FUNDACIÓN MAPFRESeguridad y Medio Ambiente

Year 31 Nº 124 2011

Effect of the muscle state on the reaction capacityERGONOMICS

This study aims to ascertain the effect of the muscle state on the rapid motor response to an external stimulus. It is known that the speed of a ballistic movement with joint displacement varies in terms of the starting muscle state. In this study we argue that the motor and premotor reaction times might be influenced by whether the muscle was in repose beforehand or in a state of fatigue-causing contraction.

Doctor in Medicine and Surgery. Tenure holding researcher, Instituto de Salud Carlos III, Madrid.
Ph.D. in Medicine. Contracted researcher. Instituto de Salud Carlos III, Madrid.
Doctor in Medicine and Surgery. Senior consultant of the Hospital Clinic, Barcelona.

In certain jobs, such as conveyor-belt, assembly-line or packaging-line working, the precision of each gesture is a basic component of the work in hand. These jobs call for low intensity physical movements constantly repeated in time. Various researchers have shown how, in conditions of fatigue, they can cause pain, strain or muscle damage. [1-4]

Our understanding of the requisites of any task is enhanced if we also understand better how we execute it and how our perceptive system collaborates in recognition of changes in the execution conditions – fatigue, attention, discrimination – to be able to adapt the automated pattern to the new conditions. Fatigue or attention lapses might therefore lead to faulty task performance and occupational accidents. In the case of physical fatigue, the prevention of these states of fatigue is paramount in terms of regulating the amount of activity to be carried out in a job and timing the rest periods. [5-7] Fatigue, like other factors, disrupts the execution of an activity.

Working from these premises we consider it of interest here to investigate three phenomena:

  • In critical situations, when a quick reaction is called for, find out whether there are differences in the motor response of rested muscles and those subjected to a previous state of contraction.
  • Find out if the previous fatigue level is relevant.
  • Find out if the history of muscle contraction impinges on the execution of a motor programme.

To answer these questions we have examined whether changes in the motor pattern induced by previous fatigue have any effect on the neural or muscle components in the ballistic response pattern under reaction time conditions.


Study scope and subjects

Sixteen subjects (9 women and 7 men aged 27 to 52) took part in the study. All were right handed with normal or corrected vision.


The subjects were standing up before a computer screen with the arms hanging close to the body. Electromyographic (EMG) signals were recorded in the right arm by means of surface electrodes fitted to the anterior deltoid (AD) and Triceps brachii (TB). An accelerometer was fitted to the epicondyle of the humerus to record arm movements and measure kinematic variables.


Each subject was studied in a single session, broken down into two blocks according to the type of contraction to be made after the control tests, either long duration (LD) or short duration (SD). The test order was random between subjects. The time between tests was over 30 minutes. The experiment began by instructing the subjects in the test conditions. In the control tests the subjects were asked to make a horizontal abduction as a rapid response upon display of an imperative signal (IS) on the computer screen. The tests were conducted following simple reaction time (RT) paradigms. [8-9] The imperative signal was keyed in by the researcher without previous warning.

In each block, after the CTs, the subjects stood next to a wall to carry out a maximum contraction against it with the right wrist by means of an abduction movement with the arm, either for 30 seconds (LD) as prolonged fatigue or for 10 seconds (SD) as brief contraction, while maintaining the same position. They were instructed to effect a maximum contraction. At the end of the contraction the subjects then had to stand in front of the computer again for carrying out the RT tests. These were conducted at specific time intervals after the 30-second contraction up to an interval of five minutes.

Before the start of the study the subjects carried out a sufficient number of attempts, without previous fatigue, with feedback from EMG readings. For the experimental situation the subjects practised the gesture of leaning against the wall without pushing against it, to avoid unnecessary fatigue. Data reading began when the subjects felt comfortable with the procedure in each condition. Eight to ten CTs were conducted before starting the experimental tests. Twenty minutes after ending each block of LD or SD tests, 8-10 final tests (FT) identical to the control tests were recorded.


Data recording and analysis

For each test we recorded the signals generated by the IS, the EMG activity and the movement. The figures were recorded on a personal computer at a frequency of 2000 Hz for subsequent analysis with specific software. To record the EMG background activity a measurement was taken of the mean amplitude of the rectified EMG signal, both of the AD and TB, and they were measured during the 200 ms interval preceding display of the IS.

The following variables were measured for all signal conditions recorded after the IS:

  • Commencement of AD as latency of EMG activity in the AD. In those tests in which muscles might be active during the IS the commencement moment was taken to be that in which rectified EMG activity changed by more than 10μV/ms within 500 ms of the IS.
  • Commencement of TB, as latency of EMG activity in the TB. Measurement criteria were the same as those described above for the AD.
  • Commencement of movement (CM). This is measured from the IS as the moment when there is a change in the accelerometer signal.
  • Electromechanical delay (EMD), as the difference between AD and CM

Ethical Aspects

The study abided by the ethical requisites of the Declaration of Helsinki. The subjects were duly informed of the study and gave their consent beforehand for participation therein.

Statistical Analysis

Parametric procedures were used. Tasks and series were compared by means of a repeated measures analysis of variance (ANOVA), carried out post hoc when differences were found. Correlations between tasks were calculated by means of the Pearson coefficient of correlation. The statistical significance was set for p<0.05.


All the subjects carried out the tests adequately. It should be pointed out here that each study was very time-consuming. Not only did the subjects have to be fitted with the equipment but they also had to be instructed in how to carry out all the parts of the task and evaluate post-effort recovery. Despite this drawback the test enabled us to ascertain the response after brief contraction and after prolonged contraction (fatigue).

The responses of the anterior deltoid and triceps brachii as reaction times are shown in figures 1 and 2, respectively. These show that there is a reduction of reaction times in both muscles for the two blocks of tests. Reaction times are also seen to recover and return to similar values to the control tests after five minutes. The effect is also seen to be greatest when the previous contraction was prolonged. By way of comparison, this descriptive analysis of the data and the subsequent statistical inference show that the anterior deltoids were more heavily affected than the triceps, with lower reaction times especially after prolonged contraction (p<0.05). The results also show that the effect on the anterior deltoid lasted longer than on the triceps in terms of this lower reaction time after prolonged contraction (figures 1 and 2), (p<0.05). The curves of figures 1 and 2 show a similar response between the conditions of simple post-contraction and fatigue (figures 1 and 2), (p>0.05).

Figure 1. Mean reaction time of all subjects for anterior deltoid (in ms) from 30 seconds to 300 seconds after the contraction. The reaction time is expressed in normalised form taking as 100% the mean control values. Red triangles: condition of fatigue; blue squares: condition of moderate post-contraction.

Figure 2. Mean reaction time of all subjects for triceps brachii (in ms) from 30 seconds to 300 seconds after the contraction. Normalised against control test; the symbols are the same as for figure 1.

Analysis of movement data shows that it was also affected by the previous muscle situation. Figure 3 shows the movement effect, which again turns out to be significant (p<0.05), in line with the effect on the anterior deltoid.

Figure 3. Mean movement time of all subjects (in ms) from 30 seconds to 300 seconds after the contraction. Normalised against control test; the symbols are the same as for figure 1.

Readings were also taken, within the movement, of the electromagnetic delay, which turns out not to change in terms of the previous muscle state (figure 4), lacking any statistical significance (p>0.05).

Figure 4. Duration of electromagnetic delay of all subjects (in ms) from 30 seconds to 300 seconds after the contraction. Normalised against control test; the symbols are the same as for figure 1.

Since both anterior deltoid and triceps muscles take part in the same movement chain within the set task, an evaluation was also made of the correlation of their reaction times. In the control situation this correlation is high (R2= 0.8), whereas the correlation is much lower in the cases of fatigue and brief post contraction (R2 = 0.4 and 0.5 respectively). The correlation between deltoid and the subsequent movement was high under all conditions (R2 from 0.8-0.9).


This study shows the transition of muscle responses in two progressive thresholds of previous muscle activity. In the first minute no differences were observed in premotor time, motor time or electromagnetic delay, but there was a notable shortening of premotor time and motor time without changes in the electromagnetic delay between 90 and 150 ms in the tests vis-à-vis the controls. The differences ceased to be significant after 300 seconds.


The influence of the muscle state on reaction times has already been studied under various arrangements and with variable results. Some studies focused on the premotor time [10-12]; others reported an absence of change [13-15], while a third group found reductions in both times [16-20]. The differences might be due to the varying protocols used. Those that observed reductions in the premotor times also observed differences in the electromagnetic delay. Yeung et al [20] reported a delay therein and consider it to be a compensation to bring the task to completion. Li et al [19] investigated the effect of imagined movements (motor imagery) on the response. Castellote et al [17] used a model in which the muscle was already in isometric activity or rapid oscillations. Etnyre and Kinugasa [18] also investigated the effect of previous isometric contraction but of very brief duration. In all these studies the responses were measured just after the conditioning activity; in our study, however, we have lengthened the measurement period until return to the initial conditions after five minutes. This phenomenon, as a result of the previous activity, bore no direct relationship with the level of this activity, suggesting that it is not so much the initial state but rather the muscle tone and condition that impinge on the response. Similar effects have been observed after vibratory stimuli, suggesting that it might be a functional state of afferent pathways of the spinal cord that effects the voluntary cortical response of motor action. The phenomenon has been observed both in proximal musculature and medial musculature of the limb, although the effects are more notable in the former set of postural muscles (anterior deltoid). The movement effects were along the same lines, suggesting that they are secondary to the premotor effect, in view of the fact that there were no changes in electromagnetic delay considered as a whole.

In view of all the above we consider it worthwhile for future studies to look into the activities of people working in jobs involving repetitive strain, who adopt awkward postures or carry out quick movements using the same muscles over prolonged periods of time, since any lack in postural precision may be detrimental to the work or cause occupational illnesses. [21, 22] Certain conveyor-belt tasks, like carrying products from the belt to crates, cleaning fruit, food packaging or processing, not only require precise and rapid movements but also an interiorisation of the action to be carried out, since this action has to be performed with some precision and proper timing to pick up and put down delicate objects; it might sometimes even call for decision-taking, as in the case of selecting fruit. The fact that the gestures are repetitive already reduces the optimum initial muscle state if any event should call for a quick response. One gesture follows another, and although the work is routine there is little leeway for any loss of attention. The worker always needs to keep aware of the work he or she is doing, and may at times modify this awareness [23]. Workers also need a visual reaction capacity against external requirements [24], and this capacity may be limited under conditions of muscle fatigue. Extreme examples are seen in the operation of machinery and driving of vehicles. [25]


These results therefore suggest that the subject’s motor system modulates the ulterior rapid responses. Subjects should not be allowed to reach states of high fatigue in their work since they must be ready at any time to respond to any internal or external alteration. They therefore need to remain attentive and suitably anticipate any change in conditions or response [17], both in terms of precision and speed. In risk situations a mistake might not only bring about an accident but may even have legal consequences.


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