Lab 1

Question 1

Introduction

Answer 1

When skeletal muscles are excited by the nervous system, they convert chemical energy into
mechanical energy. This leads to muscle contraction. The production of muscle tension (and thus
muscle force) is accomplished by interactions between the myosin and actin filaments within
skeletal muscle cells. When a muscle contracts, and the intended movement (e.g., elbow flexion)
is unconstrained (i.e., not isometric) the production of force also creates a velocity around the joint. These interactions, referred to as the “crossbridge”, involve a cycle of phases whereby the myosin head: i) detaches from actin; ii) rotates to a high energy state; iii) reattaches to actin; and iv)releases from its higher energy state to pull actin filaments in a way that shortens the muscle fibres (i.e., the “sliding filament theory”). The greater the number of crossbridge cycles occurring at the same time within a muscle, the greater the force it can produce.

Two functional measures of muscle contraction are the tension that it produces (i.e., force) and
how quickly it shortens (i.e., velocity). When humans attempt to move an object by applying force (e.g., lifting furniture from the ground, pushing a cart, curling a barbell), the speed with which the object moves will depend on its mass. The heavier the object, the more force the muscle needs to apply to overcome gravity and inertia. The more force the muscle needs to apply, the slower the muscle can shorten to move the object. Stated differently, a fully activated muscle will shorten at a slow velocity when it contracts against a heavy load. This occurs due to the muscle “force-velocity” relationship.

When skeletal muscle concentrically contracts (i.e., in “shortening” conditions), the relationship
between muscle contraction force and velocity is characterized by a hyperbolic function (see
Figure 1). In this relationship, force declines as velocity increases. At the extremes of this
relationship are the “maximal isometric force” (P0) or the point on the curve at which velocity
equals “0” (i.e., at the y-intercept) and the “maximal shortening velocity” (Vmax) the point on the
curve at which force equals “0” (i.e., at the x-intercept). Typically, force is expressed in units of
“Newtons” (N). One N is the force required to cause a mass of 1 kg to accelerate at a rate of 1 m/s^2 in the absence of other force-producing effects. Velocity is usually expressed in m/s.

Under carefully controlled experimental conditions, the shape of force-velocity relationship can
be reproduced when assessed using single muscle fibre preparations, whole muscle, and with
groups of muscles performing dynamic movements (e.g., squat, bench press, biceps curl). In any
of these conditions, muscles can only operate on or below the force-velocity curve (and not above it). Thus, this relationship sets the theoretical limit of muscle performance.

The force that a muscle produces at various velocities can be explained, at a basic level, by
crossbridge cycling and the sliding filament theory of muscle contraction. Maximal force is
primarily determined by the number of formed crossbridges, the force per crossbridge, and calcium availability (i.e., calcium release). Conversely, maximal velocity is primarily determined by crossbridge detachment rates (i.e., ADP release rate). Therefore, determinants of force and velocity are governed by separate basic mechanisms.

The force-velocity relationship can tell us a lot about how the structure of muscle relates to its
function. For example, the “high force” region of the curve (near P0) will depend on the cross-
sectional area of the muscle and the arrangements of its muscle fibres. Both factors will increase
the number of sarcomeres that are in parallel with one another and the overall strength of the
muscle. In addition, muscle fibre type composition (type II versus type I muscle fibres) will
influence the entire curve (including both P0 to Vmax).

Figure 2 illustrates the force-velocity relationship of two muscles composed of primarily type I
versus primarily type II muscle fibres. Note the differences in the shape of the curve and the
relative positions of P0 and Vmax. Muscle with a greater proportion of type II muscle fibres can
produce greater force at any given velocity and greater velocity at any given force. This is because type II fibres have a higher myosin ATPase content (i.e., more chemical energy available) and activity (i.e., higher rate of energy delivery) and much higher rate of calcium release and reuptake from the sarcoplasmic reticulum (i.e., can contract and relax faster).

It is important to note that the force-velocity relationship for a muscle is not fixed and will change depending on many factors. One of the main factors is time or duration of exercise. As we will learn later in the course, when a muscle is maximally activated, its ability to generate force (or power) will decrease hyperbolically over time. This occurs because the rate at which our muscle metabolic systems can deliver chemical energy wanes over time. Thus, if the force-velocity relationship of a muscle was compared before versus immediately after any duration of exercise, the relationship would be shifted downwards and to the left. How much the relationship changes will depend on the duration and intensity of exercise. These two factors will also influence the level of fatigue within the muscle.

Muscle fatigue may be defined as a loss in the capacity of the muscle to develop force and/or
velocity resulting from muscle activity under load. Muscle fatigue is reversible by rest. The factors that cause a loss in muscle force or velocity development are multi-factorial and may originate at different levels of the motor pathway. Typically, fatigue is divided into two components: central and peripheral fatigue. Peripheral fatigue is produced by changes at or downstream of the neuromuscular junctions (e.g., impairments in excitation-contraction coupling, or crossbridge effectiveness) and central fatigue originates in the central nervous system (e.g., decreased cortical motor drive, impaired efferent transmission).

The force-velocity relationship can also help determine the maximal power that a muscle can
produce under any loading condition. Mechanical power is the product of force and velocity (e.g., N x m/s = Nm/s). When power is plotted against velocity, the curve exhibits a parabolic function (see Figure 3). The peak of the parabola identifies the shortening velocity at which “maximal power” may be achieved and mechanical power is zero at either end of the force-velocity relationship. The “maximal power” can provide information regarding the contraction velocity that is most mechanically efficient (i.e., can generate the most power at a given load). Because of the separate mechanisms governing force and velocity (see above) “maximal power” is achieved at an approximate middle point between maximal force and velocity. Thus, there is a trade off between the two to produce “maximal power”. To achieve a high velocity crossbridge detachment rate must be quick, however, this impairs force production of individual crossbridges because there is inadequate time for optimal force generation. Furthermore, only the fastest myosin heads (i.e., Type II fibers) can consistently form crossbridges during high velocities as this high speed increases the probability that myosin will “miss” the binding site on actin. This reduces the total number of crossbridge interactions and thus the total force produced by the muscle. Therefore, to produce a high velocity contraction two primary determinants of maximal force production (i.e., force per crossbridge, and number of formed crossbridges) is impaired. This trade off explains the force-velocity and power-velocity relationships (see Figure 3).

In experimental conditions, the force-velocity relationship is determined by measuring velocity of movement against a given load. The outcome is a single data point that, along with other data
points derived from different loads, can be graphed to produce the “force-velocity” relationship.
For example, Figure 4 below shows the average velocity of an individual performing a double arm overhead triceps extension. Each data point represents a single, concentric contraction. The
individual completed the 6 contractions using dumbbells of 1, 2, 3, 5, 8 and 10 kg. During each
contraction, the average velocity was recorded (total displacement, in metres divided by seconds;
m/s). Force, in Newtons was determined by multiplying the load by the gravitational acceleration
constant (9.81 m/s2).