So far in our study of skeletal muscle we have looked in a fair amount of detail at the mechanisms which are responsible for the contraction of the individual muscle fibres. However, behaviourally it is the control over the force generated by the muscle which is really critical. For example, your biceps brachii is largely responsible for the flexion of your forearm whether you are sculling a schooner of Coopers in a busy bar or doing forearm curls with 100 Kg of weight in the gym. If the force generated when raising your schooner was too large, you might end up throwing the Coopers over your shoulder (in which case it might be just as well that you had spent that time in the gym).
Skeletal muscle is of course also known as voluntary muscle because it is directly under the control of motoneurones. In addition to this neural regulation, however, the mechanics of the muscle itself plays an important role in the control over the force generated. In this section we will take a fairly brief look at some of the factors (both neural and mechanical) which act to modify the force of skeletal muscle contraction.
First of all we need to appreciate that skeletal muscles are actually capable of two major types of contraction, as described below. Note that the term contraction in the context of muscle physiology does not necessarily mean 'shortening'. Rather, it simply refers to activation of the contractile machinery of muscle fibres - the cross-bridges. Following contraction, this contractile machinery is switched off, and tension declines, allowing the muscle fibre to relax.
Isotonic Contraction: This is where the tension the muscle remains constant as the muscle shortens. This is the type of contraction that occurs (for example) when you lift a glass off and move it towards your mouth. The mass of the glass doesn't change during the movement (so the tension in your muscle doesn't change) BUT the length of the biceps brachii muscle decreases as you flex your forearm. Because the tension remains the same, the contraction is considered to be isotonic. This is the type of contraction which actually produces movement.
Isometric Contraction: This is where the contraction of the muscle produces tension but the muscle's length does not change. An example of this type of contraction is when you put your hand under a fixed bench (palm up) and try to lift it. Your biceps brachii is generating plenty of tension but the length of the muscle remains the same (hence the name isometric). This is the type of contraction you use when you want to hold something still, and is also vital for the maintenance of posture.Isometric contractions are also sometimes referred to as static contractions.
It is important to realise that the mechanisms for producing isometric and isotonic contractions are exactly the same (i.e. cross-bridge interactions between the myofilaments). The only difference is that isotonic contractions involve sarcomere shortening whereas isometric contractions don't. There are in fact a large number of factors which regulate the force of contraction produced by a skeletal muscle. Time does not permit an exhaustive consideration of these here but we will deal with three of the more physiologically important ones.
A. Motor Unit Recruitment
You will recall from a previous section that a motor unit consists of a single motoneurone plus all the muscle fibres that it innervates. Consequently for a very weak contraction of the muscle, only a few motor units are recruited (activated) whereas more and more motor units are recruited when progressively larger contractions are required. This is known as the principle of orderly recruitment.
Interestingly, during sustained contractions of skeletal muscles (e.g. when you hold your arm out in front of you for a long time) the motor units take turns at doing the work. This progressive change in motor unit recruitment allows some motor units to rest whilst others continue doing to work. This is of course all carefully regulated by the neural circuits within the central nervous system that control movement.
A mechanism that may partially explain the principle of orderly recruitment is the size principle, which states that the order of recruitment of motor units is directly related to the size of their motoneurone. Motor units with smaller motoneurones will be recruited first.
B. Action Potential Frequency
Another way that the tension produced by a skeletal muscle fibre can be increased is by increasing the action potential frequency in the motor units. In the same way that excitatory post synaptic potentials exhibit temporal summation at synapses, so too can the force generated by action potentials in skeletal muscle fibres. This is possible because of the relatively long time that the tension generated by a single action potential lasts (up to 100 ms).
The diagram opposite shows that the tension (red) generated in a skeletal muscle fibre in response to a single action potential (blue) last much longer than the action potential. The smallest contractile response of a muscle fibre or a motor unit to a single electrical stimulus is termed a twitch.
When the action potentials in the muscle fibre are sufficiently far apart (in time) the tension produced by the second action potential is the same as the first. 
However, if the time between the two action potentials is decreased (you can simulate this by clicking on the +/- buttons below) then the tension produced by the second action potential increases before the tension produced by the first has declined and so we get temporal summation resulting in increased tension. 
If the action potential frequency is increased then you observe a progressive increase in the force generated due to this summation. Eventually increasing the action potential frequency even more produces no further increase in the tension produced. This is referred as a tetanic contraction.
C. Length-Tension Relationship
Another very important factor which alters the amount of force a muscle can generate is the length of the muscle prior to contraction.
There is an optimal length of each muscle fibre relative to its ability to generate force. Recall that a muscle fibre is comprised of sarcomeres connected end to end and that these sarcomeres are composed of both thick and thin filaments.
The optimal sarcomere length (and therefore muscle length) is that length where there is optimal overlap of the thick and thin filaments, thus maximising cross-bridge interaction. This can be demonstrated fairly easily in an experiment where you measure the force generated during a tetanic contraction of a single muscle at different starting lengths.
The diagram opposite shows the apparatus used to investigate the length-tension relationship in a skeletal muscle. The muscle is attached to a micromanipulator which allows the length of the muscle to be varied. The nerve supplying the muscle is stimulated at a frequency that which produces a tetanic contraction and the force generated during this contraction is recorded using a tension gauge.
The type of result you get when you plot starting length against tetanic tension in this type of experiment is shown opposite. As you can see, the maximal force which can be generated is around the the normal resting length of the muscle (point B on the graph) but that it drops off very quickly if the muscle is too short (point A) OR stretched (point C) prior to stimulation.
One explanation for this is illustrated by the cartoons on the right that illustrate the postulated appearance of a sarcomere at these three different points on the length-tension relationship.
At the normal resting range (B) there is a maximum overlap between the thin and thick filaments so the maximal tension can be generated.
However when the muscle is very short the thin filaments overlap (A) whilst stretching the muscle only permits interactions between the very ends of the thick filaments and the thin filaments (C). In both instances, the opportunities for cross-bridge interactions are greatly reduced, and consequently so is the force that can be generated.
This rather neat explanation remains fairly speculative, but it does support the sliding-filament mechanism on one hand, and also explains (to some degree) why the most difficult phase of a pull-up is the initial 2-3 cm of movement (from a dead-hang) (i.e., when the muscles are at their longest length) and the phase where you have to get your chin over that bar (i.e., when the muscles are at their shortest length).