Real-time feedback enhances forward propulsion during walking in old adults

BACKGROUND

Reduced propulsive function during the push-off phase of walking plays a central role in the deterioration of walking ability with age. We used real-time propulsive feedback to test the hypothesis that old adults have an underutilized propulsive reserve available during walking.

METHODS

8 old adults (mean [SD], age: 72.1 [3.9] years) and 11 young adults (age: 21.0 [1.5] years) participated. For our primary aim, old subjects walked: 1) normally, 2) with visual feedback of their peak propulsive ground reaction forces, and 3) with visual feedback of their medial gastrocnemius electromyographic activity during push-off. We asked those subjects to match a target set to 20% and 40% greater propulsive force or push-off muscle activity than normal walking. We tested young subjects walking normally only to provide reference ground reaction force values.

FINDINGS

Walking normally, old adults exerted 12.5% smaller peak propulsive forces than young adults (P<0.01). However, old adults significantly increased their propulsive forces and push-off muscle activities when we provided propulsive feedback. Most notably, force feedback elicited propulsive forces that were equal to or 10.5% greater than those of young adults (+20% target, P=0.87; +40% target, P=0.02). With electromyographic feedback, old adults significantly increased their push-off muscle activities but without increasing their propulsive forces.

INTERPRETATION

Old adults with propulsive deficits have a considerable and underutilized propulsive reserve available during level walking. Further, real-time propulsive feedback represents a promising therapeutic strategy to improve the forward propulsion of old adults and thus maintain their walking ability and independence.

Forces and mechanical energy fluctuations during diagonal stride roller skiing; running on wheels?

Mechanical energy is conserved during terrestrial locomotion in two ways; the inverted pendulum mechanism for walking and the spring-mass mechanism for running. Here, we investigated if diagonal stride cross-country roller skiing (DIA) utilizes similar mechanisms. Based on previous studies, we hypothesized that running and DIA would share similar patterns of kinetic energy (KE), gravitational potential energy (GPE) and elastic energy fluctuations as if roller skiing is “running on wheels”. Experienced skiers (N=9) walked, ran and roller skied with DIA at 1.25m/s and 3 m/s on a level dual-belt treadmill that recorded perpendicular and parallel forces. We calculated the KE and GPE of the COM from the force recordings. As expected, the KE and GPE fluctuated with an out-of-phase pattern during walking and an in-phase pattern during running. Unlike walking, during DIA, the KE and GPE fluctuations were in-phase as they are in running. However, during the glide phase, KE was dissipated as frictional heat and could not be stored elastically in the tendons like in running. Elastic energy storage and return epitomize running and thus we reject our hypothesis. Diagonal stride cross-country skiing is a biomechanically unique movement that only superficially resembles walking or running.

Dynamic stability of running: the effects of speed and leg amputations on the maximal Lyapunov exponent

In this paper, we study dynamic stability during running, focusing on the effects of speed, and the use of a leg prosthesis. We compute and compare the maximal Lyapunov exponents of kinematic time-series data from subjects with and without unilateral transtibial amputations running at a wide range of speeds. We find that the dynamics of the affected leg with the running-specific prosthesis are less stable than the dynamics of the unaffected leg and also less stable than the biological legs of the non-amputee runners. Surprisingly, we find that the center-of-mass dynamics of runners with two intact biological legs are slightly less stable than those of runners with amputations. Our results suggest that while leg asymmetries may be associated with instability, runners may compensate for this effect by increased control of their center-of-mass dynamics.

Activity and functions of the human gluteal muscles in walking, running, sprinting, and climbing

It has been suggested that the uniquely large gluteus maximus (GMAX) muscles were an important adaptation during hominin evolution based on numerous anatomical differences between humans and extant apes. GMAX electromyographic (EMG) signals have been quantified for numerous individual movements, but not across the range of locomotor gaits and speeds for the same subjects. Thus, comparing relative EMG amplitudes between these activities has not been possible. We assessed the EMG activity of the gluteal muscles during walking, running, sprinting, and climbing. To gain further insight into the function of the gluteal muscles during locomotion, we measured muscle activity during walking and running with external devices that increased or decreased the need to control either forward or backward trunk pitch. We hypothesized that 1) GMAX EMG activity would be greatest during sprinting and climbing and 2) GMAX EMG activity would be modulated in response to altered forward trunk pitch demands during running. We found that GMAX activity in running was greater than walking and similar to climbing. However, the activity during sprinting was much greater than during running. Further, only the inferior portion of the GMAX had a significant change with altered trunk pitch demands, suggesting that the hip extensors have a limited contribution to the control of trunk pitch movements during running. Overall, our data suggest that the large size of the GMAX reflects its multifaceted role during rapid and powerful movements rather than as a specific adaptation for a single submaximal task such as endurance running.

How does age affect leg muscle activity/coactivity during uphill and downhill walking?

Walking uphill and downhill can be challenging for community-dwelling old adults. We investigated the effects of age on leg muscle activity amplitudes and timing during level, uphill, and downhill walking. We hypothesized that old adults would exhibit smaller increases in ankle extensor muscle activities and greater increases in hip extensor muscle activities compared to young adults during uphill vs. level walking. We also hypothesized that, compared to level walking, antagonist leg muscle coactivation would be disproportionately greater in old vs. young adults during downhill walking. Ten old (72±5yrs) and ten young (25±4yrs) subjects walked at 1.25m/s on a treadmill at seven grades (0°, ±3°, ±6°, ±9°). We quantified the stance phase electromyographic activities of the gluteus maximus (GMAX), biceps femoris (BF), rectus femoris (RF), vastus medialis (VM), medial gastrocnemius (MG), soleus (SOL), and tibialis anterior (TA). Old adults exhibited smaller increases in MG activity with steeper uphill grade than young adults (e.g., +136% vs. +174% at 9°). A disproportionate recruitment of hip muscles led to GMAX activity approaching the maximum isometric capacity of these active old adults at steep uphill grades (e.g., old vs. young, 73% MVC vs. 33% MVC at +9°). Neither uphill nor downhill walking affected the greater coactivation of antagonist muscles in old vs. young adults. We conclude that the disproportionate recruitment of hip muscles with advanced age may have critical implications for maintaining independent mobility in old adults, particularly at steeper uphill grades.

The metabolic and mechanical costs of step time asymmetry in walking

Animals use both pendular and elastic mechanisms to minimize energy expenditure during terrestrial locomotion. Elastic gaits can be either bilaterally symmetric (e.g. run and trot) or asymmetric (e.g. skip, canter and gallop), yet only symmetric pendular gaits (e.g. walk) are observed in nature. Does minimizing metabolic and mechanical power constrain pendular gaits to temporal symmetry? We measured rates of metabolic energy expenditure and calculated mechanical power production while healthy humans walked symmetrically and asymmetrically at a range of step and stride times. We found that walking with a 42 per cent step time asymmetry required 80 per cent (2.5 W kg(-1)) more metabolic power than preferred symmetric gait. Positive mechanical power production increased by 64 per cent (approx. 0.24 W kg(-1)), paralleling the increases we observed in metabolic power. We found that when walking asymmetrically, subjects absorbed more power during double support than during symmetric walking and compensated by increasing power production during single support. Overall, we identify inherent metabolic and mechanical costs to gait asymmetry and find that symmetry is optimal in healthy human walking.

Advanced age affects the individual leg mechanics of level, uphill, and downhill walking

Advanced age brings biomechanical changes that may limit the uphill and/or downhill walking ability of old adults. Here, we investigated how advanced age alters individual leg mechanics during level, uphill, and downhill walking. We hypothesized that, compared to young adults, old adults would exhibit: (1) reduced trailing leg propulsive ground reaction forces (GRFs) and positive work rates during uphill walking, and (2) reduced leading leg braking ground reaction forces and negative work rates during downhill walking. We calculated the individual leg mechanical work performed by 10 old (mean±SD, age: 72±5 yrs) and 11 young (age: 26±5 yrs) adults walking at 1.25 m/s on a dual-belt force-measuring treadmill at seven grades (0° and ±3°, ±6°, ±9°). As hypothesized, old adults exhibited significantly reduced propulsive GRFs (e.g., -21% at +9°) and average trailing leg positive work rates (e.g., -26% at +9°) compared to young adults during both level and uphill walking. Old adults compensated by performing greater positive work than young adults during the subsequent single support phase. In contrast, we reject our second hypothesis. We found no differences in braking GRFs or negative work rates between old and young adults. However, old adults exhibited significantly reduced second peak perpendicular GRFs during downhill walking compared to young adults. Our findings most notably identify how advanced age may impair uphill walking ability and thus independence and quality of life.

The role of elastic energy storage and recovery in downhill and uphill running

In level running, humans and other animals store and recover elastic energy during each step. What role does elastic energy play during downhill and uphill running? We measured the fluctuations of the mechanical energy of the center of mass (CoM) of 15 human participants running at 3 m s(-1) on the level, downhill and uphill on a force-measuring treadmill mounted at 3, 6 and 9 deg. In level running, nearly symmetrical decreases and increases of the combined gravitational potential and kinetic (GPE+KE) energy of the CoM indicated equal possible elastic energy storage and recovery. However, asymmetrical fluctuations during hill running indicate reduced maximum possible elastic energy storage and return. We analyzed mechanical energy generation and dissipation during level and hill running by quantifying the anatomically estimated elastic energy storage (AEEE) in the arch and Achilles' tendon using peak ground reaction forces and anatomical characteristics. AEEE did not change with grade. At shallow downhill grades, the body must generate mechanical energy, though it dissipates more than it generates. At steeper downhill grades, little to no energy generation is required and only mechanical energy dissipation must occur. The downhill grade at which mechanical energy must no longer be generated occurs at approximately -9 deg, near the metabolically optimal running grade. At shallow uphill grades, mechanical energy must be generated to raise the CoM, and at steeper grades, additional energy must be generated to offset reduced elastic energy storage and return.

Energetic Cost and Preferred Speed of Walking in Obese vs. Normal Weight Women

OBJECTIVE

We tested the hypotheses that walking is more expensive for obese women, and they prefer slower walking speeds that minimize the gross energy cost per distance despite a greater relative aerobic effort [percent of maximal oxygen uptake (Vo(2max))/kg].

RESEARCH METHODS AND PROCEDURES

Twenty adult women, 10 obese (BMI = 34.1 +/- 3.2 kg/m(2)) and 10 normal weight (BMI = 20.4 +/- 2.1 kg/m(2)) volunteered. To determine the metabolic rate and energy cost per distance vs. speed relationships, we measured Vo(2) and V(CO(2)) while subjects walked on a treadmill at six speeds (0.50, 0.75, 1.0, 1.25, 1.5, and 1.75 m/s; 5-minute trials, with a 5-minute rest period between trials). We measured preferred walking speed on a 50-m section of level sidewalk and Vo(2max) using a modified Balke treadmill protocol.

RESULTS

Walking was 11% more expensive for the obese subjects, but they preferred to walk at similar speeds as normal weight subjects (1.40 vs. 1.47 m/s, p = 0.07). Both groups preferred walking speeds at which their gross energy cost per distance was almost minimized. Obese subjects had a smaller Vo(2max)/kg, so they required a greater relative aerobic effort at the preferred speed (51% vs. 36%, p = 0.001).

DISCUSSION

Obese women preferred a walking speed that minimized energy cost per distance, even though this strategy required a greater relative aerobic effort than walking more slowly. Our results suggest that walking slower for a set distance may be an appropriate exercise recommendation for a weight management prescription in obese adults.

Exercise efficiency of low power output cycling

Exercise efficiency at low power outputs, energetically comparable to daily living activities, can be influenced by homeostatic perturbations (e.g., weight gain/loss). However, an appropriate efficiency calculation for low power outputs used in these studies has not been determined. Fifteen active subjects (seven females, eight males) performed 14, 5-min cycling trials: two types of seated rest (cranks vertical and horizontal), passive (motor-driven) cycling, no-chain cycling, no-load cycling, cycling at low (10, 20, 30, 40 W), and moderate (50, 60, 80, 100, 120 W) power outputs. Mean delta efficiency was 57% for low power outputs compared to 41.3% for moderate power outputs. Means for gross (3.6%) and net (5.7%) efficiencies were low at the lowest power output. At low power outputs, delta and work efficiency values exceeded theoretical values. In conclusion, at low power outputs, none of the common exercise efficiency calculations gave values comparable to theoretical muscle efficiency. However, gross efficiency and the slope and intercept of the metabolic power vs mechanical power output regression provide insights that are still valuable when studying homeostatic perturbations.

Leg stiffness of sprinters using running-specific prostheses

Running-specific prostheses (RSF) are designed to replicate the spring-like nature of biological legs (bioL) during running. However, it is not clear how these devices affect whole leg stiffness characteristics or running dynamics over a range of speeds. We used a simple spring-mass model to examine running mechanics across a range of speeds, in unilateral and bilateral transtibial amputees and performance-matched controls. We found significant differences between the affected leg (AL) of unilateral amputees and both ALs of bilateral amputees compared with the bioL of non-amputees for nearly every variable measured. Leg stiffness remained constant or increased with speed in bioL, but decreased with speed in legs with RSPs. The decrease in leg stiffness in legs with RSPs was mainly owing to a combination of lower peak ground reaction forces and increased leg compression with increasing speeds. Leg stiffness is an important parameter affecting contact time and the force exerted on the ground. It is likely that the fixed stiffness of the prosthesis coupled with differences in the limb posture required to run with the prosthesis limits the ability to modulate whole leg stiffness and the ability to apply high vertical ground reaction forces during sprinting.

The effects of grade and speed on leg muscle activations during walking

Compared to level walking, additional muscle actions are required to raise and lower the center of mass during uphill and downhill walking, respectively. However, it remains unclear which muscle recruitment strategies are employed at typical grades when walking over a range of speeds. Based on previous reports, we hypothesized that, across a range of walking speeds, hip, knee, and ankle extensor muscle activations would increase with steeper uphill grade, but only knee extensor muscle activations would increase with steeper downhill grade. We also hypothesized that these changes in muscle activations with grade would become more pronounced at faster walking speeds. To test these hypotheses, 10 young adults (5M/5F) walked on a standard treadmill at seven grades (0°, ± 3°, ± 6°, and ± 9°) and three speeds (0.75, 1.25, and 1.75 ms(-1)). We quantified the stance phase electromyographic activities of the gluteus maximus (GMAX), biceps femoris (BF), rectus femoris (RF), vastus medialis (VM), medial gastrocnemius (MG), and soleus (SOL) muscles. On average, compared to level walking, hip (BF: 635%, GMAX: 345%), knee (RF: 165%, VM: 366%), and ankle (MG: 175%, SOL: 136%) extensor muscle activities increased to walk up 9°, but only knee (RF: 310%, VM: 246%) extensor muscle activities increased to walk down 9°. Further, these changes in muscle activations with grade became greater with faster walking speed. We conclude that people employ distinct uphill (hip, knee, and ankle extensors) and downhill (knee extensors) muscle recruitment strategies generally across walking speeds and progressively with steeper grade.

Mechanical work performed by the individual legs during uphill and downhill walking

Previous studies of the mechanical work performed during uphill and downhill walking have neglected the simultaneous negative and positive work performed by the leading and trailing legs during double support. Our goal was to quantify the mechanical work performed by the individual legs across a range of uphill and downhill grades. We hypothesized that during double support, (1) with steeper uphill grade, the negative work performed by the leading leg would become negligible and the trailing leg would perform progressively greater positive work to raise the center of mass (CoM), and (2) with steeper downhill grade, the leading leg would perform progressively greater negative work to lower the CoM and the positive work performed by the trailing leg would become negligible. 11 healthy young adults (6 M/5 F, 71.0±12.3 kg) walked at 1.25 m/s on a dual-belt force-measuring treadmill at seven grades (0, ±3, ±6, ±9°). We collected three-dimensional ground reaction forces (GRFs) and used the individual limbs method to calculate the mechanical work performed by each leg. As hypothesized, the trailing leg performed progressively greater positive work with steeper uphill grade, and the leading leg performed progressively greater negative work with steeper downhill grade (p<0.005). To our surprise, unlike level-ground walking, during double support the leading leg performed considerable positive work when walking uphill and the trailing leg performed considerable negative work when walking downhill (p<0.005). To understand how humans walk uphill and downhill, it is important to consider these revealing biomechanical aspects of individual leg function and interaction during double support.

The energetic cost of maintaining lateral balance during human running

To quantify the energetic cost of maintaining lateral balance during human running, we provided external lateral stabilization (LS) while running with and without arm swing and measured changes in energetic cost and step width variability (indicator of lateral balance). We hypothesized that external LS would reduce energetic cost and step width variability of running (3.0 m/s), both with and without arm swing. We further hypothesized that the reduction in energetic cost and step width variability would be greater when running without arm swing compared with running with arm swing. We controlled for step width by having subjects run along a single line (zero target step width), which eliminated any interaction effects of step width and arm swing. We implemented a repeated-measures ANOVA with two within-subjects fixed factors (external LS and arm swing) to evaluate main and interaction effects. When provided with external LS (main effect), subjects reduced net metabolic power by 2.0% (P = 0.032) and step width variability by 12.3% (P = 0.005). Eliminating arm swing (main effect) increased net metabolic power by 7.6% (P < 0.001) but did not change step width variability (P = 0.975). We did not detect a significant interaction effect between external LS and arm swing. Thus, when comparing conditions of running with or without arm swing, external LS resulted in a similar reduction in net metabolic power and step width variability. We infer that the 2% reduction in the net energetic cost of running with external LS reflects the energetic cost of maintaining lateral balance. Furthermore, while eliminating arm swing increased the energetic cost of running overall, arm swing does not appear to assist with lateral balance. Our data suggest that humans use step width adjustments as the primary mechanism to maintain lateral balance during running.

Why is walker-assisted gait metabolically expensive?

Walker-assisted gait is reported to be ∼200% more metabolically expensive than normal bipedal walking. However, previous studies compared different walking speeds. Here, we compared the metabolic power consumption and basic stride temporal-spatial parameters for 10 young, healthy adults walking without assistance and using 2-wheeled (2W), 4-wheeled (4W) and 4-footed (4F) walker devices, all at the same speed, 0.30m/s. We also measured the metabolic power demand for walking without any assistive device using a step-to gait at 0.30m/s, walking normally at 1.25m/s, and for repeated lifting of the 4F walker mimicking the lifting pattern used during 4F walker-assisted gait. Similar to previous studies, we found that the cost per distance walked was 217% greater with a 4F walker at 0.30m/s compared to unassisted, bipedal walking at 1.25m/s. Compared at the same speed, 0.30m/s, using a 4F walker was still 82%, 74%, and 55% energetically more expensive than walking unassisted, with a 4W walker and a 2W walker respectively. The sum of the metabolic cost of step-to walking plus the cost of lifting itself was equivalent to the cost of walking with a 4F walker. Thus, we deduce that the high cost of 4F walker assisted gait is due to three factors: the slow walking speed, the step-to gait pattern and the repeated lifting of the walker.

The effects of step width and arm swing on energetic cost and lateral balance during running

In walking, humans prefer a moderate step width that minimizes energetic cost and vary step width from step-to-step to maintain lateral balance. Arm swing also reduces energetic cost and improves lateral balance. In running, humans prefer a narrow step width that may present a challenge for maintaining lateral balance. However, arm swing in running may improve lateral balance and help reduce energetic cost. To understand the roles of step width and arm swing, we hypothesized that net metabolic power would be greater at step widths greater or less than preferred and when running without arm swing. We further hypothesized that step width variability (indicator of lateral balance) would be greater at step widths greater or less than preferred and when running without arm swing. Ten subjects ran (3m/s) at four target step widths (0%, 15%, 20%, and 25% leg length (LL)) with arm swing, at their preferred step width with arm swing, and at their preferred step width without arm swing. We measured metabolic power, step width, and step width variability. When subjects ran at target step widths less (0% LL) or greater (15%, 20%, and 25% LL) than preferred, both net metabolic power demand (by 3%, 9%, 12%, and 15%) and step width variability (by 7%, 33%, 46%, and 69%) increased. When running without arm swing, both net metabolic power demand (by 8%) and step width variability (by 9%) increased compared to running with arm swing. It appears that humans prefer to run with a narrow step width and swing their arms so as to minimize energetic cost and improve lateral balance.

Optimal choice

We present a classification and theoretical analysis of discrete-trial and free-operant choice procedures in which reinforcement is assigned to one alternative only, or independently to both, is either always available or conditionally available, and is either "held" or not from trial to trial. Momentary-maximizing and (globally) optimal choice sequences are defined in terms of initializing and marker events. Free-operant choice is analyzed in terms of a clock space whose axes are the times since the last A and B choices. The analysis shows that most molar matching data are derivable from momentary maximizing, and that the momentary-maximizing hypothesis has not been adequately tested in either discrete-trial or free-operant situations.

Running-specific prostheses limit ground-force during sprinting

Running-specific prostheses (RSP) emulate the spring-like behaviour of biological limbs during human running, but little research has examined the mechanical means by which amputees achieve top speeds. To better understand the biomechanical effects of RSP during sprinting, we measured ground reaction forces (GRF) and stride kinematics of elite unilateral trans-tibial amputee sprinters across a range of speeds including top speed. Unilateral amputees are ideal subjects because each amputee's affected leg (AL) can be compared with their unaffected leg (UL). We found that stance average vertical GRF were approximately 9 per cent less for the AL compared with the UL across a range of speeds including top speed (p < 0.0001). In contrast, leg swing times were not significantly different between legs at any speed (p = 0.32). Additionally, AL and UL leg swing times were similar to those reported for non-amputee sprinters. We infer that RSP impair force generation and thus probably limit top speed. Some elite unilateral trans-tibial amputee sprinters appear to have learned or trained to compensate for AL force impairment by swinging both legs rapidly.

The fastest runner on artificial legs: different limbs, similar function?

The recent competitive successes of a bilateral, transtibial amputee sprint runner who races with modern running prostheses has triggered an international controversy regarding the relative function provided by his artificial limbs. Here, we conducted three tests of functional similarity between this amputee sprinter and competitive male runners with intact limbs: the metabolic cost of running, sprinting endurance, and running mechanics. Metabolic and mechanical data, respectively, were acquired via indirect calorimetry and ground reaction force measurements during constant-speed, level treadmill running. First, we found that the mean gross metabolic cost of transport of our amputee sprint subject (174.9 ml O2·kg−1·km−1; speeds: 2.5–4.1 m/s) was only 3.8% lower than mean values for intact-limb elite distance runners and 6.7% lower than for subelite distance runners but 17% lower than for intact-limb 400-m specialists [210.6 (SD 13.2) ml O2·kg−1·km−1]. Second, the speeds that our amputee sprinter maintained for six all-out, constant-speed trials to failure (speeds: 6.6–10.8 m/s; durations: 2–90 s) were within 2.2 (SD 0.6)% of those predicted for intact-limb sprinters. Third, at sprinting speeds of 8.0, 9.0, and 10.0 m/s, our amputee subject had longer foot-ground contact times [+14.7 (SD 4.2)%], shorter aerial [−26.4 (SD 9.9)%] and swing times [−15.2 (SD 6.9)%], and lower stance-averaged vertical forces [−19.3 (SD 3.1)%] than intact-limb sprinters [top speeds = 10.8 vs. 10.8 (SD 0.6) m/s]. We conclude that running on modern, lower-limb sprinting prostheses appears to be physiologically similar but mechanically different from running with intact limbs.

Modulation of leg muscle function in response to altered demand for body support and forward propulsion during walking

A number of studies have examined the functional roles of individual muscles during normal walking, but few studies have examined which are the primary muscles that respond to changes in external mechanical demand. Here we use a novel combination of experimental perturbations and forward dynamics simulations to determine how muscle mechanical output and contributions to body support and forward propulsion are modulated in response to independent manipulations of body weight and body mass during walking. Experimentally altered weight and/or mass were produced by combinations of added trunk loads and body weight support. Simulations of the same experimental conditions were used to determine muscle contributions to the vertical ground reaction force impulse (body support) and positive horizontal trunk work (forward propulsion). Contributions to the vertical impulse by the soleus, vastii and gluteus maximus increased (decreased) in response to increases (decreases) in body weight; whereas only the soleus increased horizontal work output in response to increased body mass. In addition, soleus had the greatest absolute contribution to both vertical impulse and horizontal trunk work, indicating that it not only provides the largest contribution to both body support and forward propulsion, but the soleus is also the primary mechanism to modulate the mechanical output of the leg in response to increased (decreased) need for body support and forward propulsion. The data also showed that a muscle's contribution to a specific task is likely not independent of its contribution to other tasks (e.g., body support vs. forward propulsion).

Low metabolic cost of locomotion in ornate box turtles, Terrapene ornata

Evolution has produced a wide range of body plans, but for a given body mass, the energetic cost of transport (COT) of terrestrial animals falls in a relatively narrow range. Previous research indicates that the COT depends on the proficiency of minimizing mechanical work performed, efficiency of performing that work, and cost of generating force to support weight. Turtles are unique in that their protective shell and shoulder-girdle articulation may eliminate the need for the ;muscular sling'. In addition, turtles have slower, more efficient muscles than other vertebrates. However, slow locomotion may raise the COT by confounding mechanical-energy conservation via the inverted-pendulum mechanism. Our goal was to determine the metabolic COT and efficiency of a terrestrial turtle species during locomotion. We studied 18 ornate box turtles, Terrapene ornata. Walking speed was extremely slow (0.07+/-0.005 m s(-1)). The average minimum COT was 8.0+/-0.70 J kg(-1) m(-1) attained at approximately 0.1 m s(-1). Ornate box turtles consume only half the energy predicted by the allometric relationship for all terrestrial animals (15.9+/-0.35 J kg(-1) m(-1)), and, thus, appear to be very economical walkers. When walking up a 24 deg. incline turtles moved significantly slower (0.04+/-0.004 m s(-1)), but performed the extra work required to walk uphill with very high efficiencies (>49%). It appears that the co-evolution of a protective shell, the associated shoulder morphology, and very slow, efficient muscles produce both economical level walking and efficient uphill walking.

Independent effects of weight and mass on plantar flexor activity during walking: implications for their contributions to body support and forward propulsion

The ankle plantar flexor muscles, gastrocnemius (Gas) and soleus (Sol), have been shown to play important roles in providing body support and forward propulsion during human walking. However, there has been disagreement about the relative contributions of Gas and Sol to these functional tasks. In this study, using independent manipulations of body weight and body mass, we examined the relative contribution of the individual plantar flexors to support and propulsion. We hypothesized that Gas and Sol contribute to body support, whereas Sol is the primary contributor to forward trunk propulsion. We tested this hypothesis by measuring muscle activity while experimentally manipulating body weight and mass by 1) decreasing body weight using a weight support system, 2) increasing body mass alone using a combination of equal added trunk load and weight support, and 3) increasing trunk loads (increasing body weight and mass). The rationale for this study was that muscles that provide body support would be sensitive to changes in body weight, whereas muscles that provide forward propulsion would be sensitive to changes in body mass. Gas activity increased with added loads and decreased with weight support but showed only a small increase relative to control trials when mass alone was increased. Sol activity showed a similar increase with added loads and with added mass alone and decreased in early stance with weight support. Therefore, we accepted the hypothesis that Sol and Gas contribute to body support, whereas Sol is the primary contributor to forward trunk propulsion.

Running with horizontal pulling forces: the benefits of towing

Towing, or running with a horizontal pulling force, is a common technique used by adventure racing teams. During an adventure race, the slowest person on a team determines the team's overall performance. To improve overall performance, a faster runner tows a slower runner with an elastic cord attached to their waists. Our purpose was to create and validate a model that predicts the optimal towing force needed by two runners to achieve their best overall performance. We modeled the effects of towing forces between two runners that differ in solo 10-km performance time and/or body mass. We calculated the overall time that could be saved with towing for running distances of 10, 20, and 42.2-km based on equations from previous research. Then, we empirically tested our 10-km model on 15 runners. Towing improved overall running performance considerably and our model accurately predicted this performance improvement. For example, if two runners (a 70 kg runner with a 35 min solo 10-km time and a 70-kg runner with a 50-min solo 10-km time) maintain an optimal towing force throughout a 10-km race, they can improve overall performance by 15%, saving almost 8 min. Ultimately, the race performance time and body mass of each runner determine the optimal towing force.

Changing the demand on specific muscle groups affects the walk-run transition speed

It has been proposed that muscle-specific factors trigger the human walk-run transition. We investigated if changing the demand on trigger muscles alters the preferred walk-run transition speed. We hypothesized that (1) reducing the demand on trigger muscles would increase the transition speed and (2) increasing the demand on trigger muscles would decrease the transition speed. We first determined the normal preferred walk-run transition speed (PTS) using a step-wise protocol with a randomized speed order. We then determined PTS while subjects walked with external devices that decreased or increased the demand on specific muscle groups. We concurrently measured the electromyographic activity of five leg muscles (tibialis anterior, soleus, rectus femoris, medial and lateral gastrocnemius) at each speed and condition. For this study, we developed a dorsiflexor assist device that aids the dorsiflexor muscles. A leg swing assist device applied forward pulling forces at the feet thus aiding the hip flexors during swing. A third device applied a horizontal force near the center of mass, which impedes or aids forward progression thus overloading or unloading the plantarflexor muscles. We found that when demand was decreased in the muscles measured, the PTS significantly increased. Conversely, when muscle demand was increased in the plantar flexors, the PTS decreased. However, combining assistive devices did not produce an even faster PTS. We conclude that altering the demand on specific muscles can change the preferred walk-run transition speed. However, the lack of a summation effect with multiple external devices, suggests that another underlying factor ultimately determines the preferred walk-run transition speed.

Effects of independently altering body weight and body mass on the metabolic cost of running

The metabolic cost of running is substantial, despite the savings from elastic energy storage and return. Previous studies suggest that generating vertical force to support body weight and horizontal forces to brake and propel body mass are the major determinants of the metabolic cost of running. In the present study, we investigated how independently altering body weight and body mass affects the metabolic cost of running. Based on previous studies, we hypothesized that reducing body weight would decrease metabolic rate proportionally, and adding mass and weight would increase metabolic rate proportionally. Further, because previous studies show that adding mass alone does not affect the forces generated on the ground, we hypothesized that adding mass alone would have no substantial effect on metabolic rate. We manipulated the body weight and body mass of 10 recreational human runners and measured their metabolic rates while they ran at 3 m s(-1). We reduced weight using a harness system, increased mass and weight using lead worn about the waist, and increased mass alone using a combination of weight support and added load. We found that net metabolic rate decreased in less than direct proportion to reduced body weight, increased in slightly more than direct proportion to added load (added mass and weight), and was not substantially different from normal running with added mass alone. Adding mass alone was not an effective method for determining the metabolic cost attributable to braking/propelling body mass. Runners loaded with mass alone did not generate greater vertical or horizontal impulses and their metabolic costs did not substantially differ from those of normal running. Our results show that generating force to support body weight is the primary determinant of the metabolic cost of running. Extrapolating our reduced weight data to zero weight suggests that supporting body weight comprises at most 74% of the net cost of running. However, 74% is probably an overestimate of the metabolic demand of body weight to support itself because in reduced gravity conditions decrements in horizontal impulse accompanied decrements in vertical impulse.

Effects of Obesity on the Biomechanics of Walking at Different Speeds

PURPOSE

Walking is a recommended form of exercise for the treatment of obesity, but walking may be a critical source of biomechanical loads that link obesity and musculoskeletal pathology, particularly knee osteoarthritis. We hypothesized that compared with normal-weight adults 1) obese adults would have greater absolute ground-reaction forces (GRF) during walking, but their GRF would be reduced at slower walking speeds; and 2) obese adults would have greater sagittal-plane absolute leg-joint moments at a given walking speed, but these moments would be reduced at slower walking speeds.

METHODS

We measured GRF and recorded sagittal-plane kinematics of 20 adults (10 obese and 10 normal weight) as they walked on a level, force-measuring treadmill at six speeds (0.5-1.75 m.s(-1)). We calculated sagittal-plane net muscle moments at the hip, knee, and ankle.

RESULTS

Compared with their normal-weight peers, obese adults had much greater absolute GRF (N), stance-phase sagittal-plane net muscle moments (N.m) and step width (m).

CONCLUSIONS

Greater sagittal-plane knee moments in the obese subjects suggest that they walked with greater knee-joint loads than normal-weight adults. Walking slower reduced GRF and net muscle moments and may be a risk-lowering strategy for obese adults who wish to walk for exercise. When obese subjects walked at 1.0 versus 1.5 m.s(-1), peak sagittal-plane knee moments were 45% less. Obese subjects walking at approximately 1.1 m.s(-1) would have the same absolute peak sagittal-plane knee net muscle moment as normal-weight subjects when they walk at their typical preferred speed of 1.4 m.s(-1).

The effects of adding mass to the legs on the energetics and biomechanics of walking

PURPOSE

The metabolic cost of walking increases when mass is added to the legs, but the effects of load magnitude and location on the energetics and biomechanics of walking are unclear. We hypothesized that with leg loading 1) net metabolic rate would be related to the moment of inertia of the leg (I(leg)), 2) kinematics would be conserved, except for heavy foot loads, and 3) swing-phase sagittal-plane net muscle moments and swing-phase leg-muscle electromyography (EMG) would increase.

METHODS

Five adult males walked on a force-measuring treadmill at 1.25 m.s(-1) with no load and with loads of 2 and 4 kg per foot and shank, 4 and 8 kg per thigh, and 4, 8, and 16 kg on the waist. We recorded metabolic rate and sagittal-plane kinematics and net muscle moments about the hip, knee, and ankle during the single-stance and swing phases, and EMG of key leg muscles.

RESULTS

Net metabolic rate during walking increased with load mass and more distal location and was correlated with I(leg) (r2 = 0.43). Thigh loading was relatively inexpensive, helping to explain why the metabolic rate during walking is not strongly affected by body mass distribution. Kinematics, single-stance and swing-phase muscle moments, and EMG were similar while walking with no load or with waist, thigh, or shank loads. The increase in net metabolic rate with foot loading was associated with greater EMG of muscles that initiate leg swing and greater swing-phase muscle moments.

CONCLUSIONS

Distal leg loads increase the metabolic rate required for swinging the leg. The increase in metabolic rate with more proximal loads may be attributable to a combination of supporting (via hip abduction muscles) and propagating the swing leg.

Limitations to maximum running speed on flat curves

Why is maximal running speed reduced on curved paths? The leading explanation proposes that an increase in lateral ground reaction force necessitates a decrease in peak vertical ground reaction force, assuming that maximum leg extension force is the limiting factor. Yet, no studies have directly measured these forces or tested this critical assumption. We measured maximum sprint velocities and ground reaction forces for five male humans sprinting along a straight track and compared them to sprints along circular tracks of 1, 2, 3, 4 and 6 m radii. Circular track sprint trials were performed either with or without a tether that applied centripetal force to the center of mass. Sprinters generated significantly smaller peak resultant ground reaction forces during normal curve sprinting compared to straight sprinting. This provides direct evidence against the idea that maximum leg extension force is always achieved and is the limiting factor. Use of the tether increased sprint speed, but not to expected values. During curve sprinting, the inside leg consistently generated smaller peak forces compared to the outside leg. Several competing biomechanical constraints placed on the stance leg during curve sprinting likely make the inside leg particularly ineffective at generating the ground reaction forces necessary to attain maximum velocities comparable to straight path sprinting. The ability of quadrupeds to redistribute function across multiple stance legs and decouple these multiple constraints may provide a distinct advantage for turning performance.

Mechanical energy fluctuations during hill walking: the effects of slope on inverted pendulum exchange

Humans and other animals exchange gravitational potential energy (GPE) and kinetic energy (KE) of the center of mass during level walking. How effective is this energy exchange during downhill and uphill walking? Based on previous reports and our own reasoning, we expected that during downhill walking, the possibility for mechanical energy exchange would be enhanced and during uphill walking, the possibility for exchange would be reduced. We measured the fluctuations of the mechanical energies for five men and five women walking at 1.25 m s-1. Subjects walked on the level, downhill, and uphill on a force measuring treadmill mounted at 3°, 6° and 9°. We evaluated energy exchange during the single support period based on the GPE and KE fluctuation factors of phase relationship, relative magnitude and extent of symmetry. As expected, during level walking, the GPE and KE curves were out of phase, of similar magnitude, and nearly mirror images so that the fluctuations in combined (GPE+KE) energy were attenuated. During downhill walking, the fluctuations in the combined energy of the center of mass were smaller than those on the level, i.e. mechanical energy exchange was more effective. During uphill walking, the fluctuations in the combined energy of the center of mass were larger than those on the level, i.e. mechanical energy exchange was less effective. Mechanical energy exchange occurred during downhill, level and uphill walking, but it was most effective during downhill walking.

The locomotor kinematics of Asian and African elephants: changes with speed and size

For centuries, elephant locomotion has been a contentious and confusing challenge for locomotion scientists to understand, not only because of technical difficulties but also because elephant locomotion is in some ways atypical of more familiar quadrupedal gaits. We analyzed the locomotor kinematics of over 2400 strides from 14 African and 48 Asian elephant individuals (body mass 116-4632 kg) freely moving over ground at a 17-fold range of speeds, from slow walking at 0.40 m s-1 to the fastest reliably recorded speed for elephants, 6.8 m s-1. These data reveal that African and Asian elephants have some subtle differences in how size-independent kinematic parameters change with speed. Although elephants use a lateral sequence footfall pattern, like many other quadrupeds, they maintain this footfall pattern at all speeds, shifting toward a 25% phase offset between limbs (singlefoot) as they increase speed. The duty factors of elephants are greater for the forelimbs than for the hindlimbs, so an aerial phase for the hindquarters is reached at slower speeds than for the forequarters. This aerial phase occurs at a Froude number of around 1,matching theoretical predictions. At faster speeds, stance and swing phase durations approach asymptotes, with the duty factor beginning to level off,concurrent with an increase in limb compliance that likely keeps peak forces relatively low. This increase of limb compliance is reflected by increased compression of the hindlimbs. Like other tetrapods, smaller elephants are relatively more athletic than larger ones, but still move very similarly to adults even at &lt;500 kg. At any particular speed they adopt greater relative stride frequencies and relative stride lengths compared to larger elephants. This extends to near-maximal locomotor performance as well - smaller elephants reach greater Froude numbers and smaller duty factors, hence likely reach relatively greater peak loads on their limbs and produce this force more rapidly. A variety of lines of kinematic evidence support the inference that elephants change their mechanics near a Froude number of 1 (if not at slower speeds), at least to using more compliant limbs, if not spring-like whole-body kinetics. In some ways, elephants move similarly to many other quadrupeds,such as increasing speed mainly by increasing stride frequency (except at fast speeds), and they match scaling predictions for many stride parameters. The main difference from most other animals is that elephants never change their footfall pattern to a gait that uses a whole-body aerial phase. Our large dataset establishes what the normal kinematics of elephant locomotion are, and can also be applied to identify gait abnormalities that may signal musculoskeletal pathologies, a matter of great importance to keepers of captive elephants.

Effects of obesity and sex on the energetic cost and preferred speed of walking

The metabolic energy cost of walking is determined, to a large degree, by body mass, but it is not clear how body composition and mass distribution influence this cost. We tested the hypothesis that walking would be most expensive for obese women compared with obese men and normal-weight women and men. Furthermore, we hypothesized that for all groups, preferred walking speed would correspond to the speed that minimized the gross energy cost per distance. We measured body composition, maximal oxygen consumption, and preferred walking speed of 39 (19 class II obese, 20 normal weight) women and men. We also measured oxygen consumption and carbon dioxide production while the subjects walked on a level treadmill at six speeds (0.50–1.75 m/s). Both obesity and sex affected the net metabolic rate (W/kg) of walking. Net metabolic rates of obese subjects were only ∼10% greater (per kg) than for normal-weight subjects, and net metabolic rates for women were ∼10% greater than for men. The increase in net metabolic rate at faster walking speeds was greatest in obese women compared with the other groups. Preferred walking speed was not different across groups (1.42 m/s) and was near the speed that minimized gross energy cost per distance. Surprisingly, mass distribution (thigh mass/body mass) was not related to net metabolic rate, but body composition (% fat) was ( r2= 0.43). Detailed biomechanical studies of walking are needed to investigate whether obese individuals adopt novel energy saving mechanisms during walking.

Energy cost and muscular activity required for leg swing during walking

To investigate the metabolic cost and muscular actions required for the initiation and propagation of leg swing, we applied a novel combination of external forces to subjects walking on a treadmill. We applied a forward pulling force at each foot to assist leg swing, a constant forward pulling force at the waist to provide center of mass propulsion, and a combination of these foot and waist forces to evaluate leg swing. When the metabolic cost and muscle actions were at a minimum, the condition was considered optimal. We reasoned that the difference in energy consumption between the optimal combined waist and foot force trial and the optimal waist force-only trial would reflect the metabolic cost of initiating and propagating leg swing during normal walking. We also reasoned that a lower muscle activity with these assisting forces would indicate which muscles are normally responsible for initiating and propagating leg swing. With a propulsive force at the waist of 10% body weight (BW), the net metabolic cost of walking decreased to 58% of normal walking. With the optimal combination, a propulsive force at the waist of 10% BW plus a pulling force at the feet of 3% BW the net metabolic cost of walking further decreased to 48% of normal walking. With the same combination, the muscle activity of the iliopsoas and rectus femoris muscles during the swing phase was 27 and 60% lower, respectively, but the activity of the medial gastrocnemius and soleus before swing did not change. Thus our data indicate that ∼10% of the net metabolic cost of walking is required to initiate and propagate leg swing. Additionally, the hip flexor muscles contribute to the initiation and propagation leg swing.