Sprint Biomechanics Notes

I found this text on my harddrive. I think this may have been on an old speed training page which is no longer online. It may be of interest to some of you.

a. Global Observations

Ground forces and the velocities created

a) Horizontal Velocity
(1) Horizontal distance traveled by the C of M in a prescribed time
(2) Horizontal velocity is affected by Horizontal Acceleration during ground contact
(a) Ground contact is composed of an eccentric phase and a concentric phase

i. Eccentric phase: initiated at ground contact and ends when the C of M is directly over the foot
The eccentric phase is where the sprinter must work against forces that create negative horizontal acceleration

ii. Concentric phase: initiated when the C of M is over the foot and ends at the start of leg recovery
The concentric phase contributes little if any to maximum velocity sprinting

The concentric phase contributes greatly to the acceleration phase
Horizontal velocity is changed via the type of Horizontal Acceleration which occurs during ground contact

(a) Horizontal deceleration decreases horizontal velocity
i. Created through excessive stride length due to poor mechanics
ii. Created through excessive positive foot and leg speed at ground contact

b) Vertical Velocity
(1) Vertical distance traveled by the C of M in a prescribed time
(2) Vertical velocity is affected by Vertical Acceleration during ground contact
(a) The eccentric phase possesses various amounts of vertical deceleration during sprinting

i. Greatest in the last part of the race at distances over 80m
(b) The concentric phase possesses various amounts of vertical acceleration during sprinting
i. Greatest in the early portion of the race prior to reaching maximum speed
c) Relationship of Horizontal and Vertical Velocities

(1) Both Vertical and Horizontal velocities are altered during ground contact

(a) The C of M is projected in a direction resulting from both horizontal and vertical velocities
(b) As vertical velocities increase, horizontal velocities decrease
© Eccentric Phase: Deceleration in both Horizontal and Vertical velocities occur in this phase
i. Moving through this phase in a shorter time while minimizing vertical deceleration yields: A lower takeoff angle

Greater stride frequency = Reduced flight time

  1. Path of the Center of Mass (C of M)

a) The path describes a sinusoidal curve when viewing the saggital plane (from the side)
(1) C of M is at a high point on the curve during the flight phase
(2) C of M is at a low point on the curve during the support phase

b) The amplitude of the sinusoidal curve provides indications as to the quality and efficiency of the sprinter (1) Amplitude increases due to poor mechanics

(a) Ground contact time and flight time increase

i. Results in increased vertical velocity through inefficient causes (b) Stride frequency decreases

© Amplitude increases are often directly related to stride length increases and directly related to vertical velocities generated

i. Vertical displacement of the C of M from ground contact to mid-support indicates the ability to resist

absorption of forces

(2) Amplitude due to good mechanics and sprinting qualities has the following characteristics: (a) Flight times are longer without increasing ground contact times (b) Ground contact times are shorter for identical flight times © High negative foot and leg speed at ground contact

(d) Efficient positive foot distance (distance of foot ahead of C of M) at touchdown

(e) Relative and absolute strength of hip flexors, knee extensors, and plantar flexors to resist force absorption

and stabilize are present

(f) Neuromuscular timing and coordination to allow for anticipatory muscle contractions and proper motor

sequencing is well developed

b. Stride Length Factors

  1. Anthropometric Factors a) Leg length

(1) Measured from the crest of the greater trochanter to the floor (without shoes)

(2) Optimal stride length generally should be 2.3 -2.5 times leg length for women and 2.5-2.7 for men

b) Body composition.

(1) The less lean the sprinter the shorter will be the effective stride length 2) Strength and Power

a) Maximum strength contributes to joint stabilization at high velocities which aids in elastic strength expression b) At higher velocities maximum strength becomes less important except as it relates to elastic force production

  1. Elasticity

a) Energy from a falling body is absorbed by a contracted muscle, forcibly lengthening it which in turn rebounds with great force

(1) Recently Bosco found in a drop jump that muscle actually shortened a small amount as the whole system lengthened. This implies that connective tissue is the elastic part of the system. This means that max strength is even more important. The muscle is almost static so ATP is not required. Thus the energy cost in elastic force production is very low.

(2) The elastic component augments the contractile component for force production

b) When utilized, contributes to much longer stride lengths without compromising sound technique c) The ability to anticipate the need for force production is important in elastic force production 4) Neuromuscular Integration

a) Neuromuscular Sequencing or Motor Program

(1) The specific sequential firing order of the muscles involved b) Neuromuscular Timing

(1) The anticipation of the forthcoming action and the sending of the proper message to the brain to fire the muscles

c) Neuromuscular Coordination

(1) Coordinating the agonist and antagonist muscles to work in concert together

  1. Mechanical Efficiency

a) Closely related to Neuromuscular Integration and Coordination

b) At max velocities the relationship between C of M, joint angles, and effective muscle strengths and power output come into play

  1. Dynamic Mobility

a) The ability to move a limb segment through a greater range of motion in the same time or a prescribed range in a shorter time
b) May be limited by individual anatomical factors 7) Training Age
a) As a young athlete matures their productive stride length should increase 8) Factors which can be positively influenced through training a) Strength and Power
b) Neuromuscular Integration and Coordination c) Mechanical Efficiency d) Elasticity
e) Dynamic Mobility
c. Stride Frequency Factors

  1. Anthropometric Factors
    a) Thigh length effects the speed at which the leg may be recovered 2) Neuromuscular Integration
    a) See Stride Length Factors (above)
    (1) The specific sequential firing order of the muscles involved 3) Mechanical Efficiency
    a) Short levers aid in enhancing the speed of leg recovery
    b) Forces must be applied through the greatest distance in the shortest time 4) Strength
    a) Agonist and antagonist muscle groups must be balanced in strength to “abruptly” decelerate the moving limb (1) Eliminates dead time between recovery phase and preparation for support
    b) Maximum strength levels must be high enough to allow for little absorption of force on ground contact
    (1) Strength to stabilize joints so that little force is absorbed in the knee, ankle, and hip upon ground contact (2) Ensures a responsive ground contact which is short in duration and high in force production 5) Power
    a) The greater the muscular power the greater the values of angular acceleration and angular velocity in the limb
    b) The muscle must have high power output so as to be able to move the C of M through ground contact as fast as possible
    (1) This minimizes braking forces 6) Genetics
    a) Nerve conductance velocities
    b) Muscle contraction velocities and fiber type
    d. Relationship between Stride Length and Stride Frequency
  2. Stride Frequency is the larger limiting factor in sprint performance
    a) Mann points out that most better sprinters improve their performance through improved stride rate 2) Both Length and Frequency are improved by increasing leg strength a) Results in necessary ground force production more quickly
  3. There is an inverse relationship between stride length and stride frequency
  4. There appears to be an inverse relationship between stride frequency and ground contact time
  5. There appears to be an inverse relationship between stride frequency and the amplitude of the curve described by the C of M
  6. Evaluating Stride Length
    a) Perform a flying run through a 10m zone sprinkled with sawdust or flour and measure the distance from toe to toe b) Use a fixed camera and measure (knowing the length of the foot) the number of foot lengths from one toeoff to the next
  7. Evaluating Stride Frequency
    a) Perform a flying run through a 10m zone
    b) Use a fixed camera and count the number of video frames from one toe-off for 2-4 strides to the last toe-off. Divide this number by 30 to get the time. Divide the number of strides measured (2-4) by the time to get # strides/sec.

c) Manual measurement

  1. Preparation for support

a. Rapid acceleration at the thigh down and back by the hip extensors.

  1. Quick change of direction and acceleration downward

a) Works along with the quick-anticipated recovery of the other leg (support leg)
b) Increases stride frequency by “cutting out” the “dead time” during the change from flexion to extension
b. Continued dorsiflexion of the foot along with extension at the hip joint and flexion at the knee joint 1) Toe remains up -keeping the gastrocnemuis on stretch
2) Foot is pulled “actively” under the C of M to yield “active landing” a) Hamstrings and gluts act as primary movers during hip extension
(1) Hip extensors contract violently to maximize force production on ground contact (2) Upper leg speed (back) at takeoff is critical
3) Ankle is “casted” or set at approximately 90 degree.s just prior to touchdown
a) Casting is created by co-contraction of plantar and dorsi flexors to yield a rigid ankle joint (1) Minimizes absorption of power into the track and maximizes elastic force production 4) Quadriceps are contracted just prior to ground contact
a) This action is automatic and acts to stabilize the knee and produce greater elastic forces at touchdown (1) Helps project the body forward at ouchdown thus yielding greater stride length
3. SupportPhase
a. Eccentric Phase

  1. The outside of the forefoot contacts the ground
    a) C of M is slightly behind foot initially and is over the foot ultimately
    b) Muscle contractions must be anticipated and coordinated so to minimize loss of velocity c) Elastic force production is essential
    d) The shorter the time in this phase, the more efficient is the force application and the shorter the ground contact time
    b. Concentric Phase
  2. The foot is in support under the Center of Mass a) Gluts continue to extend the thigh
    b) A period of short driving action which plays a smaller role at maximum velocity than in acceleration
  1. The Recovery Phase
    a. Support foot breaks contact with the ground
  1. Incomplete extension of hip, knee, and ankle joints a) Better sprinters have less extension at the hip
    b) Maximum extension at the hip and knee contribute little to force production at maximum velocity
    c) Results in an earlier recovery which advances the knee swing further d) Results in more hip flexion of the swing leg at takeoff
    e) Excessive extension may indicate lack of strength and coordination
    b. Thigh is flexed at the hip as soon as possible along with the ground

  2. Anticipate the recovery while the foot is in contact with the ground.
    a) Immediate contraction of the stretched hip flexors aids in the forward and upward swing earlier in the recovery phase
    (1) Puts the leg in a better position to apply force at the next ground contact (2) May contribute to stride frequency 2) Tight flexion of the lower leg
    a) The lower leg must be flexed tightly as soon as possible
    (1) Demands continuous contraction of the hamstrings until the thigh swings to full flexion at the hip (2) Demands strength in the knee flexors 3) Dorsiflexion of the foot at ankle
    a) The flexion of the foot toward the knee puts the gastrocnemius on stretch (1) Aids in flexion at the knee
    (2) Holds the lower leg flexed to the thigh during periods in the swing where the hamstring and sartorius are not
    as effective

  3. Ankle of the swing leg passes above the knee of the support leg
    a) Ankle above the knee keeps the leg lever short for as long as possible
    (1) Decreases the moment of inertia and increases the angular velocity of the thigh about the hip
    (2) Keeps the hamstring in a position of weakness so it cannot work to decelerate the thigh too soon
    c. Swing leg thigh is decelerated

  4. Leg begins to extend (open up)
    a) Unfolding occurs at the knee and extension begins at the hip
    (1) Extension is due to transfer of momentum, not reaching or quadricep contraction
    (2) Momentum is transferred to the support leg to aid in power production for stride length

  1. General considerations during Maximum Velocity Sprinting.
    a. Posture
  1. Trunk Erect 2) Head level 3) Hips tall
    b. Arm Action
  2. Used to balance the forces created by the legs 2) Initiates the actions of the legs
  3. Elbow angle varies from approx. 60-140 degrees (front to back position) 4) Emphasis placed on driving the arms backward
  4. Arms are recovered elastically by the stretch of muscles in the shoulder
    a) Flexion in both directions due to concentric contraction at the shoulder is difficult to produce at high stride frequencies
  5. Lateral deviation beyond the saggital plane is undesirable

so … triple extension during acceleration is good, but during maximum velocity triple extension is a sign of inefficiency and/or lack of strength?

if true, then what would be some coaching cues to use during maximum velocity?

Extention is a matter of power and more power sooner causes the leg to extend closer under the C/M as you go faster. Many people have the misconception that extention only happens if it is behind the C/M.
Extention during the acceleration phase is also a straight line through the hips but on a different overall angle.

The faster you go, the more power you can produce, the closer to the bottom dead center of the athlete , the toe-off occurs!
Vertical Force production in this phase, is most important and is often the limiting factor.
The closer to the BDC toe-off occurs, indicates that the athlete is able to keep his hips high(is strong enough), and also has pronounced frontside mechanics.
Triple extension (toe-off), always happens behind the C.M of the the athlete, right?
In the acceleration phase (where horizontal force production matters the most), the foot lands slightly behind the C.M .
In maxV , the foot lands directly (ideally) under the C.M and in this phase relaxation and maximization of vertical force production is vital.
Do you agree, Charlie?

It sounds right except when fully upright at top speed, foot strike is very slightly ahead of BDC, contributing to pre-stretch along with vertical forces. In the top speed phase, horizontal forces are too fast to be perceptable and cannot be influenced voluntarily.

if verticle forces are key, how is driving the arm BACK the way to go?? how is it possible NOT to reduce force if you keep the foot dorsiflected at all times?? Look at photos of the top athletes and look at their foot position as they leave the ground. Comments??

Is there an inverse relationship between stride frequency and ground contact time? A shorter GCT yields more force and potentially more flight time- but more than the reduction in GCT? This is interesting because there may be two answers depending on the level of the athlete. A rank beginner will certainly increase frequency because the reduction in GCT will be marked but the top athlete may not once the difference becomes very small.
this also opens up the question of WHERE frequency changes. We already know that frequency quickens for the top sprinters over the first few steps and the final segment but does the rest of the frequency curve remain static or shift? If so, why would that be?

exactly right, humans will always land SLIGHTLY ahead of the BDC, no matter how strong we are … We just cant overcome our physiology. The goal would be to land exactly under the C.M, imo

I believe that the arm swing (driving the elbows back etc) DOES NOT actually enhance the horizontal force production (in maxV).
The horizontal force capabilities of the arms are very limited due to the
simultaneous forward-backward action of contralateral arms. That is, although the
forward swinging arm has the ability to generate horizontal propulsive forces, any benefit
is cancelled out by the opposite action of the contralateral arm moving backwards.
The arm swing does, however, serve two important roles. The first of these is to
counterbalance the rotary momentum of the legs (Hinrichs et al., 1987; Mann &
Hermann, 1985). If it were not for the action of the arms, an athlete would not be able to
control the rotation of their trunk caused by the unilateral action of the legs. The second
role that the arm swing serves is to enhance vertical propulsive forces. Research evidence
indicates that the arms may contribute up to 10% of the total vertical propulsive forces an
athlete is capable of applying to the ground (Hinrichs, 1987). This is because unlike the
spatial phase difference of the arm swing in the forward-backward direction, both arms
are synchronized in their upward and downward movement. As a result, there is no
cancellation of their affect in the vertical direction and the synchronized upward
movement of both arms is able to contribute to the vertical propulsive forces an athlete
can apply to the ground. In light of these considerations, an optimal arm swing is one
which is symmetrical and roughly matches the timing and magnitude of movement of the
legs. Efficient sprinters exhibit an arm swing that originates from the shoulder and has a
flexion and extension action at the shoulder and elbow that is commensurate to the
flexion and extension occurring at the ipsilateral shoulder and hip.

On the other question, i think keeping your feet dorsiflexed all the time through the gait cycle, will potentially negatively affect your mechanics and force production.
I think you must keep your feet dorsiflexed (sth that will come naturally) during the last stages of the flight, and this has more than one beneficial effect

Are you sure about this bit? Often at max v top sprinters have less leg movement (more frontside mechanics) and so would exhibit less arm action based on this thinking. In reality to appears that they exhibit more. Frans Bosch has some interesting things to say about this one but I can’t remember them off the top of my head so I’ll come back to it later.

max V cues?

I think arms do play a role in horizontal force production at max. v. but not as a direct result of their stroke or momentum, but as a pre-tensor to the posterior chain. The fascia from the lats crosses the lower back and sacrum to form a train with the glutes. Driving the elbow down activates the lats and pre-tenses the glute fascia giving it a firmer anchor to pull against when the foot hits the ground and it is required to generate more power. This is why the down stroke of the arm is so important rather than the backward lift at the end of the arm drive.

Dorsiflexion is advantageous to begin just after toe off (not be dorsiflexed but in the process of achieving it) as it initiates a gross motor program that contracts the hip flexors, however it probably just begins as a result of the SSC the tib ant undergoes whilst toeing off.

The foot cannot stay dorsiflexed throughout the entire stride. If it does there can be no elastic storage of energy when the foot makes contact with the gound - the foot must be extended and prepared to resist flexion in order to store the energy and utilise the SSC.

This is copied directly, word for word, from a recent article published in Track Coach.

Maybe he forgot to reference the source.

I like this thought. It matches some of the ideas I’ve had on this correlation.

Oh yes , definately. The article was from Top Speed Mechanics from Mr.Young . You can download it from elitetrack.com
I think his work on maxV mechanics is spot-on.

Here is a video of M.Young from elitetrack, giving some tips on maxV

This is similar to the idea described by Frans Bosch who believes that the arms create torque in the body which enhance the stretch the abdominal this in turn cases anterior pelvic rotation which in turn causes eccentric contraction of the glutes. This stiffness added by the arms to the stomach muscles allow the glutes to contract SLOWER which allows them to impart more force when running!

Biomechanics involves direct relationships between momentum, force, or impulse etc. Any cause and effect relationship requires a direct relationship. This pre-tensor argument for the posterior chain has no direct relationship between arms and force development at MV.

The fascia from the lats crosses the lower back and sacrum to form a train with the glutes. Driving the elbow down activates the lats and pre-tenses the glute fascia giving it a firmer anchor to pull against when the foot hits the ground and it is required to generate more power. This is why the down stroke of the arm is so important rather than the backward lift at the end of the arm drive.

Pre-tensing glut fascia??

Fascia is tissue that is elastic, meaning that it has no nerve innervation. And it is passive not active. Passive elastic tissues don’t have any motor units to innervate them. You can’t pre- tense the glut fascia like this.

I believe there are direct and indirect relationships with all functions, though for our purposes it has more value to identify what to do and how best to do it.
My understanding is that the facial areas are rich in innervation which in part explains how acupuncture in this area can stimulate a positive response- via indirect means. If you run your hands along the facial area, you can feel it appear to ‘melt’ after treatment (the best description I can come up with)