Programming of time-to-peak force for brief isometric force pulses: effects on reaction time

According to the parallel force unit model (PFUM) the programming of an isometric force pulse requires the specification of the number of force units and force unit duration. The programming of a force pulse with minimal time-to-peak force is an exception, however, as force unit duration is limited by the minimal possible value, which should be easier to adjust than larger force unit durations. Therefore, the duration of the programming process should be shorter for these force pulses and hence should result in shorter reaction time (RT). Four experiments assessed this prediction using a response precueing procedure. In each experiment the participants produced isometric flexions with their left or right index finger, and time-to-peak force was manipulated within a block. The results are consistent with the predictions of PFUM. The results, however, are at variance with alternative accounts which assume that RT depends primarily on response duration or rate of force production.     

Programming of time-to-peak force for brief isometric force pulses: Effects on reaction time

According to the parallel force unit model (PFUM) the programming of an isometric force pulse requires the specification of the number of force units and force unit duration. The programming of a force pulse with minimal time-to-peak force is an exception, however, as force unit duration is limited by the minimal possible value, which should be easier to adjust than larger force unit durations. Therefore, the duration of the programming process should be shorter for these force pulses and hence should result in shorter reaction time (RT). Four experiments assessed this prediction using a response precueing procedure. In each experiment the participants produced isometric flexions with their left or right index finger, and time-to-peak force was manipulated within a block. The results are consistent with the predictions of PFUM. The results, however, are at variance with alternative accounts which assume that RT depends primarily on response duration or rate of force production.     

Impulse and Movement Space-Time Variability

In 3 experiments, the authors examined movement space-time variability as a function of the force-time properties of the initial impulse in a movement timing task. In the range of motion and movement time task conditions, peak force, initial rate of force, and force duration were manipulated either independently or in combination across a range of parameter values. The findings showed that (a) impulse variability is predicted well by the elaboration of the isometric force variability scaling functions of L. G. Carlton, K. H. Kim, Y. T. Liu, and K. M. Newell (1993) to movement, and (b) the movement spatial and temporal outcome variability are complementary and well predicted by an equation treating the variance of force and time in Newton's 2nd law as independent random variables. Collectively, the findings suggest that movement outcome variability is the product of a coherent space-time function that is driven by the nonlinear scaling of the force-time properties of the initial impulse.     

Force and timing variability in rhythmic unimanual tapping

In 3 experiments the interdependencies between timing and force production in unimanual paced and self-paced rhythmic tapping tasks were examined as participants (N = 6 in each experiment) tapped (a) to 1 of 3 target periods (333 ms, 500 ms, and 1,000 ms), while they simultaneously produced a constant peak force (PF) over a 50-s trial; (b) to produce 1 of 3 target forces (5, 10, and 15 N) at their preferred frequency, while keeping their rhythm as invariant as possible; and (c) to all combinations of target force and period. The results showed that (a) magnitudes of force and period were largely independent; (b) variability in timing increased proportionally with tapping period, and the variability in force increased with peak force; (c) force variability decreased at faster tapping rates; and (d) timing variability decreased with increasing force levels. (e) Analysis of tap-to-tap variability revealed adjustments over sequences of taps and an acceleration in the tapping rate in unpaced conditions. The interdependencies of force and time are discussed with respect to the challenges they provide for an oscillator-based account.     

Distinguishing between haloperidol's and decamethonium's disruptive effects on operant behavior in rats: use of measurements that complement response rate.

The behavioral effects of haloperidol (0.04 to 0.16 mg/kg) and nonparalytic doses of decamethonium (0.2 to 0.8 mg/kg) were studied with operant methods that permitted the measurement of response rate, peak force of response, duration of response, and duration of the rat's head entry into the reinforcement dipper well. Type of operant response topography (forelimb press or forelimb grasp-and-pull) and peak force (low or high) required for reinforcement delivery were independent variables. The low-force, press-topography condition yielded qualitatively different profiles for the two drugs. Haloperidol increased peak force and duration of operant response, increased maximum head entry duration, and temporally dissociated forelimb and head entry behavior. Decamethonium decreased force and duration of operant response, did not appreciably affect maximum head entry duration, and did not influence the normal temporal coupling of forelimb and head entry responses. The haloperidol effects were seen as reflections of pseudo-Parkinsonism, not muscle weakness, which appeared to be the primary source of decamethonium's behavioral effects.     

Force and Timing Variability in Rhythmic Unimanual Tapping

In 3 experiments the interdependencies between timing and force production in unimanual paced and self-paced rhythmic tapping tasks were examined as participants (N = 6 in each experiment) tapped (a) to 1 of 3 target periods (333 ms, 500 ms, and 1,000 ms), while they simultaneously produced a constant peak force (PF) over a 50-s trial; (b) to produce 1 of 3 target forces (5, 10, and 15 N) at their preferred frequency, while keeping their rhythm as invariant as possible; and (c) to all combinations of target force and period. The results showed that (a) magnitudes of force and period were largely independent; (b) variability in timing increased proportionally with tapping period, and the variability in force increased with peak force; (c) force variability decreased at faster tapping rates; and (d) timing variability decreased with increasing force levels, (e) Analysis of tap-to-tap variability revealed adjustments over sequences of taps and an acceleration in the tapping rate in unpaced conditions. The interdependencies of force and time are discussed with respect to the challenges they provide for an oscillator-based account.     

On the Relationship Between Peak Force and Peak Force Variability in Isometric Tasks

An experiment is reported documenting the relationship between peak force and peak force variability with a fixed criterion time to peak force for an isometric task requiring activation of the elbow flexors. The results show that maximum peak force increases with increments in time to peak force and that peak force variability increases with increments of peak force in an exponential type function. Furthermore, despite the presence of peak force and time to peak force feedback, subjects systematically shifted time to peak force as a function of the percentage of peak force being produced. This temporal modulation changes the percentage of peak force represented by any given peak force criterion. When peak force is made proportional to the degree of departure from the criterion time to peak force, a linear relationship is found between peak force and peak force variability. These findings suggest that time to peak force and rate of force production are parameters that influence veridical estimates of the force variability function.     

On the relationship between peak force and peak force variability in isometric tasks

An experiment is reported documenting the relationship between peak force and peak force variability with a fixed criterion time to peak force for an isometric task requiring activation of the elbow flexors. The results show that maximum peak force increases with increments in time to peak force and that peak force variability increases with increments of peak force in an exponential type function. Furthermore, despite the presence of peak force and time to peak force feedback, subjects systematically shifted time to peak force as a function of the percentage of peak force being produced. This temporal modulation changes the percentage of peak force represented by any given peak force criterion. When peak force is made proportional to the degree of departure from the criterion time to peak force, a linear relationship is found between peak force and peak force variability. These findings suggest that time to peak force and rate of force production are parameters that influence veridical estimates of the force variability function.     

A minicomputer system for recording the dynamic properties of individual operant responses

A PDP-12 is used to conduct operant experiments in which peak force, duration, time integral of force, and interresponse time may each serve as the criterion response property for reinforcement, and all four properties are simultaneously recorded as dependent variables. Calibration, acquisition and control, and data analysis programs are described.     

Preload and isometric force variability

Two experiments are reported, which examined the relative contributions of preload (resting force level), change of force, and the time taken to achieve the force in determining isometric force variability. The findings showed that change of force is the strongest determiner of peak force variability but that preload and time to peak force have smaller though systematic effects. A formula that predicts peak force variability is proposed, with preload as an additive effect to the ratio between the change of force level and the square root of time to peak force. These findings confirm that these three impulse variables are significant in predicting force variability and that the impact of rate of force on peak force variability is nonlinear.     

Preload and Isometric Force Variability

Two experiments are reported, which examined the relative contributions of preload (resting force level), change of force, and the time taken to achieve the force in determining isometric force variability. The findings showed that change of force is the strongest determiner of peak force variability but that preload and time to peak force have smaller though systematic effects. A formula that predicts peak force variability is proposed, with preload as an additive effect to the ratio between the change of force level and the square root of time to peak force. These findings confirm that these three impulse variables are significant in predicting force variability and that the impact of rate of force on peak force variability is nonlinear.     

Impulse variability in isometric tasks

An isometric elbow flexion task was used in two experiments that examined the influence of force-production characteristics on impulse variability. Impulse size was held constant while peak force, time to peak force, rate of force, and, hence, the shape of the criterion force-time curve were manipulated. The results indicated that changes in the force-time curve under conditions of equal impulse bring about systematic changes in impulse variability, and this effect is more pronounced for larger impulse conditions. The inability of existing functions to account for the peak force variability findings led to the generation of a new predicted force variability function. The proposed function accounts for changes in the standard deviation and coefficient of variation of peak force, impulse, and rate of force over a range of force-time conditions.     

Rhythmic performance during a whole body movement: dynamic analysis of force-time curves

The purpose of this study was to investigate rhythmic performance during two-legged hopping in place. In particular, it was tested whether (a) timing control is independent of force control, (b) a dynamic timer model explains rhythmic performance, and (c) it is a force related parameter that carries the timing information. Eleven participants performed two-legged hopping at their preferred hopping frequency (PHF) and at two hopping frequencies set by an external rhythmic stimulus as lower (LHF) and higher (HHF) than their PHF, respectively. A force plate was used to record the ground reaction force (GRF) time curves during two-legged hopping. The primary temporal and force related parameters determined from the GRF-time curves were the durations of the cycle of movement (t(cycle)), of the contact phase (t(contact)), of the flight phase (t(flight)), the magnitude of peak force (Fz(peak)) and the rate of peak force development (RFD). Control of t(cycle) was independent of force control as shown by the non-significant correlations between t(cycle) and the force parameters of the GRF-time curve. Lag 1 autocorrelations of t(cycle) were not significant in any of the HF, thereby a dynamic timer model is considered to explain the timing of t(cycle) during two-legged hopping. RFD varied more than any other GRF-time curve parameter, exhibited consistent significant strong correlations with the GRF-time curve parameters and significant negative lag 1 autocorrelations in PHF, thus, it was highlighted as the potent timing control parameter. Finally, we provide a practical application for the optimization of rhythmic performance.     

Comparison of ankle kinematics and ground reaction forces between prospectively injured and uninjured collegiate cross country runners

Biomechanical comparative studies on running-related injuries have included either currently or retrospectively injured runners. The purpose of this study was to prospectively compare ankle joint and ground reaction force variables between collegiate runners who developed injuries during the cross country season and those who did not. Running gait analyses using a motion capture system and force platform were conducted on 19 collegiate runners prior to the start of their cross country season. Ten runners sustained running-related injuries and 9 remained healthy during the course of the season. Strike index, peak loading rate of the vertical ground reaction force, dorsiflexion range of motion (ROM), eversion ROM, peak eversion angle, peak eversion velocity, and eversion duration from the start of the season were compared between injury groups. Ankle eversion ROM and peak eversion velocity were greater in uninjured runners while peak eversion angle was greater in injured runners. Greater ankle eversion ROM and eversion velocity with lower peak eversion angle may be beneficial in reducing injury risk in collegiate runners. The current data may only be applicable to collegiate cross country runners with similar training and racing schedules and threshold magnitudes of ankle kinematic variables to predict injury risk are still unknown.     

Multiple time scales in serial production of force: a tutorial on power spectral analysis of motor variability

We present a tutorial on a power spectral approach to variability in serial motor performance, describing as a case study two experiments on the form of the variance in two force production tasks. In Experiment 1 we examine grip force and load force in repetitive unimanual pulling; in Experiment 2, we describe repetitive bimanual pressing. In both experiments log-log plots of power spectral density of peak force of the responses in each stream against frequency (i.e. periodicity or repetition cycle defined with respect to the ordered succession of responses) were approximately linear with negative slopes which varied systematically with test conditions. We propose the two response streams in each experiment are associated with different levels of sustained attention and state a simple model involving summation of moving average processes on multiple time scales as a qualitative account of the changes in 1/f slope.     

Fractionated reaction time and the rate of force development

The relationship between the rate of force development and components of fractionated reaction time were investigated in the present study. Subjects (N=9) were administered extensive practice before being required to produce 98N of isometric force on a hand dynamometer at a maximal rate, at 20% slower than maximal, and at 40% slower than maximal. Repeated measures analysis of variance followed by non-orthogonal Dunn planned comparisons demonstrated that pre-motor time and reaction time increased as similar peak forces were produced over longer durations. No significant differences in motor times were revealed. These data suggested that the manner in which force is expressed relates to the complexity of motor programming. The increased requirement of coordinating alpha-gamma coactivation, as well as the increased need for rate coding as a process underlying force development at slower contraction rates, are discussed in relation to programming complexity.     

Force and Timing Components of the Motor Program

Three experiments were undertaken to assess the effects of variations of force and time on both simple and choice reaction time. The first two experiments demonstrated that although latency did not vary as a function of force, timing variations, such as requiring that a response be maintained, led to consistent changes in reaction time. These results led to the development of a model of motor programming in which force and timing are dissociated as separate components. However, the data also indicated that the force component may be further analyzed into two subcomponents—force activation and force deactivation. The model predicts that the latter subcomponent may be programmed on-line provided that sufficient time elapses between the implementation of the two subcomponents. A different pair of movements was used in Experiment 3 to further demonstrate that force activation and deactivation may be preprogrammed into a single component. These results support the aspect of the proposed model that makes a distinction between operations required for program construction from those necessary for program implementation.     

Force and timing components of the motor program

Three experiments were undertaken to assess the effects of variations of force and time on both simple and choice reaction time. The first two experiments demonstrated that although latency did not vary as a function of force, timing variations, such as requiring that a response be maintained, led to consistent changes in reaction time. These results led to the development of a model of motor programming in which force and timing are dissociated as separate components. However, the data also indicated that the force component may be further analyzed into two subcomponents-force activation and force deactivation. The model predicts that the latter subcomponent may be programmed on-line provided that sufficient time elapses between the implementation of the two subcomponents. A different pair of movements was used in Experiment 3 to further demonstrate that force activation and deactivation may be preprogrammed into a single component. These results support the aspect of the proposed model that makes a distinction between operations required for program construction from those necessary for program implementation.     

On the locus of speed-accuracy trade-off in reaction time: inferences from the lateralized readiness potential

Lateralized readiness potentials (LRPs) were used to determine the stage(s) of reaction time (RT) responsible for speed-accuracy trade-offs (SATs). Speeded decisions based on several types of information were examined in 3 experiments, involving, respectively, a line discrimination task, lexical decisions, and an Erikson flanker task. Three levels of SAT were obtained in each experiment by adjusting response deadlines with an adaptive tracking algorithm. Speed stress affected the duration of RT stages both before and after the start of the LRP in all experiments. The latter effect cannot be explained by guessing strategies, by variations in response force, or as an indirect consequence of the pre-LRP effect. Contrary to most models, it suggests that SAT can occur at a late postdecisional stage.     

Generalization and response latency

The present experiments investigated generalization in a reaction time situation where the generalization stimulus, a tone, preceded the reaction time signal, a light. The hypotheses under investigation were that the duration of the cue stimulus would determine the degree of generalization (Experiment I) and that the response latency independent of the stimulus duration would be related to the amount of generalization (Experiment II). A particular generalization test stimulus (a tone of 40, 45, 50, 60, 65, or 70 dB) was presented only once always following two bar-pressing responses to training stimulus (tone of 55 dB) under each of two conditions of stimulus duration in Experiment I and under each of two conditions of response latency in Experiment II. It was found that under the condition of short response latency generalization was broader.