First published online January 30, 2009
Journal of Experimental Biology 212, 523-534 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.024927
Control and function of arm swing in human walking and running
Herman Pontzer1,*,
John H. Holloway, 4th1,
David A. Raichlen2 and
Daniel E. Lieberman3
1 Department of Anthropology, Washington University, 119 McMillan Hall, Saint
Louis, MO 63130, USA
2 Department of Anthropology, University of Arizona, 1009 E. South Campus Drive,
PO Box 210030, Tucson, AZ 85721, USA
3 Department of Anthropology, Harvard University, 11 Divinity Avenue, Cambridge,
MA 02138, USA
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Fig. 1. Schematic diagram of passive and active arm swing hypotheses. (A) Simple
mass damper (see Soong and Dargush,
1997 ). Oscillating forces applied by a controller (red element) to
the principle Mass 1 will tend to move it (solid line in position plot); the
attachment of an auxiliary Mass 2 using a damped spring can decrease the
amplitude of movement of Mass 1 (dashed line in position plot); the
effectiveness of the damping is a function of the spring stiffness k
and damping constant c, and is proportional to Mass 2. (B) In the
passive arm swing model, oscillating moments from the swinging legs tend to
accelerate the pelvis and other body segments in turn; all energy in the
system is generated by the legs. The arms act as an auxiliary mass which damps
movement of the torso (and head). Shoulder and arm accelerations are predicted
to increase with angular displacement of the trunk (y) and shoulder
(x), respectively. (C) In the active arm swing model, energy into the
system comes from both the swinging legs and the shoulder muscles driving the
arms. Accelerations of the pelvis and torso are expected to be negatively
correlated (i.e. in opposition). Since forces of the shoulder muscles will
accelerate both the arm and torso masses, albeit in opposing directions, arm
acceleration is predicted to be negatively correlated with shoulder
acceleration. In both passive and active models, oscillation of the torso and
head will increase if arms are removed. Note that these systems (B and C) are
rotational in nature, but are rendered as linear systems here for clarity.
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Fig. 2. Schematic diagram of the reference frame and kinematic variables. Rotation
of the head, shoulders and pelvis in the transverse (x–y) plane
about the vertical (z) axis was measured using reflective markers
(gray circles) with reference to the x-axis; arrows indicate positive
rotation. Trunk torsion was measured as the rotation of the pelvis relative to
the shoulders. Arm rotation was measured in the sagittal (y–z)
plane using the reconstructed arm center of mass (*) and shoulder
relative to vertical; arrow indicates positive rotation. Angular displacement
of the shoulder ( ) was defined as negative when the arm was retracted
(as shown), positive when protracted. Step width was measured as the
difference in x-position of successive heel strikes.
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Fig. 4. Kinematic results. (A–D) Predictions of the passive arm swing
hypothesis (see Fig. 2B);
(E–H) active arm swing predictions (see
Fig. 2C). Plots are
representative results for walking and running and list the subject (Sub.) and
speed shown. Histograms are Pearson's R-values for all speeds and
subjects, walking and running combined. Hatched areas in histograms indicate
predicted values for passive (B,D) or active (F,H) hypotheses.
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Fig. 5. Representative anterior and posterior deltoid activity for (A) arm pump,
(B) 1.5 m s–1 walking, and (C) 3.0 m s–1
running trials. EMG data have been processed as described in the text and
normalized to the maximum activation within a trial. The subject from whom
data were obtained is listed.
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Fig. 6. Representative angular velocity (red line) and angular acceleration (blue
line) for the arm at the shoulder, overlaid on normalized anterior and
posterior deltoid activity, during (A) walking at 1.5 m s–1
and (B) running at 3.0 m s–1. Deltoid activity is processed
and shown as in Fig. 5. Periods
of apparent eccentric contraction are indicated (red arrows), as are periods
in which shoulder acceleration is in opposition to prevailing muscle activity
(blue arrows) or occurs without substantial deltoid activity (black arrows).
Not all such periods are indicated.
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Fig. 7. Mean ± s.d. values for walking (1.5 m s–1) and
running (3.0 m s–1) for (A) step width variation and (B)
locomotor cost. *Significant difference compared with control
trials (P<0.05).
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© The Company of Biologists Ltd 2009
Significant difference compared with
arm weights trials (P<0.05). 
Significant
difference compared with both arm weights and control trials
(P<0.05).