Endurance time, muscle activation, and mean arterial pressure were measured during two types of submaximal fatiguing contractions that required each subject to exert the same net muscle torque in the two tasks. Sixteen men and women performed isometric contractions at 15% of the maximum voluntary contraction (MVC) force with the elbow flexor muscles, either by maintaining a constant force while pushing against a force transducer (force task) or by supporting an equivalent inertial load while maintaining a constant elbow angle (position task). The endurance time for the force task (1402 +/- 728 s) was twice as long as that for the position task (702 +/- 582 s, P < 0.05), despite a similar reduction in the load torque at exhaustion for each contraction. The rate of increase in average electromyographic activity (EMG, % peak MVC value) for the elbow flexor muscles was similar for the two tasks. However, the average EMG was greater at exhaustion for the force task (22.4 +/- 1.2%) compared with the position task (14.9 +/- 1.0%, P < 0.05). In contrast, the rates of increase in the mean arterial pressure, the rating of perceived exertion, anterior deltoid EMG, and fluctuations in motor output (force or acceleration) were greater for the position task compared with the force task (P < 0.05). Furthermore, the rate of bursts in EMG activity, which corresponded to the transient recruitment of motor units, was greater for the brachialis muscle during the position task. These results indicate that the briefer endurance time for the position task was associated with greater levels of excitatory and inhibitory input to the motor neurons compared with the force task.

To gain insight into the mechanical determinants of walking energetics, we investigated the effects of aging and arm swing on the metabolic cost of stabilization. We tested two hypotheses: 1) elderly adults consume more metabolic energy during walking than young adults because they consume more metabolic energy for lateral stabilization, and 2) arm swing reduces the metabolic cost of stabilization during walking in young and elderly adults. To test these hypotheses, we provided external lateral stabilization by applying bilateral forces (10% body weight) to a waist belt via elastic cords while young and elderly subjects walked at 1.3 m/s on a motorized treadmill with arm swing and with no arm swing. We found that the external stabilizer reduced the net rate of metabolic energy consumption to a similar extent in elderly and young subjects. This reduction was greater (6-7%) when subjects walked with no arm swing than when they walked normally (3-4%). When young or elderly subjects eliminated arm swing while walking with no external stabilization, net metabolic power increased by 5-6%. We conclude that the greater metabolic cost of walking in elderly adults is not caused by a greater cost of lateral stabilization. Moreover, arm swing reduces the metabolic cost of walking in both young and elderly adults likely by contributing to stability.

A human walker vaults up and over each stance limb like an inverted pendulum. This similarity suggests that the vertical motion of a walker's center of mass reduces metabolic cost by providing a mechanism for pendulum-like mechanical energy exchange. Alternatively, some researchers have hypothesized that minimizing vertical movements of the center of mass during walking minimizes the metabolic cost, and this view remains prevalent in clinical gait analysis. We examined the relationship between vertical movement and metabolic cost by having human subjects walk normally and with minimal center of mass vertical movement ("flat-trajectory walking"). In flat-trajectory walking, subjects reduced center of mass vertical displacement by an average of 69% (P = 0.0001) but consumed approximately twice as much metabolic energy over a range of speeds (0.7-1.8 m/s) (P = 0.0001). In flat-trajectory walking, passive pendulum-like mechanical energy exchange provided only a small portion of the energy required to accelerate the center of mass because gravitational potential energy fluctuated minimally. Thus, despite the smaller vertical movements in flat-trajectory walking, the net external mechanical work needed to move the center of mass was similar in both types of walking (P = 0.73). Subjects walked with more flexed stance limbs in flat-trajectory walking (P < 0.001), and the resultant increase in stance limb force generation likely helped cause the doubling in metabolic cost compared with normal walking. Regardless of the cause, these findings clearly demonstrate that human walkers consume substantially more metabolic energy when they minimize vertical motion.

Elderly adults consume more metabolic energy during walking than young adults. Our study tested the hypothesis that elderly adults consume more metabolic energy during walking than young adults because they perform more individual limb work on the center of mass. Thus we compared how much individual limb work young and elderly adults performed on the center of mass during walking. We measured metabolic rate and ground reaction force while 10 elderly and 10 young subjects walked at 5 speeds between 0.7 and 1.8 m/s. Compared with young subjects, elderly subjects consumed an average of 20% more metabolic energy (P=0.010), whereas they performed an average of 10% less individual limb work during walking over the range of speeds (P=0.028). During the single-support phase, elderly and young subjects both conserved approximately 80% of the center of mass mechanical energy by inverted pendulum energy exchange and performed a similar amount of individual limb work (P=0.473). However, during double support, elderly subjects performed an average of 17% less individual limb work than young subjects (P=0.007) because their forward speed fluctuated less (P=0.006). We conclude that the greater metabolic cost of walking in elderly adults cannot be explained by a difference in individual limb work. Future studies should examine whether a greater metabolic cost of stabilization, reduced muscle efficiency, greater antagonist cocontraction, and/or a greater cost of generating muscle force cause the elevated metabolic cost of walking in elderly adults.

I-RFA is a protective factor associated with less severe acute symptoms and shorter recovery after SRC. Conveying this message to athletes, coaches, and others involved in the care of athletes may promote timely injury reporting.