HIP Cycle Research Articles

Interruptions of fictive scratch motor rhythms by activation of cutaneous flexion reflex afferents in the turtle

A low-spinal immobilized turtle displays a fictive scratch reflex in hindlimb muscle nerves in response to mechanical stimulation of specific regions of the shell (Robertson et al., 1985). There are 3 forms of the scratch reflex: the rostral, the pocket, and the caudal; each exhibits rhythmic activation of hindlimb motor neurons. Cutaneous stimulation of the distal hindlimb elicits a fictive flexion reflex that exhibits tonic excitation of hip protractor (flexor) motor neurons and tonic inhibition of knee extensor motor neurons (Stein et al., 1982). In the present study, we describe the motor pattern blends that resulted from transient activation of either the ipsilateral or the contralateral flexion reflex pathway during ongoing scratch motor patterns. Two types of blends were observed: (1) insertions of a flexion reflex synergy into an interrupted scratch cycle and (2) deletions of parts of a scratch cycle. Associated with each type of motor pattern blend was a permanent reset of the ongoing scratch rhythm. The sign of the reset (phase-advance or phase-delay) could be predicted for all forms of the scratch based on the location of the foot stimulus (ipsi- or contralateral) and its timing relative to the hip protractor/retractor cycle. The timing of knee extensor activity within the hip cycle is different for each form of the scratch (Robertson et al., 1985); thus, the sign of the reset cannot be predicted from the timing of the stimulus relative to the knee extensor cycle. These results indicate the importance of the hip rhythm in determining the overall timing of the scratch reflex.(ABSTRACT TRUNCATED AT 250 WORDS)

High cycle fatigue of weld repaired cast Ti-6AI-4V

In order to determine the effects of weld repair on fatigue life of titanium-6Al-4V castings, a series of specimens was exposed to variations in heat treatment, weld procedure, HIP cycle, cooling rate, and surface finish. The results indicate that weld repair is not detrimental to HCF properties as fatigue cracks were located primarily in the base metal. Fine surface finish and large colony size are the primary variables improving the fatigue life. The fusion zone resisted fatigue crack initiation due to a basketweave morphology and thin grain boundary alpha. Multipass welds were shown not to affect fatigue life when compared with single pass welds. A secondary HIP treatment was not detrimental to fatigue properties, but was found to be unnecessary.