Power and Strength-The Physical Parameters and Relationships that ExistBy Nathan WilliamsFor www.EliteFTS.comSize theory of motor unit recruitment The motor unit consists of a nerve, referred to as a motor neurone, originating from the spinal cord and all the individual muscle fibres that nerve innervates. There are between 100 and 1000 motor units in each muscle that control the contraction/relaxation of the individual muscle fibres. The motor units themselves consist of between three and 800 fibres of the same fibre type, dependent on the level of precision control needed by the muscle. The finer the movements required by a muscle, the fewer fibres are contained in each motor unit. Slow twitch fibres tend to innervate between ten and 180 fibres while fast twitch motor units are larger with motor neurones innervating between 300 and 800 fibres. When a muscle is placed under tension, motor units are recruited based on size with the smaller slow twitch motor units firing first followed by progressively larger fast twitch motor units further increasing the tension. The muscle generates maximum force when all motor units are recruited and are firing at their maximum rate. The suggested training application of this is that fast twitch fibres will only be recruited when there is the requirement to generate large forces. Hence, resistance training with light weights will not generate the recruitment of large numbers of fast twitch fibres. Near maximal weights will ensure large forces are required, thus recruiting all available motor units (Wilson 1994). Neural adaptations to resistance training There are a number of commonly cited neural adaptations that occur as a result of resistance training including:
Other adaptations not discussed in this article include musculo-tendonus stiffness (increases producing more force), selective motor unit recruitment, changes to muscle architecture (increased angle of pennation), and changes in enzyme concentration. Disinhibition is the process of “turning off” the inhibitory signals that are part of the body’s feedback system designed to protect it from injury. These signals, primarily from the Golgi tendon organs, prevent the recruitment of further motor units and prevent motor units firing at maximal rates, thereby limiting the amount of force generated by a muscle. Constant exposure to large levels of tension can reduce the sensitivity of the Golgi tendon organs allowing greater forces to be produced through greater motor unit recruitment and greater firing rates. Motor unit synchronisation involves neural adaptation allowing motor units within a muscle to fire simultaneously or in a more coordinated fashion than usually occurs when a muscle is activated. Generally, when a muscle is under tension and force production is required, motor units are fired randomly while following the size theory of motor unit recruitment. However, strength training has been shown to enhance motor unit synchronisation, which in itself is beneficial to increasing strength (Wilson 1994). The learning effect helps overcome the lack of coordination between agonists, stabilisers, and antagonist muscle groups when performing new resistance training exercises. Nowhere is this lack of muscular coordination displayed more prominently than when beginner resistance trainers attempt to push a barbell in a straight line on the bench press! Wilson (1994) states, “As the neuromuscular system becomes increasingly proficient with the performance of an exercise, the coordination of the muscles improves, facilitating performance. The above examples of neural adaptations to resistance training are the reasons that strength gains can be made without hypertrophy occurring. This type of gain is particularly evident in athletes beginning resistance-training programmes for the first time.” Length-tension relationship The normal resting length of a muscle is its optimal length for producing muscular tension due to the efficiency of myosin and actin binding at this length. As the muscle contracts, cross-bridge linkage formations are impaired because of the amount of overlap of the filaments. Hence, the muscular contraction becomes less effective. A decrease in the effectiveness of contraction is also seen when the muscle is stretched beyond its normal resting state. This time it’s due to the reduced overlap of filaments causing a decreased number of available binding sites. This is partially offset by some additional tension produced by the elastic connective tissue, which lies in parallel to the muscle fibres (Wilson 1994). The length-tension relationship is displayed in figure 1.
Figure 1: Muscle length-tension curve (adapted from Bloomfield, Ackland, Elliot 1992). Force-velocity relationship The force generated during an isometric contraction is relatively high. However, it is the muscles undergoing rapid eccentric movements that generate the highest forces. Maximal eccentric force is approximately 30 percent greater than maximal force produced by concentric movements. As the speed of a concentric movement increases, the amount of tension the muscle can produce decreases. This is outlined in figure 2.
Figure 2: Force-velocity relationship for muscle (Bloomfield, et al. 1992). One application of this relationship is seen in heavy eccentric resistance training where athletes attempt to enhance strength by taking advantage of the large forces that can be produced during these eccentric movements (Wilson 1994). Power-load relationships Given that the force-velocity relationship states velocity and force are inversely related for concentric movements (see figure 2), it is clear that maximum power can’t be generated through maximizing both speed and force at the same time. Wilson (1994) suggests that maximal power (power = force X velocity) could theoretically be achieved in three ways:
The latter has been shown to maximize power production, which peaks at around 30–45 percent as shown in figure 3. Between 30–50 percent 1RM is the desired range for optimal power development. However, there has been further research indicating that 80 percent can produce peak power in trained athletes.
Figure 3: Power-load relationship of muscle (Wilson 1992). References Bloomfield J, Ackland TR, Elliot BC (1994) Applied Anatomy and Biomechanics in Sport. Victoria, Australia: Blackwell Scientific Publications. Wilson GJ (1994) Strength and Power in Sport. In: Bloomfield J, Ackland TR, Elliot BC (1994) Applied Anatomy and Biomechanics in Sport. Victoria, Australia: Blackwell Scientific Publications, 110–19. Elite Fitness Systems strives to be a recognized leader in the strength training industry by providing the highest quality strength training products and services while providing the highest level of customer service in the industry. For the best training equipment, information, and accessories, visit us at www.EliteFTS.com. |
