Elbow Rotation Research Articles

Children With Elbow Extension Forearm Rotation Limitation: Functional Outcomes Using the Forearm Rotation Elbow Orthosis

In Brief A common upper limb deformity in a child with cerebral palsy or obstetric brachial plexus palsy is a pronation or a supination contracture of the forearm in association with a flexion contracture of the elbow. Management of the involved upper limb is critical in preventing further contractures and in optimizing overall function. Use of the forearm rotation elbow orthosis (FREO) improves hand-arm alignment and function by increasing active and passive joint range of motion (ROM). Furthermore, consistent night use of the FREO can eliminate the need for repeated serial casting of the upper limb, because ROM gains are maintained over time. The following case study describes the design and the use of the FREO with two children with the elbow and forearm contractures and presents preliminary data that illustrate the potential benefits of using this orthosis as an adjunct to current occupational therapy treatment. A common upper-limb deformity in children with cerebral palsy or obstetric brachial plexus palsy is a pronation or supination contracture of the forearm in association with a flexion contracture of the elbow. This case study presents preliminary data showing the potential benefits of the Forearm Rotation Elbow Orthosis (FREO) used as an adjunct to occupational therapy treatment. The FREO improves hand-arm alignment and function by increasing active and passive joint range of motion, and consistent night use can eliminate the need for repeated serial castings of the upper limb.

Résultat de la butée coracoïdienne type Latarjet pour instabilité antérieure chronique de l’épaule: À propos de 74 cas

Purpose of the study Results of the Latarjet procedure for chronic anterior shoulder instability using a coracoid block are known in terms of recurrence, but not in terms of apprehension. We studied a prospective consecutive series of shoulders treated with the Latarjet method in order to determine whether patients forget their shoulder or whether they are still bothered, particularly during sports activities. Material and methods The series included 74 patients with chronic anterior shoulder instability treated with a coracoid block according to the Latarjet procedure. Sixty-nine were available for review and 66 had complete radiographic explorations (93.2%). The Duplay score was used to classify sports activities. Mean age was 26.5 years; 90% of the patients practiced sports. The surgical procedure was performed by the same operator for 78% of patients. The Duplay score and the Constant score were determined and standard x-rays (four views) were obtained. Statistical analysis was performed with the chi-square test. Multivariate analysis was then applied to the subpopulation presenting persistent apprehension. Results Follow-up was at least 24 months, average follow-up 50 months. Four patients presented secondary dislocation and two subluxation; 85% of patients were satisfied, 6% were hesitant and 9% were dissatisfied. The Duplay score was: excellent (18.8%), good (49.9%), fair (20.2%) and poor (10.1%). External elbow rotation (RE1) was limited by 17.69̊ compared with the other side. Eighty-seven percent of patients resumed their sports activity five months postoperatively on average; 34% presented persistent apprehension. This subpopulation was examined separately. Multivariate analysis demonstrated two significantly independent factors of persistent apprehension: recovery of RE1 at 30 days postop, and total recovery of R1 at last follow-up. The radiographies demonstrated degenerative lesions in 10.6% of patients. Discussion The satisfaction rate of 85% and the 9% failure rate are similar to earlier reports. Our series was however exceptional in terms of an unusually high rate of persistent apprehension. Statistical analysis did not demonstrate a significant link between the presence of hyperlaxity and persistent apprehension. Nevertheless, in patients with hyperlaxity associated with chronic anterior shoulder instability, we associate Latarjet coracoid block with Neer capsuloplasty.

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Introduction: Varus posteromedial rotational elbow injury is a recently described and incompletely characterized pattern of traumatic elbow instability. Methods: A single surgeon participated in the treatment of sixteen patients with a varus posteromedial rotational injury pattern. All of the injuries were characterized by varus angulation of the elbow without dislocation and fracture of the anteromedial facet of the coronoid process. All but three patients had avulsion of the origin of lateral collateral ligament complex from the lateral epicondyle (two patients with olecranon fractures and one with a concomitant fracture of the base of the coronoid process did not have LCL injury).

Arthroscopic electrothermal shrinkage of chronic posterolateral elbow instability: Good or moderate outcome in 21 patients followed for an average of 2.5 years

Background The lateral collateral ligament complex is the key structure involved in recurrent elbow instability. Treatment is surgical, by repair or reconstruction of the lateral collateral ligament complex. We evaluated the effect of arthroscopic electrothermal shrinkage for treatment of chronic posterolateral rotator elbow instability.Methods 21 patients, median age 32 (24–50) years, suffering from chronic lateral elbow instability underwent arthroscopic electrothermal ligament shrinkage with a bipolar shrinkage probe. All patients were available for follow-up after median 30 (8–48) months.Results No complications were seen. The Morrey score increased from 40 to 77 points. The result was moderate (50–80 points) in 10 patients, and in the other patients a good result was achieved (80–95 points). The manual stress radiography showed a mean lateral joint opening of 13 (8–18) mm preoperatively. During follow-up, it decreased to 2 (1–4) mm.Interpretation Our findings suggest that arthroscopic bipolar ligament shrinkage is sufficient for the treatment of chronic posterolateral rotator elbow instability.

Intrinsic joint kinematic planning. II: Hand-path predictions based on a Listing’s plane constraint

This study was aimed at examining the assumption that three-dimensional (3D) hand movements follow specific paths that are dictated by the operation of a Listing's law constraint at the intrinsic joint level of the arm. A kinematic model was used to simulate hand paths during 3D point-to-point movements. The model was based on the assumption that the shoulder obeys a 2D Listing's constraint and that rotations are about fixed single-axes. The elbow rotations were assumed to relate linearly to those of the shoulder. Both joints were assumed to rotate without reversals, and to start and end rotating simultaneously with zero initial and final velocities. Model predictions were compared to experimental observations made on four right-handed individuals that moved toward virtual objects in "extended arm", "radial", and "frontal plane" movement types. The results showed that the model was partially successful in accounting for the observed behavior. Best hand-path predictions were obtained for extended arm movements followed by radial ones. Frontal plane movements resulted in the largest discrepancies between the predicted and the observed paths. During such movements, the upper arm rotation vectors did not obey Listing's law and this may explain the observed discrepancies. For other movement types, small deviations from the predicted paths were observed which could be explained by the fact that single-axis rotations were not followed even though the rotation vectors remained within Listing's plane. Dynamic factors associated with movement execution, which were not taken into account in our purely kinematic approach, could also explain some of these small discrepancies. In conclusion, a kinematic model based on Listing's law can describe an intrinsic joint strategy for the control of arm orientation during pointing and reaching movements, but only in conditions in which the movements closely obey the Listing's plane assumption.

Modeling 3D object manipulation: synchronous single-axis joint rotations?

In the present paper we introduce a movement planning model that is capable of predicting object manipulation movements in three dimensions. A basic assumption of this model is that the joint kinematics of the movement are optimized, which implies that joint rotations are synchronous. Synchronous joint rotations can be considered as a simplifying strategy to control arm movements, thus controlling the timing of several segments as a whole rather than for each joint separately. We will discuss evidence for synchronous joint rotations in 2D and explain why 3D synchrony is much more complex to substantiate. Different joint-angle representations and measures of asynchrony yield conflicting results. After showing that our model predicts realistic hand paths for various movement directions (the center-out task), we focus on a task that involves re-orientation of a hand-held cylinder, thus especially zooming in on those degrees of freedom not taken into account in 2D models. The more the cylinder needs to be rotated, the more curved the hand path is. With respect to 3D synchrony, a representation of shoulder and elbow rotations as single-axis rotations comes closest to synchronous joint rotations, which suggests that the brain plans a movement in joint space as a single postural transition.

Effect of slow, small movement on the vibration-evoked kinesthetic illusion

The study reported in this paper investigated how vibration-evoked illusions of joint rotation are influenced by slow (0.3 degrees /s), small (2-4 degrees ) passive rotation of the joint. Normal human adults (n=15) matched the perceived position of the left ("reference") arm with the right ("matching") arm while vibration (50 pps, 0.5 mm) was applied for 30 s to the relaxed triceps brachii of the reference arm. Both arms were constrained to rotate horizontally at the elbow. Three experimental conditions were investigated: (1) vibration of the stationary reference arm, (2) slow, small passive extension or flexion of the reference arm during vibration, and (3) slow, small passive extension or flexion of the reference arm without vibration. Triceps brachii vibration at 50 pps induced an illusion of elbow flexion. The movement illusion began after several seconds, relatively fast to begin with and gradually slowing down to a stop. On average, triceps vibration produced illusory motion at an average latency of 6.3 s, amplitude of 9.7 degrees , velocity of 0.6 degrees /s, and duration of 16.4 s. During vibration, slow, small ( approximately 0.3 degrees /s, 1.3 degrees ) passive rotations of the joint dramatically enhanced, stopped, or reversed the direction of illusory movement, depending on the direction of the passive joint rotation. However, the subjects' perceptions of these passive elbow rotations were exaggerated: 2-3 times the size of the actual movement. In the absence of vibration, the subjects accurately reproduced these passive joint rotations. We discuss whether the exaggerated perception of slow, small movement during vibration is better explained by contributions of non muscle spindle Ia afferents or by changes in the mechanical transmission of vibration to the receptor.

Disruptions in joint control during drawing arm movements in Parkinson’s disease

Impairments in control of multi-joint arm movements in Parkinson's Disease (PD) were investigated. The PD patients and age-matched elderly participants performed cyclical arm movements, tracking templates of a large circle and four differentially oriented ovals on a horizontal table. The wrist was immobilized and the movements were performed with shoulder and elbow rotations. The task was performed with and without vision at a cycling frequency of 1.5 Hz. Traces of the arm endpoint, joint-motion parameters represented by range of motion and relative phase, and joint-control characteristics represented by amplitude and timing of muscle torque were analyzed. The PD patients provided deformations of the template shapes that were not observed in movements of elderly controls. The deformations were consistent for each shape but differed across the shapes, making quantification of impairments in the endpoint movement difficult. In contrast, the characteristics of joint control and motion demonstrated systematic changes across all shapes in movements of PD patients, although some of these changes were observed only without vision. A specification of the PD influence was observed at the level of joint control and it was not distinguishable in joint and endpoint motion, because of the property of multi-joint movements during which control at each joint influences motion at the other joints. The results suggest that inability of PD patients to provide fine muscle torque regulation coordinated across the joints contributes to the altered endpoint trajectories during multi-joint movements. The study emphasizes the importance of the torque analysis when deficits in multi-joint movements are investigated, because specific impairments that can be detected in joint-control characteristics are difficult to trace in characteristics of joint and endpoint kinematics, because of interactions between joint motions.

Interaction between discrete and rhythmic movements: reaction time and phase of discrete movement initiation during oscillatory movements

This study investigates a task in which discrete and rhythmic movements are combined in a single-joint elbow rotation. Previous studies reported a tendency for the EMG burst associated with the discrete movement to occur around the expected burst associated with the rhythmic movement (e.g., [Exp. Brain Res. 99 (1994) 325; J. Neurol. Neurosurg. Psychiatry 40 (1977) 1129; Hum. Mov. Sci. 19 (2000) 627]). We document this interaction between discrete and rhythmic movements in different task variations and suggest a model consisting of rhythmic and discrete pattern generators that reproduces the major results. In the experiment, subjects performed single-joint elbow oscillatory movements (2 Hz). Upon a signal, they initiated a movement that consisted of a shift in the midpoint of the oscillation (MID), a shift in the amplitude of the oscillation (AMP), or a combination of both (MID + AMP). These shifting movements were performed either in a reaction time or in a self-paced fashion. The tendency for the EMG bursts associated with the discrete and rhythmic movements to synchronize was found similarly in all three tasks and instruction conditions, but the synchronization was most pronounced in the self-initiated discrete movement. Reaction time was increased for the combined task (MID + AMP), indicating higher control demands due to a combination of discrete and rhythmic components. This EMG burst synchronization was reproduced in a model based on a half-center oscillator with activation signals that produce either rhythmic or discrete activity. This activity was interpreted as torques driving a simple limb model. Summation of discrete and rhythmic activation signals of the pattern generators was sufficient to simulate the EMG burst synchronization. Further, simulation data reproduced the modulation of the reaction time as a function of the phase of the discrete movement.

Influence of biomechanical constraints on horizontal arm movements.

Influence of mechanical interactions between the shoulder and elbow on production of different coordination patterns during horizontal arm movements is investigated in the present study. Subjects performed cyclical movements along a circle and along lines of 4 different orientations. Cycling frequency was manipulated to highlight control features responsible for interactive torque regulation. When the shoulder was involved in motion, torque analysis revealed that this joint was controlled similarly during all movement types. At the elbow, however, each movement type required a specific pattern of regulation of interactive torque with muscle torque. When interactive torque acted in the direction of the required elbow rotation, the demands for active control were lower than when the interactive torque resisted elbow motion and had to be actively suppressed. Kinematic analysis demonstrated that increases in cycling frequency systematically deformed the fingertip path. The amount of these deformations differed across movement types, being more pronounced for movements where the interactive torque resisted joint motion. It appears that interactive torque can assist or resist movement at the joints, making control of some movement types more difficult than others.

Interaction of discrete and rhythmic movements over a wide range of periods.

This study investigates a complex task in which rhythmic and discrete components have to be combined in single-joint elbow rotations. While previous studies of similar tasks already reported that the initiation of the discrete movement is constrained to a particular phase window of the ongoing rhythmic movement, interpretations have remained contradictory due to differences in paradigms, oscillation frequencies, and data analysis techniques. The present study aims to clarify these findings and further elucidate the bidirectional nature of the interaction between discrete and rhythmic components. Participants performed single-degree-of-freedom elbow oscillatory movements at five prescribed periods (400, 500, 600, 800, 1,000 ms). They rapidly switched the midpoint of oscillation to a second target after an auditory signal that occurred at a random phase of the oscillation, without stopping the oscillation. Results confirmed that the phase of the discrete movement initiation is highly constrained with respect to the oscillation period. Further, the duration, peak velocity, and the overshoot of the discrete movement varied systematically with the period of the rhythmic movement. Effects of the discrete-onto-rhythmic component were seen in a phase resetting of the oscillation and a systematic acceleration after the discrete movement, which also varied as a function of the oscillation period. These results are interpreted in terms of an inhibitory bidirectional coupling between discrete and rhythmic movement. The interaction between discrete and rhythmic movement elements is discussed in comparison to sequential and gating processes suggested previously.

Contribution of primate magnocellular red nucleus to timing of hand preshaping during reaching to grasp.

Magnocellular red nucleus (RNm) is involved in controlling goal-directed limb movements such as reaching to grasp. We tested two hypotheses related to RNm's role in controlling reach-to-grasp movements. One hypothesis is that forelimb RNm neurons are grasp specific, and the other is that they specify the timing of metacarpi-phalangeal (MCP) extension to preshape the hand during the appropriate phase of the reach. We recorded single-unit discharge while monkeys performed two behavioral tasks that elicited similar reaches but differed in grasp. One task consisted of a reach with a precision grasp that elicited independent use of thumb and forefinger; the other included a whole-hand grasp that elicited concerted use of the four fingers. Most RNm neurons tested were engaged strongly during both the whole-hand and precision tasks, and the magnitude of discharge modulation did not differ between tasks. Thus most RNm neurons are not grasp specific but, instead, may contribute to behavioral features common to the two tasks. Two methods were used to investigate relations between single-unit discharge and kinematic data from the same individual trials of the whole-hand and precision tasks for a subset of forelimb RNm neurons. One method focused on correlations between parameters of RNm discharge and the duration, amplitude, and velocity of rotation of forelimb joints for each of the tasks. The second method compared between-task differences in times of peak neuronal discharge to between-task differences in times of rotations of forelimb joints. Parameters of reach-related RNm discharge were more frequently correlated with parameters of MCP extension than with parameters of rotation of wrist, elbow, and shoulder joints. Analyses of temporal relations between discharge and kinematic data during both the whole-hand and precision tasks indicate that discharge was time locked most frequently to MCP extension and, to a lesser extent, elbow extension during both tasks. We conclude that RNm may command muscle synergies that provide a basic preshape of the hand at the appropriate phase of limb transport. In addition, the timing of RNm's contribution to hand preshaping varies with the behavioral requirements of the task.

Initial muscle activity in planar ballistic arm movements with varying external force directions.

We have investigated, in fast movements, the hypothesis that bi-articular muscles are preferentially selected to control the direction of force exerted on the environment, while mono-articular muscles are selected to control both this exerted force direction as well as the movement direction. Fourteen subjects performed ballistic arm movements involving shoulder and elbow rotations in the horizontal plane, either with or without an external force applied at the wrist. Joint torques required to counteract the external force were in the same order of magnitude as those required to overcome the inertial load during movements. EMG was recorded from mono- and bi-articular flexors and extensors of the elbow and shoulder. Signals were rectified and integrated (IREMG) over 100 ms following the first detected activity. MANOVA revealed that, contrary to the hypothesis, IREMG of bi-articular muscles varied with movement direction just as that of the mono-articular muscles. It was concluded that the present data do not support the hypothesis mentioned above. A second finding was that movement effects on IREMG were much stronger than external force effects. This could not be explained using Hill's force-velocity relationship. It may be an indication that in the initiation of fast movements, IREMG is not only tuned to movement dynamics and muscle contractile properties, but also to the dynamics of the build up of an active state of the muscle.

Forearm Torque Strength and Endurance for Elbow and Forearm Angles

This study investigated the effect of elbow and forearm rotation angle on forearm torque strength and endurance. Twenty-four male subjects participated in the study. The results revealed that supination torques were stronger overall with a mean maximum value of 16.2 Nm recorded for the forearm 75% prone. Maximum pronation torque was recorded as 13.5 Nm for a neutral forearm with the elbow flexed 45°. Endurance times were longer for supination torques with the elbow flexed 135° and 90° but reduced below the times of pronation for the elbow fully extended. Supination torque endurance times were affected more by elbow and forearm angles than were pronation torque endurance times.

During slow wrist movements, distance covered affects EMG at a given external force.

This study investigates the hypothesis that EMG measured from a muscle at a given force, length, and low-shortening velocity depends on the contraction history, specifically the distance over which the muscle has shortened. Slow linear horizontal wrist movements (3 cm/s) involving shoulder and elbow rotations towards a test position of 90 degrees elbow flexion were performed. REMG was measured at the test position after wrist displacements over 6.5 and 13 cm. Muscle contraction speed was below 1% of maximum. A constant force (25 N) causing flexion torque in the elbow was exerted by the wrist. Inertial load was minimal. Two main elbow flexors (biceps caput longum and breve) showed significantly higher (14 and 24%) concentric REMG after 13-cm wrist movement than after 6.5-cm. Eccentric EMG did not differ between the 6.5- and 13-cm conditions. It is concluded that adaptation of muscle activation is required to counteract the effects of contraction history on the force producing capacity of the muscle.

The anterolateral intertubercular approach to the shoulder with osteotomy of the lesser tubercle of the humerus

The authors propose an original route of approach to the shoulder that allows direct access to this articulation. Thanks to this route, complex fractures of the proximal end of the humerus can be treated by ostosynthesis or prosthesis, and shoulder arthroses, whether centered or not by prosthesis. Two technical methods are used: first, the creation of an anterior digastric trapezio-deltoid muscle flap, and then, in cases of elective prosthetic surgery, osteotomy of the lesser tubercle to open the articulation and provide direct access to the humeral head and the glenoid. The patient is installed in the semiseated position, with the apex of the shoulder projecting widely from the operating table. The incision is anterolateral, in the direction of the fibers of the deltoid and measures 8–10 cm from the anterolateral angle of the acromion (ALAA), which constitutes a convenient surgical landmark. One third of the incision is proximal; the other two-thirds are distal. The trapezio-deltoid digastric muscle flap is created: the deltoid is divided in the direction of its fibers between the anterior and middle bundles, straddling the ALAA. The acromial periosteum is incised vertically. The incision is extended upwards into the trapezius. The digastric muscle flap thus created is reflected forward together with the coracoacromial ligament. An acromioplasty can be performed. This approach by itself allows access to the upper end of the humerus in complex fractures and allows osteosynthesis if called for. In cases of elective prosthetic surgery, and if the rotator cuff is intact, complete access to the articulation is obtained by osteotomy of the lesser tubercle in a plane parallel to its humeral base. This allows reflection of the subscapularis muscle with the tubercular fragment and opening of the articulation. Retropulsion of the elbow and lateral rotation displays the humeral head, which is osteotomized, and in this way access to the glenoid is immediate. The different stages of prosthetic surgery can then be performed. Closure is made most simply by reattachment of the lesser tubercle with a stout transosseous suture. The trapezio-deltoid digastric flap is closed by interrupted sutures without tension.

Long-term loading and regional bone mass of the arm in female volleyball players.

In the present study, we compared the bone mineral content (BMC) and bone mineral density (BMD) in the arms of 11 female volleyball players (mean age 22.0 +/- 2.6 years) training for about 8 hours/week, and 11 nonactive females aged 24.6 +/- 3.1 years (mean +/- SD) not participating in regular or organized sport activity. Using dual X-ray absorptiometry (DXA), BMC was measured in the proximal and distal humerus, and BMD in the distal radius. Isokinetic concentric peak torque (highest value attained during 5 or 10 repetitions) of the rotator muscles of the shoulder and flexor and extensor muscles of the elbow were measured using an isokinetic dynamometer. The volleyball players had significantly higher BMC (P < 0.05) at the proximal humerus of the dominant arm compared with the nonactive group, but there were no differences between the groups in BMC of the distal humerus and BMD of the distal radius. In the volleyball players, BMC was significantly higher at the proximal humerus, at the distal humerus, and at the distal radius in the dominant compared with the nondominant arm. In the nonactive group, there were no significant differences in BMC and BMD between the dominant and nondominant arm at any site measured. Except for shoulder internal rotation strength and elbow flexion strength at 90 degrees/second that was higher in the dominant arm in the volleyball players, there were no significant differences in muscle strength of the rotator muscles of the shoulder and flexor and extensor muscles of the elbow between the dominant and nondominant arm in the volleyball players and nonactive controls. In the volleyball players, but not in the nonactive controls, there were several significant relationships between shoulder and elbow strength and BMC at the distal humerus of the dominant and especially the nondominant arm. These results show that young female volleyball players have a higher bone mass in the proximal humerus, distal humerus, and distal radius in the dominant compared with the nondominant arm, and a higher bone mass in the proximal humerus compared with nonactive controls. Muscle strength of the rotator muscles of the shoulder is not related to the higher bone mass in the proximal humerus of the dominant arm. Theoretically, the observed differences in bone mass can be related to the type of loading the skeleton undergoes when playing volleyball.

Proprioceptive coordination of discrete movement sequences: mechanism and generality

A "discrete" movement sequence is defined as a movement with a single goal that involves a series of overlapping joint rotations. Reaching-and-grasping and throwing are examples of discrete movement sequences. The central nervous system (CNS) can use reafferent proprioceptive information from one joint rotation in a sequence to coordinate subsequent rotations at other joints. The experiments reported in this paper demonstrate how the human CNS uses proprioceptive information to coordinate discrete movement sequences. We examined the mechanism (at an information processing level) underlying proprioceptive coordination and the generality (i.e., the boundary conditions) of these mechanisms as they apply to everyday movement sequences. Adult human subjects performed a discrete movement sequence that resembles backhand throwing: elbow extension followed by hand opening. The task was to open the hand as the elbow passed through a prescribed "target" angle. We eliminated visual information and made the arrival time at the target angle unpredictable so that the available kinematic information was provided exclusively by proprioception. The subjects were capable of performing this motor task with a high degree of precision, thereby demonstrating that the nervous system can use proprioceptive input to coordinate discrete movement sequences. Our data indicate that precise coordination is achieved by extracting kinematic information related to both the velocity of elbow rotation as well as the elbow position during movement (i.e., "dynamic position"). Dynamic position information appears to be encoded as both absolute joint angle and angular distance, although more precisely as angular distance. Although our experiments were conducted under rather restrictive laboratory conditions, this mechanism of motor coordination might also apply to everyday movement. Our results suggest that this mechanism could be employed for passive as well as active movement sequences, with and without opposing loads; it could exert its influence in discrete movement sequences as brief as 210 ms or as long as 1.5 s; and it does not involve any significant degree of learning (this proprioceptive mechanism appears to be readily available for use on the first attempt of a novel motor task).

Two components of muscle activation: scaling with the speed of arm movement.

1. The temporal waveform of muscle activity was related to the speed of arm movement. Speed was expressed in terms of the duration of a fixed amplitude movement or the "movement time." 2. Human subjects moved their arms to targets in three-dimensional space. The right arm started at a standard initial position and moved directly to the target in a single stroke. The targets were placed in various directions in a vertical plane. The arm movements consisted of shoulder and elbow rotations. 3. Subjects were required to vary the speed of their movements. In most of the experiments, trials with different movement times were randomly ordered. One of the experiments also included randomly interspersed static trials, in which the subject held the arm still at the initial posture, the final posture, or halfway between the two extremes. 4. Electromyographic (EMG) activity was recorded from several superficial elbow and/or shoulder muscles. The time base of rectified EMG records was normalized for movement time such that records from movements with various speeds were compressed to align the ending times of the movements. 5. A principal component (PC) analysis revealed that the compressed EMG waveforms could be described by a summation of PC1 and PC2 waveforms; each individual EMG waveform was approximated by a weighted sum of these two components. 6. The PC1 weighting coefficients scaled down in a monotonic relationship with movement time such that the fastest movement corresponded to a large positive weighting coefficient and the slowest movement corresponded to a small positive weighting coefficient. The PC2 weighting coefficients exhibited a similar monotonic scaling, but the values ranged from positive to negative. Further analysis demonstrated that these two components can be mathematically transformed into a tonic waveform with a constant mathematically transformed into a tonic waveform with a constant weighting coefficient and a phasic waveform with positive weighting coefficients that scale down with movement time. 7. The amplitude scaling of EMG records cannot be described by a single component, but instead requires a summation of two separate components. The tonic component may correspond to the force element needed to counteract gravity, because the magnitude of this element does not scale with movement speed. The phasic component may correspond to the force element that scales quadratically to produce a linear increase in velocity.

Time-dependent effects of kinesthetic input.

Sensory input can be used by the nervous system to control the spatial parameters of motor responses (e.g., distance, velocity, and direction) by initializing these parameters before movement onset and then by adjusting these parameters during movement. Sensory input can also be used to trigger movements. In the experiments reported in this paper, we compared the effects of kinesthetic input on a triggered motor response when the kinesthetic input was generated at different times relative to the onset of the motor response. Human subjects responded to a visual stimulus by intentionally increasing elbow torque to a target level. Kinesthetic input was generated by unexpectedly rotating each subject's elbow 100 ms before the onset of the intentional torque response (early) or coincident with the onset of the intentional torque response (late). The effect of early kinesthetic input on the intentional torque response markedly differed from the effect of late kinesthetic input. The effect of early kinesthetic input was relatively independent of the direction of elbow rotation, had a different dependence on the amplitude of rotation, and required a shorter duration of rotation compared to the effect of late kinesthetic input. These differences in the effects of early and late kinesthetic input might be related to the initialization, triggering, and adjustment of motor responses.