Zajac FE and Gordon ME. Determining Muscle’s Force and Action in Multi-Articular Movement. Exercise and Sports Science Reviews, 1989, 17:187-230.

This article written 30 years ago provides substantial support for the Applied Functional Science® “truths of human movement” proposed by Dr. Gary Gray over the past 40 years. Many subsequent articles by researchers trained under Dr. Zajac at Stanford University have used mechanical modeling with inverse and forward dynamics to understand the role of muscles during function. The difference between the functions ascribed to muscles based purely on anatomy and the roles of muscles during activity-specific, multi-joint movements is monumentally important to training and rehabilitation.

This blog will highlight some of the points, based on research and data modeling, that bring to light the deficiencies in the structural anatomy approach to muscle function and movement. These statements illuminate many of the other blogs in the Evidence That Matters For Function series by Gray Institute®. It will be left to the reader to evaluate the authors’ statements in contrast to traditional educational training, and their own real-life experiences with patients and clients.

The following are direct quotes from this research article:

  • “For multijoint movement, however, extreme caution should be exercised when anatomy alone is used to infer the action of a muscle, a muscle acts to accelerate all joints, whether spanned or not.”
  • “Since a muscles’s action depends on the position of the body and on the muscle’s interaction with external objects (such as the ground), it can vary among motor tasks, and even during a single motor task.”
  • “However, because muscles that work together need not function identically, it may be best to study how muscles work synergistically rather than agonistically, in accomplishing the motor task.”
  • “In fact, it may be that biarticular muscles are well suited to transferring power among joints rather than accelerating joints.”
  • “Thus although their torques have opposite signs, hamstrings and rectus femoris may both act, for example in flat-footed postures, to extend the knee.”
  • “For example, the gastrocnemius can accelerate either (a) the knee into flexion and the ankle into extension (plantarflexion), (b) the knee into extension and the ankle into extension, or (c) the knee into flexion and the ankle into flexion (dorsiflexion).”
  • “Similarly, the action of a uniarticular muscle crossing a ball-and-socket joint depends on the ratio of its three torque components. Thus uniarticular muscles which are anatomical opposites may not act to accelerate the body oppositely.”
  • “Soleus’s effect on the knee and ankle angular acceleration depends on the knee angle, and thus so does the ratio of the two accelerations. Notice when the knee is flexed less than 90 (degrees), SOL accelerates the knee (into extension) more than it accelerates the ankle (into extension).”
  • “Muscle’s action is task dependent because the dynamic equations describing the relation among forces and motion depend on the interactions among the body’s segments and the environment.”

The above quotations from this article (cited above) highlight the Principles of human movement embodied in the Principles-Strategies-Techniques Process of Applied Functional Science®. The Principle of Chain Reaction®, complemented by the Principles of 3-D and Task-specific, provide Strategies for our movement programs that are in harmony with the conclusions in this research paper published 30 years ago. Our professional education has lagged behind, and so it is up to each individual to avail himself / herself of the knowledge not provided by his / her formal training!