impairment of functional capacity.

Voluntary Hind Limb Resistance Exercise with Obese Zucker Rats

2295 PURPOSE: Characteristics of fatty Zucker rats offer exercise physiologists unique avenues for research on the effects of obesity, diabetes, and hypertension on the physiological mechanisms associated with exercise-induced musculosketetal adaptation and injury. The ability to use this strain of rats in volitional exercise models can further extend the research possibilities. Because volitional exercise models usually involve operant conditioning techniques that rely on dietary restrictions to sufficiently motivate voluntary behavior, a study was conducted to determine if obese Zucker rats could be trained to perform a voluntary weight-lifting movement without altering their diet. METHODS: Using a custom-designed hind limb loading apparatus, four 6-weekold male genetically obese Zucker rats (fa/fa) were trained with positive reinforcement to enter a vertical tube, insert their head into a neck ring, and repeatedly lift and lower a 700-g load using a “heel raise” movement that targeted the plantar flexor muscle group. Training sessions were conducted 5 days per week and each lasted approximately 1-hr or until at least 60 lifting and lowering movements were performed. Across training sessions, different schedules of reinforcement were studied to compare the effects of continuous versus intermittent reinforcement of responses. Reinforcers consisted of 45-mg nutritionally complete food pellets. Food and water were available in the home cage at all times. RESULTS: As expected, mean body weights increased rapidly from 201 g at 6 weeks old to 605 g at 15 weeks old. During the same period, all four rats successfully acquired and maintained the weight-lifting movement. A sufficient level of motivation to perform the lifting and lowering movement was maintained throughout the study period, even when the rats' size appeared to interfere with their movement in and out of the apparatus and when the number of responses required for each reinforcement increased across sessions. CONCLUSIONS: Results demonstrate that genetically obese Zucker rats can be trained to perform a voluntary, effortful movement with altering their diet or level of motivation. The present approach can be applied to a wide range of investigations involving volitional resistance exercise.

Nitric Oxide Synthase In Skeletal Muscle Repair Following Strain Injury

0057 Following muscle injury, satellite cells are recruited and activated to enable muscle repair. Previous studies using nitric oxide synathase (NOS) inhibition, mdx dystrophy mice and NOS-I knock-out mice have suggested that nitric oxide release may mediate satellite cell activation. PURPOSE: To assess the presence of NOS in strain-induced muscle injury in conjunction with two muscle regulatory factors: MyoD and myogenin. METHODS: Using female Sprague Dawley rats, muscle strains were produced by manually stretching (50 repetitions) an activated plantar flexor group through its normal range of motion. Muscle biopsy samples were evaluated 48- hours after strain injury for the presence of nitric oxide synthase, MyoD, and myogenin using Western immunoblot analysis. Transcript levels for neuronal (n) NOS, endothelial (e) NOS, inducible (i) NOS, MyoD and myogenin were assessed using reverse transcriptase polymerase chain reaction (RT-PCR) and kinetic PCR. RESULTS: An increase in myogenin transcripts and a decrease in nNOS transcript expression was observed 48- hours post-strain injury by RT-PCR. Western immunoblot analysis revealed increased eNOS, MyoD, and myogenin protein levels 48- hours post-strain injury, but a decrease in nNOS protein levels. CONCLUSION: NOS has been reported to play a role in muscle repair. Although nNOS expression was decreased, eNOS expression increased providing support for other NOS isoforms in muscle repair. Supported by NIOSH R01-OH-02918.