Impact of Verbal Suggestions on Finger Flexor Activation and Strength in Healthy Individuals

Background

Perception and behavior are strongly influenced by verbal information (eg, verbal suggestions) provided by other individuals and by the learning mechanism (conditioning). The manner in which healthcare workers (including physiotherapists) inform patients about typical effects of a given treatment affects patients’ feelings and expectations as well as their subjective evaluation of treatment effects. Active interventions are more effective when patients are provided with verbal information in a manner that increases their expectations concerning positive outcomes [1]. Verbal reinforcement is a well-known phenomenon commonly used in training. As early as in 1983, Johansson et al noted an increase in isometric muscle contraction resulting from high-volume verbal commands [2]. Also, Belkhiria et al observed positive effects of verbal reinforcement on isometric force [3]. Positive verbal information associated with positive conditioning produces the placebo effect, while its negative counterpart leads to the nocebo effect. The word ‘placebo’ originates from Latin and means “I shall please.” It was used to define individuals hired to mourn during funerals. In the medical literature, it was first used in the 19th century in a medical dictionary published in 1811 [4]. The term ‘placebo’ refers to a neutral factor or activity that should not have any proven ability to produce physical or psychological effects. The placebo effect indicates a positive psychological or physical impact that can be attributed to placebo or processes (expectation or conditioning) owing to which placebo takes effect [5]. Placebo may produce tangible therapeutic benefits in numerous clinical states through the activity of different neurobiological pathways [6].

Most investigations into placebo and nocebo effects do not provide exact words used in verbal suggestions; however, large differences in effects point to the significance of particular terms. There is a strong need for further research into therapeutic effectiveness improvement using verbal suggestions. In this context, use of inappropriate words can either reduce or eliminate therapeutic effects, or even result in contrary effects. Therefore, the use of inappropriate words can affect the achievement of desired therapeutic aims [7].

It was also found that the application of a non-active drug or a therapy with expected negative outcomes (eg, increased fear, fatigue, or other symptoms) often leads to an increase in adverse symptoms. It was defined as the nocebo effect (“nocebo” means “I shall harm”) [8].

The nocebo effect occurs when information and expectations regarding negative effects of an intervention lead to negative (often significant) consequences [9]. Sports-related studies revealed that the nocebo effect can reduce sports performance [6,10,11]. It was observed that among trained athletes “subjects in a control group perform worse because they know they are in the control group” [12]. Research on placebo in athletes also showed that it can enhance their performance (also in elite athletes), and its influence goes beyond ordinary motivation of an athlete to perform better [11]. In their review, Hurst et al analyzed 32 studies that included 1513 participants, finding small to moderate placebo effects (Cohen’s d=0.36) and small to moderate nocebo effects (d=0.37) in elite athletes [11].

Electromyography (EMG) refers to the reception of the electric signal from muscles controlled by the nervous system. The EMG signal is the electric activity of motor units of a muscle. There are 2 types of EMG: surface EMG (sEMG) and intramuscular EMG. Currently, signals detected by means of sEMG are preferable in terms of obtaining data on the time and intensity of surface muscle activation. EMG signals are used as electrophysiological signals both in medicine and in engineering [13].

Hand dynamometers are widely used; it has been proved that they are valid and reliable for examining healthy individuals. These tools measure muscle peak strength. There are hydraulic and electronic dynamometers which make it possible to measure average grip strength in a given time period as well as grip strength at the end of this period [14].

Therefore, this study aimed to evaluate the effect of verbal reinforcement on the placebo effect in the context of finger flexor muscle activation measured with surface electromyography (sEMG) and hand grip strength measured with a hand dynamometer in healthy subjects.

Material and Methods

ETHICS STATEMENT:

The study was approved by the Bioethics Committee of the ABNS in Biała Podlaska (protocol code 5/2019) and was conducted in accordance with the ethical standards established in the 1964 Declaration of Helsinki.

Prior to the examinations, the participants were partly informed about the purpose of the study (they were informed that the study would focus on the effects of the applied tape on muscle strength and activation) and were made aware of being able to quit the study at any time. Moreover, they gave their written informed consent to use the data collected during the study for scientific purposes (publishing). Upon completing the study, the participants were fully informed about the study aim.

STUDY DESIGN:

This single-blinded, controlled, randomized, clinical trial was registered at Clinical Trials (NCT05206383).

The participants were randomly assigned to 1 of 3 groups: group P received positive verbal information, group N received negative verbal information, and group C (control group). Randomization was performed using opaque sealed envelopes. Each participant drew 1 envelope out of an opaque box, opened it and read out the symbol of the group (P, N, or C).

PARTICIPANTS:

A total of 91 individuals volunteered to participate in the study. However, 3 persons were excluded from the study because they did not meet the inclusion criteria (Figure 1).

Eighty-eight healthy white students aged 22.64 years (SD=5.2) took part in the study. The mean value of body mass index (BMI) was 23.5±3.22 kg/m2. Homogeneity of the groups under study is shown in Table 1.

Women constituted a slightly larger percentage of the study participants (61.4%). Detailed information is presented in Table 2. The vast majority of participants were right-handed (86.4%).

The inclusion criteria were as follows: the absence of pain or any signs of musculoskeletal dysfunction in the upper limbs, no use of any non-steroidal anti-inflammatory drugs (NSAIDs) for at least 1 week prior to study entry, age 19–24 years, written consent to participate in the study, absence of peripheral or central metabolic or neurological diseases, and lack of knowledge about kinesiology taping. The exclusion criteria were as follows: studying for a degree in physiotherapy, previous surgery on the examined upper limb, previous bone fractures in the examined upper limb, injury to the examined upper limb within 6 months before the start of the study, previous application of kinesiology taping, and allergic reaction to the tape application.

PROCEDURES:

A single-blinded (subject) study with a repeated-measure design was performed to evaluate the influence of verbal information on strength and activation of wrist and finger flexors.

The experiment was conducted in a standardized manner. Students’ dominant upper limbs were tested. Each participant sat on a chair without leaning forward, with the arm slightly adducted (15°), the elbow flexed at 90°, and the forearm in a neutral position (between supination and pronation).

The participants performed a 5-min warm-up that involved active exercises of fingers and wrists (flexion-extension, abduction-adduction, and circles, with 20 repetitions each).

Dynamometric measurements, performed with the use of the hand dynamometer KERN MAP 80K1S, were preceded by 3 warm-up attempts (duration – 3 s, interval between attempts – 1 min) that helped the participants learn how to use the dynamometer.

After a 10-min break, each participant was asked to hold the grip of the dynamometer as tight as possible for 3 s, with a 1-min rest interval between attempts. The values of 3 attempts were registered. Afterwards, paper tape was applied to the forearm of the dominating upper limb. In groups P and N, verbal information was provided, followed by an immediate request to perform the dynamometer test with maximum strength in the same standardized manner. Paper tape (Endura Fix Tape) 8 cm long was applied to the forearm in a neutral position on the lateral side in the long axis of the radius. Group P received positive verbal information – “the tape increases hand muscle strength.” Group N received negative verbal information – “the tape decreases hand muscle strength.” Group C received neutral verbal information – “the effect of the tape on hand muscle strength is unknown”.

At every stage of examination, measurements were also made using sEMG. Prior to placing electrodes on the body, the skin was cleaned with 90% alcohol solution. Ag/Ag electrodes 30 mm in diameter and with a conducting area of 16 mm (SORIMEX, Toruń, Poland) were placed on the anterolateral side of the forearm, 1 cm above the medial epicondyle of the humerus. The electrodes were placed along muscle fibers of wrist and finger flexors by experienced physiotherapists (K.Z. – author, and M.K.). All sEMG examinations were performed between 8 a.m. and 12 a.m. using the 8-channel Naroxon Ultium EMG system and mioMUSCLE system (Naroxon U.S.A., Inc.).

After completing the tests, the participants provided subjective feedback on how they felt their hand grip strength changed (definitely yes, rather yes, no change, rather not, definitely not).

DATA ANALYSES:

Statistical analysis was performed with the SPSS 17.0 software package (Atonicity, USA). The main tests used in statistical analyses were the t test for dependent samples, the t test for independent samples, and ANOVA. The tests were employed when certain requirements designed for parametric tests were met, 1 of which is the assumption of homogeneity of variance. The following dependence coefficients based on the chi-square test were used to calculate the qualitative-nominal data: Phi, V Kramer, while for variables on ordinal scales: Tau-b and Kendall’s Tau-c. Due to the non-parametric distribution (checked with the Shapiro-Wilk and Kolmogorov-Smirnov tests), quantitative variables were described by providing descriptive characteristics: mean, median, and standard deviation. The Bonferroni test was used to compare the post hoc equilibrium groups of the one-way ANOVA. The level of statistical significance was set at P<0.05.

Correlations between quantitative variables were determined using r-Pearson coefficient, which informs about the strength of a correlation as well as its direction (either positive or negative). The value obtained ranges from −1 to 1, where (−1) points to strong negative correlation, while (1) indicates strong positive correlation.

Results

DYNAMOMETRIC MEASUREMENTS:

Dynamometric measurements revealed significant changes in muscle strength in groups N and C. In groups P and C, an increase in muscle strength was noted, (P=0.067; P=0.035), whereas group N manifested a decrease in muscle strength (P=0.03).

MUSCLE ACTIVATION (SEMG):

None of the groups exhibited significant changes in muscle activation measured with sEMG (P>0.05). Detailed information is shown in Table 3.

DIFFERENCES BETWEEN THE GROUPS:

No significant differences in dynamometric and sEMG measurements between the studied groups were observed at any stage of examination (P>0.05). Detailed information is presented in Table 4.

In all groups, the highest number of participants (44.3% on average) did not feel any changes in muscle strength after the procedures were applied. No significant differences were noted between the groups in subjective evaluation of changes in muscle strength (P>0.05). Detailed information is shown in Table 5.

CORRELATIONS:

No correlations were found between subjective evaluation of muscle strength and dynamometric measurement results and sEMG in any of the examined groups (P>0.05). Detailed information is presented in Table 6.

Discussion

STUDY LIMITATIONS:

The present study has some limitations. To begin with, it only focused on effects observed immediately after taping and providing verbal suggestions. Moreover, it is important that we did not analyse the participants’ expectations, which considerably limits our interpretation of results with regard to this mechanism. Further research should be carried out taking this aspect into consideration. Also, we did not perform a double-blinded trial due to the impracticality of blinding the experimenter in our settings that included different verbal suggestions in placebo and nocebo groups.

It is worth noting that dynamometric measurements do not provide information on endurance and fatigue of grip [14]. In the measurement of muscle activation, a non-invasive method was used, in which amplitude is weakly associated to the effective neural drive to the muscles due to the effect of volume conductor, amplitude cancellation, and random positions of motor units in the muscle tissue [37].

Conclusions

The study revealed a significant decrease in the strength of finger flexors in the group with negative verbal information. It confirms that negative verbal information can cause the nocebo effect. At no stage of examination were the differences between the groups in dynamometric and sEMG measurements significant. From a practical standpoint, our findings may have serious implications for physiotherapists and coaches. For instance, if physiotherapists tell their patients that the therapy will make them feel sore, the likelihood of pain or functional state deterioration will be greater. Words associated with treatment should be chosen carefully to avoid negative consequences.