Walking at the preferred stride frequency minimizes muscle activity

Biomechanical Consequences of Walking With the Suspended Backpacks

OBJECTIVE

This article aimed to investigate the biomechanical mechanisms underlying the energetic advantages of the suspended backpacks during load carriage.

METHODS

In this study, we examined eight adults walking with a 15 kg load at 5 km/h with a designed suspended backpack, in which the load could be switched to locked and suspended with four combinations of stiffness. Mechanical work and metabolic cost were measured during load carriage.

RESULTS

The results showed that the suspended backpacks led to an average reduction of 23.35% in positive work, 24.77% in negative work, and a 12.51% decrease in metabolic cost across all suspended load conditions. Notably, the decreased mechanical work predominantly occurred during single support (averaging 84.19% and 71.16% for positive and negative work, respectively), rather than during double support.

CONCLUSION

Walking with the suspended backpack induced a phase shift between body movement and load movement, altering the human-load interaction. This adjustment caused the body and load to move against each other, resulting in flatter trajectories of the human-load system center of mass (COM) velocities and corresponding profiles in ground reaction forces (GRFs), along with reduced vertical excursions of the trunk. Consequently, this interplay led to flatter trajectories in mechanical work rate and reduced mechanical work, ultimately contributing to the observed reduction in energetic expenditure.

SIGNIFICANCE

Understanding these mechanisms is essential for the development of more effective load-carrying devices and strategies in various applications, particularly for enhancing walking abilities during load carriage.

Alterations in step frequency and muscle activities using body weight support influence the ventilatory response to sinusoidal walking in humans

The use of body weight support (BWS) can reveal important insights into the relationship between lower-limb muscle activities and the ventilatory response during sinusoidal walking. Here, healthy participants (n = 15) walked on a treadmill while 0%, 30%, and 50% of their body weight was supported with BWS. The walking speed was varied sinusoidally between 3 and 6 km h-1, and three different frequencies, and periods ranging from 2 to 10 min were used. Breath-by-breath ventilation ([Formula: see text]) and CO2 output ([Formula: see text]) were measured. The tibialis anterior (TA) muscle activity was measured by electromyography throughout the walking. The amplitude (Amp), normalized Amp [Amp ratio (%)], and phase shift (PS) of the sinusoidal variations in measurement variables were calculated using a Fourier analysis. The results revealed that the Amp ratio in [Formula: see text] increased with the increase in BWS. A steeper slope of the [Formula: see text]-[Formula: see text] relationship and greater [Formula: see text]/[Formula: see text] values were observed under reduced body weight conditions. The Amp ratio in TA muscle was significantly positively associated with the Amp ratio in the [Formula: see text] (p < 0.001). These findings indicate that the greater amplitude in the TA muscle under BWS may have been a potent stimulus for the greater response of ventilation during sinusoidal walking.

Humans trade off whole-body energy cost to avoid overburdening muscles while walking

Metabolic cost minimization is thought to underscore the neural control of locomotion. Yet, avoiding high muscle activation, a cause of fatigue, often outperforms energy minimization in computational predictions of human gait. Discerning the relative importance of these criteria in human walking has proved elusive, in part, because they have not been empirically decoupled. Here, we explicitly decouple whole-body metabolic cost and 'fatigue-like' muscle activation costs (estimated from electromyography) by pitting them against one another using two distinct gait tasks. When experiencing these competing costs, participants (n = 10) chose the task that avoided overburdening muscles (fatigue avoidance) at the expense of higher metabolic power (p < 0.05). Muscle volume-normalized activation more closely models energy use and was also minimized by the participants' decision (p < 0.05), demonstrating that muscle activation was, at best, an inaccurate signal for metabolic energy. Energy minimization was only observed when there was no adverse effect on muscle activation costs. By decoupling whole-body metabolic and muscle activation costs, we provide among the first empirical evidence of humans embracing non-energetic optimality in favour of a clearly defined neuromuscular objective. This finding indicates that local muscle fatigue and effort may well be key factors dictating human walking behaviour and its evolution.

Modular control of muscle coordination patterns during various stride time and stride length combinations

BACKGROUND

Modular organization in muscular control is generally specified as synergistic muscle groups that are hierarchically organized. There are conflicting perspectives regarding modular organization for regulation of walking speeds, with regard to whether modular organization is relatively consistent across walking speeds. This conflict might arise from different stride time (time for one stride) and stride length combinations for achieving the same walking speed.

RESEARCH QUESTION

Does the regulation of the modular organization depend on stride time and stride length (stride time-length) combinations?

METHODS

Ten healthy men walked at a moderate speed (nondimensional speed = 0.4) on a treadmill at five different stride time-length combinations (very short, short, preferred, long, and very long). Surface electromyograms from 16 muscles in the trunk and lower limb were recorded. The modular organization was modeled as muscle synergies, which represent groups of synchronously activated muscles. Muscle synergies were extracted using a decomposition technique. The number of synergies and their activation durations were analyzed.

RESULTS

The number of synergies was consistent in the preferred and quasi-preferred condition (median: 4.5 [short], 4.5 [preferred], 5 [long]), while it varied in the extreme condition (median: 4 [very short] and 6 [very long]; 0.02 ≤  p ≤ 0.09). Gait parameters (stride time, stride length, stance time, swing time, and double stance time) were significantly different for preferred and quasi-preferred conditions (p <  0.03).

SIGNIFICANCE

Our results provide additional insights on the flexibility of modular control during walking, namely that the number of synergies or activations are fine-tuned even within one walking speed. Our finding implies that a variety of walking patterns can be achieved by consistent synergies except for extreme walking patterns.

Outcome-dependent effects of walking speed and age on quantitative and qualitative gait measures

BACKGROUND

Walking speed predicts many clinical outcomes in old age. However, a comprehensive assessment of how walking speed affects accelerometer based quantitative and qualitative gait measures in younger and older adults is lacking.

RESEARCH QUESTION

What is the relationship between walking speed and quantitative and qualitative gait outcomes in younger and older adults?

METHODS

Younger (n = 27, age: 21.6) and older participants (n = 27, age: 69.5) completed 340 steps on a treadmill at speeds of 0.70 to a maximum of 1.75 m·s^-1. We used generalized additive mixed models to determine the relationship between walking speed and quantitative (stride length, stride time, stride frequency and their variability) and qualitative (stride regularity, stability, smoothness, symmetry, synchronization, predictability) gait measures extracted from trunk accelerations.

RESULTS

The type of relationship between walking speed and the majority of gait measures (quantitative and qualitative) was characterized as logarithmic, with more prominent speed-effects at speeds below 1.20 m·s^-1. Changes in quantitative measures included shorter strides, longer stride times, and a lower stride frequency, with more variability at lower speeds independent of age. For qualitative measures, we found a decrease in gait symmetry, stability and regularity in all directions with decreasing speeds, a decrease in gait predictability (Vertical, V, anterior-posterior, AP) and stronger gait synchronization (AP-mediolateral, ML, AP-V), and direction dependent effects of gait smoothness, which decreased in V direction, but increased in AP and ML directions with decreasing speeds. We found outcome-dependent effects of age on the quantitative and qualitative gait measures, with either no differences between age-groups, age-related differences that existed regardless of speed, and age-related differences in the type of relationship with walking speed.

SIGNIFICANCE

The relationship between walking speed and quantitative and qualitative gait measures, and the effects of age on this relationship, depends on the type of gait measure studied.

Effects of using a whole-body powered exoskeleton during simulated occupational load-handling tasks: A pilot study

Whole-body powered exoskeletons (WB-PEXOs) can be effective in reducing the physical demands of heavy occupational work, yet almost no empirical evidence exists on the effects of WB-PEXO use. This study assessed the effects of WB-PEXO use on back and leg muscle activities during lab-based simulations of load handling tasks. Six participants (4M, 2F) completed two such tasks (load carriage and stationary load transfer), both with and without a WB-PEXO, and with a range of load masses in each task. WB-PEXO use reduced median levels of muscle activity in the back (∼42-53% in thoracic and ∼24-43% in lumbar regions) and legs (∼41-63% in knee flexors and extensors), and mainly when handling loads beyond low-moderate levels (10-15 kg). Overall, using the WB-PEXO also reduced inter-individual variance (smaller SD) in muscle activities. Future work should examine diverse users, focus on finding effective matches between WB-PEXO use and specific tasks, and identify applications in varied work environments.

Walking cadence (steps/min) and intensity in 61-85-year-old adults: the CADENCE-Adults study

BACKGROUND

Heuristic (i.e., evidence-based, rounded) cadences of ≥100 and ≥ 130 steps/min have consistently corresponded with absolutely-defined moderate (3 metabolic equivalents [METs]) and vigorous (6 METs) physical activity intensity, respectively, in adults 21-60 years of age. There is no consensus regarding similar thresholds in older adults.

PURPOSE

To provide heuristic cadence thresholds for 3, 4, 5, and 6 METs in 61-85-year-old adults.

METHODS

Ninety-eight community-dwelling ambulatory and ostensibly healthy older adults (age = 72.6 ± 6.9 years; 49% women) walked on a treadmill for a series of 5-min bouts (beginning at 0.5 mph with 0.5 mph increments) in this laboratory-based cross-sectional study until: 1) transitioning to running, 2) reaching ≥75% of their age-predicted maximum heart rate, or 3) reporting a Borg rating of perceived exertion > 13. Cadence was directly observed and hand-tallied. Intensity (oxygen uptake [VO2] mL/kg/min) was assessed with indirect calorimetry and converted to METs (1 MET = 3.5 mL/kg/min). Cadence thresholds were identified via segmented mixed effects model regression and using Receiver Operating Characteristic (ROC) curves. Final heuristic cadence thresholds represented an analytical compromise based on classification accuracy (sensitivity, specificity, positive and negative predictive value, and overall accuracy).

RESULTS

Cadences of 103.1 (95% Prediction Interval: 70.0-114.2), 116.4 (105.3-127.4), 129.6 (118.6-140.7), and 142.9 steps/min (131.8-148.4) were identified for 3, 4, 5, and 6 METs, respectively, based on the segmented regression. Comparable values based on ROC analysis were 100.3 (95% Confidence Intervals: 95.7-103.1), 111.5 (106.1-112.9), 116.0 (112.4-120.2), and 128.6 steps/min (128.3-136.4). Heuristic cadence thresholds of 100, 110, and 120 were associated with 3, 4, and 5 METs. Data to inform a threshold for ≥6 METs was limited, as only 6/98 (6.0%) participants achieved this intensity.

CONCLUSIONS

Consistent with previous data collected from 21-40 and 41-60-year-old adults, heuristic cadence thresholds of 100, 110, and 120 steps/min were associated with 3, 4, and 5 METs, respectively, in 61-85-year-old adults. Most older adults tested did not achieve the intensity of ≥6 METs; therefore, our data do not support establishing thresholds corresponding with this intensity level.

TRIAL REGISTRATION

Clinicaltrials.gov NCT02650258 . Registered 24 December 2015.

To walk or to run - a question of movement attractor stability

During locomotion, humans change gait mode between walking and running as locomotion speed is either increased or decreased. Dynamical systems theory predicts that the self-organization of coordinated motor behaviors dictates the transition from one distinct stable attractor behavior to another distinct attractor behavior (e.g. walk to run or vice versa) as the speed is changed. To evaluate this prediction, the present study investigated the attractor stability of walking and running across a range of speeds evoking both self-selected gait mode and non-self-selected gait mode. Eleven subjects completed treadmill walking for 3 min at 0.89, 1.12, 1.34, 1.56, 1.79, 2.01, 2.24 and 2.46 m s-1 and running for 3 min at 1.79, 2.01, 2.24, 2.46, 2.68, 2.91, 3.13 and 3.35 m s-1 in randomized order while lower limb joint angles and sacrum displacements was recorded. Attractor stability was quantified by continuous relative phase and deviation phase of lower limb segment angles, and the largest Lyapunov exponent, correlation dimension and movement variability of the sacrum marker displacement and the hip, knee and ankle joint angles. Lower limb attractor stability during walking was maximized at speeds close to the self-selected preferred walking speed and increased during running as speed was increased. Furthermore, lower limb attractor stability was highest at a particular gait mode closest to the corresponding preferred speed, in support of the prediction of dynamical systems theory. This was not the case for the sacrum displacement attractor, suggesting that lower limb attractor behavior provides a more appropriate order parameter compared with sacrum displacement.

Lower limb joint angle variability and dimensionality are different in stairmill climbing and treadmill walking

The present study tested if the quadratic relationship which exists between stepping frequency and gait dynamics in walking can be generalized to stairmill climbing. To accomplish this, we investigated the joint angle dynamics and variability during continuous stairmill climbing at stepping frequencies both above and below the preferred stepping frequency (PSF). Nine subjects performed stairmill climbing at 80, 90, 100, 110 and 120% PSF and treadmill walking at preferred walking speed during which sagittal hip, knee and ankle angles were extracted. Joint angle dynamics were quantified by the largest Lyapunov exponent (LyE) and correlation dimension (CoD). Joint angle variability was estimated by the mean ensemble standard deviation (meanSD). MeanSD and CoD for all joints were significantly higher during stairmill climbing but there were no task differences in LyE. Changes in stepping frequency had only limited effect on joint angle variability and did not affect joint angle dynamics. Thus, we concluded that the quadratic relationship between stepping frequency and gait dynamics observed in walking is not present in stairmill climbing based on the investigated parameters.

Neuromuscular effort predicts walk-run transition speed in normal and adapted human gaits

Often, humans and other animals move in a manner that minimizes energy costs. It is more economical to walk at slow speeds, and to run at fast speeds. Here, we asked whether humans select a gait that minimizes neuromuscular effort under novel and unfamiliar conditions, by imposing interlimb asymmetry during split-belt treadmill locomotion. The walk-run transition speed changed markedly across different gait conditions: forward, backward, hybrid (one leg forward, one leg backward) and forward with speed differences (one leg faster than the other). Most importantly, we showed that the human walk-run transition speed across conditions was predicted by changes in neuromuscular effort (i.e. summed leg muscle activations). Our results for forward gait and forward gait with speed differences suggest that human locomotor patterns are optimized under both familiar and novel gait conditions by minimizing the motor command for leg muscle activation.