The effects of short-term and long-term experiences on co-contraction of lower extremity postural control muscles during continuous, multi-directional support-surface perturbations

The Effect of Wave Motion Intensities on Performance in a Simulated Search and Rescue Task and the Concurrent Demands of Maintaining Balance

OBJECTIVE

The purpose of this study was to examine how intensity of wave motions affects the performance of a simulated maritime search and rescue (SAR) task.

BACKGROUND

Maritime SAR is a critical maritime occupation; however, the effect of wave motion intensity on worker performance is unknown.

METHODS

Twenty-four participants (12 male, 12 female) performed a simulated search and rescue task on a six-degree-of-freedom motion platform in two conditions that differed in motion intensity (low and high). Task performance, electromyography (EMG), and number of compensatory steps taken by the individual were examined.

RESULTS

As magnitude of simulated motion increased, performance in the SAR task decreased, and was accompanied by increases in lower limb muscle activation and number of steps taken.

CONCLUSIONS

Performance of an SAR task and balance control may be impeded by high-magnitude vessel motions.

APPLICATION

This research has the potential to be used by maritime engineers, occupational health and safety professionals, and ergonomists to improve worker safety and performance for SAR operators.

The underlying mechanisms of improved balance after one and ten sessions of balance training in older adults

Training improves balance control in older adults, but the time course and neural mechanisms underlying these improvements are unclear. We studied balance robustness and performance, H-reflex gains, paired reflex depression, and co-contraction duration in ankle muscles after one and ten training sessions in 22 older adults (+65 yrs). Mediolateral balance robustness, time to balance loss in unipedal standing on a platform with decreasing rotational stiffness, improved (33%) after one session, with no further improvement after ten sessions. Balance performance, absolute mediolateral center of mass velocity, improved (18.75%) after one session in perturbed unipedal standing and (18.18%) after ten sessions in unperturbed unipedal standing. Co-contraction duration of soleus/tibialis anterior increased (16%) after ten sessions. H-reflex gain and paired reflex depression excitability did not change. H-reflex gains were lower, and soleus/tibialis anterior co-contraction duration was higher in participants with more robust balance after ten sessions, and co-contraction duration was higher in participants with better balance performance at several time-points. Changes in robustness and performance were uncorrelated with changes in co-contraction duration, H-reflex gain, or paired reflex depression. In older adults, balance robustness improved over a single session, while performance improved gradually over multiple sessions. Changes in co-contraction and excitability of ankle muscles were not exclusive causes of improved balance.

Aging Affects Lower Limb Joint Moments and Muscle Responses to a Split-Belt Treadmill Perturbation

Age-related changes cause more fall-related injuries and impede the recoveries by older adults compared to younger adults. This study assessed the lower limb joint moments and muscle responses to split-belt treadmill perturbations in two groups (14 healthy young group [23.36 ± 2.90 years] and 14 healthy older group [70.93 ± 4.36 years]) who performed two trials of unexpected split-belt treadmill perturbations while walking on a programmable split-belt treadmill. A motion capture system quantified the lower limb joint moments, and a wireless electromyography system recorded the lower limb muscle responses. The compensatory limb's (i.e., the tripped limb's contralateral side) joint moments and muscle responses were computed during the pre-perturbation period (the five gait cycles before the onset of a split-belt treadmill perturbation) and the recovery period (from the split-belt treadmill perturbation to the baseline gait relying on the ground reaction forces' profile). Joint moments were assessed by maximum joint moments, and muscle responses were quantified by the normalization (%) and co-contraction index (CCI). Joint moments and muscle responses of the compensatory limb during the recovery period were significantly higher for the YG than the OG, and joint moments (e.g., knee flexion and extension and hip flexion moments) and muscle responses during the recovery period were higher compared to the pre-perturbation period for both groups. For CCI, the older group showed significantly higher co-contraction for biceps femoris/rectus femoris muscles than the young group during the recovery period. For both groups, co-contraction for biceps femoris/rectus femoris muscles was higher during the pre-perturbation period than the recovery period. The study confirmed that older adults compensated for muscle weakness by using lower joint moments and muscle activations and increasing muscle co-contractions to recover balance after split-belt treadmill perturbations. A better understanding of the recovery mechanisms of older adults who train on fall-inducing systems could improve therapeutic regimens.

Negative Impacts of Prolonged Standing at Work on Musculoskeletal Symptoms and Physical Fatigue: The Fifth Korean Working Conditions Survey

PURPOSE

We aimed to investigate variations in the risk of low back pain (LBP), lower extremity muscle pain, and whole body fatigue according to differences in prolonged standing work hours in relation to risk factor exposure and rest frequency.

MATERIALS AND METHODS

From the fifth Korean Working Conditions Survey data collected in 2017, data for 32970 full-time workers who worked for more than 1 year at their present job were analyzed. We classified the workers according to exposure to fatigue or painful postures, carrying heavy objects, performance of repetitive movements that burden the musculoskeletal system, and how often they took a break. Relationships between time spent in a standing posture at work and risks of LBP, lower extremity muscle pain, and whole body fatigue were analyzed by multivariate logistic regression.

RESULTS

Of the full-time workers in the survey, 48.7% worked in a standing position for more than half of their total working hours. A higher odds ratio (OR) value for lower extremity muscle pain was observed in female not exposed to carrying heavy objects [OR: 3.551, 95% confidence interval (CI): 3.038-4.150] and not exposed to performing repetitive movements (OR: 3.555, 95% CI: 2.761-4.557).

CONCLUSION

Changes in work methodologies are needed to lower the number of hours spent in a prolonged standing posture at work, including being able to rest when workers want to do so, to reduce pain and fatigue.

Quantifying Segmental Contributions to Center-of-Mass Motion During Dynamic Continuous Support Surface Perturbations Using Simplified Estimation Models

Investigating balance reactions following continuous, multidirectional, support surface perturbations is essential for improving our understanding of balance control in moving environments. Segmental motions are often incorporated into rapid balance reactions following external perturbations to balance, although the effects of these motions during complex, continuous perturbations have not been assessed. This study aimed to quantify the contributions of body segments (ie, trunk, head, upper extremity, and lower extremity) to the control of center-of-mass (COM) movement during continuous, multidirectional, support surface perturbations. Three-dimensional, whole-body kinematics were captured while 10 participants experienced 5 minutes of perturbations. Anteroposterior, mediolateral, and vertical COM position and velocity were calculated using a full-body model and 7 models with reduced numbers of segments, which were compared with the full-body model. With removal of body segments, errors relative to the full-body model increased, while relationship strength decreased. The inclusion of body segments appeared to affect COM measures, particularly COM velocity. Findings suggest that the body segments may provide a means of improving the control of COM motion, primarily its velocity, during continuous, multidirectional perturbations, and constitute a step toward improving our understanding of how the limbs contribute to balance control in moving environments.

Role of a single session of ball throwing exercise on postural control in older adults with mild cognitive impairment

PURPOSE

The purpose of the study was to investigate the role of training in improvement of balance control in older adults with mild cognitive impairment.

METHODS

Older adults (mean age 78) with mild cognitive impairment (MCI) and cognitively intact older adults (mean age 72) were exposed to self-initiated perturbations while performing bilateral shoulder flexion task before and after a single training session consisting of throwing a medicine ball. EMG activity of six trunk and lower limb muscles was recorded. Muscle onsets, integrals of EMG, and muscle co-contraction (C) and reciprocal (R) activation indices were calculated and analyzed during the anticipatory and compensatory phases of postural control.

RESULTS

Anticipatory postural adjustments (APAs) were observed in both groups.  Individuals with MCI, as compared to controls, had higher level of co-contraction of muscles. The training resulted in enhancement of the generation of APAs in individuals with MCI seen as earlier onset of leg and trunk muscle activity prior to the bilateral arm flexion task. While smaller co-contraction of muscles post-training was seen in both the groups, the effect of a single training session was significant in control subjects only.

CONCLUSIONS

The outcome of the exploratory study suggests that perturbation-based training could be used to improve balance control in older adults with and without mild cognitive impairment.

Interaction Between Thoracic Movement and Lumbar Spine Muscle Activation Patterns in Young Adults Asymptomatic for Low Back Pain: A Cross-Sectional Study

OBJECTIVE

The purpose of this study was to investigate the interaction between thoracic movement and lumbar muscle co-contraction when the lumbar spine was held in a relatively neutral posture.

METHODS

Thirty young adults, asymptomatic for back pain, performed 10 trials of upright standing, maximum trunk range of motion, and thoracic movement tasks while lumbar muscle activation was measured. Lumbar co-contraction was calculated, compared between tasks, and correlated to thoracic angles.

RESULTS

Movement tasks typically exhibited greater co-contraction than upright standing. Co-contraction in the lumbar musculature was 67%, 45%, and 55% greater than upright standing for thoracic flex, thoracic bend, and thoracic twist, respectively. Generally, the thoracic movement task demonstrated greater co-contraction than the maximum task in the same direction. Co-contraction was also correlated to thoracic angles in each movement direction.

CONCLUSION

Tasks with thoracic movement and a neutral lumbar spine posture resulted in increases in co-contraction within the lumbar musculature compared with quiet standing and maximum trunk range-of-motion tasks. Findings indicated an interaction between the 2 spine regions, suggesting that thoracic posture should be accounted for during the investigation of lumbar spine mechanics.

Motor adaption during repeated motor control testing: Attenuated muscle activation without changes in response latencies

With repeated exposure to postural perturbations the human postural control system can adapt and create efficient strategies to counteract these perturbations. The Motor Control Test (MCT) is commonly used to elicit reactionary postural movements. Though this device has been assessed for possible learning effects and reliability of composite scores, yet no study has evaluated possible neuromuscular alterations repeated bouts might elicit. Twenty participants (age: 25 ± 4.73 years; height: 183.8 ± 8.5 cm; mass: 85.2 ± 15.6 kg) volunteered and, following familiarization, performed five full-randomized MCTs over six testing sessions. The first five sessions occurred on consecutive days, with the sixth occurring two days later. Electromyography (EMG) was recorded on right lower extremity knee flexors and extensors, and ankle plantarflexors and dorsiflexors. Response latencies and Mean and RMS muscle activity were calculated and analyzed using 1 × 5 (within days) and 1 × 6 (across days) RM ANOVA. Decreases in muscle activation of proximal musculature were noted between days and trials within days, however these changes were not maintained after the two-day retention period. No differences were detected for MCT scores. These results suggest repeated MCT exposure modifies neuromuscular responses to maintain similar reaction time through a postural control strategy shift.