Journal of Applied Physiology
Vol. 82, No. 1, pp. 1-2, January 1997

This Month in the Journal

BIOMECHANICS OF HOPPING ON DIFFERENT SURFACES

MICROVASCULAR ARCHITECTURE IN SKELETAL MUSCLE

O₂ TRANSPORT IN MUSCLE OF YOUNG AND OLD SUBJECTS DURING EXERCISE

PHARYNGEAL PATENCY ENHANCED BY SYNTHETIC SURFACTANT

INFANT RESUSCITATION: ORAL OR NASAL ROUTE?

STORAGE ERRORS IN BLOOD GAS MEASUREMENTS

MUSCLE TRAINING BY MECHANICAL LOADING

SOLUTE FLUXES IN HYPEROXIC LUNG INJURY

BIOMECHANICS OF HOPPING ON DIFFERENT SURFACES

When a runner moves from pavement to a soft grassy surface and back again, there may be no obvious change in gait despite the marked change in compliance of the running surface. The biomechanics of running depend on the combined stiffness of the legs and the running surface, acting as two springs in series. Ferris and Farley (p. 15) hypothesized that the stiffness of the leg spring might vary reciprocally with the stiffness of the surface, thus offsetting the effects of altered surface stiffness on the mechanics of locomotion. This hypothesis was confirmed in studies of human subjects, hopping in place on different surfaces. The mechanism whereby leg stiffness is adjusted to accommodate changes in surface stiffness remains unknown. The paper is discussed in an Invited Editorial by Alexander (p. 13).

MICROVASCULAR ARCHITECTURE IN SKELETAL MUSCLE

The organization of capillary perfusion along individual muscle fibers was examined by Emerson and Segal (p. 42), who used elegant microanatomic techniques in hamster retractor muscle. The organization and size of microvascular units (MVUs), each defined as a terminal arteriole and the group of capillaries that it supplies, were assessed in relation to the orientation of parallel muscle fibers. Alignment of MVUs along muscle fibers was characterized as largely, but not completely, random. The findings indicate that an increase in blood flow along muscle fibers requires the perfusion of many MVUs and suggest that vasodilation is coordinated among the parent arterioles that serve the corresponding MVUs.

O₂ TRANSPORT IN MUSCLE OF YOUNG AND OLD SUBJECTS DURING EXERCISE

Does aging alter the delivery and utilization of O₂ in exercising muscle? Chilibeck and associates (p. 63) examined this question by studying gastrocnemius muscle capillarization and O₂ uptake kinetics during brief bouts of plantar-flexion exercise in young and old subjects. The measured variables were similar in both age groups, suggesting that age has little influence on O₂ delivery or utilization during exercise of a muscle group accustomed to everyday activity.

PHARYNGEAL PATENCY ENHANCED BY SYNTHETIC SURFACTANT

Van Der Touw et al. (p. 78) measured the anterior-posterior pharyngeal diameter (X-ray fluoroscopy) and its closing and reopening pressures in five awake supine subjects in response to an intraluminal pressure decrease from 0 to 20 cmH₂O during voluntary glottic closure. Measurements were made after the application of saline or a synthetic lung surfactant into the supraglottic airway. Surfactant increased pharyngeal diameter, and both closing and reopening pressures were more negative after surfactant application. Genioglossal electromyographic activity did not change. Thus surface and adhesive forces appear to act on intraluminal surfaces of the upper airway and may influence upper airway patency.

INFANT RESUSCITATION: ORAL OR NASAL ROUTE?

According to current recommendations, infants are to be resuscitated by blowing air into both the nose and mouth. This is often not feasible, as it is difficult or impossible to blow air into both airways without special equipment. To examine the relative effectiveness of nose vs. mouth pressure in resuscitation, Wilson-Davis et al. (p. 152) measured tracheal and esophageal air entry by using nose, combined nose and mouth, and mouth routes in eight infants postmortem. Tracheal air entry occurred at lower pressures when air was blown into the nose rather than into the mouth and nose or mouth alone. These findings indicate that the nasal route of air entry is the most effective. In addition, neck flexion was found to impede air entry, as might be expected.

STORAGE ERRORS IN BLOOD GAS MEASUREMENTS

Samples of arterial blood are customarily collected anaerobically in plastic syringes, which are then stored on ice pending analysis for PO₂ and PCO₂. Cooling reduces the metabolic activity of the blood but also reduces the PO₂ and PCO₂ during storage, thus altering the gradients for diffusive gas exchange with ambient air through the plastic syringe barrels. Wu et al. (p. 196) studied these complex processes by using tonometered blood samples stored in several types of syringes for up to 6 h. Lengthy cold storage in small plastic syringes can lead to a significant overestimation of both PO₂ and PCO₂, with consequent underestimation of the calculated alveolar-to-arterial PO₂ difference.

MUSCLE TRAINING BY MECHANICAL LOADING

Caiozzo et al. (p. 211) utilized a rat medial gastrocnemius muscle preparation in an experimental design that allowed for separation of the effects of muscle force and muscle contraction frequency on myosin heavy chain (MHC) expression. Muscles were "trained" on alternate days for 1 mo under standardized load conditions by using concentric contractions elicited by 100-Hz stimulation or eccentric contractions elicited using 25-Hz stimulation. Both training paradigms elicited similar reductions in the type IIB and increases in the type IIX MHC isoform contents of gastrocnemius muscles. These data suggest that load, rather than stimulation frequency, is an important determinant of MHC protein isoform expression.

SOLUTE FLUXES IN HYPEROXIC LUNG INJURY

Ventilation with 100% O₂ produces lung injury, including increased permeability of the alveolocapillary membrane and pulmonary edema. One physiological countermeasure in alveolar edema is active transport of solutes from the alveolar fluid by the alveolar epithelium. Zheng et al. (p. 240) asked the question: Is active solute transport from alveolar to vascular space impaired in response to hyperoxia? They found that passive sodium and glucose fluxes were increased in hyperoxic rats but that hyperoxia had no significant effects on active absorption of sodium or glucose. These results suggest that impairment of active transport of these solutes is not an important contributor to the pulmonary edema of hyperoxia.