Power and Strength: The Physical Parameters and Relationships That Exist

By Nathan Williams

For www.EliteFTS.com

The motor unit consists of a nerve, referred to as a motor neuron, originating from the spinal cord and all of the individual muscle fibers that the nerve innervates. There are between 100 and 1000 motor units in each muscle, and they control the contraction/relaxation of the individual muscle fibers. The motor units themselves consist of between three and 800 fibers of the same fiber type, depending on the level of precision control needed by the muscle. The finer the movements required by a muscle, the fewer fibers contained in each motor unit. Slow twitch fibers tend to innervate between ten and 180 fibers while fast twitch motor units are larger with motor neurons innervating between 300 and 800 fibers.

When a muscle is placed under tension, motor units are recruited based on size. The smaller, slow twitch motor units fire first followed by progressively larger, fast twitch motor units, which further increase 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 fibers will only be recruited when large forces are required. Hence, resistance training with light weights won’t generate the recruitment of large numbers of fast twitch fibers. Near maximal weights will ensure that large forces are required, thus recruiting all available motor units (Wilson 1994).

There are a number of commonly cited neural adaptations that occur as a result of resistance training including:

• disinhibition
• motor unit synchronization
• learning effect

Other adaptations 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 designed to protect the body from injury as part of the body’s feedback system. These signals, primarily from the Golgi tendon organs, prevent the recruitment of further motor units and prevent motor units from 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 synchronization involves neural adaptation. This allows motor units within a muscle to fire simultaneously or in a more coordinated fashion than what 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 synchronization, which in itself is beneficial to increasing strength (Wilson 1994).

The learning effect helps overcome the lack of coordination between agonists, stabilizers, and antagonist muscle groups when performing new resistance training exercises. This lack of muscular coordination is displayed most prominently 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.” These examples of neural adaptations to resistance training are the reasons that strength gains can be made without the occurrence of hypertrophy. This type of gain is particularly evident in athletes beginning resistance training programs for the first time.

A muscle’s normal resting length is its optimal length for producing muscular tension because of the efficiency of myosin and actin binding at this length. As the muscle contracts, cross-bridge linkage formation is impaired. This is due to the amount of overlap of the filaments. Thus, the muscular contraction becomes less effective.

A decrease in the effectiveness of a contraction is also seen when the muscle is stretched beyond its normal resting state. This time the reduced overlap of filaments causes a decrease in the 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 fibers (Wilson 1994). The length-tension relationship is displayed in figure 1.

Figure 1: Muscle length-tension curve (adapted from Bloomfield et al. 1992).

The force generated during an isometric contraction is relatively high. However, the muscles undergoing rapid eccentric movements 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. In this type of training, athletes attempt to enhance strength by taking advantage of the large forces that can be produced during these eccentric movements (Wilson 1994).

Given that the force-velocity relationship states that 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:

1) high force X low speed

2) low force x high speed

3) moderate force X moderate speed

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, further research has indicated 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, and Elliot BC (1994) Applied Anatomy and Biomechanics in Sport. Victoria, Australia: Blackwell Scientific Publications, pgs. 110–119.