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Biomedical subjects

J F Kahn

Publications and source records attributed to J F Kahn.

22 records · Page 2Linked to original sources
PubMed1985

Comparison of two modalities when exerting isometric contractions.

In order to verify whether the method employed to obtain an isometric contraction has any effects on the time for which the contraction can be sustained as well as the consequent cardiocirculatory responses, nine male subjects, from 23 to 42 years of age, carried out isometric contractions of the elbow flexor muscles against either a suspended weight (W) or a fixed strain gauge (SG). The maximum voluntary contraction (MVC) appears to be identical in both instances; and, for a given percentage of MVC, the limit-time of the isometric contraction and its accompanying heart rate changes reveal no significant differences in the two cases.

Adult↗
PubMed1984

A study of fatigue during repetitive static work performed in two different segmental positions.

Sixteen subjects, nine women and seven men, aged between 19 and 35 years, performed three series of isometric contractions of the flexor muscles of the forearm at 40% MVC. Each series consisted of four isometric contractions sustained until exhaustion, and separated from each other by an incomplete rest interval of 5 min. The position of the arm (either the lower or the upper position) was variable according to the series. Under these conditions, study of heart rate (HR), systolic blood pressure (SBP) and limit-time showed that: HR and SBP were not affected by the arm position even when fatigue was important. For a given load with a progressive appearance of fatigue, the limit-time in female subjects was higher. In both groups, in spite of the fatigue appearing after several isometric contractions performed in the upper position, movement of the arm to the lower position was sufficient to significantly increase the time-limit of another isometric contraction at 40% MVC without concomitant increase in cardiac work.

Adult↗
PubMedDate not supplied

Complementary roles of central command and muscular reflex in the regulation of heart rate during submaximal isometric contraction.

During submaximal isometric contraction, the heart rate (HR) and the electromyographic activity (EMG) increase continuously. Although activation of the muscle and the cardiovascular center is placed partly under the common control of the central command, the nature of the relationship that may exist between HR and the integrated electromyogram (iEMG) is seldom studied. Seventeen healthy men, 22.4 +/- 0.5 years of age (M +/- SE), performed isometric contractions with the right elbow flexors. Forces of 25, 40, 50 and 65% of the maximum voluntary contraction (MVC) were used, and the contractions were sustained until (isotonic isometric contraction: IIC) and beyond exhaustion (anisotonic isometric contraction: AIC). During IIC, a linear relationship exists between HR and iEMG; the slope of this relationship is independent of the relative force developed, which is in favor of a predominant role played by the central command in HR increase. The increase in the ratio iEMG/HR at the approach of local muscular exhaustion would indicate that at the end of IIC there is an increase in the relative part furnished by the information of peripheral origin in HR regulation. During AIC, the force (F) decreases in an exponential manner and stabilizes at around 25% MVC from tAIC = 70 s on. The iEMG and HR change independently: iEMG decreases like F such that iEMG/F remains constant; HR continues to increase in the first phase corresponding to the rapid decrease in F and iEMG, then in a second phase, it decreases linearly with respect to time. Our results suggest that the action of the central command is dominant during stage 1 of AIC, while during stage 2 the relative part furnished by the muscle reflexes increases. Beyond tAIC = 70 s, there seems to be a certain degree of central fatigue.

Adolescent↗