Dynamic arm swinging in human walking

Abstract

Humans tend to swing their arms when they walk, a curious behaviour since the arms play no obvious role in bipedal gait. It might be costly to use muscles to swing the arms, and it is unclear whether potential benefits elsewhere in the body would justify such costs. To examine these costs and benefits, we developed a passive dynamic walking model with free-swinging arms. Even with no torques driving the arms or legs, the model produced walking gaits with arm swinging similar to humans. Passive gaits with arm phasing opposite to normal were also found, but these induced a much greater reaction moment from the ground, which could require muscular effort in humans. We therefore hypothesized that the reduction of this moment may explain the physiological benefit of arm swinging. Experimental measurements of humans (n = 10) showed that normal arm swinging required minimal shoulder torque, while volitionally holding the arms still required 12 per cent more metabolic energy. Among measures of gait mechanics, vertical ground reaction moment was most affected by arm swinging and increased by 63 per cent without it. Walking with opposite-to-normal arm phasing required minimal shoulder effort but magnified the ground reaction moment, causing metabolic rate to increase by 26 per cent. Passive dynamics appear to make arm swinging easy, while indirect benefits from reduced vertical moments make it worthwhile overall.

Figure 1.

(a) Dynamic walking model with free-swinging arms, powered only by descending a gentle slope (see electronic supplementary material for detailed schematics and animations). (b) Simulation results for peak vertical ground reaction moment and vertical angular momentum in the Normal, Bound and Anti-Normal arm-swinging modes. Normal and Anti-Normal refer to passive dynamic gaits with the arms moving similar to normal human walking and the opposite phasing, respectively. Bound refers to a model walking with the arms mechanically constrained against rotation. Units are dimensionless, using body mass M, leg length L and gravitational acceleration g as normalizing factors. (c) Frame-by-frame rendering of the Normal gait (walking from left to right).

Figure 2.

Artificial arms made of wood (left) or rope (right) provide physical demonstrations of passive dynamic arm swinging during walking. The artificial arms were allowed to swing freely from a yoke (inset) worn by individuals as they walked with their arms bound to their sides (see electronic supplementary material for videos).

Figure 3.

Walking conditions tested experimentally. Human subjects walked with arms swinging normally (Normal), physically bound to the sides (Bound), volitionally held to the body (Held) and volitionally swinging with phase opposite to normal (Anti-Normal). Gait mechanics and metabolic energy expenditure were measured under these conditions.

Figure 4.

Experimental measurements of the mechanical effects of arm swinging during human walking. (a) Vertical ground reaction moment about the centre of pressure of the stance foot plotted versus time and (b) the peak moments over a stride; (c) the arms' contribution to vertical angular momentum versus time and (d) corresponding peak values; (e) whole-body angular momentum about the vertical versus time and (f) corresponding peak values. Trajectories show mean across subjects per condition, and bar graphs show peaks averaged across subjects. Error bars show 1 s.d. and asterisks indicate statistical significance (p < 0.05). Double support is denoted by a shaded region in plots. In (c), the band labelled ‘rest of body’ represents the vertical angular momentum of the body not including the arms; the range contains all mean trajectories, which were found to be dominated by the legs. Arm angular momentum increased in the order Normal, Bound/Held, Anti-Normal as expected while angular momentum of the rest of the body remained roughly constant, resulting in significant increases in whole-body angular momentum. Increased peak vertical moments were necessitated by increased rates of change in whole-body vertical angular momentum. (a,c,e) Light grey, Normal; mid grey, Bound; dark grey, Held; black, Anti-Normal.

Figure 5.

Net metabolic rate increased significantly in the order of Normal, Bound, Held and Anti-Normal. Bars indicate mean energy expenditure rate across subjects. Error bars denote s.d.'s, and asterisks indicate statistical significance (p < 0.05).

Figure 6.

Upper-limb joint mechanics. (a) Shoulder and (b) elbow angles versus time (left) and corresponding peak angles (bars at right). (c) Shoulder and (d) elbow torque versus time (left) and corresponding peak torques (bars at right). (e) Shoulder and (f) elbow joint power versus time (left) and corresponding positive work rate (bars at right). Peak torques and work rates were quite small, and did not significantly differ in magnitude between Normal and Anti-Normal conditions, suggesting that arm motions were predominantly passive. Light grey, Normal; dark grey, Held; black, Anti-Normal; dashed lines, ±s.d.