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.     

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.     

Effects of variations in force on fractionated reaction time in simple and choice conditions

The present study examined the effects of force output on fractionated reaction time under simple and choice conditions. 20 subjects were required to react and produce a designated force as soon as possible after a visual stimulus. Five different levels of force were 10, 30, 50, 70, and 90% of the maximum grip strength of the subjects. Analysis showed that reaction time (RT) changed as a function of force in both conditions, with the longest RT occurring at the 70% condition. The same pattern was also evident for premotor time. These findings suggest that the changes in RT with increases in force are mediated predominantly by central rather than peripheral processes.     

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.     

Effects of speed and accuracy of exertion of force on the relationship between force output and fractionated reaction time

The present study was designed to investigate the effect of speed and accuracy of force exertion on the relationship between force output and fractionated reaction time. Subjects exerted their force (10% or 40% of maximum isometric contraction) on "accurate" and "fast" tasks as rapidly as possible at the light signal. On the "fast" task, premotor time for the 40% target was lengthened in comparison with that for the 10% target, and motor time was shortened with an increase of force output. On the "accurate" task, on the other hand, premotor time was independent of magnitude of force, and no relation between motor time and force output was found. These findings show that the relationship between force output and fractionated reaction time may be affected by the effort to exert force accurately.     

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.     

A recruitment theory of force-time relations in the production of brief force pulses: the parallel force unit model

A theory, the parallel force unit model, is advanced in which the buildup and decline of force in rapid responses of short duration are assumed to reflect variability in timing of several parallel force units. Response force is conceived of as being a summation of a large number of force units, each acting independently of one another. Force is controlled by either the number of recruited force units or the duration each unit contributes its force. Several predictions are derived on the basis of this theory and are shown to be in qualitative agreement with empirical findings about both the mean and variability of brief force impulses. The model also has consequences for the temporal properties of a response. For example, under certain circumstances, it predicts a reciprocal relation between reaction time and response force. Although the theory is proposed as a psychological account, relations between the assumptions and basic principles in neurophysiology are considered. Possible future applications and generalizations of the theory are discussed.     

Reaction time and response dynamics

The experiments reported were designed to examine the relationship between reaction time and the response dynamics of a finger-press task. Experiments 1 and 2 manipulated force duration and peak force level in both simple and choice reaction-time paradigms. Experiment 3 constrained both force duration and peak force, leading to independent changes in the rate of force production. The findings from all three experiments suggest that the rate of force production, rather than force duration, is the key response parameter determining reaction time. Reaction time decreased as an exponential function of rate of force production independent of force duration and peak force.     

Effort in isometric muscular contractions related to force level and duration

The degree of perceived force involved in squeezing a handgrip dynamometer is shown to grow as a power function of the force of isometric contraction and also as a power function of the duration of the squeeze. The exponent for force turns out to be more than twice that for duration. These two power functions are able to predict measurements of muscle endurance, i.e., of the maximum length of time that contraction of any constant level of force can be sustained.     

Intermittent visual information and the multiple time scales of visual motor control of continuous isometric force production

In an experiment, we examined the effect of intermittency (from 25.6 Hz to 0.2 Hz) of visual information on continuous isometric force production as a function of force level (5%, 10%, 25%, and 50% of maximal voluntary contraction [MVC]). The amount of force variability decreased and the irregularity of force output increased as a function of increased visual intermittency rate. Vision was found to have an influence on the frequency structure of force output up to 12 Hz, and the 25% MVC force level had more high-frequency modulations with higher rates of visual information. The effective use of intermittent visual information is mediated nonlinearly by force level, and there are multiple time scales of visual control (range, approximately 0 - 12 Hz) that are postulated to be a function of both feedback and feedforward control processes.     

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.     

Fractioned reaction time as a function of response force

The relationship between fractionated reaction time components and response force was studied in a simple reaction time task. Subjects squeezed a force transducer between the right thumb and index finger. Three conditions with 5, 25, and 50% of the maximum voluntary isometric force were investigated in a counterbalanced order. The results showed that premotor reaction time was negatively related to peak force amplitude, while motor reaction time remained constant across force conditions. An interpretation of the effect on premotor reaction time in terms of a shift in the speed-accuracy trade-off function was refuted. Although the data were consistent with a two-stage programming model, it was concluded that differences in motor nerve fiber conduction velocity as a function of response force could explain the results obtained.     

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.     

Force Increase in a Repetitive Motor Task Inducing Motor Fatigue

To evaluate task induced motor fatigue in a well-established finger tapping task, we analyzed tapping parameters and included the time course of measures of force. We hypothesized that a decline in tapping force would reflect task induced motor fatigue, defined by a lengthening of inter-tap intervals (ITI). A secondary aim was to investigate the reliability of tapping data acquisition with the force sensor. Results show that, as expected, tapping speed decreased linearly over time, due to both an increase of ITI and tap duration. In contrast, tapping force increased non-linearly over time and was uncorrelated to changes in tapping speed. Force data could serve as a measure to characterize task induced motor fatigue. Force sensors can assess a decline in tapping speed as well as an independent increase of tapping force. We argue that the increase of force reflects central compensation, i.e. perception of fatigue, due to an increase in task effort and difficulty.     

A force/displacement analysis of muscle testing

Manual muscle testing procedures are the subject of a force and displacement analysis. Equipment was fabricated, tested, and employed to gather force, displacement, and time data for the purpose of examining muscle-test parameters as used by clinicians in applied kinesiology. Simple mathematical procedures are used to process the data to find potential patterns of force and displacement which would correspond to the testing of strong and weak muscles of healthy subjects. Particular attention is paid to the leading edge of the force pulses, as most clinicians report they derive most of their assessment from the initial thrust imparted on the patient's limb. An analysis of the simple linear regression of the slope (distance vs force) of the leading edge of a force pulse indicates that a significantly large slope is indicative of weak muscles (as perceived by the clinician), and a small slope is indicative of strong muscles. Threshold criteria for slopes are specified to create a model that may discriminate between strong and weak muscles. The model is accurate 98% of the time compared to judgments of clinicians with more than 5 years of experience but is considerably lower for clinicians with less than five years of experience (64%). this accuracy rate indicates that the model is reliable in predicting the clinician's perception of muscle strength, and it also indicates that the testing procedure for muscle strength used by experienced clinicians in applied kinesiology are reliable. The experiment lays the groundwork for studies of the objectivity of muscle-strength assessment in applied kinesiology.     

Force variability in isometric responses

In the present study we examined the contribution of different impulse parameters to peak force variability in an isometric task. Five experiments are reported that each held constant a different impulse parameter while allowing the other impulse parameters to vary. The results indicate that change in force level is the parameter that has the greatest effect on peak force variability, although time to peak force and preload also systematically influence response variability. A formula that accommodates the relation between impulse parameters and force variability is proposed. The data suggest that even in isometric tasks, it is the force-time properties of the impulse, rather than discrete parameters such as peak force, that determine the outcome variability.     

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.