Academic journal article Research Quarterly for Exercise and Sport

Neuromuscular Characteristics of Endurance- and Power-Trained Athletes

Academic journal article Research Quarterly for Exercise and Sport

Neuromuscular Characteristics of Endurance- and Power-Trained Athletes

Article excerpt

In response to chronic physical training, the human neuromuscular system undergoes significant and specific adaptations. More importantly, these influences are the result of the type and quantity of physical activity. One of the simplest neuromuscular mechanisms is the spinal stretch reflex. The reflex system was previously viewed as inflexible, with a relatively fixed response that could vary only slightly. However, more recent data have identified an adaptive plasticity in the reflex system. In this respect, the reflex system can be used to assess training and aging adaptations of the human neuromuscular system. Due to their methodological simplicity, both the tendon-tap reflex and the electrically evoked Hoffmann reflex (H-reflex) can be used to assess training adaptations of the human neuromuscular system. The purpose of this paper is to review briefly the tendon-tap and H-reflex paradigms and delineate the research findings pertaining to changes in the reflex system with physical training. For purposes of clarity, this discussion will be divided into the following: (a) differences observed in the tendon-tap reflex, (b) differences observed in the H-reflex, and (c) role of interneurons in mediating these changes.

Key words: neural plasticity, training

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In response to chronic physical training, the human neuromuscular system undergoes significant and specific adaptations. More importantly, these influences are the result of the type and quantity of physical activity. The adaptations that occur in the neuromuscular system can be assessed in various ways. For example, strength acquisition with training has long been shown to consist of both a neural component (e.g., quick increase in strength gains) and a muscular component (e.g., muscle hypertrophy; for review see Hakkinen, 1994). In this paper, neural changes occurring in the spinal stretch reflex loop as a result of chronic training will be examined. As the spinal stretch reflex represents one of the simplest involuntary responses, physical training on a one-synapse structure can uncover much information pertaining to these adaptations.

Perhaps the simplest neuromuscular mechanism is the spinal stretch reflex, which is elicited when a mechanical stretch of the tendon or muscle is detected by a specialized sensory receptor located within the muscle, the muscle spindle. The amplitude and rate of this length change are coded and relayed to the central nervous system via the Ia afferent pathway. Sudden muscle stretch will increase the firing rate of the Ia afferents, which transfer information from the muscle to the spinal cord. Once in the spinal cord, this information is then passed monosynaptically to the alpha motoneurons. Although connections from Ia fibers to the motor neurons are diffuse throughout the motor pool, the small diameter motoneurons (e.g., slow-twitch muscle fibers) are more easily depolarized by the Ia input due to their membrane properties. As a result, this powerful stimulus usually leads to the initiation of a motor volley to the neuromuscular junction, resulting in a corrective contraction of the stretched muscle. This neural circuitry for the spinal stretch reflex is shown in Figure 1.

[FIGURE 1 OMITTED]

The primary function of the stretch reflex is to compensate for nonlinear mechanical muscle properties and provide the central nervous system with the means to regulate muscle stiffness over a range of background muscle forces and lengths (Houk, 1977, 1979). The entire stretch reflex response can consist of two-three components, with a short latency component (the spinal stretch reflex; 30-60 ms) followed by one to two longer-latency responses (> 70 ms; e.g., the functional stretch reflex and triggered reactions). Typically, the spinal stretch reflex for the quadriceps muscles (patellar reflex) and the triceps surae muscles (Achilles reflex) have been examined. This distinction is important, because the quadriceps muscle consists of fast-and slow-twitch fibers, whereas the triceps surae (with special reference to the soleus muscle) is predominantly a slow-twitch muscle. …

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