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Kathryn Harvat, Natalie Parker, William Berg
Dept. of Kinesiology, Nutrition, & Health, Miami University
Abstract
Abstract
Anticipatory postural adjustments (APAs) are critical for maintaining postural stability during voluntary movements. While it is well-established that these mechanisms rely on sensory input, the role of vision in modulating APA characteristics remains unclear. The purpose of this study was to investigate the effect of vision on APAs using electromyography (EMG) to monitor muscle activity in the semitendinosus and lumbar erector spinae during a rapid bilateral arm-raising task. 40 participants performed 20 trials: 10 with normal vision, and 10 while blindfolded. Results demonstrated that APAs in the semitendinosus muscle occurred significantly earlier in the blindfolded condition, while no significant difference was observed in the lumbar erector spinae. These findings suggest that APAs in the semitendinosus may act earlier in the absence of visual cues.
Introduction
Introduction
Postural stability is essential for performing effective voluntary movements and is modulated by the central nervous system (CNS) through the integration of sensory inputs such as vision, proprioception, and vestibular information. One mechanism that is employed by the CNS to maintain balance during movement is anticipatory postural adjustments (APAs). APAs are muscle contractions that precede the initiation of voluntary movement and work in a feed-forward manner to allow the CNS to counteract impending postural disturbances (Aruin & Latash, 1995). For example, trunk and lower extremity muscle activation has been shown to precede movement of the arms in order to stabilize the body (Bouisset and Zattara, 1990). One way to study self-induced APAs is by using a bilateral arm-raising task, in which participants rapidly raise both arms parallel to the ground, inducing muscle contractions in the lumbar erector spinae, semitendinosus, and soleus muscles. Characteristics of APAs have been shown to adapt to various factors, such as fatigue (Strang and Berg, 2007), and neurological disorders such as Parkinson’s disease (Aruin et al., 1996). However, the role of vision in regulating APAs remains unclear.
The purpose of this study was to determine the effect of vision on characteristics of APAs. For example, we predict that APAs may act more conservatively to account for uncertainty about information such as body position, external environment, and spatial orientation in the absence of vision, by initiating muscle activation earlier. The findings of this research will help contribute to our understanding of the mechanisms used by the CNS to modulate postural control, and how the presence or absence of visual input may affect individuals’ postural stability.
Methods
Methods
Participants
This experiment involved 40 participants, 19 male and 17 female, between the ages of 18 and 28. All participants reported normal or corrected-to-normal vision and no injury or pathology of the shoulder. Each participant was paid $10 for participating in the study. The study was approved by the Miami University Institutional Review Board, and all participants provided informed consent.
Procedures
EMG electrodes were placed on participants to measure muscle activity in the anterior deltoid, lumbar erector spinae, semitendinosus, and soleus muscles on the participant’s dominant side. The electrode sites were cleaned and shaved before placing the electrodes. Cover-all tape and flex tape were used to secure the electrodes and sensors. Wrist weights (1 lb for females, 2 lbs for males) were attached to both hands using flex tape.
Participants wore a belt with a switch on their dominant side, positioned so the distal radioulnar joint naturally rested against the switch when the participant stood relaxed with arms at the sides. Participants were positioned with their feet shoulder-width apart, one arm length from an infrared beam at the height of the participant’s axilla to detect completion of the arm raising task. Participants were instructed to remain relaxed with both arms resting at their sides, and at the sound of the auditory stimulus, raise both arms to shoulder height as quickly as possible. The time between the initiation of the trial and the auditory stimulus varied between 1-6 seconds.
The study included two conditions: vision and blindfolded. Participants performed 3 practice trials for each condition, followed by 10 trials per condition. Condition order was counterbalanced across participants by sex. Participants rested 30-60 seconds between trials and 2 minutes between condition blocks.
Measurement and Data Analysis
EMG data was used to measure APA onset, by determining the time of muscle activation relative to the initiation of the anterior deltoid. Onset times were calculated for the semitendinosus and the lumbar erector spinae, while the soleus was omitted due to insufficient frequencies of APAs. For the purposes of this experiment, activity of the semitendinosus or erector spinae was considered an APA when activation onset was 100 ms prior to 50 ms after the onset of the anterior deltoid. Reaction time, movement time, and response time were also calculated for each trial. A paired t-test was used to compare muscle contraction onset relative to the onset of the anterior deltoid between the vision and blindfolded conditions.
Results
Results
APA Timing
For the semitendinosus muscle, APAs occurred significantly earlier in the blindfolded condition compared to the vision condition (p = 0.0029). The mean onset of contraction occurred at 0.001 seconds before contraction of the anterior deltoid in the blind condition, compared to 0.007 seconds after anterior deltoid contraction in the vision condition (Figure 1), indicating an earlier anticipatory response when visual input was removed.
Conversely, the lumbar erector spinae showed no significant differences between conditions (p=0.934). In both vision and blind conditions, muscle contraction occurred approximately 0.004 seconds before anterior deltoid contraction (Figure 1).
Reaction and Movement Time
Participants demonstrated significantly faster reaction times in the vision condition (p=0.043), and movement times were faster in the blindfolded condition (p=0.013). Total response time did not significantly differ between conditions (p=0.614).
Conclusion
Conclusion
The findings of this study advance our understanding of how the CNS uses sensory input to adjust the onset of anticipatory postural adjustments. These results suggest that visual input influences the timing of APAs in the semitendinosus muscle, providing insight into how the CNS compensates for sensory limitations by adjusting postural control.
Career Readiness Skills
Career Readiness Skills
Career & Self-Development: Learned how to design and conduct scientific research, developing foundational skills for a future in academic medicine.
Communication: Presented findings in written reports and oral presentation to communicate data in clear, accessible language for various audiences.
Critical Thinking: Applied scientific reasoning to develop a hypothesis, analyze data, and draw conclusions about anticipatory postural adjustments.
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