DIFFERENT REACTIVITY PATTERNS OF PHYSIOLOGICAL VARIABLES
FOR STRESSORS OF A DIFFERENT NATURE: HEART RATE AND HEART RATE IRREGULARITY BEFORE
PARACHUTE JUMPING AND DURING FLIGHT SIMULATOR PRACTICE.

Haylitt Retief and C.H.J.M.Opmeer
Laboratory of Ergonomic Psychology of the Organization
for Health Research.TNO, .
Zuiderzeoweg 10 - Amsterdam, The Netherlands


INTRODUCTION.

The objective assessment of a subjects workload is a complex problem (Opmeer/Retief; Retief/Opmeer,1972).
When physiological measures are taken under .the assumption that they reflect an aspect of workload one enters the realm of stress research.
Seleye defined stress by a measurable shift in physiological variables. Basically this shift would be such that an increase in stress corresponds to an increase in the ergotropic/trophotrophic quotient ( Gellhorn).

In the variety of definitions covering this vague concept part of the confusion is caused by the tendency to differentiate between stress as defined by its physiological equivalent (e.g. Seley) and stress as an experienced state (e . g. Cofer & Appley).
Another question that has been raised concerns the possibility of a relation between the nature of the stress inducing stimulus and the type of physiological response (Levi).
This experiment is part of a project investigating the specific sensitivity of certain physiological parameters for stress induced by qualitatively different agents(e.g. energetic versus emotional versus cognitive stimuli).
It is suggested by the authors that in empirical research to this day no convergence exists pointing to any physiological measure (or combination of measures) as parameters with specific sensitivity for conditions as mentioned above. The organism seems to react the same whether aroused by stimuli constituting tasks with cognitive demands, energetic demands or stimuli provoking "emotions".
It would be of considerable importance, however, if physiological measures would be found that could be used as differential criteria for the one condition or the other.
In 1967 Kalsbeek and Ettema explored the sensitivity of the sinus arrhythmia as a measure of stress caused by a predominantly cognitive task (a choice task between auditory stimuli). They found that sinus arrhythmia was more sensitive to levels of difficulty of the task than heart-rate (a commonly employed stress measure). They generalized their findings to other forms of stress caused by predominantly cognitive tasks, suggesting that sinus arrhythmia might be a specific measure for "mental load".
In 1968, Retief and Opmeer, working at Kalsbeek's laboratory, evaluated the sensitivity of sinus arrhythmia for levels of cockpit workload (flight simulators).

They found that heart-rate was also very sensitive to these load levels, though sinus arrhythmia had the greater sensitivity (W² value). Increase in heart-rate, regardless of cause,, is generally assumed to suppress sinus arrhythmia. Thus the question became whether suppression of sinus arrhythmia would be different when heart-rate was increased by stimuli causing cognitive demands. energetic demands or provoking "emotions".
To investigate this problem Opmeer conducted an experiment where heart-rate was varied on a bicycle ergo-meter (Opmeer,19 ) The design also incorporated conditions with added cognitive load (auditory binary choices).
The present experiment investigates the variability of sinus arrhythmia breathing And heart-rate in conditions of variable "emotional" load; cognitive and energetic demands being suppqs1edly minimal (trainee parachutists waiting before the jump).
Inducting from the Kalsbeek and Ettema findings it is hypothesized that under these conditions heart-rate will be a more sensitive measure than sinus arrhythmia this last measure being more sensitive for workload levels like e.g. cockpit workload ( Hypotheses l).
It is further hypothesized that heart rate, sinus arrhythmia and breathing rate will differentiate between the four stress levels defined in this experiment (hypotheses 2). Hypotheses 3 predicts a correlation between physiological variables and subjective experience.

METHOD.

Four ordinally increasing levels of emotional stress were postulated as criterion, against which the differentiating sensitivity of SA, HF and RR were assessed: 1."Rest"; 2. "Next stick"; 3. "IP1"; 4. "IP2" . This order supposedly represents increasing stress.
Description of sample periods- 1. "Rest" was measured at the end of the day of jumping. Subjects knew that they would not be permitted to make any further jump after this registration. Thus reporting for registration has stimulus value defining the end of the days jumping. 2. "Next stick". A "stick" is the term used for a group of para's which Will be airborne together for the purpose of jumping. During registration in this phase subject was certain that he would be airborne within 15 minutes. He could hear the aircraft with the proceeding stick during the entire registration.
Subject was instructed to watch the take off of the aircraft with the proceeding stick and registration commenced immediately afterwards. 3. In plane 1 ("IPI"). The airborne para's were dropped in two runs of three to prevent their landing outside the field. The experimental subject was the last to be dropped. Registration of "IPI" starts after subject is seated and terminates when the first para of the first run is dropped. 4. In Plane 2 (IP2). Sample period starts at the end of previous one and terminates at the moment of egress. Duration of period ig approximately 3 minutes. Stimulus marking start of period is exit of first para of preceding lot.
The experimenters took great care that the sample periods were always defined by these same stimuli., for all subjects and sessions. Also, interference of any form of energetic or cognitive load was avoided during registration. It was felt that a high degree of experimental control was achieved in these respects.
Data collection and apparatus - BR was registered on a Mosely strip chart recorder from a signal derived from a Siemens breathing rate meter activated by a nose thermistor. HF and SA were stored on electromagnetic tape which registered needle pulses derived from the ECG R top by a cardio-tacho-meter. In the aircraft the full ECG was recorded on a small portable tape recorder from an IC unit incorporating a
modulator and pre-amplifier.(This.combination was also used by us to record ECG during a 40 second free fall).
Data processing - Cardiac raw data were fed into an electronic device (SINBA) which processed these data to render 3 output quantities:
1. a digit representing average HF over 1 minute periods,
2. a digit representing the sum of positive differences between successive R-top interval times during 1 minute ("S"),
3. a digit representing the number of waves per minute, a wave being an excursion from a reversal point in excess of 11 milliseconds ("n"). A reversal point is a point were R-top interval times start decreasing after increasing or visa versa.
In this experiment the value defining SA is derived by deviding "S" by "n" for each minute (v.d. Maas, 19)
BR was calculated from the strip chart recording by counting the number of waves of which the amplitude exceeded half the amplitude of the proceeding one (one minute values).
Statistics - Analyses of variance was done to verify the predictions concerning the effect of stress.levels and sessions. A 2x4x12 design (incorporating the airborne samples) and a 2x2x12 design (comparing ground samples only) were used. (Wiener 1962. page 289). .1
The criterion to verify predicted selective physiological reactivity was the magnitude of W 2 (Hayes,1966).

(parachap1.gif)

To evaluate the subjective stress experience in each stress level a 7 point rating Scale was constructed (Torgersoni 1962, page 72).
The values thus obtained were correlated with the relevant physiological quantities.
Subjects - were 12 male para's, average age 21 years. Each was registered 2 times under similar conditions. First registration was after 2 jumps has been made, second registration after approximately 6 jumps has been made.
The aircraft was an 8 seated Brousard. Jumping (all static's) was from an altitude of 1.500 ft. towards a pebble circle in a large grass airfield.

RESULTS.

Figure 1 shows relevant values of HF and SA with increasing stress (average over all subjects).
Dotted line.displays SA, values, solid line HF values. First session is marked by circles,
second session by full dot.

Table 1: displays relevant ANOVA values with magnitude of W² in last column.
Source of 		f			p		    W²
variation 	HF	SA	RR	HF	SA	RR	HF	SA
sessions	8.54	9.11	3.16	0.025	0.025	-ns>	0.02	0.02
stress		123.95	36.86	0.04	0.001	0.001	ns>	0.71	0.46

Table 2: represents ANOVA values from comparison of the 2 conditions on the ground. viz. before 
jump and rest value at the end of the day.
Source of 		f			p		    W²
variation 	HF	SA	RR	HF	SA	RR	HF	SA
sessions	12.21	3.85	3.16	0.005	0.10	ns	0.07	0.03
stress		34.32 	1.52 	0.04	0.001	ns	ns	0.20

Quantification of subjects subjective rating of experienced stress in each of the four situations results in the following values (Torgersons 1967, page 72):
R =1.64 ; BJ = 2.82 ; IP1 = 4.09 ; IP2 = 4.45.
These values represent quantities on a 7 point scale were a value of 0 had the subjective equivalent of." no tension at all" and the value of 7" the greatest tension I have ever experienced".
Values 0 and 7 are hardly ever used by subjects in this type of procedure ( the ''end effect". Torgerson,1967).
Correlation between experienced stress and physiological variables is significant for HF only (r =0.,97 ; p = 0.025).
For SA a correlation of -0.88 was found but since only two degrees of freedom existed this high value was not significant.

CONCLUSIONS.

Hypotheses 1. Compared with BR and SA, HF seems to be the most sensitive measure for emotional stress as defined in this paper W²) 2 values for this parameter are highest for all stress levels (table 1). Also, when,comparing ground conditions only, HF is the only variable that differentiates significantly between stress levels (and also between sessions)(Table 2.). Finally HF appears to be the only variable correlating significantly with the subjects experienced stress. These findings all point in the same direction. Hypotheses 1. must therefore be accepted.
Hypotheses 2. The hypotheses that all physiological variables will differentiate between the 4 levels as defined in this experiment is accepted for HR and SA and rejected.for RR. Hypotheses 3. Hypotheses 3, relating to the correlation between subjective experience and physiological variables is accepted for HF and.rejected for SA and BR.

DISCUSSION.

With regard to the non significant difference.of RR at rest and before jump it is felt that considerable inter- and intra individual differences influence this result.
Diagram 1 displays individual reactivity of RR. The values shown represent change in RR before and after jump. It may be noted that some subjects have a high RR before jump in the first session and a lower in the second. Also, some subjects react with a decrease in RR with increased stress, others have an opposite tendency. With regard to this individual variability a comparable finding is mentioned by Reid et al.(1971). With regard to HF may be mentioned that the general tendency is increases towards egress. Average in graph 1 is over the last 3 minutes. This average obscures the fact that values at the moment of egress are much higher. The average HF per minute equivalent, computed from the interval between the last 2 beats before jump was 140 (s ) Values relate to average over 12 subjects and 2 sessions. Highest individual values were 157 in session 1 and 171 in session 2 (standard deviations 11,5 and 16,5 respectively).
The reader is reminded that the present experiment was done as a follow-up of research centered around the evaluation of physiological variables as cockpit workload criteria.
It was suggested that with cockpit workload, information handling is the major stressor . For that type of stress SA was suggested to be a more sensitive measure, when compared

Curve 1 A displays cardiotachogram of pilot flying DC7 simulator B in simple flying configuration. Curve 1 shows same subject in final approach phase. Sudden change in curve 1 is caused by subject being unexpectedly instructed to initiate approach (GCA). Pilot is unfamiliar with DC7 and executing single crew operation. Curve 2 shows cardiotachogram of a parachute jumper in last three minutes before jump.
Irregularity of line indicates degree of sinus arrhythmia. As can be seen, suppression of SA, when compared with the more relaxed condition, is almost complete in the flying task. With parachute jump SA remains relatively unaffected in both situations.
HR is high in both situations. Peaks in 1a are sharp in comparison to curve 2, but this is a result of a difference in chart speed. The extreme suppression of SA in curve 1b is a "school example". Also, curves 1 and 2 relate to different individuals.
Quantification of these phenomena (12 pilots, 12 para's) with subsequent statistical analyses renders the following W² values.

			HR 	SA 	RR
Flight simulator 	0.04 	0.24	0.29
parachute jumping	0.71  	0.46 	0.00

The magnitude of W² can be used as a criterion for the sensitivity of the relevant.variable in differentiating between the selected stress levels.
The authors feel that the positively verified predictions in the parachute experiment viewed in relation with their flight simulator research suggest that a selective physiological reactivity may exist for the relevant stressors. It may be worth-while to consider this possibility when physiological variables are measured in these fields.

SUMMARY.

The authors conducted some research.concerning the evaluation of physiological variables as cockpit workload criteria. It was recognized that a change in these variables could be caused by tasks requiring information handling but also by "emotional" factors.
The experiment described in this paper.was conducted to investigate whether it would be possible to differentiate between these different stressors.
It was hypothesized that heart rate would react more strongly to "emotional".stressors (like parachute
jumping) than sinus arrhythmia, which is considered to react more than heart rate in cognitive tasks (like flying).
In this paper the variability of these variables was studied in 4 levels of "emotional" stress (parachute
jumping) .
Results are compared with physiological Variability in flight simulators and found to be in concurrence with the expectations..

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