| Neuromuscular Adaptations to Conditioning |
| Neuromuscular Anatomy and Physiology - All-or-none law o When a motor neuron fires, all the fibers that it serves are simultaneously activated and develop force. o A motor neuron excites the muscle fiber or fibers that it innervates by chemical transmission. o The action potential causes release of a chemical, acetycholine, which diffuses across the neuromuscular junction, causing excitation of the sarcolemma. o Once an action potential causes release of sufficient acetycholine to activate the sarcolemma, the fiber contracts. - Twitch o Each action potential traveling down a motor neuron results in a short period of activation of the muscle fibers within the motor unit. o If a second twitch is elicited from the motor unit before the fibers completely relax, force from the two twitches summates, and the resulting force is greater than that produced by a single twitch. o Decreasing the time interval between the twitches results in greater summation of force. - Tetanus o The stimuli may be delivered at so high a frequency that the twitches completely fuse o This is the maximal amount of force the motor unit can develop. - Variable Fast-twitch (Type I) fiber Slow-twitch (Type II) fiber Contraction speed Fast Slow Power output High Low Endurance Low High Aerobic enzymes Low High Anaerobic enzymes High Low Fatigue resistance Low High |
| Motor Unit Recruitment Patterns During Exercise - The force output of a muscle can vary over a wide range, a gradation that is essential for smooth, coordinated patterns of movement. Muscular force can be graded in two ways. o Frequency of activation § Increasing frequency of firing of the individual motor units increases force output of the whole muscle. o Recruitment § Varying the number of motor units activated |
| Role of Proprioceptors in Learning Physical Skills - Proprioceptors o Specialized sensory receptors located inside muscles, joints, and tendons that monitor the length and tension of the musculotendonous complex. o They provide the central nervous system with information concerning kinesthetic sense, or conscious appreciation of the body in three-dimensional space. Such neural input maintaining muscle tone and body posture. - Muscle spindles o Consist of several modified muscle fibers enclosed in a sheath of connective tissue. o These receptors provide information on muscle length and the rate of change in length. When the muscle lengthens, spindles are stretched. This deformation activates the sensory neuron of the spindle, which sends an impulse to the spinal cord, where it synapses (connects) with alpha motor neurons. This results in the activation of motor neuron that innervate the same muscle. - Golgi tendon organs (GTOs) o Located in tendons near the myotendinous junction are in series, that is, attached end to end, with extrafusal muscle fibers. o These receptors are sensitive to stretch, but much less so than muscle spindles. o They transmit information concerning tension rather than muscle length. Whereas spindles facilitate activation of the muscle, neural input from GTOs inhibits muscle activation, a process that is thought to provide a protective mechanism from development of excessive tension. Thus, when an extremely heavy load is placed on the muscle, the GTOs cause the muscle to relax. The ability to override this inhibition may be one of the fundamental adaptations to heavy resistance training. |
| Neuromuscular Adaptations to Exercise Conditioning Adaptations to Resistance Training - Hypertrophy o One of the fundamental adaptations to resistance training is an increase in muscle mass. This occurs by enlargement of muscle fibers, not by an increase in their number. o The benefit of an increased cross-sectional area of the muscle fibers is an increased ability to develop force. o Muscle fiber hypertrophy does not occur uniformly between the two major fiber types. o Conventional resistance training that fast-twitch fibers show greater increases in size than slow-twitch fibers. o Ultimate potential for hypertrophy may reside in relative proportion of fast-twitch fibers may have limited potential to markedly increase muscle mass with resistance training. o Females show the same or greater relative hypertrophy after resistance training o Increases in strength during the first 1 to 2 months of training performed by previously sedentary people are usually not accompanied by muscle fiber hypertrophy. Consequently, it has been hypothesized that neural factors must adapt in some way to allow for increased expression of strength. After this period, muscle fiber hypertrophy becomes obvious and contributes to increased strength. It is more difficult to demonstrate marked increases in muscle size and strength in weight-trained athletes. o Neural factors must account for the improved performance. Accordingly, the contribution of training to optimal performance can only be realized if training intensity is maximal. - Atrophy o 1-month of detraining results in minimal muscle fiber atrophy and strength loss. o After this period, decreases in strength occur at a greater rate than decreases in fiber size. ? It is the loss of the neural adaptations to resistance training that is mainly responsible for the decrease in strength over 2 to 3 months of detraining. o 1-month of detraining should not be seriously compromised. - Type of resistance training performed o Explosive training - speed of movement (Olympic weightlifters) § Substantial improvement in maximal power output § This occurs because of muscle fiber hypertrophy, but more significantly because of improvements in the maximal rate at which force can be developed. o Conventional resistance training - lifting heavy loads § Greater hypertrophy and increases in strength but does not improve maximal power output as much. o Significant increases in muscle strength and mass of the thighs can be induced by: § 2 to 3 intense training sessions per week, each no longer than 30 min. § 3 to 5 sets of 2 or 3 exercises with 6 to 12 repetitions per set are more than sufficient § Even once weekly, this regime is sufficient to maintain muscular strength and mass. Thus, it should be possible to conduct in-season training in such a way muscle mass and strength do not decrease, thereby maintaining the gains that were made in the summer or winter during off-season or preseason periods. - Resistance training does not enhance maximal aerobic power or aerobic power of muscle tissue; neither does it appear to impair these variables. In fact, concurrent performance of resistance and endurance training does not abate the extent of the positive adaptations to endurance training. When competitive distance runners added lower body resistance training to their conditioning programs, several aspects of short-term endurance actually improved. Although the mechanisms responsible for the response have not been identified, it appears that resistance training may increase a distance runner’s or cyclist’s ability to sprint at the end of the race. |
| Adaptations to Endurance Training - Increase in the aerobic capacity of the trained musculature. - Increase aerobic power and oxidative capacity of skeletal muscle - This adaptation occurs as a result of glycogen sparing (less glycogen use during exercise) within the muscle, which prolongs performance and reduces lactic acid buildup. Moreover, increased fat utilization contributes to glycogen sparing. - The concurrent performance of intense endurance and resistance training compromises the extent of the increase in strength that would have occurred if only resistance training had been performed. - Performance of endurance training: o 3 days/week o 30 min/day o Intensity = heart rate of at least 70% of the maximum o Frequency, daily duration, and intensity need to be increased |