Healthy older adults have insufficient hip range of motion and plantar flexor strength to walk like healthy young adults

Limited plantar flexor strength and hip extension range of motion (ROM) in older adults are believed to underlie common age-related differences in gait. However, no studies of age-related differences in gait have quantified the percentage of strength and ROM used during gait. We examined peak hip angles, hip torques and plantar flexor torques, and corresponding estimates of functional capacity utilized (FCU), which we define as the percentage of available strength or joint ROM used, in 10 young and 10 older healthy adults walking under self-selected and controlled (slow and fast) conditions. Older adults walked with about 30% smaller hip extension angle, 28% larger hip flexion angle, 34% more hip extensor torque in the slow condition, and 12% less plantar flexor torque in the fast condition than young adults. Older adults had higher FCU than young adults for hip flexion angle (47% vs. 34%) and hip extensor torque (48% vs. 27%). FCUs for plantar flexor torque (both age groups) and hip extension angle (older adults in all conditions; young adults in self-selected gait) were not significantly <100%, and were higher than for other measures examined. Older adults lacked sufficient hip extension ROM to walk with a hip extension angle as large as that of young adults. Similarly, in the fast gait condition older adults lacked the strength to match the plantar flexor torque produced by young adults. This supports the hypothesis that hip extension ROM and plantar flexor strength are limiting factors in gait and contribute to age-related differences in gait.     

Effect of squat depth and barbell load on relative muscular effort in squatting

ABSTRACT: Bryanton, MA, Kennedy, MD, Carey, JP, and Chiu, LZF. Effect of squat depth and barbell load on relative muscular effort in squatting. J Strength Cond Res 26(10): 2820-2828, 2012-Resistance training is used to develop muscular strength and hypertrophy. Large muscle forces, in relation to the muscle's maximum force-generating ability, are required to elicit these adaptations. Previous biomechanical analyses of multi-joint resistance exercises provide estimates of muscle force but not relative muscular effort (RME). The purpose of this investigation was to determine the RME during the squat exercise. Specifically, the effects of barbell load and squat depth on hip extensor, knee extensor, and ankle plantar flexor RME were examined. Ten strength-trained women performed squats (50-90% 1 repetition maximum) in a motion analysis laboratory to determine hip extensor, knee extensor, and ankle plantar flexor net joint moment (NJM). Maximum isometric strength in relation to joint angle for these muscle groups was also determined. Relative muscular effect was determined as the ratio of NJM to maximum voluntary torque matched for joint angle. Barbell load and squat depth had significant interaction effects on hip extensor, knee extensor, and ankle plantar flexor RME (p < 0.05). Knee extensor RME increased with greater squat depth but not barbell load, whereas the opposite was found for the ankle plantar flexors. Both greater squat depth and barbell load increased hip extensor RME. These data suggest that training for the knee extensors can be performed with low relative intensities but require a deep squat depth. Heavier barbell loads are required to train the hip extensors and ankle plantar flexors. In designing resistance training programs with multi-joint exercises, how external factors influence RME of different muscle groups should be considered to meet training objectives.     

Power output and work in different muscle groups during ergometer cycling

The aim of this study was to calculate the magnitude of the instantaneous muscular power output at the hip, knee and ankle joints during ergometer cycling. Six healthy subjects pedalled a weight-braked bicycle ergometer at 120 watts (W) and 60 revolutions per minute (rpm). The subjects were filmed with a cine camera, and pedal reaction forces were recorded from a force transducer mounted in the pedal. The muscular work at the hip, knee and ankle joint was calculated using a model based upon dynamic mechanics described elsewhere. The mean peak concentric power output was, for the hip extensors, 74.4 W, hip flexors, 18.0 W, knee extensors, 110.1 W, knee flexors, 30.0 W and ankle plantar flexors, 59.4 W. At the ankle joint, energy absorption through eccentric plantar flexor action was observed, with a mean peak power of 11.4 W and negative work of 3.4 J for each limb and complete pedal revolution. The energy production relationships between the different major muscle groups were computed and the contributions to the total positive work were: hip extensors, 27%; hip flexors, 4%; knee extensors, 39%; knee flexors, 10%; and ankle plantar flexors 20%.     

The neuromuscular demands of toe walking: a forward dynamics simulation analysis

Toe walking is a gait deviation with multiple etiologies and often associated with premature and prolonged ankle plantar flexor electromyographic activity. The goal of this study was to use a detailed musculoskeletal model and forward dynamical simulations that emulate able-bodied toe and heel-toe walking to understand why, despite an increase in muscle activity in the ankle plantar flexors during toe walking, the internal ankle joint moment decreases relative to heel-toe walking. The simulations were analyzed to assess the force generating capacity of the plantar flexors by examining each muscle's contractile state (i.e., the muscle fiber length, velocity and activation). Consistent with experimental measurements, the simulation data showed that despite a 122% increase in soleus muscle activity and a 76% increase in gastrocnemius activity, the peak internal ankle moment in late stance decreased. The decrease was attributed to non-optimal contractile conditions for the plantar flexors (primarily the force-length relationship) that reduced their ability to generate force. As a result, greater muscle activity is needed during toe walking to produce a given muscle force level. In addition, toe walking requires greater sustained plantar flexor force and moment generation during stance. Thus, even though toe walking requires lower peak plantar flexor forces that might suggest a compensatory advantage for those with plantar flexor weakness, greater neuromuscular demand is placed on those muscles. Therefore, medical decisions concerning whether to reduce equinus should consider not only the impact on the ankle moment, but also the expected change to the plantar flexor's force generating capacity.     

Lower extremity control during turns initiated with and without hip external rotation

The pirouette turn is often initiated in neutral and externally rotated hip positions by dancers. This provides an opportunity to investigate how dancers satisfy the same mechanical objectives at the whole-body level when using different leg kinematics. The purpose of this study was to compare lower extremity control strategies during the turn initiation phase of pirouettes performed with and without hip external rotation. Skilled dancers (n=5) performed pirouette turns with and without hip external rotation. Joint kinetics during turn initiation were determined for both legs using ground reaction forces (GRFs) and segment kinematics. Hip muscle activations were monitored using electromyography. Using probability-based statistical methods, variables were compared across turn conditions as a group and within-dancer. Despite differences in GRFs and impulse generation between turn conditions, at least 90% of each GRF was aligned with the respective leg plane. A majority of the net joint moments at the ankle, knee, and hip acted about an axis perpendicular to the leg plane. However, differences in shank alignment relative to the leg plane affected the distribution of the knee net joint moment when represented with respect to the shank versus the thigh. During the initiation of both turns, most participants used ankle plantar flexor moments, knee extensor moments, flexor and abductor moments at the push leg׳s hip, and extensor and abductor moments at the turn leg׳s hip. Representation of joint kinetics using multiple reference systems assisted in understanding control priorities.     

Walking with increased ankle pushoff decreases hip muscle moments

In a simple bipedal walking model, an impulsive push along the trailing limb (similar to ankle plantar flexion) or a torque at the hip can power level walking. This suggests a tradeoff between ankle and hip muscle requirements during human gait. People with anterior hip pain may benefit from walking with increased ankle pushoff if it reduces hip muscle forces. The purpose of our study was to determine if simple instructions to alter ankle pushoff can modify gait dynamics and if resulting changes in ankle pushoff have an effect on hip muscle requirements during gait. We hypothesized that changes in ankle kinetics would be inversely related to hip muscle kinetics. Ten healthy subjects walked on a custom split-belt force-measuring treadmill at 1.25m/s. We recorded ground reaction forces and lower extremity kinematic data to calculate joint angles and internal muscle moments, powers and angular impulses. Subjects walked under three conditions: natural pushoff, decreased pushoff and increased pushoff. For the decreased pushoff condition, subjects were instructed to push less with their feet as they walked. Conversely, for the increased pushoff condition, subjects were instructed to push more with their feet. As predicted, walking with increased ankle pushoff resulted in lower peak hip flexion moment, power and angular impulse as well as lower peak hip extension moment and angular impulse (p<0.05). Our results emphasize the interchange between hip and ankle kinetics in human walking and suggest that increased ankle pushoff during gait may help to compensate for hip muscle weakness or injury and reduce hip joint forces.     

Empirical evaluation of gastrocnemius and soleus function during walking

Distinguishing gastrocnemius and soleus muscle function is relevant for treating gait disorders in which abnormal plantarflexor activity may contribute to pathological movement patterns. Our objective was to use experimental and computational analysis to determine the influence of gastrocnemius and soleus activity on lower limb movement, and determine if anatomical variability of the gastrocnemius affected its function. Our hypothesis was that these muscles exhibit distinct functions, with the gastrocnemius inducing limb flexion and the soleus inducing limb extension. To test this hypothesis, the gastrocnemius or soleus of 20 healthy participants was electrically stimulated for brief periods (90 ms) during mid- or terminal stance of a random gait cycle. Muscle function was characterized by the induced change in sagittal pelvis, hip, knee, and ankle angles occurring during the 200 ms after stimulation onset. Results were corroborated with computational forward dynamic gait models, by perturbing gastrocnemius or soleus activity during similar portions of the gait cycle. Mid- and terminal stance gastrocnemius stimulation induced posterior pelvic tilt, hip flexion and knee flexion. Mid-stance gastrocnemius stimulation also induced ankle dorsiflexion. In contrast mid-stance soleus stimulation induced anterior pelvic tilt, knee extension and plantarflexion, while late-stance soleus stimulation induced relatively little change in motion. Model predictions of induced hip, knee, and ankle motion were generally in the same direction as those of the experiments, though the gastrocnemius? results were shown to be quite sensitive to its knee-to-ankle moment arm ratio.     

Age causes a redistribution of joint torques and powers during gait.

At self-selected walking speeds, elderly compared with young adults generate decreased joint torques and powers in the lower extremity. These differences may be actual gait-limiting factors and neuromuscular adaptations with age or simply a consciously selected motor pattern to produce a slower gait. The purpose of the study was to compare joint torques and powers of young and elderly adults walking at the same speed. Twelve elderly and fourteen young adults (ages 69 and 21 yr) walked at 1.48 m/s over a force platform while being videotaped. Hip, knee, and ankle torques and powers were calculated from the reaction force and kinematic data. A support torque was calculated as the sum of the three joint torques. Extensor angular impulse during stance and positive work at each joint were derived from the torques and powers. Step length was 4% shorter and cadence was 4% higher in elderly adults (both P < 0.05) compared with young adults. Support angular impulse was nearly identical between groups, but elderly adults had 58% greater angular impulse and 279% more work at the hip, 50% less angular impulse and 39% less work at the knee, and 23% less angular impulse and 29% less work at the ankle compared with young adults (t-test, all P < 0.05). Age caused a redistribution of joint torques and powers, with the elderly using their hip extensors more and their knee extensors and ankle plantar flexors less than young adults when walking at the same speed. Along with a reduction in motor and sensory functions, the natural history of aging causes a shift in the locus of function in motor performance.     

Effect of fatigue on single-leg hop landing biomechanics

The objective of this study was to measure adaptations in landing strategy during single-leg hops following thigh muscle fatigue. Kinetic, kinematic, and electromyographic data were recorded as thirteen healthy male subjects performed a single-leg hop in both the unfatigued and fatigued states. To sufficiently fatigue the thigh muscles, subjects performed at least two sets of 50 step-ups. Fatigue was assessed by measuring horizontal hopping ability following the protocol. Joint motion and loading, as well as muscle activation patterns, were compared between fatigued and unfatigued conditions. Fatigue significantly increased knee motion (p = 0.012) and shifted the ankle into a more dorsiflexed position (p = 0.029). Hip flexion was also reduced following fatigue (p = 0.042). Peak extension moment tended to decrease at the knee and increase at the ankle and hip (p = 0.014). Ankle plantar flexion moment at the time of peak total support moment increased from 0.8 (N x m)/kg (SD, 0.6 [N x m]/kg) to 1.5 (N x m)/kg (SD, 0.8 [N x m]/kg) (p = 0.006). Decreased knee moment and increased knee flexion during landings following fatigue indicated that the control of knee motion was compromised despite increased activation of the vastus medialis, vastus lateralis, and rectus femoris (p = 0.014, p = 0.014, and p = 0.017, respectively). Performance at the ankle increased to compensate for weakness in the knee musculature and to maintain lower extremity stability during landing. Investigating the biomechanical adaptations that occur in healthy subjects as a result of muscle fatigue may give insight into the compensatory mechanisms and loading patterns occurring in patients with knee pathology. Changes in single-leg hop landing performance could be used to demonstrate functional improvement in patients due to training or physical therapy.     

Mechanical energy analysis identifies compensatory strategies in disabled elders' gait

Current concepts in disablement emphasize the importance of identifying mobility impairments in aging humans to enable timely intervention and, ultimately, prevent disability. Because mobility impairments are likely to result in compensatory movement strategies, recognizing and understanding those strategies may be critical in designing effective interventions for preventing disability. We sought to determine if mechanical energy methods are useful for identifying and understanding lower extremity compensatory movement strategies due to disabilities. Aleshinski's method was used to compute mechanical energy expenditure (MEE) and mechanical energy compensation (MEC) for the sagittal plane stance leg and low-back joints of healthy elders (HE) and disabled elders (DE) during preferred speed and paced (120 steps min(-1)) gait. DE subjects expended less ankle energy in late-stance and more low-back energy in mid-stance than did the HE subjects. When controlling for walking speed, the difference in ankle MEE disappeared, but mid-stance hip MEE was significantly higher for the DE subjects. Despite increased hip and low-back MEE, the DE subjects compensated hip and low-back muscles greater then HE subjects by increasing energy transferred into the pelvis, particularly when walking faster than their self-selected speed. Increased energy transfers into the pelvis during mid-stance may be a strategy used to assist in advancing and controlling the contralateral limb's swing phase. Increased trunk energy, however, may compromise dynamic stability and increase the risk of falling. We conclude that mechanical energy methods are useful for identifying and understanding compensatory movement strategies in elders with disabilities.     

Implications of increased medio-lateral trunk sway for ambulatory mechanics

The purposes of this study was to test a mechanism to reduce the knee adduction moment by testing the hypothesis that increased medio-lateral trunk sway can reduce the knee adduction moment during ambulation in healthy subjects, and to examine the possibility that increasing medio-lateral trunk sway can produce similar potentially adverse secondary gait changes previously associated with reduced knee adduction moments in patients with knee osteoarthritis. Nineteen healthy adults performed walking trials with normal and increased medio-lateral trunk sway at a self-selected normal walking speed. Standard gait analysis was used to calculate three-dimensional lower extremity joint kinematics and kinetics. Knee and hip adduction moments were lower (-65.0% and -57.1%, respectively) for the increased medio-lateral trunk sway trials than for the normal trunk sway trials. Knee flexion angle at heel-strike was 3 degrees higher for the increased than for the normal trunk sway trials. Knee and hip abduction moments were higher for the increased medio-lateral trunk sway trials, and none of the other variables differed between the two conditions. Walking with increased medio-lateral trunk sway substantially reduces the knee adduction moment during walking in healthy subjects without some of the adverse secondary effects such as increased axial loading rates at the major joints of the lower extremity. This result supports the potential of using gait retraining for walking with increased medio-lateral trunk sway as treatment for patients with degenerative joint disease such as medial compartment knee osteoarthritis.     

Age-related differences in muscle power during single-step balance recovery

The purpose of this study was to investigate age-related differences in muscle power during a surrogate task of trip recovery. Participants included 10 healthy young men (19-23 years old) and 10 healthy older men (65-83). The task involved releasing participants from a forward-leaning posture. After release, participants attempted to recover their balance using a single step of the right foot. Muscle power at the hip, knee, and ankle of the stepping limb were determined from the product of joint angular velocity and joint torque. Muscle powers during balance recovery followed a relatively consistent pattern in both young and older men, and showed effects of both lean and age. Interestingly, the effects of age did not always involve smaller peak power values in the older men as expected from the well-documented loss of muscle power with aging. Older men exhibited smaller peak muscle power at the knee and larger peak muscle power at the ankle and hip compared to young men. The increases in muscle power at the ankle and hip may result from a neuromuscular adaptation aimed at improving balance recovery ability by compensating for the age-related loss of muscle function.     

A comparison of dorsal and heel plate foot tracking methods on lower extremity dynamics

The primary method to model ankle motion during inverse dynamic calculations of the lower limb is through the use of skin-mounted markers, with the foot modeled as a rigid segment. Motion of the foot is often tracked via the use of a marker cluster triad on either the dorsum, or heel, of the foot/shoe. The purpose of this investigation was to evaluate differences in calculated lower extremity dynamics during the stance phase of gait between these two tracking techniques. In an analysis of 7 subjects, it was found that sagittal ankle angles and sagittal ankle, hip and knee moments were strongly correlated between the two conditions, however, there was a significant difference in peak ankle plantar flexion and dorsiflexion angles. Frontal ankle angles were only moderately correlated and there was a significant difference in peak ankle eversion and inversion, resulting in moderate correlations in frontal plane moments and a significant difference in peak hip adductor moments. We demonstrate that the technique used to track the foot is an important consideration in interpreting lower extremity dynamics for clinical and research purposes.     

Biomechanical characterization and clinical implications of artificially induced toe-walking: differences between pure soleus, pure gastrocnemius and combination of soleus and gastrocnemius contractures

The purpose of this study was to characterize biomechanically three different toe-walking gait patterns, artificially induced in six neurologically intact subjects and to compare them to selected cases of pathological toe-walking. The subjects, equipped with lightweight mechanical exoskeleton with elastic ropes attached to the left leg's heel on one end and on shank and thigh on the other end in a similar anatomical locations where soleus and gastrocnemius muscles attach to skeleton, walked at speed of approximately 1m/s along the walkway under four experimental conditions: normal walking (NW), soleus contracture emulation (SOL), gastrocnemius contracture emulation (GAS) and emulation of both soleus and gastrocnemius contractures (SOLGAS). Reflective markers and force platform data were collected and ankle, knee and hip joint angles, moments and powers were calculated using inverse dynamic model for both legs. Characteristic peaks of averaged kinematic and kinetic patterns were compared among all four experimental conditions in one-way ANOVA. In the left leg SOL contracture mainly influenced the ankle angle trajectory, while GAS and SOLGAS contractures influenced the ankle and knee angle trajectories. GAS and SOLGAS contractures significantly increased ankle moment during midstance as compared to SOL contracture and NW. All three toe-walking experimental conditions exhibited significant power absorption in the ankle during loading response, which was absent in the NW condition, while during preswing significant decrease in power absorption as compared to NW was seen. In the knee joint SOL contracture diminished, GAS contracture increased while SOLGAS contracture approximately halved knee extensor moment during midstance as compared to NW. All three toe-walking experimental conditions decreased hip range of motion, hip flexor moment and power requirements during stance phase. Main difference in the right leg kinematic and kinetic patterns was seen in the knee moment trajectory, where significant increase in the knee extensor moment took place in terminal stance for GAS and SOLGAS experimental conditions as compared to SOL and NW. The kinetic trajectories under SOL and GAS experimental conditions were qualitatively compared to two selected clinical cases showing considerable similarity. This implies that distinct differences in kinetics between SOL, GAS and SOLGAS experimental conditions, as described in this paper, may be clinically relevant in determining the relative contribution of soleus and gastrocnemius muscles contractures to toe-walking in particular pathological gait.     

Obesity is not associated with increased knee joint torque and power during level walking

While it is widely speculated that obesity causes increased loads on the knee leading to joint degeneration, this concept is untested. The purpose of the study was to identify the effects of obesity on lower extremity joint kinetics and energetics during walking. Twenty-one obese adults were tested at self-selected (1.29m/s) and standard speeds (1.50m/s) and 18 lean adults were tested at the standard speed. Motion analysis and force platform data were combined to calculate joint torques and powers during the stance phase of walking. Obese participants were more erect with 12% less knee flexion and 11% more ankle plantarflexion in self-selected compared to standard speeds (both p<0.02). Obese participants were still more erect than lean adults with approximately 6 degrees more extension at all joints (p<0.05, for each joint) at the standard speed. Knee and ankle torques were 17% and 11% higher (p<0.034 and p<0.041) and negative knee work and positive ankle work were 68% and 11% higher (p<0.000 and p<0.048) in obese participants at the standard speed compared to the slower speed. Joint torques and powers were statistically identical at the hip and knee but were 88% and 61% higher (both p<0.000) at the ankle in obese compared to lean participants at the standard speed. Obese participants used altered gait biomechanics and despite their greater weight, they had less knee torque and power at their self-selected walking speed and equal knee torque and power while walking at the same speed as lean individuals. We propose that the ability to reorganize neuromuscular function during gait may enable some obese individuals to maintain skeletal health of the knee joint and this ability may also be a more accurate risk indicator for knee osteoarthritis than body weight.     

Propulsive adaptation to changing gait speed

Understanding propulsion and adaptation to speed requirements is important in determining appropriate therapies for gait disorders. We hypothesize that adaptations for changing speed requirements occur primarily at the hip. The slow, normal and fast gait of 24 healthy young subjects was analyzed. The linear power was analyzed at the hip joint. The anterior-posterior and vertical induced accelerations of the hip were also determined. Linear power and anterior-posterior-induced acceleration (IA) analyses of the hip reveal that the lower limb joint's moments contribute to body forward propulsion primarily during late swing and early stance. Propulsive adaptations to speed changes occur primarily at the hip and secondarily at the ankle. These analyses show that hip muscles, particularly the hip extensors, are critical to propulsion. They also show that ankle function is primarily for support, but is important to propulsion, especially at slow speeds.     

Adaptations in the gait pattern with experimental hamstring pain

Pain changes movement but most studies have focused on basic physiological adaptations during non-functional movement tasks. The existing studies on how pain affects lower extremity gross movement biomechanics have primarily involved movements in which the quadriceps is the primary muscle and little attention has been given to how pain in other muscles affects functional movement. The purpose of this study was to investigate the changes in the gait patterns of healthy subjects that occur during experimental muscle pain in the biceps femoris.In a cross-over study design, 14 healthy volunteers underwent EMG assisted 3D gait analyses before, during and after experimental biceps femoris pain induced by intramuscular injections of hypertonic saline. Isotonic saline injections were administered as a non-painful control.The experimental biceps femoris pain led to reductions in hip extensor moments, knee flexor and lateral rotator moments. No changes in lower extremity kinematics and EMG activity in any of the recorded muscles were observed.It is concluded that experimental muscle pain in the biceps femoris leads to changes in the gait pattern in agreement with unloading of the painful muscle. The changes are specific to the painful muscle. The present study provides support to the theory that musculoskeletal pain is a protective signal leading to changes in movement patterns that serve to unload the painful tissue.     

Effects of added trunk load and corresponding trunk position adaptations on lower extremity biomechanics during drop-landings

Although both trunk mass and trunk position have the potential to affect lower extremity biomechanics during landing, these effects are not well understood. Our overall hypothesis stated that both trunk mass and trunk position affect lower extremity biomechanics in landing. Thus, our purpose was to determine the effects of an added trunk load and kinematic trunk adaptation groups on lower extremity joint kinematics, kinetics, and energetics during drop-landings. Twenty-one recreationally active subjects were instrumented for biomechanical analysis. Subjects performed two sets of eight double-limb landings with and without 10% body weight added to the trunk. On lower extremity dependent variables, 2(condition: no load, trunk load)x2(group: trunk extensors vs. trunk flexors) ANOVAs were performed. Condition by group interactions at the hip showed differing responses to the added trunk load between groups where the trunk extensor group decreased hip extensor efforts ( downward decrease 11-18%) while the trunk flexor group increased hip extensor efforts ( upward increase 14-19%). The trunk load increased biomechanical demands at the knee and ankle regardless of trunk adaptation group. However, the percent increases in angular impulses and energy absorption in the trunk extensor group were 14-28% while increases in the trunk flexor group were 4-9%. Given the 10% body weight added to the trunk, the 14-28% increases at the knee and ankle in the trunk extensor group were likely due to the reduced hip extensor efforts during landing. Overall these findings support our overall hypothesis that both trunk mass and trunk position affect lower extremity biomechanics during vertically oriented landing tasks.     

Predicting the metabolic cost of incline walking from muscle activity and walking mechanics

The goal of this study was to identify which muscle activation patterns and gait features best predict the metabolic cost of inclined walking. We measured muscle activation patterns, joint kinematics and kinetics, and metabolic cost in sixteen subjects during treadmill walking at inclines of 0%, 5%, and 10%. Multivariate regression models were developed to predict the net metabolic cost from selected groups of the measured variables. A linear regression model including incline and the squared integrated electromyographic signals of the soleus and vastus lateralis explained 96% of the variance in metabolic cost, suggesting that the activation patterns of these large muscles have a high predictive value for metabolic cost. A regression model including only the peak knee flexion angle during stance phase, peak knee extension moment, peak ankle plantarflexion moment, and peak hip flexion moment explained 89% of the variance in metabolic cost; this finding indicates that kinematics and kinetics alone can predict metabolic cost during incline walking. The ability of these models to predict metabolic cost from muscle activation patterns and gait features points the way toward future work aimed at predicting metabolic cost when gait is altered by changes in neuromuscular control or the use of an assistive technology.