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View Full Version : Re: Two Peaks at heel strike



Gaspar Morey
10-08-1998, 09:28 PM
Clifford Larkins wrote:

> Dear group,
>
> Recently, there was a message posted about "two peaks at heel
> strike". I am sorry it has taken me so long to respond to this message,
> but I had to dig out old data that I haven't looked at in 4 years. As I
> read the discussions regarding this topic, I remembered that I had observed
> this double peak vertical force pattern while collecting ground reaction
> forces in the triple jump. About 4 years ago I built a portable runway
> that could house three force plates. Using this setup I was able to
> collect ground reaction forces while the jumper was performing the three
> phases of the triple jump in succession. I collected data on myself and
> two of the jumpers that I was coaching at the time. The female jumper
> displayed this pattern of double vertical peak forces that you described
> primarily during her step phase. The pattern for the hop and jump phases
> were quite different. This was a surprise to me since I had never seen
> this type of force pattern in the running literature (and to my knowledge
> no other force data exists which measures all three phases of the triple
> jump in succession).
> More surprising to me, however, was the horizontal force curve for
> all three jumpers which went positive first before it went negative. In
> other words, the jumpers generated propulsive forces for a very short
> period of time at heel strike before they generated the expected breaking
> impulse. This was a real shock! I would be interested in knowing if any
> one else has observed this pattern in either walking, running, or jumping?
>
> I hope this adds some worthwhile information to the discussion.
>
> Sincerely,
>
> Clifford Larkins, Ph.D.
>

A few years ago I also measured german elite triple jumpers. I just recorded
the GRF during the transition between the HOP and the STEP jumps.
Regarding the vertical forces, sometimes i also recorded a small peak before
main impact. As I did also the kinematics, this peak is atributed to the
forefoot landing. The athletes landing is almost in foot flat position, when
the forefoot lands slightly before the rearfoot you will get this small peak
before impact.
About the positive peak in the horizontal forces: This peak indicates that the
athlete foot and leg are moving faster backwards (seeking actively the floor)
than the athlete himself is moving forward (his center of mass). This positive
peak happens when the leg is almost unloaded, the braking impulse starts as
soon as the load rises. Afterwards we get an accelerating impulse again.
Note: Its important to take into account that when the athlete is slower
(horizontal speed of his center of mass), it is easier for him to achieve a
lower braking impulse.

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