## Forcemotion Principle

Another way to modify human movement is to change the application of forces. The Force-Motion Principle states that it takes unbalanced forces (and the subsequent torques they induce) to create or modify our motion. To know what size and direction of force to change, recall that a free-body diagram of the biomechanical system is usually employed. A major limitation of functional anatomical analysis was the limited nature of the forces and structures being considered. We are not in a position to perform quantitative calculations to determine the exact motion created at this point in the text, but this section will provide examples of the qualitative application of the Force-Motion Principle in improving human movement. Later on, in chapters 6 and 7, we will explore Newton's laws of motion and the major quantitative methods used in biomechanics to explore the forces that create human movement.

Kinesiology professionals often work in the area of physical conditioning to improve function. Function can be high-level sport performance or remediation of the effects of an injury, disuse, or aging. If muscle forces are the primary motors (hip extensors in running faster) and brakes (plantar flexors in landing from a jump), the Force-Motion Principle suggests that muscle groups that primarily contribute to the motion of interest should be trained. Remember that this can be a more complex task than consulting your anatomy book. How can we know what exercises, technique (speed, body position), or load to prescribe?

Imagine a physical education teacher working with students on their upper body muscular strength. A particular student is working toward improving his score on a pull-up test in the fitness unit. The forces in a pull-up exercise can be simplified into two vertical forces: the downward gravitation force of bodyweight and an upward force created by concentric muscle actions at the elbows, shoulders, and back. The considerable isometric actions of the grip, shoulder girdle, and trunk do not appear to limit this youngster's performance. You note that this student's bodyweight is not excessive, so losing weight is not an appropriate choice. The teacher decides to work on exercises that train the elbow flexors, as well as the shoulder adductors and extensors. The teacher will likely prescribe exercises like lat pulls, arm curls, and rowing to increase the student's ability to pull downward with a force larger than his body-weight.

Suppose a coach is interested in helping a young gymnast improve her "splits" position in a cartwheel or other arm support stunt (Figure 3.17). The gymnast can easily overcome the passive muscular tension in the hip adductors to create a split in

Figure 3.17. The Force-Motion Principle can be applied in a situation where a gymnast is having difficulty in performing inverted splits. The two forces that may limit the split are the passive tension resistance of the hip muscles or inadequate strength of the hip abductors. The coach must decide which forces limit this athlete's performance.

Figure 3.17. The Force-Motion Principle can be applied in a situation where a gymnast is having difficulty in performing inverted splits. The two forces that may limit the split are the passive tension resistance of the hip muscles or inadequate strength of the hip abductors. The coach must decide which forces limit this athlete's performance.

a seated position, but the downward force creating this static position is large (weight of the upper body) compared to the weight of the leg that assists the split in the inverted body position. The Force-Motion Principle suggests that the balance of forces at the hips must be downward to create the split in the dynamic action of the stunt. In other words, the forces of gravity and hip abductors must create a torque equal to the upward torque created by the passive tension in the hip adductors. If the gymnast is having trouble with this stunt, the two biome-chanical solutions that could be considered are stretching the hip adductors (to decrease passive muscle tension resistance) and increase the muscular strength or activation of the hip abductors.

The examples of the Force-Motion Principle have been kept simple for several reasons. First, we are only beginning our journey to an understanding of biomechan-ics. Second, the Force-Motion Principle deals with a complex and deeper level of mechanics (kinetics) that explains the causes of motion. Third, as we saw in this chapter, the complexity of the biomechanical

Interdisciplinary Issue:Variability

Scientists from a variety of disciplines have been interested in the variability of human movement performance. Biome-chanical studies have often documented variability of kinematic and kinetic variables to determine the number of trials that must be analyzed to obtain reliable data (Bates, Osternig, Sawhill, & Janes, 1983; Rodano & Squadrone, 2002;Win-ter, 1984). Motor learning studies have focused on variability as a measure of neuromuscular control (Davids et al., 2003; Slifkin & Newell, 2000).The study of variability has also indicated that variability may play a role in potential injury (James, Dufek, & Bates, 2000). Multiple biomechanical measurements of kinematics and kinetics may provide important contributions to interdisciplinary studies of human movement variability.

and neuromuscular system makes inference of muscle actions complicated. The variability in the forces and kinematics of human movement, therefore, have been of interest to a variety of scholars (see the Interdisciplinary Issue on Variability). The rest of the book will provide challenges to the perception that the causes of and solutions to human movement problems are simple and introduce you to the main areas of biomechanics that are used to answer questions about the causes of movement.

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