Written by Geoff Strauss
Academic Staff, Biomechanics and Exercise Physiology, School of Physiotherapy, Curtin University, 1980 to 2013
Course Coordinator, BSc (Exercise, Sports and Rehabilitation Science), 2012/1
(Draft Version of 14Sep12)
There are many health professionals who prescribe resistance exercise to individuals. Irrespective of whether the purpose is rehabilitation following injury or surgical intervention, for a healthy and active lifestyle or for elite performance, the scientific basis for exercise prescription is rarely evident. Professionals can fail to apply even basic principles of resistance exercise prescription in the years after their training on the principles and their practical implementation. Many professionals will remember students who load the client with the nearest weight and instruct them to do 10 repetitions. No rationale is provided for the choice of resistance load (determining the intensity of the exercise), its point of application to the patient, the number of repetitions to be performed or the end point of a set of repetitions (efforts), nor a demonstration or explanation of the technique that the client should replicate.
In 2006 Toigo and Boutellier (2006) reiterated the six “classic” mechano-biological determinants of resistance exercise stimuli (or principles of resistance exercise prescription) and proposed seven new descriptors. The “classic” determinants were:
x1 load magnitude
x2 number of repetitions
x3 number of sets
x4 rest in-between sets ([s] or [min])
x5 number of exercise interventions (per [d] or week)
x6 duration of the experimental period ([d] or weeks)
The additional (new) descriptors were:
x7 fractional and temporal distribution of the contraction modes per repetition and duration (s) of one repetition
x8 rest in-between repetitions ([s] or [min])
x9 time under tension
x10 volitional muscular fatigue
x11 range of motion
x12 recovery time in-between exercise sessions ([h] or [d])
x13 anatomical definition of the exercise (exercise form)
While the theme of the article by Toigo and Boutellier (2006) was the molecular and cellular mechanisms by which training adaptations occurred, the determinants were obvious criteria enabling specific exercise prescription. All 13 determinants have been included in the exercise physiology curriculum taught to students in entry-level exercise and sports science, and entry-level physiotherapy degrees at Curtin University. This material is taught in lectures and reinforced in practical sessions. Over the 3 teaching periods that these determinants have been used, modifications that have been made include changes to the order of presentation of the determinants, the development of new determinants, and the development of spreadsheets within an Excel workbook for Resistance Exercise Assessment and Training. These spreadsheets accompany the practical sessions and allow students to implement a strength assessment, and train and reassess their strength over a period of 4 to 6 weeks. The spreadsheets include automatic calculations of time under tension and relative training volume, and determine new 1RM estimations under certain conditions.
The aim of the practical sessions is not for students to achieve a significant increase in strength, power or endurance, but to experience the effort that they will be asking clients to make, to implement valid assessments and to prescribe resistance training appropriately for the goal they are setting. Most students who are inexperienced in resistance exercise training in a gym setting are set the goal of increased strength in a particular muscle group or movement.
As a moderately equipped rehabilitation gymnasium is available for use, the Excel workbook is used to document the exercise machine used, and the prime mover muscle group(s) and muscle action sequence used in practical sessions. Students are required to add to this workbook by providing more detailed information on the active muscles or muscle groups (stabilizers, etc).
In addition to the exercise machines, a stopwatch or (clock), a metronome and recording sheets are essential items for each assessment and exercise session. Items that are required at different times include a goniometer or other instrument to measure joint angle (a digital inclinometer, at assessment sessions), some tape and a pen. A still or video camera is also useful for both evaluating assessments and for feedback during or after training sessions.
All of the descriptors must be specified for resistance exercise prescription, but fewer descriptors are required for assessment. However, students soon learn that a well-constructed training program also provides assessment data! The overarching aim of the framework is to provide sufficient information so that a different student (or professional) can read the documentation and implement an identical assessment or training session with the client.
The Title of the Therapist and Client Record
Titles and header information for both the Assessment and Training records should include the client’s and therapist’s name, the type of assessment or training, the muscle group and the exercise or exercise machine. For example, an Assessment Record might be called Strength Assessment of the Knee Extensors on the “Brand X” Knee Extension Machine, while the Training Record might be called Strength Training the Knee Extensors on the “Brand X” Knee Extension Machine.
Curtin Descriptor #1 – Description of the Exercise
(Toigo and Boutellier Descriptor x13 – anatomical definition and description of the exercise (exercise form))
The muscle group and exercise machine included in the title can be repeated but it is more useful to include images or figures to illustrate how the client exercises on the exercise machine. In particular the images should illustrate the Start Position, and the Stop Position. The Start Position should display how the client is positioned on the exercise machine prior to commencement of the first muscle action, and a statement of what that muscle action is.
Exercises are comprised of repetitions of a cycle consisting of two phases. For example, on the knee extension machine, the start position has the leg flexed prior to the first phase involving a concentric muscle action of the knee extensors that moves the leg to an extended position (perhaps near full knee extension). The second phase begins from the phase one finish position (Stop Position) and involves a controlled return to the start position with the knee extensors working eccentrically. In the bench press exercise, the start position has the upper extremities extended straight holding the bar and weights. The first phase involves eccentric muscle actions as the bar is lowered to touch the chest (the phase one finish position or stop position). The second phase involves an upwards press from the chest to the extended upper extremities position using concentric muscle actions of the involved muscle groups.
As the overarching aim of the framework is to provide sufficient information so that a different student (or clinician) can read the documentation and implement an identical assessment or training session with the client, detailed information should be provided. For example, if the seat is adjustable, what notch is the seat adjusted to? How far is the centre of the resistance pad from the axis of rotation of the exercise machine? Are straps used to stabilize the client?
Professionals prescribing resistance exercise will require repetitions of the exercise to be performed with “good technique”, and this typically means “with control through the range of motion”. Aspects of what constitutes a controlled performance with good technique should be stated (operationally defined), including the exercise cadence and expected time to complete the repetitions prescribed. “Trick movements” or substitutions to avoid may be useful information to convey to some clients. A demonstration of the exercise is often useful.
The instructions that are given to the patient should also be written in full into the spreadsheet. This information should cover the purpose of the exercise, and be stated in terms that can be easily understood by the client.
The instructions should reinforce all of the descriptors that follow; the range of motion required, the intensity of exercise, the speed of the repetitions, the number of repetitions expected, when the participant may appropriately stop performing the exercise, and when and how long they may rest. Finally, any relative contraindications to continued performance of the exercise should be provided.
For students, the course assessment can include identification of more than just the prime movers for the exercise. They may be asked to identify synergistic muscles, and muscles acting as stabilisers.
Curtin Descriptor #2 – Position(s), or Range of Motion [ROM]
(Toigo and Boutellier Descriptor x11 – range of motion)
The range of motion (ROM) must be specified. Specify the start and stop positions as “physiotherapy joint angles” and note the method of assessment of the angles. Considerations such as whether there is a mechanical limit at one end of the movement ROM should also be noted.
A very important consideration is whether the client completes the specified ROM. This is often critical because the design of many isolated joint exercise machines, and fatigue, results in the client failing to achieve the specified ROM. In isolated joint exercises such as exercising the knee extensors on a knee extension machine, failure occurs in the weakest part of the ROM, at full knee extension. Movement through the first phase occurs when the moment of force produced by the muscle group acting concentrically exceeds the moments of force due to resistance loads. When the angle-specific muscle moment equals the resistance moment, the muscle is only able to produce an isometric muscle action. As the muscle fatigues, or as motivation and activation is reduced, the isometric muscle action (stop angle) is achieved earlier in the ROM (further from end of the passive ROM).
For assessments of muscle strength it is essential that the movement occurs through the specified ROM for a repetition to be valid. Practically, the stop angle is rarely the end of the ROM. An operational definition of the position at the end of range may be varied, but for knee extension, 10 degrees of knee flexion has been sufficient in my experience. This value can also be justified for training as muscle strength carryover from trained to untrained angles has been reported as around 10 degrees ()ref.
For multiple joint exercises, there may be a “sticking point” within the ROM in addition to (or rather than) a weaker range at the end of ROM (e.g. in a bench press). Again, achieving the specified ROM is critical to determining if a repetition is valid, and therefore to the determination of a “repetition maximale” (RM). The section on Descriptor #3 – Load Magnitude will expand on the RM concept.
One convenient way to establish the end of the operational ROM is to place a mark on the weight stack tracking frame (that is equivalent to the end of the operational ROM) that must be reached. For example, in the “Lat Pulldown”exercise, the end of ROM may be set as pulling the unloaded (or lightly loaded) bar down until it is level with the xyphoid process. The height of the top of the weight stack is marked on tape that is affixed to the weight stack tracking frame (Figure 1). When increased loads are added for assessment or training, a repetition is only counted as complete if the weight stack rises to this level. Two incomplete repetitions or volitional cessation of the exercise validates the number of completed repetitions. Provided the client’s starting position does not change between sessions, this mark on the weight stack tracking frame is used in future assessments and training sessions.
For training, the therapist needs to determine how large the ROM should be. For example, if the stop angle is set within 10 degrees of the true end of ROM, fewer complete (valid) repetitions will be able to be performed compared with training where “incomplete repetitions” are also performed. The therapist may determine that “time under tension” or training volume is a more important consideration than tension generated within the weakest part of the ROM. This may even be viewed as desirable if an aim is to work to performance failure (Descriptor 10).
Curtin Descriptor #3 – Pretraining Assessed, Predicted or Estimated 1RM Load (kg) [Pred 1RM]
The rationale for this descriptor is that the load magnitude (Curtin Descriptor #4) should never be an arbitrary choice, but be based on a prior assessment. While there may be a number of ways in which muscle performance may be assessed, a common approach is to use the “repetition maximale” (RM) concept proposed by DeLorme (1945). A 1RM effort is the load magnitude that may be can be moved through the specified ROM with good technique once (but not twice). If a client can perform 4 repetitions through the specified ROM with good technique, the load magnitude would be termed a 4RM load, or a 4RM exercise. Generally the RM concept is valid for between 1 and 10 maximal efforts (1RM to 10RM). This is because 1RM is moderately to strongly (linearly) related to 2 to 10RM efforts. After 10 repetitions the relationship becomes curvilinear, with less accurate prediction of 1RM strength. A simple formula allowing the prediction of the 1RM load magnitude is:
% of a 1RM load = 100 – (2.5 * ?RM)
where ?RM is the number of repetitions completed according to the exercise criteria before performance failure. For example, if a client achieved 4RM (completes 4 repetitions, but could not complete a fifth repetition) against an external resistance of 32.5 kg of force, the formula would be:
% of a 1RM load = 100 – (2.5 * 4RM)
i.e. % of a 1RM load = 100 – (10)
i.e. % of a 1RM load = 90 %
The next step in predicting 1RM is to state the following:
If 32.5 kg of force is the resistance for 90% of 1RM
How many kg of force is the resistance for 100% of 1RM
# kg of force = 32.5 * 100/90
i.e. # kg of force = 36.1 kg
This formula is reasonably accurate for most muscle groups, however more specific formulae can be found in the literature. Increased accuracy can be facilitated by more sensitive adjustments in the resistance loads. In the strength and conditioning gym we have sets of small weights that can be added to the plates in the weight stack (Figure 2a). Some exercise machines have small incremental weights that can be added to the stack (Figure 2b).
Curtin Descriptor #4 – Load Magnitude (kg) [LM]
(Toigo and Boutellier Descriptor x1 – load magnitude)
The intensity of effort is the single most important factor in determining the magnitude and rate of change in strength. It is thought that a greater training volume of a lower intensity of exercise may improve the strength gain, so the training volume is also important.
An absolute value of the resistance load magnitude (eg. 30 kg) provides limited information on the intensity of the efforts that will be required to perform the exercise. It is useful to know both the absolute load and the relative intensity that the load requires. A concept to provide a relative intensity value is provided in the simple formula described in Descriptor 3.
Thus, if “a 6RM load of 30 kg” were stated, the relative intensity of the efforts can also be determined (6RM = 100% – (6 * 2.5 %) = ~85% (of the weakest point in the concentric phase of movement).
Some exercise programs report loads as percentages of an ?RM load; for example, 75% of a 10RM load. The implication of this description is much clearer when re-calculated as a proportion of a 1RM load, even when 1RM load has not been assessed but has been estimated or predicted. Thus, 75% of a 10RM load would be recalculated as 75% of a (100 – (2.5 * 10) % of a 1RM load, that is, 75% of 75% of a 1RM load, or 56% of predicted 1RM load.
While it is often reported that someone exercises at 1RM, or at 100% of an 8RM load (i.e. 8RM), is a 1RM really maximal? In fact, very little of the time spent in exercise is at 100% of the muscle’s capacity. A muscle only works at 100% when it generates a maximal effort moment that is matched by an equal (but opposite in direction) resistance moment. The muscle moment is a function of the muscle tension produced. The tension generated varies according to its activation, the length (joint angle) of the muscle (length-tension relationship) and the speed of the movement (force-velocity relationship).
In addition, the first of 8 repetitions of an 8RM exercise is at 80% of 1RM, the second repetition at a slightly higher relative effort, etc, increasing to the eighth repetition when the effort is maximal by definition (it is the eight repetition with an 8RM load), but only maximal at the weakest position in the ROM.
Curtin Descriptor #5 – Tempo [Te]
(Toigo and Boutellier Descriptor x7 – fractional and temporal distribution of the contraction modes per repetition and duration (s) of one repetition
Dynamic repetitions involve at least two phases; a concentric and an eccentric phase. At their simplest, a repetition is either a concentric effort followed by an eccentric effort (e.g. knee extension exercise), or an eccentric effort followed by a concentric effort (e.g. bench press exercise). The isometric phase that must exist between the phases as the movement changes direction would be considered too brief to be of significance. The times taken for the concentric and eccentric phases are then recorded.
Alternatively, isometric phases between the concentric and eccentric phases, and between the eccentric and concentric phases may be specified. They typically occur at the beginning of the concentric and eccentric phases. An exercise could contain four phases; isometric, concentric, isometric and eccentric phases. The times taken for all phases are then recorded.
A further variation could occur on an exercise machine where one end of the range has a mechanical limit that allows the “start position” to be held while the person rests. Again, the knee extension machine is a good example where the client may take the opportunity for a brief rest, or a rest duration may be incorporated in the repetition (see Curtin Descriptor #6).
Timing of exercises is aided by using a metronome that will count out beats at the set cadence. A recent resistance exercise trend is to program an eccentric phase that is twice as long as the concentric phase. The participant might perform the exercise with a two beat concentric phase, followed by a four beat eccentric phase, at a cadence of 72 beats per minute.
Knowledge of the temporal distribution allows a more accurate estimation of the intensity of effort and the energy systems contributions that will be required for performance.
Curtin Descriptor #6 – Rest in-between Repetitions, and Repetition W:R Ratio [RbwR, RbwRR]
(Toigo and Boutellier Descriptor x8 – rest in-between repetitions ([s] or [min])
In the event where a muscle group performs the phases of movement in a single repetition and can then rest, a further criterion must be defined. For example, on the upright row free weight exercise, the bar and weights hang down and can be maintained by tension within passive muscle and joint structures and very little muscle activation. Similarly, on the knee extension machine, the knee extension start position is usually defined by a mechanical limit (that doesn’t allow further knee flexion). The mechanical limit enables the participant to rest between each effort (repetition), and the time taken for this rest phase should be recorded.
This brief rest is probably under-utilized in resistance exercise prescription. When the muscle is able to relax, muscle blood flow returns. Re-oxygenation and diffusion of metabolites from the involved muscle can then occur.
During a continuous effort when the muscle is always active, the tension within the muscle compresses the vascular bed and restricts or stops muscle blood flow. The muscle therefore becomes ischemic and is forced to work anaerobically. Systolic blood pressure increases to drive blood flow to the active muscle, while diastolic blood pressure increases as there is no flow through the muscle. The volume of blood held in vessels proximal to the vascular occlusion caused by the muscle compression increases. A brief rest (muscle relaxation period) allows a brief period of blood flow through the muscle, and will ameliorate the diastolic blood pressure response.
If there is a rest period between repetitions then a Work:Rest ratio for repetitions (W:R reps) can be defined. The ratio should be described by both as a ratio of the times and as a 1:X or Y:1 ratio. For example, a repetition on a knee extension machine may take 3s. If a 2s rest at the start angle is specified, then the temporal details should be specified as W 3s : R 2s, and the W:R ratio as 1.5 : 1.
If there is no rest period then this descriptor is not applicable (NA) and should be recorded as NA.
Curtin Descriptor #7 – Number of Repetitions per Set [R]
(Toigo and Boutellier Descriptor x2 – number of repetitions)
The relative load magnitude allows an estimation of the number of repetitions. For example a 6RM load suggests that the client should achieve 6 repetitions. A lack of motivation or earlier fatigue may mean that 6 repetitions cannot be completed, while a training adaptation may mean that the client is now able to achieve 7 or 8 repetitions before performance failure (Curtin Descriptor #10) or volitional muscular fatigue (Toigo and Boutellier Descriptor x10).
If the exercise prescription specifies a constant number of repetitions, the repetitions can be written into the training record ahead of their performance. If however the repetitions are performed to performance failure, the performance must occur so that the number of repetitions can then be recorded.
If an instruction to “perform the maximum number of repetitions you can” is given for the first set, then this performance also serves as an assessment. That is, if the training exercise is carried out using the same instructions and to the same specifications as an assessment of strength, then it can suffice for an assessment. For example, if a resistance load estimated to be a 6RM is prescribed and the client performs 8 repetitions before performance failure in their first training set with this load, then that load is actually an 8RM load. The absolute resistance weight in kg can be adjusted up for the next training occasion (requiring 6RM efforts) can be calculated by multiplying by 0.85/0.80. The calculated absolute resistance weight will be the new 6RM resistance load.
The number of repetitions may be set as a constant, or may be allowed to vary with fatigue.
Curtin Descriptor #8 – Time Under Tension [TUT]
(Toigo and Boutellier Descriptor x9 – time under tension)
While the intensity of effort is the single most important factor in determining the magnitude and rate of change in magnitude of strength training gains, a greater training volume of a lower intensity of exercise would be expected to increase the strength gain. Traditionally, training volume has been defined as a function of the number of sets and repetitions performed in a single training period or over a specified period. A relatively recent addition to the resistance exercise terminology is the “time under tension” (TUT) concept that can be used as a more precise indicator of training volume. It can be calculated as the sum of the exercise periods for each set. Concentric and eccentric muscle activation times can be calculated as TUTconc and TUTecc.
Curtin Descriptor #9 – Average Time per Repetition [ATR]
This descriptor is really a calculated value that is quite useful in reviewing the exercise performance. The calculation in the spreadsheet takes the Time Under Tension and divides this by the Number of Repetitions per Set for each set. If efforts are performed at the set tempo then the average time per repetition should be the same! An average repetition time can be derived from the data for each set.
Curtin Descriptor #10 – Rest in-between Sets, and Set W:R Ratio [RbwS, RbwSR]
(Toigo and Boutellier Descriptor x4 – rest in-between sets)
The rest time in-between sets is determined from the exercise time. It is important to consider the intensity of exercise, the energy system involvement, whether fatigue is expected during the session, and how much fatigue is desired. The rest time or rest period is often described in a Work:Rest ratio for sets (W:R sets). For strength programs, this ratio is often 1:6.
Alternatively, a minimum time can be stipulated and might be allowed to vary depending on the participant’s perception of their preparedness for the next set. This procedure would minimize fatigue in the later sets. The rest time should be recorded.
If there is a rest period provided between sets then a Work:Rest ratio for sets (W:R sets) can be defined. Either a specific recovery time (important for energy systems, fatigue) or a minimum recovery time can be specified and the ratio is then either described by a 1:X or Y:1 ratio.
Curtin Descriptor #11 – Number of Sets [S]
(Toigo and Boutellier Descriptor x3 – number of sets)
Once a decision has been made about the type of resistance training program to be designed and implemented, and an assessment of muscle strength completed, the training intensity can be specified. A high intensity, low repetition strength training program is usually implemented with a higher number of sets so that an effective training volume may be achieved, for example, 7 sets of 4RM.
Conversely, a lower intensity, high repetition strength training program is implemented with a lower number of sets so that an effective training volume may be achieved, for example, 4 sets of 8RM.
A low intensity, high repetition endurance training program is implemented with a lower number of sets so that an effective training volume may be achieved, for example, 3 sets of 20RM.
Curtin Descriptor #12 – Was Performance Failure Achieved? [PF]
(Toigo and Boutellier Descriptor x10 – volitional muscular fatigue)
This criterion seeks to identify the cause of endpoint of the exercise and is recorded as a Yes or a No. If the answer is Yes, then fatigue is likely the reason for not being able to complete further repetitions. If no, then factors other than fatigue could be the reason. Some sets of exercise are designed with a set number of repetitions and without performance failure. For example, a client whose 1RM load is 50kg exercises with a 40 kg load (an 8RM load) may only perform sets with 6 repetitions. The client should not be fatigued in the last repetition and is therefore not working with maximal effort in any of the repetitions. Worse still, as a strength training prescription, is the client who exercises with a 30RM load and is instructed to complete 10 repetitions!
A Yes, or No answer to the question “was performance failure achieved” can be elicited from the participant for each set. Programs may be designed where the participant exercises to fatigue in every set. Other program designs may require the participant to exercise to fatigue in only the first set, or in only the last set.
Curtin Descriptor #13 – Relative Training Volume [RTV]
Typically no quantitative account of the intensity of the training is incorporated into the calculation of training volume. This framework proposes that a relative training volume can be determined by multiplying the TUT by an intensity factor. An intensity ratio is determined by multiplying the training session predicted 1RM load magnitude divided by the pre-training 1RM load magnitude.
Curtin Descriptor #14 – New Assessed or Predicted 1RM (kg) [New Pred 1RM]
Under certain conditions a training session can also be an assessment condition. When a strength training session incorporates a set (usually the first set) where 2 to 8RM (an appropriate load) of an exercise are performed good technique until performance failure, this suffices as a strength assessment. This assessment of ?RM strength enables the reassessment of a new (predicted) 1RM strength load magnitude.
Curtin Descriptor #15 – Number of Exercise Sessions
(Toigo and Boutellier Descriptor x5 – number of exercise interventions (per [d] or week)
This descriptor is self-explanatory and describes the number of sessions per day or sessions per week.
Curtin Descriptor #16 – Recovery Time In-between Sessions [Rec]
(Toigo and Boutellier Descriptor x12 – recovery time in-between exercise sessions ([h] or [d]))
This descriptor specifies the time in-between sessions. Some training programs specify a time in days, or simply a minimum time. If the program involves alternating the muscle group focus (eg. leg sessions with arm and trunk sessions), then this also may be noted.
Curtin Descriptor #17 – Duration of the Training Period [TP]
(Toigo and Boutellier Descriptor x6 – duration of the experimental period ([d] or weeks)
Typically how many weeks or months the program is expected to take. The word “experimental” may be substituted for “training” in a research study.
There is sometimes a need for additional notes to accompany the information recorded against in the 17 descriptors. For example, if the client was experiencing muscle soreness prior to commencement of the session, this might be noted and even quantified using a visual analogue scale (VAS). Other significant issues to note may include general fatigue (perhaps due to previous heavy exercise or reduced sleep), or illness.
DeLorme TL, Watkins AL (1948) Technics of progressive resistance exercise. Archives of Physical Medicine and Rehabilitation 263-273.
DeLorme TL (1945) Restoration of muscle power by heavy-resistance exercises. Journal of Bone and Joint Surgery (American)
Toigo and Boutellier (2006)