It is reasonable to break down sprinters into the three groups but I’m not sure how to break down the teaching of motor tasks as stated. Skill development is important at all stages, and must not only be learned, but re-learned along the way. I think the learning curve moves more towards individual drills as the athlete advances. Any suggestions as to an approach to each of the three groups?
As for the part that will drive me nuts, I think it is easy to show that effort may increase force BUT sprinting is cyclical and repetitive and no single step tells the story.
Get faster in practice, where you have many opportunities to work on skills- AND faulty input can be corrected on the next rep- without a disastrous meet result that you must “chalk up to learning”
I’ve got it now. Thanks.
Here are a set of notes I had put together for a course I had set up as an independent study. Portions are taken directly from certain authors. My apologies to any of the authors if credit has not been given properly, this is taken from my laptop and at the current time I am unable to reference the notes vs. the source texts.
Stretch Reflex
The two main functions of muscle are to generate power and to react to perturbations. Muscle needs to be springlike in order to react appropriately; the stretch reflex helps the muscle achieve this capability. When a muscle experiences a brief, unexpected increase in length (a stretch), the response is known as the stretch reflex.
Figure 5.26 from Enoka 1994
One component is the short-latency response (M1), which is thought to be mediated by a neural circuit limited to the spinal cord. The second component (M2) has a long latency and a more complex origin that may involve the motor cortex in the brain. A third component (M3) is occasionally observed.
Although the latencies for the stretch-reflex components vary, the M1 component generally has a latency of about 30 ms, the M2 has a latency of around 50 to 60 ms, and the earliest voluntary activity (EMG) begins at 170 ms (Enoka 1994).
The sensory receptor involved in the stretch reflex includes at least the muscle spindle, both its Group Ia and Group II afferents.
Figure 5.27 from Enoka 1994
Action potentials are generated in the muscle spindle afferents in response to muscle stretch and are propagated centrally to the spinal cord, where they elicit synaptic potentials in the motor neurons innervating the muscle in which the spindle is located. In this instance, the motorneurons are described a homonymous, because they innervate the same muscle in which the sensory receptor (muscle spindle) is located. If the stretch is adequate, a sufficient number of synaptic potentials are generated in the motor neuron to elicit an action potential that results in a muscle contraction. The stretch reflex seems most capable of accommodating small disturbances in muscle length (Enoka 1994). The net effect of this input-output circuit is that the stretch (stimulus) elicits a contraction (response) that minimizes the stretch; this type of negative-feedback circuit has also been referred as a resistance reflex.
The stretch reflex, however, does not function only as a stereotyped negative-feedback response but also as a flexible response that can adapt to changing conditions (Hammond, Merton, & Sutton, 1956). Because the stretch reflex can actually be elicited whenever there is an unexpected change in muscle length, it can also occur during movement. If, during movement, there is a mismatch between the expected and the actual muscle lengths, this difference may be sufficient to elicit a stretch reflex (Enoka 1994). Under these conditions, however, tendon organs may also be activated, and the Group Ib afferents will transmit their input (inhibitory) to the spinal cord (Matthews, 1990). In human movement, therefore, the unexpected stretch of a muscle can trigger input from many receptors, including the Group Ia, Ib, and II afferents. The net effect is an afferent barrage that can vary substantially from trial to trial, even to the extent of failing to elicit a short-latency response (such as occurs when a platform on which a subject is standing is suddenly displaced and a rapid response is required to restore balance) (Horak & Nashner, 1986; Nashner, 1977). Furthermore, because of the involvement of the cortex in the stretch reflex (M2) response), the nervous system is able to modulate the response and spread it to muscles that have not been stretched, even to antagonists, if the activity is mechanically appropriate (Matthews, 1991). The net effect of this flexibility is that both the short- and long-latency components of the stretch reflex can be modified to meet the demands of the task.
The stretch reflex components are expected to play an important role in situations where stretching loads are high or efficient stretch-shortening behavior is necessary. Under these conditions muscle stiffness must be well coordinated to meet external loading conditions. An example of this is from the study performed by from Gollhofer et al. (1987). They found the reflex contribution to sustain the repeated stretch loads became enhanced, especially when measured during the maximal drop-test condition before, and immediately after, the fatigue loading. Thus in a non-fatigue state, the muscles are able to damp the impact in the SSC by a smooth joint motion. However, repeated damping movements followed by the concentric action may eventually become so fatiguing that the neuromuscular system changes its ‘stiffness’ regulation. This change is characterized especially by a high impact force peak followed by a rapid temporary force decline (Komi 1991). The enhanced stretch reflex contribution during the fatigue could be interpreted to imply attempts of the nervous system to compensate by increasing activation of the loss of the muscles’ contractile force to resist repeated impact loads.
Figure 6.58 from Komi 1991
It may not be surprising that the mechanism of SSC fatigue is not well understood. What is astonishing, however, is the fact that the literature lacks comprehensive coverage of the mechanistic events, which occur when the muscles are trained for several weeks or months by controlled SSC exercises. There is no doubt that special jumping exercises, which utilize the SSC, have beneficial effects on strength and power (e.g. Komi et al. 1982). In addition to metabolic stimuli in the muscular tissue, training with SSC exercise specifically loads the components related to stiffness regulation. One of the main purposes of strength and power training is to improve muscle stiffness, especially in the explosive type of force production. It has been proposed (Komi 1986) that the influence of the length-feedback component (facilitatory reflex), which originates from the muscle spindles, can be enhanced through training. This would improve muscle stiffness during the important stretching phase of the SSC. When the role the inhibitory force-feedback component (from the Golgi tendon organs) can be simultaneously decreased, the final result is a further increase in muscle stiffness. This would allow the muscle to tolerate greater stretch loads, possibly store more elastic energy, and improve power as well as mechanical efficiency (ME).
Programming involves the conversion of an idea into the proper strength and pattern of muscle activity necessary for a desired movement. The major suprasegmental centers involved in programming include the association cortex (premotor), motor cortex, basal ganglia, and cerebellum (Enoka 1994). The motor cortex provides the brain with access to most motor neurons. In addition the basal ganglia, cerebellum, and cerebral cortex can independently influence motor neurons via the brain stem. The neural output that emerges from the process of programming is known as the central command and is transmitted both to lower neural centers (brain stem and spinal cord) and back to the suprasegmental centers involved in the development of the program. The signal that is transmitted back to the suprasegmental centers is known as the corollary discharge and provides a reference that enables the system to interpret incoming afferent signals.
The central command activation of the lower neural centers initiates the execution phase of a movement. This phase involves the activation of motor neurons, both those in muscles directly associated with the movement and those in other muscles that must provide postural support to stabilize the body. Despite the many possible activation sequences of motor units that may be used for a given movement, it appears that learning results in the development of a specific activation sequence for each task (Enoka 1994). A stereotyped sequence of commands sent from the spinal cord to the muscles to elicit a specific behavior is known as a motor program (Enoka 1994). The motor program is the consequence of interactions between the programming activities of suprasegmental centers, spinal networks, and afferent feedback.
In addition to the efferent commands transmitted by the motor neurons, the execution phase involves the modification of movements by feedback from the sensory receptors (e.g., muscle spindle, tendon organ, joint receptor, cutaneous mechanoreceptors). The activation of these receptors results in afferent input that acts at the segmental level and also traverses in ascending pathways to suprasegmental centers (Enoka 1994).
Figure 7.1 from Enoka 1994
Although this input can be potent enough to initiate movement (e.g., reflexes), it is typically used to ensure consistency between the movement and the surroundings. This matching of input and output is accomplished by networks of interneurons that transform afferent input onto relevant motor neurons so that the output is appropriate for the conditions detected by the sensory receptors. If a particular afferent-efferent transformation occurs frequently enough, the neural network can learn to become more economical and can be activated with a minimum of input. Neural networks that are capable of generating behaviorally relevant patterns of output in the absence of external timing cue (by afferent input) are known as central pattern generators.
Neural Integration
The performance of a movement requires the activation of the motor system, those elements of the nervous system involved in controlling muscles. In general, motor systems must be capable of three basic requirements in order to produce useful movements (Enoka 1988): (a) information from sensory receptors must be channeled to motoneurons; that is, sensory information from the different parts of the body must affect the operation of the appropriate muscles. (b) the nervous system must be able to control accurately the level of force exerted by a muscle—this involves an interaction between the control of motor-unit activity and feedback from muscle receptors; and © the activity of different muscles must be coordinated.
Tripartite Model
The millions of cells of the nervous system communicate with one another and with target cells by two means: local-graded potentials and action potentials. The tripartite model was largely developed from research on locomotion (Wetzel & Stuart, 1977). The model assumes that the neuromuscular processes associated with movement belong to one of three compartments: high-level controller, low-level controller, and peripheral factors (Feldman & Grillner, 1983; Hasan et al., 1985). These compartments represent general functions that the nervous system performs during movement.
Figure 1 from Hasan et al. 1985
The critical element in terms of performance of this model is the interaction between the low-level controller and the muscles (Enoka 1988). The nervous system activates the appropriate muscles in different sequences for different movements. According to Roberts (1976), the nervous system uses six basic modes or strategies of activation:
- Set. A part of the body is moved freely from one position to another without encountering resistance (e.g., rotating the head about a vertical axis).
- Hold. A part of the body resists displacement due to external forces (e.g., holding a cup that is being filled).
- Drive. A limb segment is shifted in position, despite an opposing resistance, by exerting sufficient force to overcome the resistance (e.g., thrusting a hand into a glove).
- Punch. A limb segment acquires momentum due to forces much greater than are necessary to move the limb from one position to another. Some of the momentum of the limb is transferred to an object as the result of an impact (e.g., hitting a punching bag, kicking a football).
- Catch. The muscles of a limb perform negative work and absorb power from an external agent (e.g., catching a ball, landing from a drop)
- Throw. The muscles of a limb perform positive work on the limb segment, including some object that may be held; subsequently, the muscles do negative work to slow down the limb (e.g., when throwing a ball, momentum is imparted to the arm as well as to the ball, but the arm momentum is absorbed after ball release).
The set of commands sent out by the low-level controller for a particular motor-control strategy (sequence of muscle activity) is also known as a motor program (Enoka 1988). The neural circuitry responsible for the motor program comprises collections of motoneurons and interneurons and is located in the low-level controller, which may (depending upon the task) correspond anatomically to the spinal cord and brain stem. The low level controller is an extremely capable component; for example, the basic features of muscle activation necessary for locomotion are generated not by the brain, but by the low-level controller (Enoka 1988).
The low-level controller cannot function alone and needs the other two components to provide an on-off switch and adaptability to a variable environment. The low-level controller is activated and sustained by descending commands (central command) from the high-level controller (supraspinal centers). The central command from the high-level controller serves to activate a motor program in the low-level controller. Afferent feedback from peripheral receptors (e.g., muscle spindle, tendon organ) can modify the output from both the high- and low-level controllers to adapt to changing conditions (Enoka 1988).
An additional feature of the tripartite model deserves emphasis. The high-level controller actually represents a complex interaction among many different supraspinal structures. A copy of the central command, which represents the output of these interactions, is led back to the high-level controller so that the various structures are made aware of the final output (Enoka 1988). This copy is referred to as the efference copy. It appears that the supraspinal structures determine the effort involved in a particular activity on the basis of this efference copy. Monitoring of the efference copy for this purpose is known as the sense of effort (Enoka 1988).
The utility of the tripartite model is considered in the topic of reflexes. Previously, we have considered the traditional notion that a reflex is an input-output relationship. The input is provided by some sensory mechanism (e.g., muscle spindle, tendon organ), and the output occurs over the final common pathway, the alpha motoneuron. Between the input and output stages, however, there exist many interneurons.
Any change in the level of excitation of the interneurons will readily affect the coupling between the input and output stages of a reflex. Many interneurons are under the direct influence of the high-level controller. Consequently, as the output from the high-level controller changes during an activity, the input-output coupling associated with a reflex may also change. It is not surprising; therefore that we describe reflexes as phase- and state-dependent responses. This means that the response depends on the state of excitation within the low-level controller.
<>You cannot alter your cognitive processes if I am understanding you. You cannot ‘train’ the process by which your brain learns movements. <>
Are you sure? What do you base this on?
I didn’t phrase that properly, I just meant you cannot prevent the fact that your brain will learn what you are doing, beneficial or not
<> Unless you are consciously working on changing an aspect of a skill, it will become more and more a habit (if performed consistently).<>
Definitely, that’s one of the things I’m concerned with, I don’t want to engrain my current speed, but rather get faster.
<>And remember to only work on one aspect of technique at a time, or you will have the problem with ‘juggling’ that you mentioned. <>
Um, apparently you didn’t get the bit about me being a juggler. That’s what I’m training.
:o Oops, I should read a bit more carefully, when I read I assumed juggling in the sense of aspects of spriniting technique. : )
<> The problem is to choose which problem to work on first! While you work on technique flaw A, technique flaw B becomes more deeply ingrained, and harder to undo later! I’m sure all coaches can relate to this? <>
Not sure it becomes more deeply engrained, but I agree that you can only focus on one element at a time.
<>For example, when I play golf, I usually have about 5-6 swing faults to work on when I am out of practice. Theory and experience tells me I can only work on one fault at a time (over the period of days) and succesfully improve. So I choose what I consider to be the biggest flaw, and work on that, then when that is fixed, I work on the second biggest flaw, and eventually I am hitting that ball as I want each time. Is any of this helping?<>
OK, that makes sense, but the difficulty is that I have to develop progressively higher throws, while increasing throw frequency, as I climb the ladder of numbers. Doing my current number over and over will only, IMO, engrain that speed and keep me from getting faster/more powerful. So I’m wondering how to “tell” myself to let it become automatic? Drills, okay. Anything else?
Hmm, I’m not a juggling expert (or even novice). But to tell yourself to let it become automatic, do you assume it will not become automatic? I can’t suggest anything new I don’t think. I can only imagine that you would have to increase the difficulty somehow as gradually as possible.
Thanks for the input.[/QUOTE]
There is a consensus in the literature of motor learning dating back to the 1970s, that beginners or athletes who are still in the cognitive stage of motor learning benefit more from feedback that is (KR) knowledge of results rather then (KP) knowledge of performance. KR is information about the success with respect to the goal state- usually represented as measurements of time /distance, while the latter is information about the learners overall technique.
IT does seem contrary to intuition that beginners learn more from feedback that is not technical based. This does raise the question why?
According to Schmidt beginners are unable to effectively use feedback on technique because the learner lacks a perceptual trace- a reference of correctness which evaluates the correctness of the movement. In practical terms, identifying errors in technique lacks meaning to the learner as they lack a kinestic sense for the movement. Giving feedback in (KR) on each trial reinforces the learner’s perceptual trace. As the learner progresses through the learning stages, they start to develop a accurate representation of how a correct movement feels. For novice sprinters developing a feel for pace judgement would serve them better then advice on technique. As sprinters progress to the autonomous stage the performer is able to feel errors in the movement therefore technical advice can facilitate improvements in the technique.
It makes sense that beginners don’t respond well to feedback as they lack reference points- AND STRENGTH to assume the correct sprint positions.
This is why it is so valuable to start with strengthening exercises where the athlete is removed from the idea of sprint technique to avoid “paralysis by analysis”. (Medicine ball acceleration drills to promote required start strength and positions for example)
Another concern was expressed about patterning stagnation later on. This can be avoided by using:
1: Speed-change in sprint training, such as in and outs etc.
2: Angle changes, such as moving from hills to the flat as training progresses.(slight grade downhill runs have been tried for this purpose, but with limited success)
3: Selection of distances for speed development to reach, but not prolong, max speed (the distance would be slightly lengthened with a strong tailwind to take advantage of the potential for a new higher speed).
4: Advancement of surface- from smooth grass to the track over the early weeks of training in a given phase.
Remember, all these tactics work best where there is the most room for improvement and the duration of exposure should be judged accordingly.
How about adding to this list and breaking the drills down by the three identified performance groups?