Nis Hjortskov

The effect of mental stress on heart rate variability and blood pressure during computer work

The aim was to evaluate the cardiovascular and subjective stress response to a combined physical and mental workload, and the effect of rest. Twelve females who had no prior experience of laboratory experiments participated in the study. Computer-work-related mental stressors were either added to or removed from a standardized computer work session in the laboratory. Beat-to-beat blood pressure and electrocardiogram (ECG) were recorded continuously during the experiment. The participants reported subjective experiences of stress in six categories using an 11-point scale before and at the end of the work. Heart rate variability (HRV) variables were calculated from the ECG recordings, and a reduction in the high-frequency component of HRV and an increase in the low- to high-frequency ratio were observed in the stress situation compared to the control session. No changes were seen in the low-frequency component of HRV. The stressors induced an increase in blood pressure compared to baseline that persisted, and for the diastolic pressure it even increased in the subsequent control session. No differences were observed for subjective experience of stress with the exception of a time trend in the exhaustion scale, i.e. a progression in reported exhaustion with time. The results—and the dissociation between HRV and blood pressure variables—indicate that HRV is a more sensitive and selective measure of mental stress. It could be speculated that heart rate-derived variables reflect a central pathway in cardiovascular control mechanisms (“central command”), while the blood pressure response is more influenced by local conditions in the working muscles that partly mask the effect of changes in mental workloads. In the rest period after each work session, HRV and blood pressure variables were partly normalized as expected. However, an 8-min period of rest was insufficient to restore blood pressure to resting values.

The effect of delayed-onset muscle soreness on stretch reflexes in human low back muscles

The aim of the study was to investigate the effect of moderate delayed‐onset muscle soreness (DOMS) on the short latency stretch reflex (SLR) and long latency stretch reflex (LLR) response i.e. electromyographic (EMG) onset latencies and EMG amplitudes in erector spinae (ES). Nine males with muscle soreness (DOMS group) were tested 24, 48 h, and 7 days post‐exercise. Eight males (control group) were tested likewise. EMG was measured from ES bilaterally at the level of L3/L4. The SLR was elicited by mechanically tapping the ES at L3/L4, and the LLR was elicited by sudden loadings of the spine. Significant reductions in force during maximal voluntary contractions and range of motion, and a significant increase in muscle soreness (measured by pressure algometry) and subjective experience of soreness in the low back indicated DOMS 24, and 48 h post‐exercise in the DOMS group. No changes were observed in the control group. The SLR and LLR response were unaffected by DOMS, i.e. no changes in EMG latencies and amplitudes were observed. In conclusion, despite changes in DOMS indicators, the reflex system protecting the stability of the lumbar spine is apparently capable of maintaining an appropriate triggering of SLR and LLR.

Short latency stretch reflex in human lumbar paraspinal muscles

The aim of the study was to measure stretch reflex latencies of the lumbar paraspinal muscles. An electromechanical tapping system was constructed enabling an accurate estimation of short latencies by utilizing a new technique combining results for different tapping durations. Latency parameters (onset, peak and zero-crossing of EMG signal) were obtained for the paraspinal muscles at the L3/L4 level for 10 male subjects. Detection of EMG onset, which was determined by a threshold criterion (2.5 S.D. of pre-activity), yielded 7.4+/-1.4 ms corresponding to a physiological short latency onset of 6.5 ms, which is considerably shorter than previously reported. However, it is shown to be consistent with the expected latency value for a monosynaptic stretch reflex for the paraspinal muscles of the low back.

Lumbar position sense acuity during an electrical shock stressor

BackgroundOptimal motor control of the spine depends on proprioceptive input as a prerequisite for co-ordination and the stability of the spine. Muscle spindles are known to play an important role in proprioception. Animal experiments suggest that an increase in sympathetic outflow can depress muscle spindle sensitivity. As the muscle spindle may be influenced by sympathetic modulation, we hypothesized that a state of high sympathetic activity as during mental stress would affect the proprioceptive output from the muscle spindles in the back muscles leading to alterations in proprioception and position sense acuity. The aim was to investigate the effect of mental stress, in this study the response to an electrical shock stressor, on position sense acuity in the rotational axis of the lumbar spine.MethodsPassive and active position sense acuity in the rotational plane of the lumbar spine was investigated in the presence and absence of an electrical shock stressor in 14 healthy participants. An electrical shock-threat stressor lasting for approximately 12 minutes was used as imposed stressor to build up a strong anticipatory arousal: The participants were told that they were going to receive 8 painful electrical shocks however the participants never received the shocks. To quantify the level of physiological arousal and the level of sympathetic outflow continuous beat-to-beat changes in heart rate (beats*min-1) and systolic, diastolic and mean arterial blood pressure (mmHg) were measured. To quantify position sense acuity absolute error (AE) expressed in degrees was measured. Two-way analysis of variance with repeated measurements (subjects as random factor and treatments as fixed factors) was used to compare the different treatments.ResultsSignificant increases were observed in systolic blood pressure, diastolic blood pressure, and heart rate during the stress sessions indicating elevated sympathetic activity (15, 14 and 10%, respectively). Despite pronounced changes in the sympathetic activity and subjective experiences of stress no changes were found in position sense acuity in the rotational plane of the lumbar spine in the presence of the electrical shock stressor compared to the control period.ConclusionThe present findings indicate that position sense acuity in the rotational plane of the spine was unaffected by the electrical shock stressor.

Changes in physical performance among construction workers during extended workweeks with 12-hour workdays

PurposeTo investigate changes of physical performance during long working hours and extended workweeks among construction workers with temporary accommodation in camps.MethodsNineteen construction workers with 12-h workdays and extended workweeks participated. Physical performance in the morning and evening of the second and eleventh workdays was tested by endurance, ability to react to a sudden load, flexibility of the back, handgrip strength and sub-maximal HR during a bicycle test. HR was registered throughout two separate workdays.ResultsHR during each of the two separate workdays corresponded to a relative workload of 25%. Sub-maximal HR was lower, reaction time faster and handgrip strength higher in the end of each test day. In the end of the work period, sub-maximal HR was lower, reaction time faster and sitting balance was better.ConclusionNo trends of decreased physical performance were found after a workday or a work period.