Effect of eccentric muscle contractions on Golgi tendon organ responses to passive and active tension in the cat

The effects of biofeedback training for efficient activation of infraspinatus on proprioception and EMG activity during shoulder external rotation

The purpose of this study investigated which biofeedback (BF) training enables efficient activation of the infraspinatus muscle that affect joint position sense (JPS) and force sense (FS) of the shoulder joint. Twenty healthy males participated and performed three external rotation (ER) exercises under three randomly assigned training conditions: 1) non-biofeedback (NBF), 2) BF and 3) force biofeedback (FBF). Each exercise was performed at intervals of one week between training conditions. After performed the ER exercise under each training condition, the relative error (RE) was calculated at shoulder ER 45° and 80°, and then shoulder ER force were measured to determine the JPS error and FS error, respectively. Muscle activity of infraspinatus and posterior deltoid were measured and compared between training conditions. The RE of shoulder ER 45° and 80° were significantly lower under the FBF conditions than other training conditions (P < 0.05). The RE of shoulder ER force were also significantly lower under the FBF conditions compared to those under the other training conditions (P < 0.05). The activity of the infraspinatus muscle was significantly higher under the FBF conditions during all three ER exercises than other training conditions (p < 0.05). We suggest that BF trainings can be useful to improve the proprioception of shoulder joint as well as activation of infraspinatus muscle while performing the ER exercises.

Knee position sense and knee flexor neuromuscular function are similarly altered after two submaximal eccentric bouts

PURPOSE

This study examined eccentric-induced fatigue effects on knee flexor (KF) neuromuscular function and on knee position sense. This design was repeated across two experimental sessions performed 1 week apart to investigate potential repeated bout effects.

METHODS

Sixteen participants performed two submaximal bouts of KF unilateral eccentric contractions until reaching a 20% decrease in maximal voluntary isometric contraction force. Knee position sense was evaluated with position-matching tasks in seated and prone positions at 40° and 70° of knee flexion so that KF were either antagonistic or agonistic during the positioning movement. The twitch interpolation technique was used to assess KF neuromuscular fatigue. Perceived muscle soreness was also assessed. Measurements were performed before, immediately (POST) and 24 h after (POST24) each eccentric bout.

RESULTS

No repeated bout effect on neuromuscular function and proprioceptive parameters was observed. At POST, central and peripheral factors contributed to the force decrement as shown by significant decreases in voluntary activation level (- 3.8 ± 4.8%, p < 0.01) and potentiated doublet torque at 100 Hz (- 10 ± 15.8%, p < 0.01). At this time point, position-matching errors significantly increased by 1.7 ± 1.9° in seated position at 40° (p < 0.01). At POST24, in presence of muscle soreness (p < 0.05), although KF neuromuscular function had recovered, position-matching errors increased by 0.6 ± 2.6° in prone position at 40° (p < 0.01).

CONCLUSION

These results provide evidence that eccentric-induced position sense alterations may arise from central and/or peripheral mechanisms depending on the testing position.

Visual guidance can help with the use of a robotic exoskeleton during human walking

Walking is an important activity that supports the health-related quality of life, and for those who need assistance, robotic devices are available to help. Recent progress in wearable robots has identified the importance of customizing the assistance provided by the robot to the individual, resulting in robot adaptation to the human. However, current implementations minimize the role of human adaptation to the robot, for example, by the users modifying their movements based on the provided robot assistance. This study investigated the effect of visual feedback to guide the users in adapting their movements in response to wearable robot assistance. The visual feedback helped the users reduce their metabolic cost of walking without any changes in robot assistance in a given time. In a case with the initially metabolic expensive (IMExp) exoskeleton condition, both training methods helped reduce the metabolic cost of walking. The results suggest that visual feedback training is helpful to use the exoskeleton for various conditions. Without feedback, the training is helpful only for the IMExp exoskeleton condition. This result suggests visual feedback training can be useful to facilitate the use of non-personalized, generic assistance, where the assistance is not tuned for each user, in a relatively short time.

FATIGUE OF KNEE EXTENSOR MUSCLES DOES NOT ALTER KNEE JOINT POSITION SENSE

The aim of this study is to investigate the effect of knee extensors fatigue on joint position sense. Fifteen healthy participants, all males, with no history of previous musculoskeletal lesions were recruited. Evaluation of the knee joint position sense and the muscle fatigue protocol had been performed using an isokinetic dynamometer. Fatigue was considered when the maximum torque was reduced by 50%. The joint position sense was analyzed by the absolute error and the variable error. The paired t-test was used to compare the mean in pre and during muscle fatigue conditions. The level of significance was 5%. Absolute and variable errors were not significantly affected by muscle fatigue. Knee joint position sense does not seem to be affected by fatigue of knee joint extensors.

Measuring the force perception in knee flexor/ extensor muscles in patients with anterior cruciate ligament injury and healthy subjects

BACKGROUND

Force perception as a contributor to the neuromuscular control of the knee joint may be altered after anterior cruciate ligament (ACL) injury.

OBJECTIVE

This study aimed to compare the force perception accuracy in the knee joints of patients with ACL injury and healthy subjects.

METHODS

Twenty-six patients with ACL injury and 26 healthy subjects participated in this case-control study. Participants were asked to produce 50% of the maximum voluntary isometric contraction of the knee muscles as a target force and reproduce it in their limbs in flexion and extension directions.

RESULTS

There were significant interactions between group and condition as well as group, condition, and limb in the force perception error respectively (P< 0.05). The highest amount of error was seen in the contralateral limb of the ACL injury group when the reference force was produced in the injured limb (P< 0.05).

CONCLUSION

The findings revealed that the force perception accuracy in the knee flexor/extensor muscles of individuals with ACL injury is impaired. Moreover, error is most evident when the patient produces force in the injured limb and replicates it in the uninjured limb in both flexion and extension directions. Therefore, the rehabilitation programs should encompass neuromuscular training in both quadriceps and hamstrings after ACL injury.

Control of Mammalian Locomotion by Somatosensory Feedback

When animals walk overground, mechanical stimuli activate various receptors located in muscles, joints, and skin. Afferents from these mechanoreceptors project to neuronal networks controlling locomotion in the spinal cord and brain. The dynamic interactions between the control systems at different levels of the neuraxis ensure that locomotion adjusts to its environment and meets task demands. In this article, we describe and discuss the essential contribution of somatosensory feedback to locomotion. We start with a discussion of how biomechanical properties of the body affect somatosensory feedback. We follow with the different types of mechanoreceptors and somatosensory afferents and their activity during locomotion. We then describe central projections to locomotor networks and the modulation of somatosensory feedback during locomotion and its mechanisms. We then discuss experimental approaches and animal models used to investigate the control of locomotion by somatosensory feedback before providing an overview of the different functional roles of somatosensory feedback for locomotion. Lastly, we briefly describe the role of somatosensory feedback in the recovery of locomotion after neurological injury. We highlight the fact that somatosensory feedback is an essential component of a highly integrated system for locomotor control. © 2021 American Physiological Society. Compr Physiol 11:1-71, 2021.

Does Compression Sensory Axonopathy in the Proximal Tibia Contribute to Noncontact Anterior Cruciate Ligament Injury in a Causative Way?-A New Theory for the Injury Mechanism

Anterior cruciate ligament injury occurs when the ligament fibers are stretched, partially torn, or completely torn. The authors propose a new injury mechanism for non-contact anterior cruciate ligament injury of the knee. Accordingly, non-contact anterior cruciate ligament injury could not happen without the acute compression microinjury of the entrapped peripheral proprioceptive sensory axons of the proximal tibia. This would occur under an acute stress response when concomitant microcracks-fractures in the proximal tibia evolve due to the same excessive and repetitive compression forces. The primary damage may occur during eccentric contractions of the acceleration and deceleration moments of strenuous or unaccustomed fatiguing exercise bouts. This primary damage is suggested to be an acute compression/crush axonopathy of the proprioceptive sensory neurons in the proximal tibia. As a result, impaired proprioception could lead to injury of the anterior cruciate ligament as a secondary damage, which is suggested to occur during the deceleration phase. Elevated prostaglandin E2, nitric oxide and glutamate may have a critical neuro-modulatory role in the damage signaling in this dichotomous neuronal injury hypothesis that could lead to mechano-energetic failure, lesion and a cascade of inflammatory events. The presynaptic modulation of the primary sensory axons by the fatigued and microdamaged proprioceptive sensory fibers in the proximal tibia induces the activation of N-methyl-D-aspartate receptors in the dorsal horn of the spinal cord, through a process that could have long term relevance due to its contribution to synaptic plasticity. Luteinizing hormone, through interleukin-1β, stimulates the nerve growth factor-tropomyosin receptor kinase A axis in the ovarian cells and promotes tropomyosin receptor kinase A and nerve growth factor gene expression and prostaglandin E2 release. This luteinizing hormone induced mechanism could further elevate prostaglandin E2 in excess of the levels generated by osteocytes, due to mechanical stress during strenuous athletic moments in the pre-ovulatory phase. This may explain why non-contact anterior cruciate ligament injury is at least three-times more prevalent among female athletes.

Passive torque influences the Hoffmann reflex pathway during the loading and unloading phases of plantar flexor muscles stretching

This study investigated the influence of passive tension on Hoffmann reflex during the loading (muscle stretched by passive joint movement) and unloading phase (joint returned to initial position) of muscle stretching. The maximal H‐reflex amplitude (H_max) was recorded in soleus in 19 young adults during the loading and unloading phases of a passive 30° dorsiflexion, from 90° ankle angle (reference position). H_max was evoked at similar angles (protocol‐1) or similar passive torque (PT; protocol‐2) during the loading and unloading phases, or during two loading phases separated by a 5‐min stretch hold at 30° ankle dorsiflexion relative to the reference position (protocol‐3). Homosynaptic depression (HD) was assessed with paired H reflexes (0.5‐s interstimulus interval) during the loading and unloading phases (protocol‐4; n=13). In protocol‐1, PT was lesser and H_max greater during the unloading than the loading phase (p < 0.001). In protocol‐2, no difference in H_max was observed between phases. In protocol‐3, PT was lesser and H_max greater during the second than the first loading phase (p < 0.001). Changes in PT during in these three protocols were associated with those in H_max (r² ≥ 0.97). In protocol‐4, HD increased and decreased during the loading and unloading phases, respectively (p < 0.001), without differing between phases. Additional experiments (n=12) showed a similar modulation of H_max in gastrocnemius medialis during loading and unloading phases, while muscle fascicle length did not differ between phases. This study indicates that the H‐reflex modulation during muscle stretching relies in part on mechanisms associated with the PT developed by the muscle‐tendon unit.

Altered Position Sense after Submaximal Eccentric Exercise-inducing Central Fatigue

PURPOSE

The purpose of this study was to concomitantly investigate the acute and delayed effects of a submaximal eccentric-induced muscle fatigue on the position sense and the neuromuscular function of the right knee extensor muscles.

METHODS

Thirteen young and physically active participants performed a unilateral isokinetic eccentric exercise of their right lower limb until a decrease in maximal voluntary isometric contraction (MVIC) of 20% was reached. Neuromuscular (i.e., MVIC, voluntary activation (VA) level, and evoked contractile properties [DB100 and DB10]) and psychophysical evaluations (i.e., bilateral position-matching task, perceived muscle soreness, and perceived fatigue) were performed at four time points: before (PRE), immediately after (POST), 24 (POST24), and 48 (POST48) the exercise.

RESULTS

The acute 20% MVIC reduction (P < 0.001) was associated with both central (i.e., -13% VA decrease, P < 0.01) and peripheral (i.e., -18% and -42% reduction of DB100 and DB10, respectively, P < 0.001) fatigue. In the following days (POST24 and POST48), VA levels had recovered despite the presence of a persisting peripheral fatigue and delayed-onset muscle soreness. Knee position sense, as revealed by position errors, was significantly altered only at POST (P < 0.05) with participants overestimating the length of their knee extensor. Position errors and VA deficits were significantly correlated at POST (r = -0.60, P = 0.03). Position errors returned to nonsignificant control values in the following days.

CONCLUSION

The acute central fatigue induced by the eccentric exercise contributes to the position sense disturbances. Central fatigue might lead to alterations in the sensory structures responsible for the integration and the processing of position-related sensory inputs.

Variation in limb loading magnitude and timing in tetrapods

Comparative analyses of locomotion in tetrapods reveal two patterns of stride cycle variability. Tachymetabolic tetrapods (birds and mammals) have lower inter-cycle variation in stride duration than bradymetabolic tetrapods (amphibians, lizards, turtles and crocodilians). This pattern has been linked to the fact that birds and mammals share enlarged cerebella, relatively enlarged and heavily myelinated Ia afferents, and γ-motoneurons to their muscle spindles. Both tachymetabolic tetrapod lineages also possess an encapsulated Golgi tendon morphology, thought to provide more spatially precise information on muscle tension. The functional consequence of this derived Golgi tendon morphology has never been tested. We hypothesized that one advantage of precise information on muscle tension would be lower and more predictable limb bone stresses, achieved in tachymetabolic tetrapods by having less variable substrate reaction forces than bradymetabolic tetrapods. To test this hypothesis, we analyzed hindlimb substrate reaction forces during locomotion of 55 tetrapod species in a phylogenetic comparative framework. Variation in species means of limb loading magnitude and timing confirm that, for most of the variables analyzed, variance in hindlimb loading and timing is significantly lower in species with encapsulated versus unencapsulated Golgi tendon organs. These findings suggest that maintaining predictable limb loading provides a selective advantage for birds and mammals by allowing energy savings during locomotion, lower limb bone safety factors and quicker recovery from perturbations. The importance of variation in other biomechanical variables in explaining these patterns, such as posture, effective mechanical advantage and center-of-mass mechanics, remains to be clarified.

Force perception at the shoulder after a unilateral suprascapular nerve block

There are two key sources of information that can be used to match forces-the centrally generated sense of effort and afferent signals from mechanical receptors located in peripheral tissues. There is currently no consensus on which source of information is more important for matching forces. The corollary discharge hypothesis argues that subjects match forces using the centrally generated sense of effort. The purpose of this study was to investigate force matching at the shoulder before and after a suprascapular nerve block. The nerve block creates a sensory and muscle force mismatch between sides when matching loads. The torque matching accuracy did not change after the nerve block was administered. Directionally, the torque error was in the direction proposed by the corollary discharge hypothesis. However, the mismatch between deltoid EMG was substantially greater compared to the changes in the torque matching error after the block. The results support that sensory information is used during force matching tasks. However, since the nerve block also created a sensory disruption between sides, it is not clear how sensory information is reweighted following the nerve block and a role for sense of effort is still implicated.

Effect of change in passive stiffness following low-intensity eccentric hamstring exercise on peak torque angle

[Purpose] The purpose of this study was to investigate the acute effect of low-intensity eccentric hamstring exercise on peak torque angle, range of motion, and passive stiffness. [Participants and Methods] Fourteen healthy young adults exercised as follows: 1) Under low-intensity eccentric hamstring exercise condition, participants performed a stiff-leg deadlift using a 20-kg barbell, 2) Under control condition with participants seated. The peak torque angle during eccentric knee flexion, hip flexion and knee extension range of motion, passive torque, and passive stiffness were measured before and after two conditions in the dominant leg. [Results] The low-intensity stiff-leg deadlift significantly increased hip flexion and knee extension range of motion and significantly decreased passive stiffness. Although the low-intensity stiff-leg deadlift did not change the peak torque angle, the changes in passive torque and passive stiffness were negatively correlated with the change in peak torque angle. [Conclusion] These results suggest that low-intensity eccentric hamstring exercise enhances flexibility, and a decrease in passive torque and passive stiffness are negatively associated with producing the eccentric peak knee flexion torque at a shorter muscle length.

Gravitational torque partially accounts for proprioceptive acuity

Proprioception of the upper extremity is commonly measured using joint position sense tasks. Recent evidence suggests heightened position sense at elevation angles in the shoulder and elbow near 90° in the sagittal plane. The influence of external torque has been suggested to play a pivotal role in the heightened acuity in elevated positions due to increased moment arm with respect to gravitational vectors. We hypothesized that the addition of a buoyance vector in opposition to this gravitational vector would reduce the influence of torque on proprioceptive acuity, resulting in consistent position sense errors with respect to elevation angle. Joint position sense was measured using an apple iPod touch using a custom application. Participants elevated their arm to 50, 70 and 90° of elevation in the sagittal plane in the absence of visual feedback. Data were collected in three conditions, normal (control) and submerged and weighted. We found angular differences between control and submerged conditions, but not between control and weighted conditions. When the arm was elevated to 90° in the submerged condition, we found participants undershot the target position by approximately -0.5° with the addition of the buoyancy force vector. Participants without this buoyancy vector at the same target position consistently overshot the target by approximately 2.0°, which suggests that external torque may be more involved in the direction of proprioceptive errors more than the magnitude of the error as the magnitude of the difference was relatively small (2.5°).

The vestibulomyogenic balance response is elevated following high-intensity lengthening contractions of the lower limb

The purpose was to investigate whether exercise-induced muscle weakness of the plantar and dorsiflexors through high-intensity lengthening contractions increases the vestibulomyogenic balance response. Nine males (∼25 years) participated in three experimental testing days to evaluate the vestibular control of standing balance and neuromuscular function of the plantar and dorsiflexors pre- and post (30 min, and 1 and 7 days) high-intensity lengthening plantar and dorsiflexions. To evaluate the vestibular-evoked balance response, participants stood quietly on a force plate while exposed to continuous, random electrical vestibular stimulation (EVS) for two 90-s trials. Relationships between EVS-antero-posterior (AP) forces and EVS-medial gastrocnemius electromyography (EMG) were estimated in the frequency domain (i.e., coherence). Weakness of the right plantar and dorsiflexors were assessed using maximal voluntary contraction (MVC) torque. The lengthening contractions induced a 13 and 24% reduction in plantar and dorsiflexor MVC torque, respectively (p < 0.05) of the exercised leg, which did not recover by 1 day post. The EVS-EMG coherence increased over a range of frequencies up to 7 days post compared to pre-lengthening contractions. Conversely, EVS-AP forces coherence exhibited limited changes. The greater EVS-EMG coherence post exercise-induced muscle weakness may be a compensatory mechanism to maintain the whole-body vestibular-evoked balance response when muscle strength is reduced.

High-threshold motor unit firing reflects force recovery following a bout of damaging eccentric exercise

Exercise-induced muscle damage (EIMD) is associated with impaired muscle function and reduced neuromuscular recruitment. However, motor unit firing behaviour throughout the recovery period is unclear. EIMD impairment of maximal voluntary force (MVC) will, in part, be caused by reduced high-threshold motor unit firing, which will subsequently increase to recover MVC. Fourteen healthy active males completed a bout of eccentric exercise on the knee extensors, with measurements of MVC, rate of torque development and surface electromyography performed pre-exercise and 2, 3, 7 and 14 days post-exercise, on both damaged and control limb. EIMD was associated with decreased MVC (235.2 ± 49.3 Nm vs. 161.3 ± 52.5 Nm; p <0.001) and rate of torque development (495.7 ± 136.9 Nm.s-1 vs. 163.4 ± 163.7 Nm.s-1; p <0.001) 48h post-exercise. Mean motor unit firing rate was reduced (16.4 ± 2.2 Hz vs. 12.6 ± 1.7 Hz; p <0.01) in high-threshold motor units only, 48h post-exercise, and common drive was elevated (0.36 ± 0.027 vs. 0.56 ± 0.032; p< 0.001) 48h post-exercise. The firing rate of high-threshold motor units was reduced in parallel with impaired muscle function, whilst early recruited motor units remained unaltered. Common drive of motor units increased in offset to the firing rate impairment. These alterations correlated with the recovery of force decrement, but not of pain elevation. This study provides fresh insight into the central mechanisms associated with EIMD recovery, relative to muscle function. These findings may in turn lead to development of novel management and preventative procedures.

Proprioception in Patients With Anterior Cruciate Ligament Tears: A Meta-analysis Comparing Injured and Uninjured Limbs

BACKGROUND

Because of a reduction in the number of mechanoreceptors or alterations of their characteristics, anterior cruciate ligament (ACL) tears lead not only to mechanical instability but also to impaired proprioception. Purpose/Hypothesis: This study analyzed whether ACL tears cause a greater decrease in proprioception in injured than in uninjured knees. The hypothesis was that knee proprioception after ACL tears would decrease more in injured than in contralateral uninjured knees, regardless of the method used to measure knee proprioception.

STUDY DESIGN

Meta-analysis.

METHODS

We identified studies comparing proprioception in ACL-injured and contralateral intact knees using threshold for detection of passive motion (TTDPM) or joint position sense (JPS) tests. JPS was assessed by measuring the reproduction of passive positioning (RPP) or active repositioning (RAP) of the knee.

RESULTS

Sixteen studies were included in this meta-analysis. The pooled results of subgroup analyses of TTDPM for both 20° and 40° of knee flexion showed that mean angle of error was 0.23° (95% CI, 0.08°-0.37°) greater in ACL-injured than in contralateral intact knees ( P = .002). Pooled data RAP and RPP subgroup analyses also showed that the mean angle of error was 0.94° higher in ACL-injured than in contralateral intact knees. The mean difference in angle of error between ACL-injured and contralateral intact knees was 0.71° greater (95% CI, 0.68°-0.74°; P < .001) by JPS than by TTDPM.

CONCLUSION

Proprioception of ACL-injured knees was decreased compared with contralateral intact knees, as determined by both joint movement (kinesthesia) and joint position. The magnitude of loss of proprioception was greater in joint position than in joint movement.

Physical and balance performance following exercise induced muscle damage in male soccer players

[Purpose] The present study aimed to determine the changes in physical and balance performance following exercise-induced muscle damage using a sport-specific protocol. [Subjects and Methods] Fifteen collegiate soccer players were asked to perform a sport-specific sprint protocol to induce muscle damage. The markers of muscle damage (soreness, range of motion, limb girth, muscle strength, creatine kinase and lactate dehydrogenase), physical performance (speed, agility and power) and balance (static and dynamic balance) were assessed at baseline and 24, 48 and 72 hours following the sprint protocol. [Results] All variables, including the markers of muscle damage, physical performance and balance showed a significant difference when assessed at the 4 time points. [Conclusion] The study demonstrated that both the physical and balance performance were affected following repeated sprint protocol in soccer players. It is recommended the balance performance of an athlete be continually assessed following exercise-induced muscle damage so as to determine the appropriate return to sport decision thereby, minimizing the risk of further injury.

The relevance of stretch intensity and position-a systematic review

Stretching exercises to increase the range of motion (ROM) of joints have been used by sports coaches and medical professionals for improving performance and rehabilitation. The ability of connective and muscular tissues to change their architecture in response to stretching is important for their proper function, repair, and performance. Given the dearth of relevant data in the literature, this review examined two key elements of stretching: stretch intensity and stretch position; and their significance to ROM, delayed onset muscle soreness (DOMS), and inflammation in different populations. A search of three databases, Pub-Med, Google Scholar, and Cochrane Reviews, identified 152 articles, which were subsequently categorized into four groups: athletes (24), clinical (29), elderly (12), and general population (87). The use of different populations facilitated a wider examination of the stretching components and their effects. All 152 articles incorporated information regarding duration, frequency and stretch position, whereas only 79 referred to the intensity of stretching and 22 of these 79 studies were deemed high quality. It appears that the intensity of stretching is relatively under-researched, and the importance of body position and its influence on stretch intensity, is largely unknown. In conclusion, this review has highlighted areas for future research, including stretch intensity and position and their effect on musculo-tendinous tissue, in relation to the sensation of pain, delayed onset muscle soreness, inflammation, as well as muscle health and performance.

Proprioceptive deficit in patients with complete tearing of the anterior cruciate ligament

Objective

To investigate the existence of proprioceptive deficits between the injured limb and the uninjured (i.e. contralateral normal) limb, in individuals who suffered complete tearing of the anterior cruciate ligament (ACL), using a strength reproduction test.

Methods

Sixteen patients with complete tearing of the ACL participated in the study. A voluntary maximum isometric strength test was performed, with reproduction of the muscle strength in the limb with complete tearing of the ACL and the healthy contralateral limb, with the knee flexed at 60°. The meta-intensity was used for the procedure of 20% of the voluntary maximum isometric strength. The proprioceptive performance was determined by means of absolute error, variable error and constant error values.

Results

Significant differences were found between the control group and ACL group for the variables of absolute error (p = 0.05) and constant error (p = 0.01). No difference was found in relation to variable error (p = 0.83).

Conclusion

Our data corroborate the hypothesis that there is a proprioceptive deficit in subjects with complete tearing of the ACL in an injured limb, in comparison with the uninjured limb, during evaluation of the sense of strength. This deficit can be explained in terms of partial or total loss of the mechanoreceptors of the ACL.

Joint-position sense is altered by football pre-participation warm-up exercise and match induced fatigue

BACKGROUND

The demands to which football players are exposed during the match may augment the risk of injury by decreasing the sense of joint position. This study aimed to assess the effect of pre-participation warm-up and fatigue induced by an official football match on the knee-joint-position sense of football players.

METHODS

Fourteen semi-professional male football players (mean age: 25.9±4.6 years old) volunteered in this study. The main outcome measures were rate of perceived exertion and knee-joint-position sense assessed at rest, immediately after a standard warm-up (duration 25 min), and immediately after a competitive football match (90 minutes duration).

RESULTS

Perceived exertion increased significantly from rest to the other assessments (rest: 8.6±2.0; after warm-up: 12.1±2.1; after football match: 18.5±1.3; p<0.001). Compared to rest, absolute angular error decreased significantly after the warm-up (4.1°±2.2° vs. 2.0°±1.0°; p=0.0045). After the match, absolute angular error (8.7°±3.8°) increased significantly comparatively to both rest (p=0.001) and the end of warm-up (p<0.001). Relative error showed directional bias with an underestimation of the target position, which was higher after the football match compared to both rest (p<0.001) and after warm-up (p<0.001).

CONCLUSIONS

The results indicate that knee-joint-position sense acuity was increased by pre-participation warm-up exercise and was decreased by football match-induced fatigue.

CLINICAL RELEVANCE

Warm-up exercises could contribute to knee injury prevention, whereas the deleterious effect of match-induced fatigue on the sensorimotor system could ultimately contribute to knee instability and injury.

Effect of exhaustive incremental treadmill effort on force generation repeatability in biathletes

The authors examined how force generation repeatability changes as the result of incremental maximal test to volitional exhaustion by well-trained (VO2/kg [mL · kg(-1) · min(-1)] 62.55 ± 5.27) individuals. 13 young biathletes (18.9 ± 1.7 years) performed isometric maximum voluntary contraction (IMVC) and submaximal targeted (98N) pushes against the force transducers by arms: elbow extension (EE), elbow flexion (EF) and legs: knee extensions (KE) in pre- and posttest conditions after incremental exhaustive test performed on treadmill. IMVC did not differ significantly between pre and posttest conditions for upper and statistically decrease in lower extremities measurements (p <.01). The mean force of 10 submaximal targeted force productions (F(mean); N) is similar for pre- and posttest measurements. Standard deviation of F(mean) (Fsd; N) and coefficient variation (CV;%) decrease statistically in elbows flexion p <.02 but not extension. The reduction of force repetition accuracy in left knee extension was noticed (p <.01). The fatigue induced by incremental running test decreases a magnitude of force production variability in specifically trained muscle groups in biathletes.

A promising approach to effectively reduce cramp susceptibility in human muscles: a randomized, controlled clinical trial

Background

To investigate if the cramp threshold frequency (CTF) can be altered by electrical muscle stimulation in a shortened position.

Methods

A total of 15 healthy male sport students were randomly allocated to an intervention (IG, n = 10) and a non-treatment control group (CG, n = 5). Calf muscles of both legs in the IG were stimulated equally twice a week over 6 weeks. The protocol was 3×5 s on, 10 s off, 150 µs impulse width, 30 Hz above the individual CTF, and was at 85% of the maximal tolerated stimulation energy. One leg was stimulated in a shortened position, inducing muscle cramps (CT), while the opposite leg was fixated in a neutral position at the ankle, hindering muscle cramps (nCT). CTF tests were performed prior to the first and 96 h after the 6^th (3 w) and 12^th (6 w) training session.

Results

After 3 w, the CTF had significantly (p<0.001) increased in CT calves from 23.3±5.7 Hz to 33.3±6.9 Hz, while it remained unchanged in nCT (pre: 23.6±5.7 Hz, mid: 22.3±3.5 Hz) and in both legs of the CG (pre: 21.8±3.2 Hz, mid: 22.0±2.7 Hz). Only CT saw further insignificant increases in the CTF. The applied stimulation energy (mA² • µs) positively correlated with the effect on the CTF (r = 0.92; p<0.001).

Conclusions

The present study may be useful for developing new non-pharmacological strategies to reduce cramp susceptibility.

Trial Registry

German Clinical Trials Register DRKS00005312

Effects of isolated locomotor muscle fatigue on pacing and time trial performance

PURPOSE

Locomotor muscle fatigue impairs exercise performance during time to exhaustion tests. However, its effect on self-regulation of power output (pacing) is unknown. The primary aim of this study was to investigate the effects of locomotor muscle fatigue on pacing and time trial performance.

METHODS

Ten healthy recreationally active men completed a 15-min time trial on a cycle ergometer 30 min after undergoing an eccentric fatiguing protocol designed to induce a substantial strength loss in the knee extensor muscles without inducing significant metabolic stress. This fatigue condition was compared with a control condition, using a randomly counterbalanced AB/BA crossover design.

RESULTS

Total work completed during the 15-min cycling time trial was significantly reduced by 4.8 % in the fatigue condition compared with the control condition. This was caused by a significant reduction in power output. Rating of perceived exertion was significantly higher in the fatigue condition compared with the control condition only during the first 3 min of the time trial. Heart rate and vastus lateralis integrated electromyogram were not significantly different between the two conditions.

CONCLUSION

The results show that participants with fatigued locomotor muscles reduce their pace but do not change their pacing strategy. As a result, there was a significant reduction in time trial performance. As predicted by the psychobiological model of exercise performance, a slower pace may be a behavioral response to compensate for the significant increase in perception of effort induced by locomotor muscle fatigue.

The proprioceptive senses: their roles in signaling body shape, body position and movement, and muscle force

This is a review of the proprioceptive senses generated as a result of our own actions. They include the senses of position and movement of our limbs and trunk, the sense of effort, the sense of force, and the sense of heaviness. Receptors involved in proprioception are located in skin, muscles, and joints. Information about limb position and movement is not generated by individual receptors, but by populations of afferents. Afferent signals generated during a movement are processed to code for endpoint position of a limb. The afferent input is referred to a central body map to determine the location of the limbs in space. Experimental phantom limbs, produced by blocking peripheral nerves, have shown that motor areas in the brain are able to generate conscious sensations of limb displacement and movement in the absence of any sensory input. In the normal limb tendon organs and possibly also muscle spindles contribute to the senses of force and heaviness. Exercise can disturb proprioception, and this has implications for musculoskeletal injuries. Proprioceptive senses, particularly of limb position and movement, deteriorate with age and are associated with an increased risk of falls in the elderly. The more recent information available on proprioception has given a better understanding of the mechanisms underlying these senses as well as providing new insight into a range of clinical conditions.

Muscle coordination is habitual rather than optimal

When sharing load among multiple muscles, humans appear to select an optimal pattern of activation that minimizes costs such as the effort or variability of movement. How the nervous system achieves this behavior, however, is unknown. Here we show that contrary to predictions from optimal control theory, habitual muscle activation patterns are surprisingly robust to changes in limb biomechanics. We first developed a method to simulate joint forces in real time from electromyographic recordings of the wrist muscles. When the model was altered to simulate the effects of paralyzing a muscle, the subjects simply increased the recruitment of all muscles to accomplish the task, rather than recruiting only the useful muscles. When the model was altered to make the force output of one muscle unusually noisy, the subjects again persisted in recruiting all muscles rather than eliminating the noisy one. Such habitual coordination patterns were also unaffected by real modifications of biomechanics produced by selectively damaging a muscle without affecting sensory feedback. Subjects naturally use different patterns of muscle contraction to produce the same forces in different pronation-supination postures, but when the simulation was based on a posture different from the actual posture, the recruitment patterns tended to agree with the actual rather than the simulated posture. The results appear inconsistent with computation of motor programs by an optimal controller in the brain. Rather, the brain may learn and recall command programs that result in muscle coordination patterns generated by lower sensorimotor circuitry that are functionally "good-enough."

Neuromuscular recovery after medial collateral ligament disruption and eccentric rehabilitation program

PURPOSE

Medial collateral ligament (MCL) rupture of the knee joint frequently occurs during sport activities. However, the optimal rehabilitation strategy after such lesion is unknown. The aim of this study was to assess the effects of progressive eccentric rehabilitation program on neuromuscular deficits induced by MCL transection.

METHODS

Rats were randomized as follows: (i) control group (C, n = 10) without any surgery; (ii) lesion groups in which neuromuscular measurements were made 1 (L1, n = 10) and 3 wk (L3, n = 9) after MCL transection by a 15- to 20-min surgery (this group was designed to determine changes induced by the MCL transection); and (iii) eccentric group (ECC, n = 7) in which rats performed a progressive 2-wk eccentric rehabilitation program beginning 1 wk after MCL transection surgery. Dynamic functional assessments were performed at weeks 1 and 3 after the MCL transection by measuring the maximal and minimal knee angles during the stance phase of the gait cycle. Neuromuscular measurements included 1) modulation of H-reflex in response to a 10-mM KCl injection, 2) analysis of the twitch relaxation properties of the quadriceps muscle, and 3) recording of metabosensitive and mechanosensitive afferents activity in response to chemical injections and to tendon patellar vibrations, respectively.

RESULTS

Our results indicated that H-reflex modulation induced by metabosensitive afferents was disturbed by MCL transection without any recovery despite rehabilitation program. Responses of both metabosensitive and mechanosensitive muscle afferents, as well as the muscle relaxation properties, were fully recovered after the eccentric rehabilitation program.

CONCLUSIONS

Our results directly indicated an influence of progressive eccentric program on muscle afferents response after MCL section but apparently not for spinal reflex modulation.

Eccentric exercise-induced muscle damage dissociates the lactate and gas exchange thresholds

We tested the hypothesis that exercise-induced muscle damage would increase the ventilatory (V(E)) response to incremental/ramp cycle exercise (lower the gas exchange threshold) without altering the blood lactate profile, thereby dissociating the gas exchange and lactate thresholds. Ten physically active men completed maximal incremental cycle tests before (pre) and 48 h after (post) performing eccentric exercise comprising 100 squats. Pulmonary gas exchange was measured breath-by-breath and fingertip blood sampled at 1-min intervals for determination of blood lactate concentration. The gas exchange threshold occurred at a lower work rate (pre: 136 ± 27 W; post: 105 ± 19 W; P < 0.05) and oxygen uptake (VO(2)) (pre: 1.58 ± 0.26 litres · min(-1); post: 1.41 ± 0.14 litres · min(-1); P < 0.05) after eccentric exercise. However, the lactate threshold occurred at a similar work rate (pre: 161 ± 19 W; post: 158 ± 22 W; P > 0.05) and VO(2) (pre: 1.90 ± 0.20 litres · min(-1); post: 1.88 ± 0.15 litres · min(-1); P > 0.05) after eccentric exercise. These findings demonstrate that exercise-induced muscle damage dissociates the V(E) response to incremental/ramp exercise from the blood lactate response, indicating that V(E) may be controlled by additional or altered neurogenic stimuli following eccentric exercise. Thus, due consideration of prior eccentric exercise should be made when using the gas exchange threshold to provide a non-invasive estimation of the lactate threshold.

Neuromuscular dysfunction with the experimental arm acting as its own reference following eccentric and isometric exercise

Eccentric exercise has been extensively used as a model to study muscle damage-induced neuromuscular impairment, adopting mainly a bilateral matching task between the reference (unexercised) arm and the indicator (exercised) arm. However, little attention has been given to the muscle proprioceptive function when the exercised arm acts as its own reference. This study investigated muscle proprioception and motor control, with the arm acting both as reference and indicator, following eccentric exercise and compared them with those observed after isometric exercise. Fourteen young male volunteers were equally divided into two groups and performed an eccentric or isometric exercise protocol with the elbow flexors of the non-dominant arm on an isokinetic dynamometer. Both exercise protocols induced significant changes in indicators of muscle damage, that is, muscle soreness, range of motion and maximal isometric force post-exercise (p < 0.05-0.001), and neuromuscular function was similarly affected following both protocols. Perception of force was impaired over the 4-day post-exercise period (p < 0.001), with the applied force being systematically overestimated. Perception of joint position was significantly disturbed (i.e., target angle was underestimated) only at one elbow angle on day 4 post-exercise (p < 0.05). The misjudgements and disturbed motor output observed when the exercised arm acted as its own reference concur with the view that they could be a result of a mismatch between the central motor command and an impaired motor control after muscle damage.

The effect of fatigue from exercise on human limb position sense

We have previously shown, in a two-limb position-matching task in human subjects, that exercise of elbow flexors of one arm led the forearm to be perceived as more extended, while exercise of knee extensors of one leg led the lower leg to be perceived as more flexed. These findings led us to propose that exercise disturbs position sense because subjects perceive their exercised muscles as longer than they actually are. In order to obtain further support for this hypothesis, in the first experiment reported here, elbow extensors were exercised, with the prediction that the exercised arm would be perceived as more flexed after exercise. The experiment was carried out under three load conditions, with the exercised arm resting on a support, with it supporting its own weight and with it supporting a load of 10% of its voluntary contraction strength. For each condition, the forearm was perceived as more extended, not more flexed, after exercise. This result was confirmed in a second experiment on elbow flexors. Again, under all three conditions the exercised arm was perceived as more extended. To explore the distribution of the phenomenon, in a third experiment finger flexor muscles were exercised. This had no significant effect on position sense at the elbow. In a fourth experiment, position sense at the knee was measured after knee flexors of one leg were exercised and, as for knee extensors, it led subjects to perceive their exercised leg to be more flexed at the knee than it actually was. Putting all the observations together, it is concluded that while the influences responsible for the effects of exercise may have a peripheral origin, their effect on position sense occurs centrally, perhaps at the level of the sensorimotor cortex.

The consequences of short-range stiffness and fluctuating muscle activity for proprioception of postural joint rotations: the relevance to human standing

Proprioception comes from muscles and tendons. Tendon compliance, muscle stiffness, and fluctuating activity complicate transduction of joint rotation to a proprioceptive signal. These problems are acute in postural regulation because of tiny joint rotations and substantial short-range muscle stiffness. When studying locomotion or perturbed balance these problems are less applicable. We recently measured short-range stiffness in standing and considered the implications for load stability. Here, using an appropriately simplified model we analyze the conversion of joint rotation to spindle input and tendon tension while considering the effect of short-range stiffness, tendon compliance, fluctuating muscle activity, and fusimotor activity. Basic principles determine that when muscle stiffness and tendon compliance are high, fluctuating muscle activity is the greatest factor confounding registration of postural movements, such as ankle rotations during standing. Passive and isoactive muscle, uncomplicated by active length fluctuations, enable much better registration of joint rotation and require fewer spindles. Short-range muscle stiffness is a degrading factor for spindle input and enhancing factor for Golgi input. Constant fusimotor activity does not enhance spindle registration of postural joint rotations in actively modulated muscle: spindle input remains more strongly associated with muscle activity than joint rotation. A hypothesized rigid alpha-gamma linkage could remove this association with activity but would require large numbers of spindles in active postural muscles. Using microneurography, the existence of a rigid alpha-gamma linkage could be identified from the correlation between spindle output and muscle activity. Basic principles predict a proprioceptive "dead zone" in the active agonist muscle that is related to the short-range muscle stiffness.

Locomotor muscle fatigue increases cardiorespiratory responses and reduces performance during intense cycling exercise independently from metabolic stress

Locomotor muscle fatigue, defined as an exercise-induced reduction in maximal voluntary force, occurs during prolonged exercise, but its effects on cardiorespiratory responses and exercise performance are unknown. In this investigation, a significant reduction in locomotor muscle force (−18%, P < 0.05) was isolated from the metabolic stress usually associated with fatiguing exercise using a 100-drop-jumps protocol consisting of one jump every 20 s from a 40-cm-high platform. The effect of this treatment on time to exhaustion during high-intensity constant-power cycling was measured in study 1 ( n = 10). In study 2 ( n = 14), test duration (871 ± 280 s) was matched between fatigue and control condition (rest). In study 1, locomotor muscle fatigue caused a significant curtailment in time to exhaustion (636 ± 278 s) compared with control (750 ± 281 s) ( P = 0.003) and increased cardiac output. Breathing frequency was significantly higher in the fatigue condition in both studies despite similar oxygen consumption and blood lactate accumulation. In study 2, high-intensity cycling did not induce further fatigue to eccentrically-fatigued locomotor muscles. In both studies, there was a significant increase in heart rate in the fatigue condition, and perceived exertion was significantly increased in study 2 compared with control. These results suggest that locomotor muscle fatigue has a significant influence on cardiorespiratory responses and exercise performance during high-intensity cycling independently from metabolic stress. These effects seem to be mediated by the increased central motor command and perception of effort required to exercise with weaker locomotor muscles.

Shoulder joint position sense improves with external load

Joint position sense (JPS) is important in the maintenance of optimal movement coordination of limb segments in functional activities. Researchers have shown that the sensitivity of musculotendinous mechanoreceptors increases as muscle activation levels increase. In the present study, when 25 participants tried to replicate the same presented position, both vector and elevation angle repositioning errors decreased linearly as the external load increased up to 40% above unloaded shoulder torque. However, external load had no effect on plane repositioning error. The results indicated that JPS increased under conditions of increasing external load but only in the direction of the applied load. That finding indicates that JPS acuity improves as muscle activation levels increase.

A musculoskeletal model of low grade connective tissue inflammation in patients with thyroid associated ophthalmopathy (TAO): the WOMED concept of lateral tension and its general implications in disease

Background

Low level connective tissue inflammation has been proposed to play a role in thyroid associated ophthalmopathy (TAO). The aim of this study was to investigate this postulate by a musculoskeletal approach together with biochemical parameters.

Methods

13 patients with TAO and 16 controls were examined. Erythrocyte levels of Zn, Cu, Ca²⁺, Mg, and Fe were determined. The musculoskeletal evaluation included observational data on body posture with emphasis on the orbit-head region. The angular foot position in the frontal plane was quantified following gait observation. The axial orientation of the legs and feet was evaluated in an unloaded supine position. Functional propioceptive tests based on stretch stimuli were done by using foot inversion and foot rotation.

Results

Alterations in the control group included neck tilt in 3 cases, asymmetrical foot angle during gait in 2, and a reaction to foot inversion in 5 cases. TAO patients presented facial asymmetry with displaced eye fissure inclination (mean 9.1°) as well as tilted head-on-neck position (mean 5.7°). A further asymmetry feature was external rotation of the legs and feet (mean 27°). Both foot inversion as well as foot rotation induced a condition of neuromuscular deficit. This condition could be regulated by gentle acupressure either on the lateral abdomen or the lateral ankle at the acupuncture points gall bladder 26 or bladder 62, respectively. In 5 patients, foot rotation produced a phenomenon of moving toes in the contra lateral foot. In addition foot rotation was accompanied by an audible tendon snapping. Lower erythrocyte Zn levels and altered correlations between Ca²⁺, Mg, and Fe were found in TAO.

Conclusion

This whole body observational study has revealed axial deviations and body asymmetry as well as the phenomenon of moving toes in TAO. The most common finding was an arch-like displacement of the body, i.e. eccentric position, with foot inversion and head tilt to the contra lateral side and tendon snapping. We propose that eccentric muscle action over time can be the basis for a low grade inflammatory condition. The general implications of this model and its relations to Zn and Se will be discussed.

Nature et substratum neurologique du sens de l’effort

INTRODUCTION

What are the nature and the neural substrate of voluntary force perception?

STATE OF ART

Experimental findings demonstrate that efferent signals related to motor command play a dominant role in perceiving voluntary muscular force. This suggests that voluntary force perception is provided through a sense of effort and not through a sense of intramuscular tension. Nevertheless, experimental data show that the contribution of sensory input to effort awareness must not be dismissed. Sensory signals are not involved in generating a signal of effort but rather in calibrating and modulating its magnitude. Neuroimaging and neuropsychological studies revealed that many cortical structures are activated during tasks of voluntary muscular force perception.

PERSPECTIVES AND CONCLUSION

In such tasks, the basal ganglia might support the coherence of cortical activity.

Models to explain fatigue during prolonged endurance cycling

Much of the previous research into understanding fatigue during prolonged cycling has found that cycling performance may be limited by numerous physiological, biomechanical, environmental, mechanical and psychological factors. From over 2000 manuscripts addressing the topic of fatigue, a number of diverse cause-and-effect models have been developed. These include the following models: (i) cardiovascular/anaerobic; (ii) energy supply/energy depletion; (iii) neuromuscular fatigue; (iv) muscle trauma; (v) biomechanical; (vi) thermoregulatory; (vii) psychological/motivational; and (viii) central governor. More recently, however, a complex systems model of fatigue has been proposed, whereby these aforementioned linear models provide afferent feedback that is integrated by a central governor into our unconscious perception of fatigue. This review outlines the more conventional linear models of fatigue and addresses specifically how these may influence the development of fatigue during cycling. The review concludes by showing how these linear models of fatigue might be integrated into a more recently proposed nonlinear complex systems model of exercise-induced fatigue.

Shoulder joint position sense improves with elevation angle in a novel, unconstrained task

Proprioception, encompassing the submodalities of kinesthesia and joint position sense, is important in the maintenance of joint stability, especially in the shoulder. The purpose of this study was to examine the effects of plane and elevation angle on unconstrained shoulder joint position sense. Twenty-two subjects (12 male, 10 female) without a history of shoulder pathology were recruited from a university campus. Subjects attempted to replicate, with respect to plane and elevation angles, various target positions. Target positions consisted of five plane angles at 90 degrees of arm elevation and five arm elevation angles in the scapular plane. All target positions were tested twice to assess the reliability of the measurement. Intraclass correlation coefficients were generally low across target positions, possibly owing to the novelty and demanding nature of the task. No differences in repositioning errors were observed between plane angles (p = 0.255). Repositioning errors decreased linearly as the elevation angle increased from 30 degrees to 90 degrees (p = 0.007) and increased again from 90 degrees to 110 degrees of elevation (p = 0.029). Our results suggest that unconstrained joint position sense may be enhanced with increased muscular activation levels. Further, afferent feedback from musculotendinous mechanoreceptors may dominate over that from capsuloligamentous sources in unconstrained movements.

An index of spindle efficacy obtained by measuring electroneurographic activity and passive tension in the rat soleus muscle

While muscle spindle afferent discharges are known to change with altered muscle use, the way in which the changes in spindle discharge are affected by modifications to the elastic properties of the muscle-tendon unit remains to analyze. This paper describes a methodology to define, in the rat, a spindle efficacy index. This index relates the spindle afferent discharges recorded from electroneurograms (ENG) due to muscle stretch to the passive elastic properties of the muscle-tendon unit quantified during the stretch imposed for the ENGs recordings. The stretches were applied to the rat soleus muscle after the Achilles tendon was severed. The spindle afferent discharges were characterized from the root mean square (RMS) values of electroneurograms (ENGs) recorded from the soleus nerve. The first step of the study was to validate the definition of dynamic and static indices (DI and SI) of spindle discharges from RMS-ENG as classically done when isolated afferents are studied. The slopes of the DI-stretch velocity or SI-stretch amplitude relationships gave the indices of spindle sensitivity under dynamic and static conditions, respectively. Incremental stiffness was calculated to describe the passive elastic properties during the dynamic and static phases of ramp and hold stretches applied at different amplitudes and velocities. The spindle efficacy index (SEI) is the ratio between the indices of spindle sensitivity and incremental stiffness values. Both spindle discharges and incremental stiffness increased with stretch amplitude under dynamic and static conditions. The corresponding SEI values were constant whatever the stretch amplitude. This result validates the relationship between spindle discharges and passive incremental stiffness. This method can be proposed to study, in the rat, the spindle function when the muscles are suspected to present changes in their neuromechanical properties.

Effect of muscle fatigue on the sense of limb position and movement

We have recently shown that in an unsupported forearm-matching task blindfolded human subjects are able to achieve an accuracy of 2-3 degrees . If one arm was exercised to produce significant fatigue and the matching task was repeated, it led subjects to make position-matching errors. Here that result is confirmed using fatigue from a simple weight-lifting exercise. A 30% drop in maximum voluntary force after the exercise was accompanied by a significant matching error of 1.7 degrees in the direction of extension when the reference arm had been fatigued, and 1.9 degrees in the direction of flexion when the indicator arm had been fatigued. We also tested the effect of fatigue on a simple movement tracking task where the reference forearm was moved into extension at a range of speeds from 10 to 50 degrees s(-1). Fatigue was found not to significantly reduce the movement-tracking accuracy. In a second experiment, movement tracking was measured while one arm was vibrated. When it was the reference arm, the subject perceived the movement to be significantly faster (3.7 degrees s(-1)) than it actually was. When it was the indicator, it was perceived to be slower (4.6 degrees s(-1)). The data supports the view that muscle spindles are responsible for the sense of movement, and that this sense is not prone to the disturbance from fatigue. By contrast, the sense of position can be disturbed by muscle fatigue. It is postulated, that the sense of effort experienced by holding the arm against the force of gravity is able to provide information about the position in space of the limb and that the increased effort from fatigue produces positional errors.

Internal models of limb dynamics and the encoding of limb state

Studies of reaching suggest that humans adapt to novel arm dynamics by building internal models that transform planned sensory states of the limb, e.g., desired limb position and its derivatives, into motor commands, e.g., joint torques. Earlier work modeled this computation via a population of basis elements and used system identification techniques to estimate the tuning properties of the bases from the patterns of generalization. Here we hypothesized that the neural representation of planned sensory states in the internal model might resemble the signals from the peripheral sensors. These sensors normally encode the limb's actual sensory state in which movement errors occurred. We developed a set of equations based on properties of muscle spindles that estimated spindle discharge as a function of the limb's state during reaching and drawing of circles. We then implemented a simulation of a two-link arm that learned to move in various force fields using these spindle-like bases. The system produced a pattern of adaptation and generalization that accounted for a wide range of previously reported behavioral results. In particular, the bases showed gain-field interactions between encoding of limb position and velocity, very similar to the gain fields inferred from behavioral studies. The poor sensitivity of the bases to limb acceleration predicted behavioral results that were confirmed by experiment. We suggest that the internal model of limb dynamics is computed by the brain with neurons that encode the state of the limb in a manner similar to that expected of muscle spindle afferents.