Some years ago while I was on the mechanical engineering staff at my former university (University of Witwatersrand, S Africa), I was asked if I could co-supervise the research of a postgraduate student from the physiotherapy department, because she had opted to study changes in strength with certain types of conditioning and one of my specialisations is the biomechanics of strength.
My first questions to her were: "What type of strength do you wish to study and how are you going to measure it?". Her response was one of amazement. After all isn't all strength the same? Isn't strength just the ability of the body to produce maximum force? Wouldn't it just be measured with some isokinetic machine like the 'Cybex'? I attempted to explain that strength is not that simple, even though everyone appears to have a very adequate intuitive grasp of the concept. Furthermore, I stressed that strength as measured isokinetically does not necessarily relate directly to strength as exhibited in real sport. During the hours that we spent discussing this issue, this poor student expressed her utter frustration that, despite having studied all about strength during her basic degree at a top university, she had never been taught any of the finer details of what strength really is.
This student's experience is by no means unique. From my involvement with thousands of other students, personal trainers and coaches throughout the world, I have found that the full scope of strength and strength training is very superficially understood and that many struggle to define strength beyond the belief that it "is the ability of the body to produce maximum force." While some may think that accurate definitions are pedantic and unnecessary, it is essential to point out that what is left out of a simplistic definition may be precisely what hinders you from fully understanding and applying any given method of training and testing.
This is why I chose to begin my contributions to this website with enough material to enable members to really understand the enormous richness and vast applications of strength in all types of personal training and sports coaching.
Let us first correct that basic definition that strength is the ability of the body to produce maximum force. It is not! Strength is the ability of the body to produce force; the ability of the body to produce maximum force is maximum strength. Even then, this implies that strength is some sort of general property of human capability, which totally ignores the fact that strength depends on the way in which it is produced or measured. After all, we frequently are exposed to arguments about who is the strongest type of person - the weightlifter, the powerlifter, the wrestler, the footballer, the manual labourer, the Highland Games contestant, a world Strongman competitor....?
If we are quite objective we have to recognise that there is no universal way of proving strength superiority because one may be strong in one test, but relatively weak in another. It is apparent that the production of "strength" depends on the test, the movements involved, motor skill, the duration of the test, the weight of the person, and several other such factors. In short, all strength is contextual or situational.
So what does one generally do when a client approaches you to develop sport specific"strength", "functional strength", "core strength" or rotator cuff strength? Well, you design a programme based upon your personal education and experience, often almost reflexively choosing to use Olympic lifting methods, circuit training, machine training, HIT ("High Intensity Training"), plyometrics and so forth, depending more often on personal bias than a thorough, objective analysis of what is involved.
Your client might be an experienced athlete like a gymnast, boxer or martial artist who challenges you with the very real point that most of the best performers in their sports never use weights, so how can doing a power clean, balancing on a ball or using a resistance machine really improve their sport specific strength? Anecdotes about all the results you have achieved may be to no avail because your client might detect that you do not fully understand the nature of the specific forms of strength (and power) required. This is yet another reason why it is vital to understand the whole spectrum of what strength really is.
Accurately speaking, strength is the result of muscular action initiated and orchestrated by electrical processes in the nervous system of the body. Classically, it may be defined as "the ability of a given muscle or group of muscles to generate muscular force under specific conditions." Thus, "maximal strength" is the ability of a particular group of muscles to produce a maximal voluntary contraction in response to optimal motivation against an external load. This strength is usually produced in competition and may also be referred to as the "competitive maximum strength." It is not the same as "absolute strength", which Dr Zatsiorsky calls "maximum maximorum strength", the maximum of all maxima, and which usually is associated with the greatest force which can be produced by a given muscle group under involuntary muscle stimulation by, for example, electrical stimulation of the nerves – supplying the muscles or recruitment of a powerful stretch reflex by sudden extremes of loading.
For practical purposes, "absolute strength" may be regarded as roughly equivalent to maximal eccentric strength, which is difficult or impractical to measure, because a maximum by definition refers to the limit point preceding structural and functional failure of the system. Thus, it is apparent that special feedback mechanisms, like governors in a mechanical engine, exist in the nervous system to prevent a muscle from continuing to produce force to the point of mechanical failure. This is why it probably would be more practical to use the "maximum explosive isometric strength" (produced under so-called maximum plyometric conditions or explosive thrusting against an isometric maximum load) as an approximation to absolute strength. To prevent confusion, we also need to note that the term "absolute strength" sometimes is used to define the maximum strength which can be produced irrespective of one's body mass.
It is also vital to recognise a "training maximum" or training 1RM (single repetition maximum), which is always less than the competition maximum in experienced athletes, because optimal motivation invariably occurs under competitive conditions. Zatsiorsky states that the training maximum is the heaviest load which one can lift without great emotional excitement, as indicated by a very significant rise in heart rate before the lift. It is noteworthy that, in the untrained person, involuntary or hypnotic conditions can increase strength output by up to 35%, but by less than 10% in the trained athlete. The mean difference between training maximum and competitive is around 12% in experienced weightlifters, with larger differences being exhibited by lifters in heavier weight classes.
The merit of identifying the different types of strength or performance maximum lies in enabling one to prescribe training intensity more efficiently. Intensity is usually defined as a certain percentage of one's maximum and it is most practical to choose this on the basis of the competitive maximum, which remains approximately constant for a fairly prolonged period. The training maximum can vary daily, so, while it may be of value in prescribing training for less qualified athletes, it is of limited value for the elite competitor.
It is relevant to note that competitions involve very few attempts to reach a maximum, yet they are far more exhausting than strenuous workouts with many repetitions, since they involve extremely high levels of psychological and nervous stress. The high levels of nervous and emotional stress incurred by attempting a competitive maximum require many days or even weeks to reach full recovery, even though physical recuperation would appear to be complete much quicker. So this type of loading is not recommended as a regular form of training.
In other words, any attempt to exceed limit weights requires an increase in nervous excitation and interferes with the athlete's ability to adapt, if this type of training is used frequently. In attempting to understand the intensity of loading prescribed by the apparently extreme Bulgarian coaches who are reputed to stipulate frequent or daily use of maximum loads in training, one has to appreciate that training with training maxima (which do not maximally stress the nervous system) is very different from training with competitive maxima (which place great stress on nervous processes).
Strength is a relative phenomenon depending on numerous factors, so it is essential that these conditions are accurately described when strength is being assessed. For instance, muscular strength varies with joint angle, joint orientation, speed of movement, muscle group and type of movement, so it is largely meaningless to speak of "absolute strength" without specifying the conditions under which it is generated. Sometimes, the term "relative strength" is introduced to compare the strength of subjects of different body mass. This is a topic we will address a little later.
It is also useful to recognise that one may define isometric, concentric and eccentric strength maxima, since every sport requires distinct levels of each one of these types of maximum. As a matter of interest, these maxima given in order of magnitude are: eccentric, isometric, concentric, which most of us already know from training experience - we can always handle between 25-40 percent more load during the eccentric phase of most movements.
STRENGTH AND FITNESS
Now that we have dissected strength in greater depth we can define "fitness" ("the ability to cope effectively with a given stress") in more detail. Fitness comprises a series of interrelated structural and functional factors, which conveniently may be referred to as the basic S-factors of fitness (Siff, "Supertraining"): Strength, Speed, Stamina (general endurance or local muscular endurance), Suppleness (flexibility), Skill (neuromuscular efficiency), Structure (somatotype, size, shape) and Spirit (psychological preparedness). Within the scope of skill, there is also a fitness quality known as Style, the individual manner of expressing a particular skill.
We can now construct a comprehensive model of physical fitness from the functional motor elements of fitness, as shown in Figure 1.
Figure 1. Pyramid Model showing the major components of musculoskeletal fitness
The diagram illustrates that strength, endurance and flexibility may be produced statically or dynamically, unlike speed, which changes along a continuum from the static to the dynamic state. However, this convenient picture could be expanded by including the "quasi-isometric" state, which can influence production of any of the motor qualities at very slow speeds. For this and other reasons, this model should be viewed as one that represents or describes rather than scientifically analyses.
The quality of flexibility has been placed at the centre of the base of the pyramid, because the ability to exhibit any of the other qualities generally depends on existence of some range of movement (ROM). It should be noted that static or dynamic flexibility refers to the maximum ROM that may be attained under static or dynamic conditions, respectively. The line joining all adjacent pairs of primary fitness factors depicts a variety of different fitness factors between each of the two extremes. The model thus allows us to identify an extended list of fitness factors, as follows (the factors bearing an asterisk are various types of special strength):
- Static strength*
- Static strength-endurance
- Dynamic strength*
- Dynamic strength-endurance*
- Speed-strength endurance*
- Strength-speed endurance*
It is sometimes convenient to identify various flexibility qualities, namely:
- Flexibility (static and dynamic)
A series of skill-related factors may also be identified, although it should be noted that skill forms an integral part of the process of exhibiting all of the above fitness or motor qualities:
All of the primary and more complex fitness factors should be viewed as convenient descriptors of qualities which are involved in different proportions in a particular physical activity. Nevertheless, this pyramidal model enables us to understand sport specific fitness and training far more effectively than with a simplistic model based only on the primary functional fitness factors of strength, endurance, speed and flexibility.
One may also consider the concept of "relative strength" (e.g., how much you lift divided by your bodyweight), especially since a client may grow stronger in terms of absolute strength, but her bodymass may also increase, so that in relative terms, she has grown weaker. The improvement in other fitness factors relative to bodymass may also be highly relevant. For instant, "relative power" (power per unit bodymass) is very important in cases where the athlete has to increase power without increasing bodymass (e.g. a weightlifter or boxer in a specific bodymass division). In sports which require the athlete to increase muscle endurance without increasing bodymass, "relative endurance" needs to be enhanced. In this case, one might even distinguish between "relative static endurance" and "relative dynamic endurance". Depending on the sport, improvement of "relative speed-strength endurance" (or relative strength endurance) under repeated cyclic or acyclic conditions, may also be relevant.
Some of the above terms may require elaboration. For example, "static strength-endurance" refers to muscle endurance under isometric conditions; "strength-speed" and "speed-strength" describe power produced under heavily loaded and very lightly loaded conditions, respectively; "speed-strength endurance" refers to the ability to produce great power continuously without serious decrement; "flexibility-speed" refers to flexibility which must be exhibited at high speed; and "speed-skill" refers to an action which must be produced skillfully at high speed.
STRENGTH FACTORS IN ACTION
So far, we have discussed different types of strength or strength qualities as components of fitness, but it is also very informative to analyse strength at the level of individual actions. This is best done by studying the curve of how the force changes with respect to time for any given movement, such as the idealised and simplified graph in Figure 2
Figure 2 A typical force-time curve describing the lifting of a free weight from a given position and returning it to it to rest. Movement occurs only when the force exceeds the weight of the object, namely over the shaded portion of the curve.
Analysis of this curve reveals several characteristics associated with the production of strength, some of which we have not discussed yet, namely:
- Starting Strength
- Rate of Force Development (RFD)
- Explosive Strength (Maximum RFD)
- Maximum Strength
- Deceleration Strength
Here, "starting-strength" refers to the ability of the muscles to develop force at the beginning of the working contraction before external movement occurs and is always produced under conditions of isometric muscle action. This fact alone has important consequences for strength training, because it dispels the opinion that the once-popular method of isometric training should be completely abandoned in modern training. On the contrary, the ability to generate starting strength rapidly can exert a profound effect on the dynamics of an entire movement, not only in terms of the magnitude of the impulse, but also regarding the psychological sensation of "lightness" that it creates during the crucial initial stage of a highly resisted movement. "Acceleration-strength", describes the ability to quickly achieve maximal external muscle force once dynamic movement has begun.
"Explosive Strength" characterizes the ability to produce maximal force in a minimal time. It is most commonly displayed in athletic movements when the contraction of the working muscles in the fundamental phases of the exercise is preceded by mechanical stretching (such as any plyometric, throwing, kicking, striking or rebounding action in many sports). In this instance, the switch from stretching to active contraction uses the elastic energy of the stretch to increase the power of the subsequent contraction. Mathematically, it is given by the maximum value of the slope of the force-time curve (where this slope is called the Rate of Force Development, RFD).
"Strength-Endurance" characterizes the ability to effectively maintain muscular activity under work conditions of long duration. In sport this refers to the ability to produce a certain minimum driving force for a prolonged period. (Examples: any longer sprint events in running, cycling, swimming for dynamic strength-endurance, and any prolonged grappling in wrestling and scrumming in rugby for static or quasi-isometric strength-endurance).
"Deceleration-Strength" refers to the ability to slow down any movement whenever necessary, especially as a joint is reaching its end of range of movement. It occurs under eccentric conditions and frequently is called into play by reflexes, which are activated to prevent injury to the joints. It is vital that this quality be adequately developed in anyone who takes part in any rapid, ballistic or powerful sports, as well as in "plyometric" or rebound training, because many injuries can result from inefficiency in slowing down or halting a forceful movement.
If the load is near maximal, then the initial slope of the Force-Time curve is small and the time taken to produce movement is prolonged. This requires the exhibition of the motor quality of "static strength-endurance", (Examples: wrestling or rugby scrumming) as opposed to “dynamic strength-endurance”, which refers to the muscle endurance required to maintain movement over a given interval (Examples: gymnastics, track running, longer sprint swimming ). This quality may be involved in carrying out a set of repetitions with a load or by maintaining cyclic work of various intensities
Suppose that we now wish to use this information to compare the performances of two different athletes in executing the same exercise. Athlete A may not be able to produce the same maximum force as athlete B, but he can produce his maximum faster than A, so that if they are to compete against one another in a contact sport, A may well defeat B in very short duration, explosive encounters. In general, if the sport concerned requires rapid Rate of Force Development, then athlete A will often have the advantage. This quality is essential in any sports that involve jumping, hitting or throwing, such as basketball, martial arts, American football and track-and-field. In this case, any training aimed at increasing B's maximal strength or bulk will be misdirected, because he needs to concentrate more on explosive strength (RFD) training. If the sport requires a high maximal force or a large amount of momentum to be exerted irrespective of time (Examples: as in powerlifting or prolonged scrummaging or strongman contests), then athlete B will prove to be superior. In such a situation, athlete A will not improve unless he trains to increase maximal strength.
THE NATURE OF STRENGTH
It is our final task to briefly examine some of the physiological and anatomical features that explain the phenomenon of strength, since the design of a successful strength training programme depends on a thorough understanding of the factors which influence strength development. The task is to determine which of these factors can be modified by physical training and which methods do so most effectively and safely. Some of these factors are structural and others, functional. Structural factors, however, only provide the potential for producing strength, since strength is a neuromuscular phenomenon, which exploits this potential to generate motor activity.
It is well known that strength is proportional to the cross-sectional area of a muscle, so that larger muscles have the potential to develop greater strength than smaller muscles. However, the fact that Olympic weightlifters can increase their strength from year to year while remaining at the same bodymass reveals that strength depends on other factors as well.
The most obvious observation is that a muscle will produce greater strength if large numbers of its fibres contract simultaneously – an event which depends on how efficiently the nerve fibres send impulses to the muscle fibres. Moreover, less strength will be developed in a movement in which the different muscles are not coordinating their efforts. It is also important to note research by Vredensky, which has shown that maximum strength is produced for an optimum, not a maximum, frequency of nerve firing. (this means that maximum strength production is not necessarily the result of activating as many muscle fibres as possible, but just the right quantity in a given situation at a given time). Furthermore, this optimal frequency changes with level of muscle fatigue.
DETERMINANTS OF STRENGTH
In general, the production of strength depends on the following major factors:
- Structural Factors
- The cross-sectional area of the muscle
- The density of muscle fibres per unit cross-sectional area
- The efficiency of mechanical leverage across the joint
- 2. Functional Factors
- The number of muscle fibres contracting simultaneously
- The rate of contraction of muscle fibres
- The efficiency of synchronisation of firing of the muscle fibres
- The conduction velocity in the nerve fibres
- The degree of inhibition of muscle fibres which do not contribute to the movement
- The proportion of large diameter muscle fibres active
- The efficiency of cooperation between different types of muscle fibre
- The efficiency of the various stretch reflexes in controlling muscle tension
- The excitation threshold of the nerve fibres supplying the muscles
- The initial length of the muscles before contraction
With reference to the concept of synchronising action among muscle fibres and groups, it is important to point out that synchronisation does not appear to play a major role in increasing the rate of strength production. Efficiency of sequentiality rather than simultaneity may be more important in generating and sustaining muscular force, especially if stored elastic energy has to be contributed at the most opportune moments into the movement process. What this means in simple terms is that it often used to used to be believed that synchronising several muscle groups to operate at the same time was more important than how the different muscle actions followed one another in producing force. Research has now shown that the way in which muscle actions follow one another in a given movement may even be of greater or equal importance in this regard. Certainly, more research has to be conducted before a definite answer can be given to the question of strength increase with increased synchronisation of motor unit firing.
I trust that this short overview of what I struggled to condense into a voluminous textbook ("Supertraining") has conveyed some of the exquisite complexity and deeper nuances of the quality of strength which has been admired since time immemorial and which has captured the attention of many of us today. It will have achieved its goals if it in some measure enables readers to improve their ability to work more effectively and appreciatively in the world of strength training and rehabilitation. My personal fascination with strength training began when I was a young student at university wishing to enhance my track and field performances, a quest which led to my becoming a competitive weightlifter and ultimately a career which wedded my sporting affection with my postgraduate studies in biomechanics and physiology. Not for one moment did I imagine that those elementary forays into Olympic weightlifting would lead to the International Olympic Council (IOC) inviting me to write a major chapter for one of its volumes, which is published to coincide with every Olympic Games. In fact, much of the information that I am sharing here is based upon that chapter ("Biomechanical Foundations of Strength and Power Training" in Biomechanics in Sport, edited by Zatsiorsky, 2000). May the quest for a better understanding and application of human strength bear similar rewards for all of you!
- Siff M C Supertraining 2000 This 500 page textbook contains all of the references upon which this article was based.
- Zatsiorsky V Science and Practice of Strength Training 1994
- Supertraining Web Forum. The archives of this list contain many articles that I and others have written on many aspects of strength as art and science. It is a free educational service, which anyone may join at: http://groups.yahoo.com/group/Supertraining/