ESTABLISHING TRAINING PARAMETERS FOR A DEEP NEURAL NETWORK TO ASSESS 2D, FRONTAL PLANE KINEMATICS

The purpose of this study was to establish the optimal training parameters to assess frontal plane, 2D kinematics using DeepLabCut. DeepLabCut is an open-source platform that allows the user to train neural networks for customized feature detection in 2D videos. Deep neural networks were trained using frontal plane videos from 41 participants who completed single- and double-leg drop landings. Networks were trained with an increasing number of training iterations (25-250k) and training frames (200-800). Our results indicate that a minimum of 175k training iterations and 400 training frames were adequate for stable network performance (training/test errors= 2.8/3.7 pixels)

Body composition changes during 8 weeks of military training are not accurately captured by circumference-based assessments

In 1981, the US military adopted body fat standards to promote physical readiness and prevent obesity. Separate circumference-based equations were developed for women and men. Both predictive equations were known to underestimate %BF. However, it was not known how well these abdominal circumference-based methods tracked changes in %BF. This study examined the validity of the circumference-based %BF equations for assessing changes in %BF in young adult recruits during Army Basic Combat Training (BCT). Dual-energy X-ray absorptiometry (DXA) and circumference-based measures of %BF were obtained in women (n = 481) and men (n = 926) at the start (pre-BCT) and end (post-BCT) of 8 weeks of BCT. Repeated-measure ANOVAs were used to assess differences between DXA and circumference pre-BCT and for the change during BCT. Pre-BCT, circumferences underestimated %BF relative to DXA, with mean errors of −6.0% ± 4.4% for women and −6.0% ± 3.5% for men (both p < 0.01), and no difference between sexes was observed (p = 0.77). DXA detected a −4.0% ± 2.4% and −3.3% ± 2.8% change in %BF for women and men in response to BCT, respectively (both p < 0.01), whereas circumference estimates of %BF indicated a 0.0% ± 3.3% (p = 0.86) change in women and a −2.2% ± 3.3% (p < 0.01) change in men (sex difference by technique p < 0.01). In conclusion, circumference-based measures underestimated %BF at the start of BCT in both sexes as compared to DXA. Circumference measures underestimated changes in %BF during BCT in men and did not detect changes in women. These findings suggest that circumference-based %BF metrics may not be an appropriate tool to track changes in body composition during short duration training

Recovery From Muscle Fatigue in Young and Older Adults: Implications for Physical Function

As adults age, skeletal muscles become smaller and weaker, which can ultimately lead to declines in physical function and disability. In general, older adults produce less isometric force and dynamic power than younger adults. The effects of this weakness are amplified following a series of muscle contractions that result in muscle fatigue. Since daily routines consist of repeated series of activity followed by rest, it is important to understand how muscle recovers from fatigue. In particular, muscle power has been shown to be related to physical function and balance. Thus, understanding the process of recovery from muscle fatigue will help in preventing declines in physical function in older adults. This dissertation consisted of two studies designed to understand how muscle recover following fatigue and the implications of that recovery on physical function. Study one examined recovery from muscle fatigue following a constrained task. Young and older adults were fatigued to a similar degree using a dynamometer, and recovery of power at 4 velocities, central activation, pre-motor signaling, neural efficiency and contractile properties were recorded over an hour. To evaluate the functional implications of the recovery, ratings of perceived exertion were collected and the amount of fatigue following a second fatigue bout was also recorded. The second study associated changes in physical function and balance with power following an ecologically-relevant fatiguing exercise. Following a 30 minute treadmill walk, chair rise time and balance were measured during the period of recovery from this task. As a result of fatigue, we saw increased power loss at high-velocities that did not recover over the course of an hour in older adults. . This finding was concurrent with other velocity specific changes in rates of force development, muscle acceleration, and pre-motor neural signaling. Functionally, we saw an increased in perceived effort during contraction in older adults, and an increased fatigue during a second fatigue bout. While chair rise didn\u27t differ as a group with fatigue, there was a significant relationship with loss of high-velocity power and change in chair rise time over the hour recovery period. Balance declined immediate post-fatigue but appeared to recover to a point of greater stability over an hour. This dissertation provides novel insight about alterations in the recovery process following an acute bout of muscle fatigue, and ultimately provides data that may be useful for developing strategies to prevent disability in older adults

Post-Fatigue Recovery of Power, Postural Control and Physical Function in Older Women

Low muscle power, particularly at high velocities, has been linked to poor physical function in older adults. Any loss in muscle power following fatiguing exercise or daily activities could impact physical function and postural control until power has fully recovered. To test the overall hypothesis that a common task such as walking can result in prolonged power loss and decreased physical function and balance, 17 healthy older (66–81 years) women completed a 32-min walking test (32MWT) designed to induce neuromuscular fatigue, followed by 60min of recovery (60R). Fatigue and recovery of knee extensor muscle power (3 velocities) were quantified by dynamometry. Function was quantified by chair rise time and postural control by measures of center of pressure (COP) range (mm) and velocity (mm·s-1) during quiet stance. Power declined at all velocities by 8–13% 2min following the 32MWT (p≤0.02) and remained depressed by 8–26% at 60R (p≤0.04). Postural control decreased following the 32MWT, indicated by increased COP range in the anterior-posterior (AP, p\u3c0.01) direction and a trend in the medial-lateral (ML) direction (p = 0.09), and returned to baseline by 60R (p≥0.10). COP velocity was unchanged immediately following the 32MWT, but at 60R was lower in ML (p = 0.03) and tended to be reduced in AP (p = 0.07). Changes in high-velocity power (270°·s-1) were associated with altered postural control (p = 0.02) and chair rise performance (p≤0.03). These results provide evidence of long-duration neuromuscular changes following fatigue in healthy older women that may place them at increased risk for functional deficits during everyday mobility tasks

In vivo oxidative capacity varies with muscle and training status in young adults

It is well established that exercise training results in increased muscle oxidative capacity. Less is known about how oxidative capacities in distinct muscles, in the same individual, are affected by different levels of physical activity. We hypothesized that 1) trained individuals would have higher oxidative capacity than untrained individuals in both tibialis anterior (TA) and vastus lateralis (VL) and 2) oxidative capacity would be higher in TA than VL in untrained, but not in trained, individuals. Phosphorus magnetic resonance spectroscopy was used to measure the rate of phosphocreatine recovery (kPCr), which reflects the rate of oxidative phosphorylation, following a maximal voluntary isometric contraction of the TA and VL in healthy untrained (7 women, 7 men, 25.7 ± 3.6 yr; mean ± SD) and trained (5 women, 7 men, 27.5 ± 3.4 yr) adults. Daily physical activity levels were measured using accelerometry. The trained group spent threefold more time (∼90 vs. ∼30 min/day; P < 0.001) in moderate to vigorous physical activity (MVPA). Overall, kPCr was higher in VL than in TA (P = 0.01) and higher in trained than in untrained participants (P < 0.001). The relationship between kPCr and MVPA was more robust in VL (r = 0.64, P = 0.001, n = 25) than in TA (r = 0.38, P = 0.06, n = 25). These results indicate greater oxidative capacity in vivo in trained compared with untrained individuals in two distinct muscles of the lower limb and provide novel evidence of higher oxidative capacity in VL compared with TA in young humans, irrespective of training status. The basis for this difference is not known at this time but likely reflects a difference in usage patterns between the muscles

Post-fatigue recovery of power, postural control and physical function in older women

<div><p>Low muscle power, particularly at high velocities, has been linked to poor physical function in older adults. Any loss in muscle power following fatiguing exercise or daily activities could impact physical function and postural control until power has fully recovered. To test the overall hypothesis that a common task such as walking can result in prolonged power loss and decreased physical function and balance, 17 healthy older (66–81 years) women completed a 32-min walking test (32MWT) designed to induce neuromuscular fatigue, followed by 60min of recovery (60R). Fatigue and recovery of knee extensor muscle power (3 velocities) were quantified by dynamometry. Function was quantified by chair rise time and postural control by measures of center of pressure (COP) range (mm) and velocity (mm·s^-1) during quiet stance. Power declined at all velocities by 8–13% 2min following the 32MWT (p≤0.02) and remained depressed by 8–26% at 60R (p≤0.04). Postural control decreased following the 32MWT, indicated by increased COP range in the anterior-posterior (AP, p<0.01) direction and a trend in the medial-lateral (ML) direction (p = 0.09), and returned to baseline by 60R (p≥0.10). COP velocity was unchanged immediately following the 32MWT, but at 60R was lower in ML (p = 0.03) and tended to be reduced in AP (p = 0.07). Changes in high-velocity power (270°·s^-1) were associated with altered postural control (p = 0.02) and chair rise performance (p≤0.03). These results provide evidence of long-duration neuromuscular changes following fatigue in healthy older women that may place them at increased risk for functional deficits during everyday mobility tasks.</p></div

Group characteristics.

<p>Group characteristics.</p

Absolute (<i>left</i>) and relative (<i>right</i>) changes in torque and power for the 4 contraction velocities.

<p>Isometric torque and power at all velocities decreased from baseline following the 32MWT (p ≤0.03, all). At 60R, the relative power deficit increased with velocity (p = 0.04), with no such differences at 2R. <i>Left</i>: mean ± SEM; <i>right</i>: mean and 95% CI for difference from baseline; dashed line indicates baseline; *p<0.05 for difference from baseline. V₇₅: Velocity at which 75% of MVIC torque was generated.</p

Associations between changes in high-velocity power and postural control.

<p>The change in high-velocity (270°·s^-1) power in response to the 32MWT at 2R was negatively associated with the change in AP COP range (<i>top</i>), indicating that fatigue was associated with instability. At 60R, the power deficit at 270°·s^-1 was positively correlated with COP Velocity (bottom), suggesting that greater residual weakness was associated with increased stability.</p