J. Hore

Disorders in Timing and Force of Finger Opening in Overarm Throws Made by Cerebellar Subjects

Abstract: Although there is agreement that an important sign of cerebellar dysfunction is disorder in timing of movement, it appears that authors who study different behaviors mean different things when they use the term “timing,” and that the underlying mechanisms are likely to be different. For overarm throwing, skilled throwers can time ball release with a precision of less than 7 ms, whereas cerebellar subjects show a large variability of 50 ms or more in this timing. Furthermore, cerebellar patients show a larger variability in the amplitude of finger opening which could either reflect a disorder in force, or result indirectly from the increased variability in timing. To determine whether timing and force of finger opening were dependent variables, the time of ball release was plotted against the amplitude of finger opening. In control subjects these two parameters were related, with early (mistimed) throws having smaller finger amplitudes. However, in cerebellar subjects the increased variability in finger amplitude could not be accounted for by the increased variability in timing. Similarly, the increased timing windows could not be explained by disorder in force at the fingers. It is concluded that the abnormal finger opening that occurs in cerebellar patients when making overarm throws results from increased variability in both the timing and force of finger extension. Whether the increased variability in timing is a disorder in triggering finger opening at the right moment, or is due to a failure to combine finger opening appropriately with the hand trajectory, remains to be determined.

Arm movement performance during reversible basal ganglia lesions in the monkey

SummaryArm motor performance of eight Cebus monkeys was examined during reversible cooling in the ventral lateral region of the putamen and globus pallidus (primarily the external segment), where neurons discharging during arm movements have been found (DeLong 1972).When attempting to hold a handle stationary during basal ganglia cooling, all monkeys developed flexion at the wrist and some developed a slow flexion drift of the arm at the elbow. The prominence of wrist flexion emphasizes that the basal ganglia may normally influence distal musculature.During basal ganglia cooling an increase in segmental stretch reflexes (15–30 ms) was sometimes observed following arm perturbations, but no consistent increase occurred in the later EMG responses (30–95 ms) in contrast to results obtained in Parkinsonian patients (Tatton and Lee 1975).No major changes were observed in the time of onset of the earliest EMG activity in the agonist muscle in a simple reaction time elbow movement task during basal ganglia cooling.Basal ganglia lesions produced major disorders in both flexion and extension movements including slowing of movements and rebound of the arm towards its initial position after onset of movement. These disorders were accompanied by an increase in tonic activity of both flexors and extensors while holding and by increased levels of cocontraction of agonists and antagonists during attempted movements.It is suggested that this basal ganglia disorder is due to a failure to achieve the correct balance of activity between agonists and antagonists that is appropriate for a particular motor act.

Timing of finger opening and ball release in fast and accurate overarm throws

How precisely does the CNS control the timing of finger muscle contractions in skilled movements? For overarm throwing, it has been calculated that a ball release window of less than 1 ms is needed for accuracy in long throws. The objective was to investigate the timing precision of ball release and finger opening for 100 overarm throws made using only the arm. Subjects sat with a fixed trunk and threw balls fast and accurately at a 6-cm-square target when it was 1.5, 3.0 and 4.5 m away. Three-dimensional angular positions in space of the clavicle, upper arm, forearm, hand and distal phalanx of the middle finger were simultaneously recorded at 1000 Hz using the magnetic-field search-coil technique. Ball release was determined by pressure-sensitive microswitches on the proximal and distal phalanges of the middle finger (proximal and distal triggers). Variability of ball release, defined in terms of the standard deviation (SD) of the means of release times, was different when synchronized to different hand kinematic parameters. It was highest to the start of movement (when the hand started rotating vertically forward and up around a space-fixed horizontal axis) and was lowest when synchronized to the moment near ball release when the hand was vertical. These values did not depend on target distance. When throws were synchronized to vertical hand position, and SDs were averaged across the 10 subjects, the average interval for 95% of the throws (4×SD) was 9.6 ms for ball release and 10.0 ms for onset of finger opening. Thus, two independent measures of timing precision gave similar results. It is concluded that for 100 fast and accurate throws made by male recreational ball players, timing of finger opening and ball release was controlled precisely but not to fractions of a millisecond.

Finger opening in an overarm throw is not triggered by proprioceptive feedback from elbow extension or wrist flexion.

Abstract Accuracy in an overarm throw requires great precision in the timing of finger opening. We tested the hypothesis that finger opening in an overarm throw is triggered by proprioceptive feedback from elbow extension or wrist flexion. The hypothesis was tested in two ways: first, by unexpectedly perturbing elbow extension or slowing wrist flexion and determining whether changes occurred in finger opening, and second, by measuring the latency from the start of these joint rotations to the start of finger opening. Subjects threw balls fast and accurately from a sitting or standing position while joint rotations were recorded with the search-coil technique. Elbow extension was unexpectedly blocked near the start of forward motion of the hand by a rope attached to the wrist that passed through a catch mechanism located behind the subject. In spite of a slowing or complete block of elbow extension, and in some cases a replacement of elbow extension by elbow flexion, finger opening always occurred and at the same latency as for normal throws. Wrist flexion was slowed in seven of eight subjects when subjects changed from throwing with a light ball (14 g, 70 mm diam.) to a heavy ball (210 g, 65 mm diam.). For the first throw with the heavy ball, this slowing was neither fully anticipated by the subject nor compensated for by the changed proprioceptive feedback associated with the slowing. Consequently, the timing of finger opening was unchanged and (to the surprise of the thrower) the ball went high. Furthermore, in unperturbed throws with tennis balls, the latency from onset of wrist flexion or elbow extension to onset of finger opening was too short for either to have triggered finger opening (across subjects means were 4 ms for wrist flexion and 21 ms for elbow extension). In additional analysis, no relation was found between the time of onset of earlier occurring rotations at the shoulder and the time of onset of finger opening. We concluded that, although a role for all proprioceptive feedback in triggering finger opening cannot be disproved by these experiments, it can be ruled out for feedback arising from elbow extension and wrist flexion, and it seems unlikely for feedback arising from events occurring very early in the throw. The more likely possibility is that finger opening in an overarm throw is triggered by a central command based on an internal model of hand trajectory.

Cerebellar saccadic dysmetria is not equal in the two eyes

SummaryDifferences in the saccadic dysmetria between the two eyes were examined in five trained monkeys. Dysmetria, produced by reversible lesions of the medial cerebellar nuclei on the left side, was qualitatively similar in the two eyes in that both eyes were hypometric or were hypermetric. Quantitatively, however, the dysmetria was different. The right eye made larger amplitude and higher velocity saccades to the right and smaller amplitude and lower velocity saccades to the left than the left eye. Differences in eye position, during a saccade trajectory, were larger than those 50 ms after saccade termination. The differences varied as a function of saccade initial position with the largest differences occurring when the saccade was directed away from the center. The results suggest that the cerebellum can compensate for differences in muscle strength by selectively adjusting the strength of the pulse-step innervation to each muscle of a yoked muscle pair. Thus, Herings law of equal innervation would appear to be a consequence of appropriate cerebellar compensation and not an invariant property of synaptic connections.

Comparison of kinematics in skilled and unskilled arms of the same recreational baseball players

Abstract We examined mechanisms of coordination that enable skilled recreational baseball players to make fast overarm throws with their skilled arm and which are absent or rudimentary in their unskilled arm. Arm segment angular kinematics in three dimensions at 1000 Hz were recorded with the search-coil technique from the arms of eight individuals who on one occasion threw with their skilled right arm and on another with their unskilled left arm. Compared with their unskilled arm, the skilled arm had: a larger angular deceleration of the upper arm in space in the forward horizontal direction; a larger shoulder internal rotation velocity at ball release (unskilled arms had a negative velocity); a period of elbow extension deceleration before ball release; and an increase in wrist velocity with an increase in ball speed. It is suggested that some of these differences in arm kinematics occur because of differences between the skilled and unskilled arms in their ability to control interaction torques (the passive torque at one joint due to motion at adjacent joints). It is proposed that one reason unskilled individuals cannot throw fast is that, unlike their skilled counterparts, they have not developed the coordination mechanisms to effectively exploit interaction torques.

Skilled throwers use physics to time ball release to the nearest millisecond

Skilled throwers achieve accuracy in overarm throwing by releasing the ball on the handpath with a timing precision as low as 1 ms. It is generally believed that this remarkable ability results from a precisely timed command from the brain that opens the fingers. Alternatively, precise timing of ball release could result from a backforce from the ball that pushes the fingers open. The objective was to test these hypotheses in skilled throwers. Angular positions of the hand and phalanges of the middle finger were recorded with the search-coil technique. In support of the backforce hypothesis, we found that when subjects made a throwing motion without a ball in the hand (i.e., without a backforce), they could not open the fingers rapidly, and they had lost the ability to time finger opening in the 1- to 2-ms range. In addition, relationships were found between the magnitude and timing of hand angular acceleration and finger (joint) extension acceleration. The results indicate that although a central command produced initial hand opening, precise timing of ball release came from a mechanism involving Newtonian mechanics, i.e., hand acceleration produced a backforce from the ball on the fingers that pushed the fingers open. In this mechanism, given the appropriate finger force/stiffness, correction for errors in hand acceleration occurs automatically because hand motion causes finger motion. We propose that skilled throwers achieve ball accuracy by computing finger force/stiffness based on state estimation of hand acceleration and that ball inaccuracy occurs when this computation is imprecise.

Deliberate utilization of interaction torques brakes elbow extension in a fast throwing motion

We tested the hypothesis that in fast arm movements the CNS deliberately utilizes interaction torques to decelerate (brake) joint rotations. Twelve subjects performed fast 2-D overarm throws in which large elbow extension velocities occurred. Joint motions were computed from recordings made with search coils; joint torques were calculated using inverse dynamics. After ball release, a large follow-through shoulder extension acceleration occurred that was initiated by shoulder extensor muscle torque. This shoulder acceleration produced a flexor interaction torque at the elbow that initiated elbow deceleration (braking). An instantaneous mechanical interaction of passive torques then occurred between elbow and shoulder, i.e., elbow extension deceleration produced a large shoulder extensor interaction torque that contributed to the shoulder extension acceleration which, simultaneously, produced a large elbow flexor interaction torque that contributed to elbow extension deceleration, and so on. Late elbow flexor muscle torque also contributed to elbow deceleration. The interaction of passive torques between shoulder and elbow was braked by shoulder flexor muscle torque. In this mechanism, shoulder musculature contributed to braking elbow extension in two ways: shoulder extensors initiated the mechanical interaction of passive torques between shoulder and elbow and shoulder flexors dissipated kinetic energy from elbow braking. It is concluded that, in fast 2-D throws, the CNS deliberately utilizes powerful interaction torques between shoulder and elbow to brake motion at the elbow.

Timing of ball release in overarm throws affects ball speed in unskilled but not skilled individuals.

We tested the hypothesis that variability in the timing of ball release in overarm throws affects ball speed. Nine unskilled and six skilled throwers made 30 throws fast and accurately from a sitting and standing position. Angular positions of finger and arm segments were recorded with search-coils at 1000 Hz; ball speed was measured with a radar gun. The time of ball release from the fingertips was measured with respect to seven arm kinematic reference points. Mean timing windows for ball release were 28 ms for unskilled throwers and 7 ms for skilled throwers. Mixed-model analyses of variance showed that a there was a statistically significant relationship between ball speed and the timing of ball release in unskilled throwers, but not in skilled throwers. This was presumably due to the difference in variability of the timing of ball release between the two groups. In contrast, skilled throwers showed a relationship between ball speed and peak forearm angular velocity (one measure of arm speed). We conclude that although variability in the timing of ball release can affect ball speed, this is only a major factor in unskilled throwers. When skilled throwers throw fast, variability in ball speed is due to variability in arm speed.

A novel shoulder–elbow mechanism for increasing speed in a multijoint arm movement

The speed of arm movements is normally increased by increasing agonist muscle activity, but in overarm throwing, an additional effect on speed may come from exploitation of interaction torques (a passive torque associated with motion at adjacent joints). We investigated how the central nervous system (CNS) controls interaction torques at the shoulder and elbow to increase speed in 2-D overarm throwing. Twelve experienced throwers made slow, medium, and fast 2-D throws in a parasagittal plane. Joint motions were computed from recordings made with search coils; joint torques were calculated using inverse dynamics. For slow and medium-speed throws, elbow extension was primarily produced by elbow muscle torque. For fast throws, there was an additional late-occurring elbow extensor interaction torque. Parceling out this elbow extension interaction torque revealed that it primarily arose from shoulder extension deceleration. Surprisingly, shoulder deceleration before ball release was not caused by shoulder flexor (antagonist) muscle torque. Rather, shoulder deceleration was produced by passive elbow-to-shoulder interaction torques that were primarily associated with elbow extension acceleration and velocity. It is concluded that when generating fast 2-D throws, the CNS utilized the arm’s biomechanical properties to increase ball speed. It did this by coordinating shoulder and elbow motions such that an instantaneous mechanical positive feedback occurred of interaction torques between shoulder and elbow before ball release. To what extent this mechanism is utilized in other fast multijoint arm movements remains to be determined.