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At Hof


Laboratory of Human Movement Analysis, University Medical Center and Center for Human Movement Sciences, University of Groningen, Groningen, The Netherlands At Hof studied Electrical Engineering at the University of Twente, Enschede, The Netherlands, arriving at the MS degree in 1972 and the PhD in 1980 on “EMG to force processing”. He was affiliated with the University of Groningen since 1973, in the departments of Medical Physics (1973-1994), Medical Physiology (1994-2001) and Center of Human Movement Science (Associate professor, from 2001). Next to this he holds an appointment at the University Hospital Groningen as Technical Director of the Laboratory of Human Movement Analysis, Department of Rehabilitation. His research interests include biomechanics, electromyography, in vivo determination of human contractile and elastic muscle properties, and human locomotion, especially balance during locomotion.

Abstract of this year's Bauman lecture:

As humans we do many of our activities in upright stance. Balance control, to remain upright while we are moving, is a precarious task because our centre of mass (CoM) is to remain above our feet, the centre of pressure (CoP), an unstable situation. Many ‘strategies’ are already known that effect this balance control in various circumstances. These strategies can conveniently be categorized into three basic mechanisms1. Mechanism 1: moving the CoP. When sufficient space is available, the major mechanism is to move the CoP. When the CoP is moved in some direction, the CoM ‘falls’ in the opposite direction. This can be done by muscle contractions of muscles acting on the foot, the ankle strategy. In this way the CoP can be moved over the foot sole: forward by soleus and gastrocnemius, medially by the peronei and laterally by the tibialis muscles. If the CoP has to move a greater distance than compatible with the dimensions of the foot, we have to make a step, the stepping strategy. A third strategy is the weight shifting strategy of two legged stance, in which the load division of left and right leg is changed by action of the hip abductors. The strategies of mechanism1 can be described by the ‘inverted pendulum’ model2. Mechanism 2: rotation around the CoM. When body segments are given a rotational acceleration around the CoM, the body as a whole gets an acceleration in the other direction1. Bauma nn Lecture : Bala nce duri ng wal king: how to hol d your hea d up Examples are the arm swing - and the hip strategy. These strategies are especially useful when the base of support is small, e.g. in one-legged stance, or when a very fast action is needed. Mechanism 3: external support. Often overlooked, but very practical, is the mechanism of external support, to make a grab at or to hold on to a hand rail, to use a cane or a rollator. To study the balance strategies used in walking, we applied pushes of known magnitude and timing to the waist of subjects who walked on a treadmill, which was equipped with force transducers to record the CoP3. When sideward pushes are given, the response is firstly a stepping strategy: in the next step the foot is placed more sideward in the direction of the push. In this way the CoM falls in the direction opposite to the push, and the perturbation is corrected. When the push is given outward, e.g. a leftward push during left stance, a crossing-over step may be required. Step time is only affected (shortened) at large perturbations. It can be shown that, in addition to the stepping strategy, also the ankle strategy is used. This strategy is restricted in magnitude, but is more precise and can be applied earlier. Muscle actions related to both strategies can be found from the EMGs. As a reaction to a backward perturbation, one would theoretically expect a shorter steplength4. This is indeed found, but it is always accompanied with a longer step duration. This is even more the case with a forward perturbation. Step time is always shortened, while steplength is sometimes lengthened, sometimes shortened, depending on the timing of the push. In patients the biomechanical abilities for balance often set bounds to the ability for walking. Examples are amputees, who cannot use the ankle strategy and who have a reduced precision of foot placement at the prosthetic side5, and CP patients, with their reduced precision of movement and, in case of equinus gait, reduced base of support.

1. Hof AL. The equations of motion for a standing human reveal three mechanisms for balance. Journal of Biomechanics 2007; 40:451-457.
2. Winter DA. Human balance and posture control during standing and walking. Gait and Posture 1995;
3:193-214. 3. Hof AL, Vermerris SM, Gjaltema WA. Balance responses to lateral perturbations in treadmill walking. In: Chiari, L. and Nardone, A. (Eds): Proceedings of the XIX conference of the International Society for Posture and Gait Research. Bologna: University of Bologna, 2009 pp. 348-349.
4. Hof AL. The `extrapolated center of mass’ concept suggests a simple control of balance in walking. Human Movement Science 21-1-2008; 27:112-125.5.
5. Hof AL, van Bockel R, Schoppen T, Postema K. Control of lateral balance in walking: Experimental findings in normal subjects and above-knee amputees. Gait & Posture 2007; 25:250-258.