Acceleration threshold detection during short anterior and posterior perturbations on a translating platform.

Balance control systems have usually been studied under two conditions, during quiet standing or under large postural perturbations of a magnitude that requires a postural adjustment to prevent falling. Between these two extremes lie perturbations that can be repeated and measured while not forcing adaptive strategies from the postural control system. Unlike other studies of postural control, we employed very short translations with varying accelerations at the edge of psychophysical detectability. These perturbations were vibration-free anterior or posterior translations of the platform on which a subject stood. Using a full Latin-square design set of perturbations in the forward or backward direction, with a smooth or jerk acceleration profile, and of length 4 or 20 mm, were presented to five subjects. Perceptual peak acceleration thresholds were determined by an iterative psychophysical method that forced the subjects to choose in which of two sequential intervals that they perceived a stimulus to have been presented. The only factor found that significantly correlated with detection was perturbation length. The 4 mm peak thresholds averaged 14.51 mm/s2 while 20 mm thresholds averaged 8.55 mm/s2. For the short perturbations employed in this study, detection of motion thus was dependent upon the magnitude of the acceleration, but it was independent of the acceleration profile (jerk versus smooth) or movement direction. By understanding the influences on the ability to perceptually detect motion underfoot, we can begin to understand what elements of the postural control system might be involved in the second-to-second control of balance.

Design, control, and characterization of a Sliding Linear Investigative Platform for Analyzing Lower Limb Stability (SLIP-FALLS).

A novel device, the Sliding Linear Investigative Platform For Analyzing Lower Limb Stability (SLIP-FALLS), has been designed to study the detection and discrimination thresholds of humans to uniaxial horizontal step, ramp, or sinusoidal translations of the surface upon which they stand or stride. The device also can be used to test the human potential for, and mechanisms of, slips and falls. The SLIP utilizes air bearing technology and a noncontact linear motor to produce ultra-low-vibration translations. The FALLS system measures the forces on four load cells, platform linear and head tri-axial accelerations, four channels of electromyographic data, motor voltage, and a subject's psychophysical response; and derives other physiological and biomechanical measures, like center-of-pressure and shear force. The effect of acceleration and shear force on the accuracy of the center-of-pressure calculations is presented. Operating ranges depend on the interactions among displacement, velocity, acceleration, and jerk parameters for linear translations, and between amplitudes and frequencies for sinusoidal translations. Displacements from 5 microm to 0.277 m, velocities from 5 microm/s to 0.3 m/s, and accelerations up to 2.5 m/s2 are achievable with precise control (i.e., without overshoot), but tradeoffs exist such that all three maxima cannot be reached simultaneously. For a 0.15 m/s linear translation at 4 m/s2, SLIP-FALLS produces substantially less vibration than the worm-driven NeuroTest system. The usefulness of having precise control over movement parameters is discussed.

The endocrine control of the initiation and growth of antlers in white-tailed deer.

Five fawn and 5 adult white-tailed deer (Odocoileus virginianus) were maintained in individual pens, fed a complete diet, and bled bi-weekly for one year. Blood serum was analyzed for testosterone (T), androstenedione (A), thyroxine (T4), calcium (Ca), phosphorus (P) and alkaline phosphatase (AP). Adults grew velvet antlers from mid-April and shed their velvet in mid-September. Hardened antlers were cast in March. Fawns grew velvet antler buttons which hardened in February and cast in March, prior to the growth of the first antlers. Adults had higher (P less than 0.05) titres of T and A over the year. Fawns had higher T4 levels and AP activity (P less than 0.05), while Ca and P levels were not different between the groups (P greater than 0.05). In the adults, T peaked in December and April and was correlated with the periods of hard antlers and the initiation of antler growth. In the fawns, T peaked in November and April and was correlated with hardened buttons and the initiation of the first antlers. Adult A peaked in June but hit nadirs in May and July and generally was not correlated with the antler cycle. In the fawns, A was similar to the T cycle. Thyroxine was relatively constant over the year in adults, but was elevated in the winter in fawns during the period of hardened buttons. Calcium and P levels were relatively constant throughout the year in both groups. AP activity was elevated over winter in adults and then decreased sharply when antlers were cast. Activity increased gradually again as new antlers grew. AP activity in fawns was elevated over the winter and spring and gradually declined as the first antlers developed.

Hormone levels and antler development in white-tailed and sika fawns.

Overall mean values of testosterone (T), androstenedione (A), thyroxine (T4), calcium (Ca), phosphorus (P), and alkaline phosphatase (AP) were (T) 2.56 +/- 2.44 ng/ml, (A) 3.16 +/- 2.58 ng/ml, (T4) 8.22 +/- 4.18 micrograms/dl, (Ca), 10.88 +/- 0.65 mg%, (P) 8.03 +/- 0.68 mg%, and (AP) 81.89 +/- 19.45 IU/l in white-tailed fawns and (T) 3.69 +/- 2.76 ng/ml, (A) 18.26 +/- 17.58 ng/ml, (T4) 4.41 +/- 1.59 micrograms/dl, (Ca) 10.08 +/- 0.80 mg%, (P) 9.42 +/- 1.69 mg% and (AP) 95.35 +/- 22.65 IU/l in sika fawns. High T titers correlated with antler button growth, and A titers peaked as buttons hardened in both groups. Higher T4 levels in late fall and early winter may have had a synergistic role for button growth in both groups. Generally higher P levels in sika fawns and relatively higher Ca levels in white-tailed fawns might be species dependent. However, relatively constant Ca and P in both groups represented mineral homeostasis. The mineralization role of AP activity was evident in both groups.

Thyroxine levels and antler growth in white-tailed deer.

1. Five normal male, 5 female, and 3 castrated fawns and 5 adult male white-tailed deer were housed in individual pens for one year to compare the relationships between thyroxine (T4) and other blood parameters and the antler cycle. 2. Biweekly serum samples were examined for T4 titers and levels of serum calcium (Ca), inorganic phosphorus (P), and alkaline phosphatase activity (AP). 3. Seasonal T4 changes were found in all deer groups, with elevated titers in the fall. Female fawns had overall lowered T4 levels. In male fawns and adult bucks, T4 seemed to play a synergistic role in antler initiation and growth. 4. Serum Ca levels remained constant throughout the year, but with lower levels in the female fawns. 5. Serum P levels were also constant seasonally, but with higher levels in the female fawns. There was no age effect on either Ca or P. 6. An age effect was evident on plasma alkaline phosphatase with lower activity in adult bucks. There was no sex effect on AP activity. 7. T4 might have an indirect association with the enzyme AP in Ca and P transport system in white-tailed deer.