Simulations suggest walking with reduced propulsive force would not mitigate the energetic consequences of lower tendon stiffness.

Aging elicits numerous effects that impact both musculoskeletal structure and walking function. Tendon stiffness (kT) and push-off propulsive force (FP) both impact the metabolic cost of walking and are diminished by age, yet their interaction has not been studied. We combined experimental and computational approaches to investigate whether age-related changes in function (adopting smaller FP) may be adopted to mitigate the metabolic consequences arising from changes in structure (reduced kT). We recruited 12 young adults and asked them to walk on a force-sensing treadmill while prompting them to change FP (±20% & ±40% of typical) using targeted biofeedback. In models driven by experimental data from each of those conditions, we altered the kT of personalized musculoskeletal models across a physiological range (2-8% strain) and simulated individual-muscle metabolic costs for each kT and FP combination. We found that kT and FP independently affect walking metabolic cost, increasing with higher kT or as participants deviated from their typical FP. Our results show no evidence for an interaction between kT and FP in younger adults walking at fixed speeds. We also reveal complex individual muscle responses to the kT and FP landscape. For example, although total metabolic cost increased by 5% on average with combined reductions in kT and FP, the triceps surae muscles experienced a 7% local cost reduction on average. Our simulations suggest that reducing FP during walking would not mitigate the metabolic consequences of lower kT. Wearable devices and rehabilitative strategies can focus on either kT or FP to reduce age-related increases in walking metabolic cost.

Quantifying mechanical and metabolic interdependence between speed and propulsive force during walking.

Walking speed is a useful surrogate for health status across the population. Walking speed appears to be governed in part by interlimb coordination between propulsive (FP) and braking (FB) forces generated during step-to-step transitions and is simultaneously optimized to minimize metabolic cost. Of those forces, FP generated during push-off has received significantly more attention as a contributor to walking performance. Our goal was to first establish empirical relations between FP and walking speed and then to quantify their effects on metabolic cost in young adults. To specifically address any link between FP and walking speed, we used a self-paced treadmill controller and real-time biofeedback to independently prescribe walking speed or FP across a range of condition intensities. Walking with larger and smaller FP led to instinctively faster and slower walking speeds, respectively, with ~80% of variance in walking speed explained by FP. We also found that comparable changes in either FP or walking speed elicited predictable and relatively uniform changes in metabolic cost, together explaining ~53% of the variance in net metabolic power and ~14% of the variance in cost of transport. These results provide empirical data in support of an interdependent relation between FP and walking speed, building confidence that interventions designed to increase FP will translate to improved walking speed. Repeating this protocol in other populations may identify other relations that could inform the time course of gait decline due to age and disease.

Muscle metabolic energy costs while modifying propulsive force generation during walking.

We pose that an age-related increase in the metabolic cost of walking arises in part from a redistribution of joint power where muscles spanning the hip compensate for insufficient ankle push-off and smaller peak propulsive forces (FP). Young adults elicit a similar redistribution when walking with smaller FP via biofeedback. We used targeted FP biofeedback and musculoskeletal models to estimate the metabolic costs of operating lower limb muscles in young adults walking across a range of FP. Our simulations support the theory of distal-to-proximal redistribution of joint power as a determinant of increased metabolic cost in older adults during walking.

Imaging and Simulation of Inter-muscular Differences in Triceps Surae Contributions to Forward Propulsion During Walking.

Forward propulsion during the push-off phase of walking is largely governed at the ankle by differential neuromechanical contributions from the biarticular medial (MG) and lateral gastrocnemii (LG) and the uniarticular soleus (SOL). However, the relative contribution of these individual muscles to forward propulsion is equivocal, with important implications for the design and control of wearable assistive devices and for targeted therapeutics. The aim of this study was to evaluate the agreement between empirical and model-predicted triceps surae (i.e., MG, LG, and SOL) contributions to forward propulsion during walking using conditions that systematically manipulated both walking speed and the mechanical demand for forward propulsion at a fixed speed-through the use of aiding and impeding forces. Ten young adults (age: 24.1 ± 3.6 years, 6M/4F) participated. We found that muscle-specific responses derived from experimental measurements (i.e., activation and fascicle behavior) were consistent with those derived from musculoskeletal simulations (i.e., muscle force and positive mechanical work) within the same subjects. In vivo, compared to walking normally, only LG muscle activation was affected by both aiding and impeding forces. Similarly, increased propulsive demand elicited greater relative fascicle shortening in the MG but not the SOL. In silico, only MG and LG force and positive mechanical work increased significantly to meet the increased demands for forward propulsion. By combining electromyography, ultrasound imaging, and musculoskeletal modeling in the same subjects, our cumulative findings suggest that the biarticular gastrocnemius muscles play a more significant role than the uniarticular soleus in governing changes in forward propulsion during the mid to late stance phase of walking.

Heart rate measures from the Apple Watch, Fitbit Charge HR 2, and electrocardiogram across different exercise intensities.

This study compared heart rate (HR) measurements for the Fitbit Charge HR 2 (Fitbit) and the Apple Watch devices with HR measurements for electrocardiogram (ECG). Thirty young adults (15/15 females/males, age 23.5 ± 3.0 years) completed the Bruce Protocol. HR measurements were recorded from the ECG and both devices every minute. Average HR for each participant was calculated for very light, light, moderate, vigorous and very vigorous intensities based on ECG-measured HR. A concordance correlation coefficient (CCC) was calculated to examine the strength of the relationship between ECG measured HR and HR measured by each device. Relative error rates (RER) were also calculated to indicate the difference between each device and ECG. An equivalence test was conducted to examine the equivalence of HRs measured by devices and ECG. The Apple Watch showed lower RER (2.4-5.1%) compared with the Fitbit (3.9-13.5%) for all exercise intensities. For both devices, the strongest relationship with ECG-measured HR was found for very light PA with very high CCC (>.90) and equivalence. The strength of the relationship declined as exercise intensity increased for both devices. These findings indicate that the accuracy of real-time HR monitoring by the Apple Watch and Fitbit Charge HR2 is reduced as exercise intensity increases.

Functional factors that are important correlates to physical activity in people with multiple sclerosis: a pilot study.

PURPOSE: Identify functional factors that are important correlates to physical activity levels among people with multiple sclerosis. METHODS: A total of eight functional tests were conducted and physical activity was objectively measured (Actigraph GT3X accelerometer) for one week in 34 people with multiple sclerosis. A corrected Akaike Information Criterion analysis was performed to identify the strongest correlates with moderate-to-vigorous physical activity, total activity and sedentary time. RESULTS: The multiple regression analysis converged on a model for moderate-to-vigorous physical activity (R2 = 0.31, F = 6.97, p= 0.003) that included total strength of the less-affected leg (partial r = 0.46, p = 0.007) and average peg test performance (partial r = -0.30, p = 0.087). The model for total activity (R2 = 0.40, F = 10.51, p < 0.001) included five times sit-to-stand performance (partial r= -0.44, p = 0.010) and total strength of the less-affected leg (partial r = 0.31, p = 0.077). The model for sedentary time (R2=0.22, F = 9.23, p = 0.005) only included total strength of the more affected leg (r= -0.47, p = 0.005). CONCLUSION: These results suggest that leg strength, manual dexterity and the ability to perform functional tasks may be important correlates with physical activity levels in people with multiple sclerosis. The findings of this pilot study can inform future investigations aiming to increase physical activity levels or develop improved rehabilitation protocols for people with multiple sclerosis. Implications for Rehabilitation Physical activity is an effective means of improving the symptoms associated with multiple sclerosis. Participation in physical activity by people with multiple sclerosis may be affected by functional factors such as leg strength, manual dexterity and the ability to rise from a seated position. Bilateral leg strength differences should be assessed and addressed in people with multiple sclerosis.