Gradation of isometric tension by different activation rates in motor units of cat flexor carpi radialis muscle

Abstract

Single motor units of the flexor carpi radialis (FCR) muscle were activated with a series of constant-rate stimulus trains to study the relation between the frequency of activation and isometric tension development (F-T relation). The tension produced by each stimulus train was expressed as a percentage of the maximum tension-time area (Amax) found for a given unit. Between 25 and 75% Amax a clear separation was seen in the rates needed to produce the same relative tension for the F-T curves of slow-twitch (type S) and fast-twitch (type F) units. Over the steepest portion of the F-T curve (25-50% Amax), where tension output was most sensitive to changes in activation rate, type F units required substantially higher stimulation rates (30 pps) to achieve the same relative tension output as type S units. Furthermore, the frequency range that corresponded to the steep portion of the curve was 2.3 times greater for type F units. For both type S and F units, twitch duration was deemed to be an important determinant of the F-T curve, as has been shown previously. A direct continuous relation was seen between the integrated twitch time (ITT) and the stimulus interval needed to produce 50% Amax (r = 0.94, P less than 0.001). Thus, units that had relatively brief twitches required higher activation rates to achieve the same relative percentage of Amax. Comparison of F-T curves from FCR with those derived by other investigators for cat hindlimb units (medial gastrocnemius and peroneus longus) revealed that significant differences in activation rates were needed to produce the same percentage of Amax throughout the midrange of the F-T curve. At 50% Amax, type F units in FCR required activation rates approximately 20 pps higher than type F units in the hindlimb. Type S units in FCR required only slightly higher rates (approximately equal to 5 pps). Based on a number of well-founded assumptions, F-T curves derived from FCR units were used to estimate the potential contribution of rate coding to total muscle tension by type S and F units. This analysis leads to the conclusion that rate modulation is a potentially important factor in the gradation of tension for the FCR muscle.

Cross-validated models of the relationships between neck muscle electromyography and three-dimensional head kinematics during gaze behavior

“…Our obtained lags were shorter than other estimations (Botterman et al 1986) and similar between the two animals but were longer for acceleration phase [M1, 14.46 Ϯ 2.84 ms (mean Ϯ SD); M2, 16.46 Ϯ 3.07 ms] vs. deceleration phase (M1, 2.15 Ϯ 1.30 ms; M2, 4.07 Ϯ 2.59 ms). This is consistent with known muscle physiology, because 1) acceleration is produced by the rate of agonist shortening, whereas deceleration is controlled by the rate of antagonist lengthening; and 2) muscles develop forces faster during active lengthening than during active contractions (Brown and Loeb 2000).…”

Contractile properties of human Thenar muscles paralyzed by spinal cord injury

“…For these movements it is vital to know the properties of parawhen excited at low rates. This has been demonstrated both for animal muscles 3,14 and for thenar lyzed intrinsic hand muscles. Furthermore, fatigue always develops more readily in response to constant motor units in intact human subjects.…”

A Within Trial Measure of the Stop Signal Reaction Time in a Head-Unrestrained Oculomotor Countermanding Task

“…Part of the difference likely arises from biomechanical differences in moving the eyes or head; for example, the head has a much greater inertia (Zangemeister and Stark 1981), and skeletal muscles take longer to develop forces compared with extraocular muscles (Barmack et al 1971;Botterman et al 1986). …”