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Abstract

BACKGROUND:

Muscle stretch reflexes are widely used to examine neural muscle function. The knowledge of reflex response in muscles crossing the shoulder is limited.

OBJECTIVE:

To quantify reflex modulation according to various subject postures and different procedures of muscle pre-activation steering.

METHODS:

Thirteen healthy male participants performed two sets of external shoulder rotation stretches in various positions and with different procedures of muscle pre-activation steering on an isokinetic dynamometer over a range of two different pre-activation levels. All stretches were applied with a dynamometer acceleration of 10 ${}^{4}$ ${}^{\circ}$ /s ${}^{2}$ and a velocity of 150 ${}^{\circ}$ /s. Electromyographical response was measured via sEMG.

RESULTS:

Consistent reflexive response was observed in all tested muscles in all experimental conditions. The reflex elicitation rate revealed a significant muscle main effect (F (5,288) $=$ 2.358, $\rho=$ 0.040; $\eta^{2}=$ 0.039; $f=$ 0.637) and a significant test condition main effect (F (1,288) $=$ 5.884, $\rho=$ 0.016; $\eta^{2}=$ 0.020; $f=$ 0.143). Reflex latency revealed a significant muscle pre-activation level main effect (F (1,274) $=$ 5.008, $\rho=$ 0.026; $\eta^{2}=$ 0.018; $f=$ 0.469).

CONCLUSION:

Muscular reflexive response was more consistent in the primary internal rotators of the shoulder. Supine posture in combination with visual feedback of muscle pre-activation level enhanced the reflex elicitation rate.

Keywords

Stretch reflex reflex modulation internal shoulder rotators external shoulder rotation subject position muscle activity feedback

1. Introduction

The stretch reflex was originally described by Liddell and Sherrington in 1924 [1]. Theoretically the stretch reflex can be evoked in any muscle resulting in up to three peaks in muscle activity [2]. The first peak coincides with the short latency stretch reflex (SLR) which is assumed to be a phasic monosynaptic response originating in Ia afferents providing homonymous excitation to the motor neurons of the stretched muscle. The SLR response is reported to begin as early as 20 ms to 50 ms after the stretch of the muscle and to last up to 40 ms (e.g. [3]). The second peak in muscle activity, first introduced by Hammond in 1954 [4], is usually defined as medium latency reflex (MLR). The MLR seems to be initiated by group II afferents with slower conduction velocity. Admittedly the exact reflex pathways of this reflex component remain mostly unknown. The MLR component of the stretch reflex is succeeded by a third considerably voluntary response. The long latency reflex (LLR) is believed to be a tonic polysynaptic muscular reaction. It is assumed that this late stretch response does not occur in relaxed muscles of healthy subjects in response to a rapid stretch [5].

Reflexive muscular responses as a complex reaction to short muscle contractions, sinusoidal movements or rapid displacement of a limb [6] are reported to be of major relevance to the involuntary control of posture and movement [7]. Moreover reflexive responses of muscles provide information that are highly supportive in diagnosing and grading of neuronal diseases such as spasticity and Parkinson [8]. Yet most of the previous findings on the stretch reflex in the literature are based on investigations of muscles around lower limb joints (the ankle [9], the knee [10] or the elbow [11]). Furthermore, the SLR component of the reflexive muscular response was mainly in the focus of previous research. So far, only few studies on the SLR were carried out on the muscles crossing the glenohumeral joint, especially on the internal rotator muscles of the shoulder [12, 13, 14]. In addition, these studies followed different and thus incomparable methodological approaches concerning the evocation of the stretch reflex resulting in a high variability in the reported results. Reflex amplitudes show a high inter-subject variability which complicates the interpretation of the results and comparison between subject groups. A strong evidence indicates that the stretch reflex is biased by several factors such as the velocity and the acceleration of the stretch, the limb position and the degree of muscle pre-activation [6]. Moreover, former research on the electrically evoked monosynaptic H-Reflex indicates that the position of the subject may also have a significant impact on the measurement outcomes [15].

The purpose of the current research was to investigate the influence of changes in subject posture, different procedures of regulating muscle pre-activation and the use of more potent shoulder abutment on the stretch reflex of the internal shoulder rotator muscles. We hypothesized that (a) external shoulder perturbation in an upright seated position, mimicking the functional position during the cocking phase in over-head sports, will yield significantly facilitated reflex response compared to a perturbation in a supine body position but (b) that the use of a visual muscle activity feedback and more potent shoulder abutment in the supine position will yield more consistent muscle pre-activation levels and consequently higher overall reflex elicitation rates than the control via a trigger set to a specific muscle pre-activation moment.

2. Methods

2.1 Participants

Thirteen healthy right-handed men (age: 24 $\pm$ 3 y, height 185 $\pm$ 6 cm, weight 82 $\pm$ 7 kg), without any pathological findings concerning their dominant shoulder within the last twelve months prior to the investigation voluntarily participated in this study. All subjects were familiar with different kinds of dynamic over-head sport (javelin throw, volleyball, handball). Before measurement all subjects were fully informed of the experiment procedures and provided written informed consent. The study’s compliance with the Declaration of Helsinki was approved by the Ethic Committee of the German Sport University Cologne beforehand. All subjects were assigned to an anonymous ID to mask individual identity and warrant data anonymity during the study.

2.2 Experimental protocol

The study consisted of one familiarisation and two test sessions. During the first test session subjects were seated in an upright position on an isokinetic dynamometer (IsoMed 2000, D&R GmbH, Hemau, Germany). Within this test condition the subject’s dominant arm was adjusted in the 90/90/90-postion (90 ${}^{\circ}$ abduction, 90 ${}^{\circ}$ external shoulder rotation, 90 ${}^{\circ}$ elbow flexion) (see Fig. 1) which was already used in a previous study [16]. We defined that the shoulder was 0 ${}^{\circ}$ externally rotated in this position. A purpose made back pad was used to stabilize the non-dominant shoulder allowing the dominant arm to mimic the functional position during the late cocking phase of a pitching motion. Individual maximum isometric moment of force and the corresponding muscle activity (MVC) of the internal shoulder rotator muscles in the 90/90/90-postion were evaluated from 3 trials with 2 minutes break in between. The highest values during a steady plateau were used for normalising pre-activation levels. Stretch reflexes were elicited by applying a positional perturbation to the dominant arm of the subjects to produce external rotation at the shoulder.

Figure 1.

A subject positioned in the dynamometer. Left: Starting position (90/90/90-position) in the upright seated test condition during a test session. Right: Starting position in the supine test condition during subject familiarisation.

During the test sessions external shoulder rotation stretches with a stretch velocity of 150 ${}^{\circ}$ /s and an acceleration of 10 ${}^{4}$ ${}^{\circ}$ /s ${}^{2}$ were applied to the subject’s dominant arm. All trials were carried out to a final stretch amplitude of 20 ${}^{\circ}$ external shoulder rotation, since former research showed no bias on relevant reflex parameters due to greater stretch amplitudes [16]. Furthermore, the relatively low maximal external stretch amplitude, in comparison to the functional position that can be observed during over-head throwing [17], contributes to the prevention of overstrain of the shoulder capsule. Ahead of each external perturbation onset subjects were instructed to pre-activate their internal rotator muscles, therefore they were requested to contract against a fixed resistance given by the isokinetic dynamometer. Two levels of muscle pre-activation were set to an internal rotation moment of either 0% MVC or 25% MVC according to the moment values of the MVC-test. The pre-activation was steered via verbal feedback. Whenever the required moment was reached subjects were asked to keep their effort on a constant level for at least 1 second before the trigger for the onset of the external shoulder rotation stretches was randomly released. This set-up results in two different testing conditions in the upright seated position. For each condition a minimum of 6 trials was accomplished with an inter trial rest period of at least 10 s to avoid bias by habituation and fatigue and the possible influence of post activation depression [18, 19].

The tests in the supine position were carried out in a second test session on a different day. The participants were placed in a supine posture on the isokinetic dynamometer with both shoulders stabilized to warrant a more potent abutment for the external shoulder stretches of their dominant arm (Fig. 1). The relative position of the dominant arm was identical to the tests in the upright seated posture (90/90/90-position). In terms of the dynamometer acceleration, velocity, muscle pre-activation level and maximal external rotation range of motion all stretches were applied under identical conditions as reported for the upright seated subject position. The muscle pre-activation level in this test session was not steered via the internal rotation moment values of the MVC-test but via a visual feedback (Feedback bar in EMGworks ${}^{\@setsize{\scriptsize}{8pt}{\viipt}{\@viipt}\textregistered}$ Software, Delsys Inc., Natick, USA) of the muscle activity of the sternal head of the pectoralis major determined by a surface electromyographic EMG sensor. The sternal head of the pectoralis was selected for the visual feedback of the muscle activity since it contracts synergistically during internal rotations of the shoulder and therefore did not react as sensitive as the clavicular head over the course of an internal shoulder rotation movement. This characteristic made it significantly easier for the participants to keep their muscle activation at the requested level for at least 1 s before onset of the external shoulder stretches. Slight variations of the timing of the perturbation onset were used to keep the perturbation unpredictable for the subjects.

2.3 Instrumentation

Surface EMG (sEMG) activity was recorded at a sampling frequency of 2000 Hz via a Bangnoli ${}^{\text{TM}}$ _8 amplifier (Delsys Inc., Natick, USA), a 16 Bits USB-6218 BNC A/D board (National Instruments, Austin, USA) and EMGworks ${}^{\@setsize{\scriptsize}{8pt}{\viipt}{\@viipt}\textregistered}$ Software (Delsys Inc., Natick, USA). Data were stored on a personal computer for further processing. Bipolar, rectangular sEMG sensors (41 mm $\times$ 20 mm) (Delsys Inc., Natick, USA) with a fixed interelectrode distance of 1 cm were used. Sensors were placed longitudinally with respect to the underlying muscle fiber arrangement and fixed to the skin above the respective muscles in accordance with the standards of the SENIAM-Project [20]. An EMG sensor was attached to each of the three muscle heads of the pectoralis (pars clavicularis, pars abdominalis and pars sternocostalis), one to the anterior deltoid (pars clavicularis), one to the lateral deltoid (pars acromialis) and one to the latissimus dorsi. The EMG sensor placement for the latissimus was carried out following the recommendations of Konrad [21] (no SENIAM recommendations for this muscle). Before EMG sensor placement the skin was shaved and cleansed with alcohol to increase the signal to noise ratio. For EMG data processing a custom-made interactive data-analysis software (MATLAB R2019a, MathWorks, Natick, USA) was used. The mechanical delay of the trials was measured by two accelerometers (2-D, 5 V signal voltage, 5 g measuring sensitivity), one was attached to the dynamometer and one was placed at the insertion of the pectoral muscle. The delay was determined by comparing the acceleration onset given by the dynamometer to those recorded by the accelerometers. The discrepancy between these data was defined as mechanical delay and therefore subtracted during reflex latency calculation.

2.4 Data processing and analysis

Surface EMG signals were demeaned and bandpass zero-lag filtered (10–450 Hz) with a digital fourth-order Butterworth filter. Apart from that the signals were full wave rectified to obtain absolute values. Subsequent to this procedure the sEMG signals were smoothed using a sixth order low pass filter with a cut-off frequency of 50 Hz. The onset of the stretch reflex response within a timeframe of 0–50 ms after perturbation was determined utilising an algorithm in accordance with the recommendations of Hodges and Bui [22]. Therefore, the point at which the magnitude of the sEMG exceeded a prespecified threshold was determined using a sliding window with a width of 25 ms (50 samples). As long as the threshold was not exceeded the window was advanced one sample at a time until an onset was found or until 50 ms after perturbation onset was reached. If no onset was found within this timeframe a missing value was recorded and this trial was excluded from further analysis. The threshold was set to 3 standard deviations of the averaged baseline muscle activity level for the 50 ms prior to the start of each perturbation. Although Hodges and Bui [22] reported that using computer based algorithmic calculations yield accurate results each trial was visually inspected to ensure that no movement artifact or other malfunctions were wrongly quantified as stretch reflex response. For between and within subject comparisons of the pre-activation activity level the Root Mean Square (RMS) within the period of 100–0 ms before perturbation onset was calculated and normalized to the RMS of the MVC trial of each subject. For each valid reflex response, the reflex gain was quantified as $\frac{\Delta\textit{stretch reflex amplitude}}{\Delta\textit{pre-activation % level}}$ . Whereby the stretch reflex amplitude was the peak-to-peak difference of the reflex signal, calculated from the non-rectified, unsmoothed sEMG signals. The pre-activation level for the reflex gain calculation was the average rectified sEMG amplitude present in the investigated muscle for a 100 ms window before perturbation onset. The reflex gain was calculated to assess the slope of the relationship between the stretch reflex and the pre-activation level. The reflex gain is known to be velocity sensitive and therefore it allows conclusions to be drawn on the number of motoneurons recruited in response to the perturbation [6]. The reflex elicitation rate was quantified by comparing the number of valid results to the maximum number of possible valid trials carried out for the relevant test condition.

2.5 Statistical analysis

All statistical data analyses were carried out on SPSS 25 (IBM Corporation, Armonk, USA). Mean values and SDs were calculated for the elicitation rate, the stretch reflex latency, the reflex gain and the muscle activity 100 ms to 0 ms prior perturbation onset of each measurement condition. Verification of naturally distributed residuals and the Leven’s test for confirmation of variance homoscedasticity were carried out as prerequisites for a repeated measure three-within factor analysis of variance (three-way ANOVA) with Bonferroni correction. If the prerequisites were not met a pairwise analysis by Wilcoxon signed-rank test was carried out. The significance level for all analyses was set at $\alpha\leqslant$ 0.05.

3. Results

Stretch reflexes were elicited in all tested muscles and in all test conditions. The elicitation rate revealed a significant muscle main effect (F (5,288) $=$ 2.358, $\rho=$ 0.040; $\eta^{2}=$ 0.039; $f=$ 0.637) which was expressed in a higher agonist response rate than synergist response rate pattern. The lateral deltoid showed the lowest reflex elicitation rate of all tested muscles throughout all test conditions, ranging from 39.85% to 73,64%. Muscle pre-activation level had a substantial impact on reflex elicitation rate. On average the reflex elicitation rate was about 30% higher in the presence of muscle pre-activation regardless of the procedure of muscle activity steering and subject positioning (F (1,288) $=$ 178.018, $\rho<$ 0.001; $\eta^{2}=$ 0.382; $f=$ 0.786). In addition, comparisons among the test conditions revealed a significant main effect of the supine subject position in combination with visual muscle pre-activation feedback compared to the upright seated position with verbal feedback (F (1,288) $=$ 5.884, $\rho=$ 0.016; $\eta^{2}=$ 0.020; $f=$ 0.143). More potent shoulder abutment for both shoulders and visual feedback of muscle pre-activation level resulted in overall higher elicitation rates.

Analysis of the muscle pre-activation revealed an interaction effect between muscle pre-activation level and the test condition (F (1,288) $=$ 12,242, $\rho=$ 0.001; $\eta^{2}=$ 0.041; $f=$ 0.207). This observation indicates that the level of muscle pre-activation varies depending on subject position and pre-activation steering. The mean muscle pre-activation level within the 0% MVC trials was 2.19 $\pm$ 0.79% of the predefined MVC muscle activation level in the supine test condition and 2.81 $\pm$ 1.33% in the upright seated condition revealing a substantial muscle resting tone in all subjects but no significant differences between the test conditions. On average subjects reached a muscle pre-activation level of 19.06 $\pm$ 3.41% of the predefined MVC muscle activation level if they were instructed to accomplish an internal shoulder rotation moment of 25% MVC. In trials with visual feedback subjects were instructed to achieve 25% of the predefined MVC muscle activation level while producing, on average, a pre-activation level of 23.17 $\pm$ 3.27% of the predefined MVC activation level.

Analysis of the reflex response revealed one identifiable component in all participants. Averaged results for all tested conditions showed that the onset of this component occurred at 29.0 $\pm$ 1.8 ms, uniform with the monosynaptic short latency stretch reflex response [11] in the upper extremities. A significant muscle pre-activation main effect was revealed for the reflex latency (F (1,274) $=$ 5.008, $\rho=$ 0.026; $\eta^{2}=$ 0.018; $f=$ 0.469). Generally, latencies were substantially shorter 28.0 $\pm$ 1.3 in the presence of muscle pre-activation compared to reflexes with no pre-activation 30.0 $\pm$ 1.9. However, the more potent shoulder abutment in the supine posture with visual pre-activation feedback and the resulting significantly higher pre-activation level in comparison to the upright seated subject positioning with muscle pre-activation steered via internal rotation moment did not reveal a significant test condition main effect on the reflex latency.

The results for the reflex gain did not meet the prerequisites for the three-way ANOVA, therefore a pairwise comparison by Wilcoxon signed-rank test was carried out. Comparisons between the tested muscles ( $n=$ 13) revealed significant differences in reflex responses in the presence of muscle pre-activation. The clavicular head of the pectoral and deltoid muscle showed greater reflex response than all the other muscles tested. Overall, only in two of the tested muscles a significant effect of the pre-activation level on the size of the reflex gain could be seen. Surprisingly the sternal head of the pectoralis ( $z=-$ 2.09, $\rho=$ 0.037, $n=$ 10; $r=$ 0.661) and the latissimus dorsi ( $z=-$ 2.981, $\rho=$ 0.003, $n=$ 12; $r=$ 0.860) showed significant higher reflex gains in no muscle pre-activation conditions in the supine position.

4. Discussion

As shown previously, our results demonstrate that a stretch reflex response could be elicited relatively consistently in all tested muscles. The main finding of this study was that the elicitation rates were significantly higher in the primary internal rotator muscles of the shoulder joint than in the affiliated synergist muscles. The pectoralis and the anterior deltoid are major internal rotators of the shoulder that contribute to shoulder stability by opposing external rotation forces [23]. Since intrafusal fibers of the muscle spindle run parallel to the extrafusal contractile elements of the muscle [24] the sensitivity of the muscle spindle could be adjusted over the entire range of motion of the shoulder joint yielding continuous signal alterations due to muscle elongation and rate of length changes. Hence, a swift and consistent reflexive response in these two muscles seems to be expected in response to a rapid external perturbation of the shoulder. In addition, higher pre-activation levels led to significant higher elicitation rates in all muscles. This observation could be explained by an increase in muscle spindle sensitivity to intramuscular lengthening [7, 23] due to higher muscle activity levels. Proske [25] reported that voluntary muscle contractions of 20–25% of the maximum strength led to a recruitment of all muscle spindles through the fusimotor system while at a muscle activity level of 5% MVC only 75% of the retrievable spindles were recruited [26]. Moreover, the test conditions influenced the consistency of the reflex response in our study. Trials in supine position caused significant higher reflex elicitation rates than those in an upright seated position. To our knowledge no evidence on reflex modulation in the thorax and shoulder muscles due to subject positioning is currently available. Nevertheless, some contrary findings to our observations on the stretch reflex in the lower limbs are reported in the literature. Shimba et al. [27] demonstrated a significantly reflexive response facilitation in the soleus muscle in a passive upright posture compared to a supine position. Several neural mechanisms were presented as possible explanations for these findings. As demonstrated by Dietz et al. [28] the addition of weight to a subject’s body under water led to an increase in the soleus stretch reflex response. The authors interpreted this observation as a reinforcing sensory information related to load, comparable to carrying one’s own bodyweight in an upright posture, enhancing the reflexive response of the soleus muscle. In our investigation we assumed a substantial impact on shoulder muscles load related proprioception through supine or upright seated subject positioning. Klein et al. [29] suggested that the proprioceptive sensitivity could vary with the direction of arm displacement relative to the gravitational force vector. This observation most likely influenced our measurement outcomes due to different alignments of the arm’s gravitational force vector with respect to the frontal plane in the upright seated position and to the transverse plane in the supine position. In addition, Suprak et al. [30] demonstrated that subject body orientation biased the kinesthetia of the shoulder joint, which might lead to body positioning dependent adjustments in the reflex threshold of the muscles crossing the shoulder. Furthermore, a potential influence of a sympathetic outflow on the muscle spindle sensitivity due to an enhancement of the sympathetic nervous systems activity resulting from a passive standing body posture has been discussed by Hjortskov et al. [31]. Some evidence revealed that the muscle sympathetic nervous activity increased successively with a progressive tilt in body position from a supine to an upright standing position [32]. Our results did not support this observation, but we cannot rule out that a switch between a supine and an upright seated posture may potentially induce an alteration in sympathetic nervous activity and therefore may have impacted our measurement outcomes in an unpredictable way. We assumed that the higher elicitation rates in the supine position could be explained by lesser variability in the muscle pre-activation level in all tested muscles evoked by the ongoing visual feedback of the current muscle activity. Since the subjects were asked to achieve a predefined muscle activity level prior perturbation onset, rather than an internal rotation moment as a product of all muscle activity surrounding the shoulder joint, the variance in activity between agonist and synergist muscles was significantly smaller in the supine test condition. Furthermore, we assumed that the more potent shoulder abutment for both shoulders in the supine position resulted in a reduction of muscle slack. As we could not exclude the incidence of small evasive trunk movements in the upright seated position these could have potentially increased the effect of muscle slack as a consequence of a convergence of muscle attachment points [33].

Although the supine test condition yielded significant higher pre-activation levels and reflex elicitation rates in comparison to the upright seated test condition, no biasing of the reflex latency could be traced to differences in the test procedure. Comparison between trials with no muscle pre-activation and with 25% of MVC, either steered via the internal shoulder rotation moment or via ongoing visual feedback of the current muscle activity, showed a significant shortening of reflex latencies in the presence of muscle pre-activation, confirming formerly reported results on the reflex latency in healthy subjects [34]. Even though we found significantly higher pre-activation levels in all tested muscles in the supine test condition compared to its uprightly seated counterpart, the difference in the muscle pre-activation levels was apparently not big enough to reveal substantial variance in reflex latency.

Analysis of the reflex gain as an indicator of the number of motoneurons recruited in response to the perturbation [6] revealed significantly higher increases in the primary internal rotators of the shoulder in comparison to the other tested muscles. This observation agreed with our expectations taking the relevance of these muscles for the maintenance of the joint stability into account. The reflex amplitude substantially increases in the presence of muscle pre-activation in all muscles. However, the observed reflex gain as a descriptor of the slope of the relationship between the stretch reflex and the pre-activation level revealed only in two of the tested muscles significant differences between pre-activated and non-pre-activated muscles. In contrast to the expected results the reflex gain was higher in the absence of muscle pre-activation in the sternal head of the pectoralis and in the latissimus dorsi in the supine position. This observation contrasted former results reported in the literature. Cronin et al. [35] revealed a modulating interrelation between muscle and tendinous tissue with variations in contraction level. The velocity at which muscle fascicles were stretched initially increased in the presence of moderate muscle activation activity and then successively decreased with an ongoing increase in the force level of the muscle resulting in a progressive reduction in reflex amplitude. However, the authors showed that this effect caused a significant reduction of the reflex amplitude only at high to very high muscle activity levels of at least more than 60% MVC leaving our results in variance with the estimated response. Individual review of the observed results of the pectoral sternal head and the latissimus dorsi in our investigation revealed two outliers which had a great impact on the averaged test values in these muscles. We assumed that in the absence of muscle pre-activation those two subjects had a potentially decreased reflex threshold resulting in an accelerated muscle spindle sensitivity for length changes. Nevertheless, the reproducibility of this observation needs to be examined in further research.

Although this study presented some interesting spheres for further research, there were some limitations to the study design. Even though we used two accelerometers to assess the mechanical delay of the measurement and therefore determined the actual perturbation onset there still may remain some undetected delay in the transmission of the external shoulder rotation motion to the adjusted shoulder muscles in both test conditions. The simultaneous acquisition of sEMG and accelerometer data of all investigated muscles would permit an even more precise evaluation of the mechanical delay of the chosen set-up resulting in a further increased accuracy in reflex detection. Furthermore, the potential bias of anticipation of the perturbation onset on the reflexive muscular response needs to be reconsidered. Strong evidence indicates an interference of perturbation predictability on the long latency reflex component [36, 37]. Even though no standardized recommendations for the optimal configuration of the pre-activation phase prior perturbation onset is reported in the literature we need to revalidate potential bias in our data. The effect of more randomised, longer pre-activation phases on the reflex latency needs to be investigated in further research.

5. Conclusion

The results of the current research indicate that in healthy subjects regardless of posture and the procedure of muscle pre-activation level steering consistent stretch reflex responses may be elicited in the presence of muscle pre-activation in the muscles crossing the shoulder joint by applying a positional perturbation to the dominant arm of the subjects using an isokinetic dynamometer to produce external rotation at the shoulder. This observation further enhanced our knowledge of the stretch reflex in the upper extremities and therefore offers an applicable method of neural muscle function examination for clinical applications and for neuromechanical analysis of human movements. Although we could not reveal a significant effect of subject position on reflex modulation, we would recommend a supine subject posture for further investigation. Supine position in combination with a visual ongoing feedback of the level of muscle activity led to a substantial increase in the reflex elicitation rate in our investigation enhancing the reproducibility of the measurement.

Footnotes

Acknowledgments

The authors thank Florian Weiser for assistance in customizing the MATLAB R2019a software for sEMG-data processing.

Conflict of interest

None to report.

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Increased shoulder muscle stretch reflex elicitability in supine subject posture

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

2. Methods

2.1 Participants

2.2 Experimental protocol

2.4 Data processing and analysis

2.5 Statistical analysis

3. Results

4. Discussion

5. Conclusion

Footnotes

Acknowledgments

Conflict of interest

References