SUMMARY1. The main purpose of this study was to examine the effects of two subtly different stimulus patterns on the force developed by fast-twitch, fatiguable motor units in a cat hindlimb muscle during control (pre-fatigue) and fatiguing contractions.2. The peak force and the force-time integral responses of nineteen high fatigue (FF) and three intermediate fatigue (FI) motor units of the tibialis posterior muscle in five deeply anaesthetized adult cats were measured at selected times during the course of a 360-s fatigue test.3. The fatigue test involved a pseudo-random alternation of two patterns of stimulation. One pattern (regular) was composed of a train of stimuli with constant interpulse intervals, set at 1-8 x the twitch contraction time of each unit (interval range, 27-51 ms), and delivered for 500 (or 400) ms. For the total (FF + FI) motorunit sample, the mean (±S.D.) stimulation frequency was 26 + 4 Hz (range, 19-37 Hz). The other stimulus pattern (optimized) consisted of three initial stimuli with short (10 ms) interpulse intervals, followed by a constant interpulse-interval train that was adjusted (interval range, 29-62 ms; frequency, 23 + 5 Hz; frequency range, 16-36 Hz) such that the total train had the same number of pulses, and the same average frequency and duration as the regular train.4. The stimulus trains were delivered at 1 s-5 for 360 s, using three-train sequences of each pattern, randomly alternating with one another. The response of the third train in each sequence was selected for the force measurements. The force profile obtained from the fatigue test was subsequently decomposed into two profiles: one attributable to regular and one to optimized stimulation.5. During the initial responses to the fatigue test, the optimized stimulus pattern produced significantly more force than the regular stimulus pattern. For FF units, the mean increase in peak force (141 %) was significantly greater than the increase in the force-time integral (59 %).6. All motor units exhibited an initial potentiation of peak force with the regular stimulation pattern, whereas peak force declined monotonically with the optimized pattern. In contrast, the force-time integral potentiated in the first 30 s for both regular and optimized stimulus patterns. L. BEVAN AND OTHERS 7. Each motor unit maintained an increased force response to optimized stimulation during the fatigue test, with the greatest relative increase occurring about 120 s into the test, well after the potentiation effect had subsided. At 360 s of stimulation, the force enhancement with optimization was still substantial (83 % for peak force and 37 % for the force-time integral), yet force was virtually non-existent in response to regular stimulation.8. These results suggest that subtle changes in the activation pattern are a potential mechanism by which the central nervous system might increase force during fatigue. Furthermore, force optimization and potentiation seem to be motorunit properties that are as fundamental as the more conventionally stu...

SUMMARY1. The main purpose of this study was to quantify the adaptation of spinal motoneurons to sustained and intermittent activation, using an extracellular route of stimulating current application to single test cells, in contrast to an intracellular route, as has been used previously. In addition, associations were tested between firing rate properties of the tested cells and other type (size)-related properties of these cells and their motor units.2. Motoneurons supplying the medial gastrocnemius muscle of the deeply anaesthetized cat were stimulated for 240 s with microelectrodes which passed sustained extracellular current at 1-25 times the threshold for repetitive firing. Many cells were also tested following a rest period with intermittent 1 s current pulses (duration 600 ms) at the same relative stimulus strength. Cell discharge was assessed from the EMG of the motor unit innervated by the test neuron. The motoneurons and their motor units were assigned to four categories (i.e. types FF, FR, S and F; where F = FF + FR) based on conventional criteria. In all, twenty F (16 FF, 4 FR) and fourteen S cells were studied with sustained stimulation. Thirty of these cells (17 F, 13 S) 4. All cells exhibited a delay from the onset of current to the first spike, followed by a brief accelerating discharge that was followed by a slower drop in firing rate. Some cells (21 of 34 with sustained activation; 20 of 32 with intermittent) exhibited doublet discharges (interspike intervals < 10 ms) that were intermingled with the more predominant singlet discharges. Doublets were more common in the S cell type.5. With sustained activation, the mean delay from the onset of current to the first spike was 2-6 + 1 1 s for F cells, and 3-2 + 1-9 s for S cells. The time required to reach peak frequency of singlet discharge following repetitive firing onset was significantly shorter for F than S cells (7-0 + 5 0 vs. 14-3 + 13-6 s) and the peak singlet frequencies also differed significantly (F, 28-0 + 7-7 Hz vs. S, 15-6 + 2-5 Hz). Subsequently, the mean magnitude of firing rate reduction from the peak to 24 s later was significantly greater for F cells than that for S cells (16-2 + 6 Hz vs. 5X8 + 3 Hz). These gradual reductions in firing frequency for both F and S cells during the course of their sustained stimulation were qualitatively similar to the late adaptation observed in previous studies that had employed intracellular stimulation.6. The time course of firing frequency for each unit with sustained activation was fitted with a double-exponential equation: the first time constant (T1) for the initial increase in frequency was relatively short (F, 2X5 + 2-1 s vs. S, 3-7 + 4-1 s). The second time constant (r2) was significantly shorter for F than S cells (130-7 + 98-4 s vs. 750 0 + 402-4 s). It is argued that the r2 values provided a quantitative description of the type of adaptation termed 'late' in previous studies.7. The responses to intermittent stimulation were qualitatively similar to those seen with sustained activat...

In the context of the European Safer Internet project EU Kids Online, the aim of this article is to address how young people deal with privacy issues in social networking sites, using Facebook as an example. The study on which it is based examined the type of personal and contact information young people disclose through their profiles. In addition, it assessed gender differences in the disclosure of personal and contact information. A hundred and thirty-one Facebook member profiles were observed, selected to fit the European Commission's youth age range of 1330. Results suggested that most people regardless of gender enter full name, facial pictures, hometown and e-mail addresses in their profiles. However, males are more likely than females to disclose mobile phone number, home address and instant messaging (IM) screen names. Consistent with the past literature, youth, especially between the ages of 18 and 22, seem unaware of the potential dangers they are facing when entering real personal and contact information in their profiles while accepting friendship requests from strangers. Recommendations for future research include investigating the levels of awareness young people have when disclosing information about themselves that can potentially harm them in more ways than one.

The purpose of this study was to quantify the changes in motor-unit action potentials (MUAP) and force during a standard motor-unit fatigue test. MUAP waveforms were characterized by the measurement of amplitude, duration, area, and shape (as reflected in a coefficient of proportionality). Fatigue-resistant motor units exhibited small, but statistically significant, changes in MUAP amplitude and area during the fatigue test, whereas fatigable motor units displayed variable changes in MUAP amplitude, duration, and area. For all motor-unit types, the coefficient of proportionality did not change, and hence the change in MUAP area was proportional to the combined changes in amplitude and duration. The between- and within-train changes in MUAP were also distinct for the fatigue-resistant and fatigable motor units. Although several mechanisms could be responsible for the changes in the MUAP as the fatigue test proceeded, the dissociation of the time courses for MUAP and force indicated that these MUAP changes were not the principal reason for the decline in force under these conditions.

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