The influence of body weight support on ankle mechanics during treadmill walking

The use of body weight support (BWS) systems during locomotor retraining has become routine in clinical settings. BWS alters load receptor feedback, however, and may alter the biomechanical role of the ankle plantarflexors, influencing gait. The purpose of this study was to characterize the biomechanical adaptations that occur as a result of a change in limb load (controlled indirectly through BWS) and gait speed during treadmill locomotion. Fifteen unimpaired participants underwent gait analysis with surface electromyography while walking on an instrumented dual-belt treadmill at seven different speeds (ranging from 0.4 to 1.6 m/s) and three BWS conditions (ranging from 0% to 40% BWS). While walking, spatiotemporal measures, anterior/posterior ground reaction forces, and ankle kinetics and muscle activity were measured and compared between conditions. At slower gait speeds, propulsive forces and ankle kinetics were unaffected by changing BWS; however, at gait speeds ≥approximately 0.8 m/s, an increase in BWS yielded reduced propulsive forces and diminished ankle plantarflexor moments and powers. Muscle activity remained unaltered by changing BWS across all gait speeds. The use of BWS could provide the advantage of faster walking speeds with the same push-off forces as required of a slower speed. While the use of BWS at slower speeds does not appear to detrimentally affect gait, it may be important to reduce BWS as participants progress with training, to encourage maximal push-off forces. The reduction in plantarflexor kinetics at higher speeds suggests that the use of BWS in higher functioning individuals may impair the ability to relearn walking.     

Effect of body weight support variation on muscle activities during robot assisted gait: a dynamic simulation study

Background and Objectives: While body weight support (BWS) intonation is vital during conventional gait training of neurologically challenged subjects, it is important to evaluate its effect during robot assisted gait training. In the present research we have studied the effect of BWS intonation on muscle activities during robotic gait training using dynamic simulations. Methods: Two dimensional (2-D) musculoskeletal model of human gait was developed conjointly with another 2-D model of a robotic orthosis capable of actuating hip, knee and ankle joints simultaneously. The musculoskeletal model consists of eight major muscle groups namely; soleus (SOL), gastrocnemius (GAS), tibialis anterior (TA), hamstrings (HAM), vasti (VAS), gluteus maximus (GLU), uniarticular hip flexors (iliopsoas, IP), and Rectus Femoris (RF). BWS was provided at levels of 0, 20, 40 and 60% during the simulations. In order to obtain a feasible set of muscle activities during subsequent gait cycles, an inverse dynamics algorithm along with a quadratic minimization algorithm was implemented. Results: The dynamic parameters of the robot assisted human gait such as joint angle trajectories, ground contact force (GCF), human limb joint torques and robot induced torques at different levels of BWS were derived. The patterns of muscle activities at variable BWS were derived and analysed. For most part of the gait cycle (GC) the muscle activation patterns are quite similar for all levels of BWS as is apparent from the mean of muscle activities for the complete GC. Conclusions: Effect of BWS variation during robot assisted gait on muscle activities was studied by developing dynamic simulation. It is expected that the proposed dynamic simulation approach will provide important inferences and information about the muscle function variations consequent upon a change in BWS during robot assisted gait. This information shall be quite important while investigating the influence of BWS intonation on neuromuscular parameters of interest during robotic gait training.     

The Combined Effects of Body Weight Support and Gait Speed on Gait Related Muscle Activity: A Comparison between Walking in the Lokomat Exoskeleton and Regular Treadmill Walking

Background

For the development of specialized training protocols for robot assisted gait training, it is important to understand how the use of exoskeletons alters locomotor task demands, and how the nature and magnitude of these changes depend on training parameters. Therefore, the present study assessed the combined effects of gait speed and body weight support (BWS) on muscle activity, and compared these between treadmill walking and walking in the Lokomat exoskeleton.

Methods

Ten healthy participants walked on a treadmill and in the Lokomat, with varying levels of BWS (0% and 50% of the participants’ body weight) and gait speed (0.8, 1.8, and 2.8 km/h), while temporal step characteristics and muscle activity from Erector Spinae, Gluteus Medius, Vastus Lateralis, Biceps Femoris, Gastrocnemius Medialis, and Tibialis Anterior muscles were recorded.

Results

The temporal structure of the stepping pattern was altered when participants walked in the Lokomat or when BWS was provided (i.e. the relative duration of the double support phase was reduced, and the single support phase prolonged), but these differences normalized as gait speed increased. Alternations in muscle activity were characterized by complex interactions between walking conditions and training parameters: Differences between treadmill walking and walking in the exoskeleton were most prominent at low gait speeds, and speed effects were attenuated when BWS was provided.

Conclusion

Walking in the Lokomat exoskeleton without movement guidance alters the temporal step regulation and the neuromuscular control of walking, although the nature and magnitude of these effects depend on complex interactions with gait speed and BWS. If normative neuromuscular control of gait is targeted during training, it is recommended that very low speeds and high levels of BWS should be avoided when possible.     

Podokinetic after-rotation in a simulated reduced gravity environment

Stepping in place on a rotating platform for a period of 15 minutes induces an adaptive response, podokinetic after-rotation (PKAR), which causes a blindfolded individual to inadvertently rotate when attempting to step in place on the floor. The purpose of this investigation was to determine whether lower extremity load receptors were involved in this adaptation. As load receptor input is critical for locomotion, we hypothesized that manipulating load via body weight support (BWS) would influence PKAR. Eleven healthy female volunteers performed 15 minutes of stepping in place on a rotating treadmill (stimulation), followed by 10 minutes of stepping in place (response) without vision on a stationary surface. Response and stimulation periods were with 50% body weight support (BWS) and without body weight support (NoBWS) in all four possible combinations (BWS-BWS, NoBWS-NoBWS, BWS-NoBWS, and NoBWS-BWS). Conditions were randomly assigned to all subjects and performed on four separate occasions at least 48 hr apart. During the 10-min PKAR response period, trunk angular velocity was calculated and plotted versus time, and exponential models were applied to the data to obtain curve-fit parameters for each condition. Despite the manipulations of BWS, no significant differences were found for any parameter value (p = 0.13-0.98). BWS applied during stimulation only, response only, or during both stimulation and response does not appear to influence PKAR. This suggests that load receptors may not play a critical role in mediating adaptive changes in locomotor trajectory control in response to walking on a rotating surface.     

Podokinetic after-rotation in a simulated reduced gravity environment

Stepping in place on a rotating platform for a period of 15 minutes induces an adaptive response, podokinetic after-rotation (PKAR), which causes a blindfolded individual to inadvertently rotate when attempting to step in place on the floor. The purpose of this investigation was to determine whether lower extremity load receptors were involved in this adaptation. As load receptor input is critical for locomotion, we hypothesized that manipulating load via body weight support (BWS) would influence PKAR. Eleven healthy female volunteers performed 15 minutes of stepping in place on a rotating treadmill (stimulation), followed by 10 minutes of stepping in place (response) without vision on a stationary surface. Response and stimulation periods were with 50% body weight support (BWS) and without body weight support (NoBWS) in all four possible combinations (BWS-BWS, NoBWS-NoBWS, BWS-NoBWS, and NoBWS-BWS). Conditions were randomly assigned to all subjects and performed on four separate occasions at least 48 hr apart. During the 10-min PKAR response period, trunk angular velocity was calculated and plotted versus time, and exponential models were applied to the data to obtain curve-fit parameters for each condition. Despite the manipulations of BWS, no significant differences were found for any parameter value (p = 0.13–0.98). BWS applied during stimulation only, response only, or during both stimulation and response does not appear to influence PKAR. This suggests that load receptors may not play a critical role in mediating adaptive changes in locomotor trajectory control in response to walking on a rotating surface.     

A novel mouse running wheel that senses individual limb forces: biomechanical validation and in vivo testing

Biomechanical data provide fundamental information about changes in musculoskeletal function during development, adaptation, and disease. To facilitate the study of mouse locomotor biomechanics, we modified a standard mouse running wheel to include a force-sensitive rung capable of measuring the normal and tangential forces applied by individual paws. Force data were collected throughout the night using an automated threshold trigger algorithm that synchronized force data with wheel-angle data and a high-speed infrared video file. During the first night of wheel running, mice reached consistent running speeds within the first 40 force events, indicating a rapid habituation to wheel running, given that mice generated >2,000 force-event files/night. Average running speeds and peak normal and tangential forces were consistent throughout the first four nights of running, indicating that one night of running is sufficient to characterize the locomotor biomechanics of healthy mice. Twelve weeks of wheel running significantly increased spontaneous wheel-running speeds (16 vs. 37 m/min), lowered duty factors (ratio of foot-ground contact time to stride time; 0.71 vs. 0.58), and raised hindlimb peak normal forces (93 vs. 115% body wt) compared with inexperienced mice. Peak normal hindlimb-force magnitudes were the primary force component, which were nearly tenfold greater than peak tangential forces. Peak normal hindlimb forces exceed the vertical forces generated during overground running (50-60% body wt), suggesting that wheel running shifts weight support toward the hindlimbs. This force-instrumented running-wheel system provides a comprehensive, noninvasive screening method for monitoring gait biomechanics in mice during spontaneous locomotion.     

Aging and respiratory muscle metabolic plasticity: effects of endurance training.

We examined the oxidative and antioxidant enzyme activities in respiratory and locomotor muscles in response to endurance training in young and aging rats. Young adult (4-mo-old) and old (24-mo-old) female Fischer 344 rats were divided into four groups: 1) young trained (n = 12), 2) young untrained (n = 12), 3) old trained (n = 10), and 4) old untrained (n = 6). Both young and old endurance-trained animals performed the same training protocol during 10 wk of continuous treadmill exercise (60 min/day, 5 days/wk). Compared with young untrained animals, the young trained group had significantly elevated (P less than 0.05) activities of 3-hydroxyacyl-CoA dehydrogenase (HADH), glutathione peroxidase (GPX), and citrate synthase (CS) in both the costal diaphragm and the plantaris muscle. In contrast, training had no influence (P greater than 0.05) on the activity of lactate dehydrogenase within the costal diaphragm in young animals. In the aging animals, training did not alter (P greater than 0.05) activities of CS, HADH, GPX, or lactate dehydrogenase in the costal diaphragm but significantly (P less than 0.05) increased CS, HADH, and GPX activities in the plantaris muscle. Furthermore, training resulted in higher activities of CS and HADH in the intercostal muscles in the old trained than in the old untrained animals. Finally, activities of CS, HADH, and GPX were significantly (P less than 0.05) lower in the plantaris in the old untrained than in the young untrained animals; however, CS, HADH, and GPX activities were greater (P less than 0.05) in the costal diaphragm in the old sedentary than in the young untrained animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Decoding the mechanisms of gait generation in salamanders by combining neurobiology, modeling and robotics

Vertebrate animals exhibit impressive locomotor skills. These locomotor skills are due to the complex interactions between the environment, the musculo-skeletal system and the central nervous system, in particular the spinal locomotor circuits. We are interested in decoding these interactions in the salamander, a key animal from an evolutionary point of view. It exhibits both swimming and stepping gaits and is faced with the problem of producing efficient propulsive forces using the same musculo-skeletal system in two environments with significant physical differences in density, viscosity and gravitational load. Yet its nervous system remains comparatively simple. Our approach is based on a combination of neurophysiological experiments, numerical modeling at different levels of abstraction, and robotic validation using an amphibious salamander-like robot. This article reviews the current state of our knowledge on salamander locomotion control, and presents how our approach has allowed us to obtain a first conceptual model of the salamander spinal locomotor networks. The model suggests that the salamander locomotor circuit can be seen as a lamprey-like circuit controlling axial movements of the trunk and tail, extended by specialized oscillatory centers controlling limb movements. The interplay between the two types of circuits determines the mode of locomotion under the influence of sensory feedback and descending drive, with stepping gaits at low drive, and swimming at high drive.     

Oral administration of a tri-therapy for central pattern generator activation in paraplegic mice: Proof-of-concept of efficacy

Spinal cord injury (SCI) is a neurological condition, for which no cure exists, typically leading to an immediate and irreversible loss of sensory and voluntary motor functions accompanied by significant health problems. We conducted proof-of-concept experiments aimed at assessing efficacy upon oral administration of a novel combination therapy for central pattern generator (CPG) activation and corresponding locomotor movement generation in completely paraplegic animals. Co-administration orally (by gavage) of buspirone, levodopa and carbidopa was found to dose-dependently induce episodes of steady weight-bearing stepping in low-thoracic (Th9/10) spinal cord-transected (Tx) mice (with no other form of assistance or training). Robust hindlimb stepping with weight-bearing capabilities was induced with the tri-therapy but not with clinically relevant doses of these compounds administered separately. These results provide evidence suggesting that this drug combination may be ideally suited to constitute a first-in-class therapy (CPG activator) for locomotor activity induction in chronic SCI individuals, given that efficacy was shown using commercially available brain-permeable small molecules, already known as safe for the treatment of various neurological indications.     

Dynamic motion planning for the design of robotic gait rehabilitation

In this paper we examine a method to control the stepping motion of a paralyzed person suspended over a treadmill using a robot attached to the pelvis. A leg swing motion is created by moving the pelvis without contact with the legs. The problem is formulated as an optimal control problem for an underactuated articulated chain. The optimal control problem is converted into a discrete parameter optimization and an efficient gradient-based algorithm is used to solve it. Motion capture data from an unimpaired human subject is compared to the simulation results from the dynamic motion optimization. Our results suggest that it is feasible to drive repetitive stepping on a treadmill by a paralyzed person by assisting in torso movement alone. The optimized, pelvic motion strategies are comparable to "hip-hiking" gait strategies used by people with lower limb prostheses or hemiparesis. The resulting motions can be found at the web site http://ww.eng.uci.edu/-chwang/project/stepper/stepper.html.     

Landing from different heights: Biomechanical and neuromuscular strategies in trained gymnasts and untrained prepubescent girls

The purpose of this study was to examine the biomechanics of the lower limb, during landing in female prepubertal gymnasts and prepubertal untrained girls, aged 9–12 years. Ten healthy participants were included in each group and performed five landings from 20, 40, and 60 cm. Kinematics, ground reaction forces (GRF) and electromyogram (EMG) from the lateral gastrocnemius, tibialis anterior, and vastus lateralis are presented. Gymnasts had higher vertical GRF and shorter braking phase during landing. Compared to untrained girls, gymnasts exhibited for all examined drop heights more knee flexion before and at ground contact, but less knee flexion at maximum knee flexion position. Especially when increasing drop heights the gymnasts activated their examined muscles earlier, and generally they had higher pre- and post landing EMG amplitudes normalized to the peak EMG at 60 cm drop height. Furthermore, gymnasts had lower antagonist EMG for the tibialis anterior compared to untrained girls, especially when landing from higher heights. It is concluded that the landing strategy preferred by gymnasts is influenced by long-term and specialized training and induces a stiffer landing pattern. This could have implications in injury prevention, which requires further investigation.     

Exercise training exacerbates tourniquet ischemia-induced decreases in GLUT4 expression and muscle atrophy in rats

The current study determined the interactive effects of ischemia and exercise training on glycogen storage and GLUT4 expression in skeletal muscle. For the first experiment, an acute 1-h tourniquet ischemia was applied to one hindlimb of both the 1-week exercise-trained and untrained rats. The contralateral hindlimb served as control. For the second experiment, 1-h ischemia was applied daily for 1 week to both trained (5 h post-exercise) and untrained rats. GLUT4 mRNA was not affected by acute ischemia, but exercise training lowered GLUT4 mRNA in the acute ischemic muscle. GLUT4 protein levels were elevated by exercise training, but not in the acute ischemic muscle. Exercise training elevated muscle glycogen above untrained levels, but this increase was reversed by chronic ischemia. GLUT4 mRNA and protein levels were dramatically reduced by chronic ischemia, regardless of whether the animals were exercise-trained or not. Chronic ischemia significantly reduced plantaris muscle mass, with a greater decrease found in the exercise-trained rats. In conclusion, the exercise training effect on muscle GLUT4 protein expression was prevented by acute ischemia. Furthermore, chronic ischemia-induced muscle atrophy was exacerbated by exercise training. This result implicates that exercise training could be detrimental to skeletal muscle with severely impaired microcirculation.     

Locomotor activity in spinal cord-injured persons.

After a spinal cord injury (SCI) of the cat or rat, neuronal centers below the level of lesion exhibit plasticity that can be exploited by specific training paradigms. In individuals with complete or incomplete SCI, human spinal locomotor centers can be activated and modulated by locomotor training (facilitating stepping movements of the legs using body weight support on a treadmill to provide appropriate sensory cues). Individuals with incomplete SCI benefit from locomotor training such that they improve their ability to walk over ground. Load- or hip joint-related afferent input seems to be of crucial importance for both the generation of a locomotor pattern and the effectiveness of the training. However, it may be a critical combination of afferent signals that is needed to generate a locomotor pattern after severe SCI. Mobility of individuals after a SCI can be improved by taking advantage of the plasticity of the central nervous system and can be maintained with persistent locomotor activity. In the future, if regeneration approaches can successfully be applied in human SCI, even individuals with complete SCI may recover walking ability with locomotor training.     

Substrate alters forelimb to hindlimb peak force ratios in primates

It is often claimed that the walking gaits of primates are unusual because, unlike most other mammals, primates appear to have higher vertical peak ground reaction forces on their hindlimbs than on their forelimbs. Many researchers have argued that this pattern of ground reaction force distribution is part of a general adaptation to arboreal locomotion. This argument is frequently used to support models of primate locomotor evolution. Unfortunately, little is known about the force distribution patterns of primates walking on arboreal supports, nor do we completely understand the mechanisms that regulate weight distribution in primates. We collected vertical peak force data for seven species of primates walking quadrupedally on instrumented terrestrial and arboreal supports. Our results show that, when walking on arboreal vs. terrestrial substrates, primates generally have lower vertical peak forces on both limbs but the difference is most extreme for the forelimb. We found that force reduction occurs primarily by decreasing forelimb and, to a lesser extent, hindlimb stiffness. As a result, on arboreal supports, primates experience significantly greater functional differentiation of the forelimb and hindlimb than on the ground. These data support long-standing theories that arboreal locomotion was a critical factor in the differentiation of the forelimbs and hindlimbs in primates. This change in functional role of the forelimb may have played a critical role in the origin of primates and facilitated the evolution of more specialized locomotor behaviors.     

Transient decreases in forelimb gait and ground reaction forces following rotator cuff injury and repair in a rat model

Due to inadequate healing, surgical repairs of torn rotator cuff tendons often fail, limiting the recovery of upper extremity function. The rat is frequently used to study rotator cuff healing; however, there are few systems capable of quantifying forelimb function necessary to interpret the clinical significance of tissue level healing. We constructed a device to capture images, ground reaction forces and torques, as animals ambulated in a confined walkway, and used it to evaluate forelimb function in uninjured control and surgically injured/repaired animals. Ambulatory data were recorded before (D–1), and 3, 7, 14, 28 and 56 days after surgery. Speed as well as step width and length were determined by analyzing ventral images, and ground reaction forces were normalized to body weight. Speed averaged 22±6 cm/s and was not affected by repair or time. Step width and length of uninjured animals compared well to values measured with our previous system. Forelimbs were used primarily for braking (?1.6±1.5% vs +2.5±0.6%), bore less weight than hind limbs (49±5% vs 58±4%), and showed no differences between sides (49±5% vs 46±5%) for uninjured control animals. Step length and ground reaction forces of the repaired animals were significantly less than control initially (days 3, 7 and 14 post-surgery), but not by day 28. Our new device provided uninjured ambulatory data consistent with our previous system and available literature, and measured reductions in forelimb function consistent with the deficit expected by our surgical model.     

Development of a robotic walking simulator for gait rehabilitation]

Restoration of gait is a major concern of rehabilitation after stroke or spinal cord injury. Modern concepts of motor learning favour a task-specific repetitive approach, i.e. "whoever wants to learn to walk again must walk." However, the physical demands this places on the therapist, is a limiting factor in the clinical routine setting. This article describes a robotic walking simulator for gait training that enables wheelchair-bound subjects to freely carry out repetitive practicing of an individually adapted gait pattern under simulation of the manual guidance of an experienced therapist. The technical principle applied makes use of programmable footplates with permanent foot/machine contact in combination with compliance control. The solution chosen comprises a planar parallel-serial hybrid kinematic system with three degrees of freedom that moves the feet in the sagittal plane. Gait analysis while floor walking and stair climbing, clinical practicability and safety aspects were the basis for the design. A variable compliance control enables man-machine interaction, ranging from purely position controlled movement to full compliance during swing phase above a virtual ground profile. In full compliance mode the robotic walking simulator behaves like a haptic device. The concept presented offers new prospects for individualized gait rehabilitation.     

Fore-Aft Ground Force Adaptations to Induced Forelimb Lameness in Walking and Trotting Dogs

Animals alter their locomotor mechanics to adapt to a loss of limb function. To better understand their compensatory mechanisms, this study evaluated the changes in the fore-aft ground forces to forelimb lameness and tested the hypothesis that dogs unload the affected limb by producing a nose-up pitching moment via the exertion of a net-propulsive force when the lame limb is on the ground. Seven healthy Beagles walked and trotted at steady speed on an instrumented treadmill while horizontal force data were collected before and after a moderate lameness was induced. Peak, mean and summed braking and propulsive forces as well as the duration each force was exerted and the time to reach maximum force were evaluated for both the sound and the lame condition. Compared with the sound condition, a net-propulsive force was produced by the lame diagonal limbs due to a reduced braking force in the affected forelimb and an increased propulsive force in the contralateral hindlimb when the dogs walked and trotted. To regain pitch stability and ensure steady speed for a given locomotor cycle, the dogs produced a net-braking force when the sound diagonal limbs were on the ground by exerting greater braking forces in both limbs during walking and additionally reducing the propulsive force in the hindlimb during trotting. Consistent with the proposed mechanism, dogs maximize their double support phases when walking. Likely associated with the fore-aft force adaptations to lameness are changes in muscle recruitment that potentially result in short- and long-term effects on the limb and trunk muscles.     

Enhanced Motor Function by Training in Spinal Cord Contused Rats Following Radiation Therapy

Weight-bearing stepping, without supraspinal re-connectivity, can be attained by treadmill training in an animal whose spinal cord has been completely transected at the lower thoracic level. Repair of damaged tissue and of supraspinal connectivity/circuitry following spinal cord injury in rat can be achieved by specific cell elimination with radiation therapy of the lesion site delivered within a critical time window, 2-3 weeks postinjury. Here we examined the effects of training in the repaired spinal cord following clinical radiation therapy. Studies were performed in a severe rat spinal cord contusion injury model, one similar to fracture/crush injuries in humans; the injury was at the lower thoracic level and the training was a combined hindlimb standing and stepping protocol. Radiotherapy, in a similar manner to that reported previously, resulted in a significant level of tissue repair/preservation at the lesion site. Training in the irradiated group, as determined by limb kinematics tests, resulted in functional improvements that were significant for standing and stepping capacity, and yielded a significant direct correlation between standing and stepping performance. In contrast, the training in the unirradiated group resulted in no apparent beneficial effects, and yielded an inverse correlation between standing and stepping performance, e.g., subject with good standing showed poor stepping capacity. Further, without any training, a differential functional change was observed in the irradiated group; standing capacity was significantly inhibited while stepping showed a slight trend of improvement compared with the unirradiated group. These data suggest that following repair by radiation therapy the spinal circuitries which control posture and locomotor were modified, and that the beneficial functional modulation of these circuitries is use dependent. Further, for restoring beneficial motor function following radiotherapy, training seems to be crucial.     

Balance control in stepping down expected and unexpected level changes

Stepping down an elevation in ongoing gait is a common task that can cause falls when the level change is unexpected. The aim of this study was to compare expected and unexpected stepping down. We hypothesized that unexpected stepping would lead to loss of control over the movement and potentially falls due to buckling of the leading leg at landing. Ten male subjects repeatedly walked over a platform on which they stepped down an expected 10-cm height difference. In 5 out of 50 trials, the height difference was encountered unexpectedly early. Kinematics and ground reaction forces under both feet were measured during the stride in which the height difference was negotiated. Stepping down involved a substantial increase in forward horizontal and angular momenta (approximately 40 N s and 20 N ms). In expected stepping down, step length was significantly increased (17%), which allowed control of these forward horizontal and angular momenta immediately following landing. In unexpected stepping down, the time between expected ground contact and actual ground contact (110 ms) appeared too short to substantially adjust leg movement and increase step length. Although buckling of the leg did not occur, presumably due to its more vertical orientation at landing, momentum could not be sufficiently attenuated at landing, but a fall was prevented by a rapid step of the trailing limb. The lack of control of momentum might cause a fall, when the capacity to make such a rapid step falls short, as in the elderly, or when the height difference is larger.     

Mechanical, morphological and biochemical adaptations of bone and muscle to hindlimb suspension and exercise

The influences of weightbearing forces on the structural remodeling, matrix biochemistry, and mechanical characteristics of the rat tibia and femur and surrounding musculature were examined by means of a hindlimb suspension protocol and highly intensive treadmill running. Female, young adult, Sprague-Dawley rats were designated as either normal control, sedentary suspended, or exercise suspended rats. For 4 weeks, sedentary suspended rats were deprived of hindlimb-to-ground contact forces, while the exercise suspended rats experienced hindlimb ground reaction forces only during daily intensive treadmill training sessions. The suspension produced generalized atrophy of hindlimb skeletal muscles, with greater atrophy occurring in predominantly slow-twitch extensors and adductors, as compared with the mixed fiber-type extensors and flexors. Region-specific cortical thinning and endosteal resorption in tibial and femoral diaphyses occurred in conjunction with decrements in bone mechanical properties. Tibial and femoral regional remodeling was related to both the absence of cyclic bending strains due to normal weightbearing forces and the decrease in forces applied to bone by antigravity muscles. To a moderate extent, the superimposed strenuous running counteracted muscular atrophy during the suspension, particularly in the predominantly slow-twitch extensor and adductor muscles. The exercise did not, however, mitigate changes in bone mechanical properties and cross-sectional morphologies, and in some cases exacerbated the changes. Suspension with or without exercise did not alter the normal concentrations of collagen, phosphorus, and calcium in either tibia or femur.