Muscular strength can be influenced by confounding factors including initial strength levels, training status and genetics. However, several morphological and neural factors underpin strength development that affect overall muscular strength in combination with the above factors. It is important to note the below physiological factors combine to increase strength, rather than any one reason presenting the underlying cause.
Muscle hypertrophy and architecture
Alterations in skeletal muscle hypertrophy can impact a muscle’s ability to produce force and power. This means the greater a muscle’s cross-sectional area, the more potential it has to produce higher levels of force due to greater force-velocity characteristics. Physiologically, an increase in the number of cross-bridge interactions between actin and myosin within existing and newly formed sarcomeres is the reasoning behind this. This can also be linked to greater muscle fibre pennation angles in muscles benefitting from greater hypertrophy, increasing the number of cross-bridge interactions within the muscle due to a greater number of muscle fascicles within a given area. It is important to note, however, muscle size and strength can vary between individuals in response to different training stimuli.
Muscular strength is inherently related to the concept of tissues expressing spring-like behaviour, which influences subsequent muscular performance. Stiffness can be described as the relationship between a given force and the amount of stretch the tissue undergoes, with increased stiffness (i.e. less stretch) leading to an improved ability to transmit force. Therefore, tendon stiffness (along with structures within the muscle such as actin, myosin, titin and connective tissues) is an important part of muscular strength and the transmission of force and related characteristics such as rate of force development (RFD) and power.
Motor unit recruitment
Motor units are recruited in a sequence from smallest to largest, meaning a task that requires low amounts of force and RFD will recruit smaller motor units that include type I (slow twitch) fibres, whilst tasks requiring large amounts of force and RFD will recruit large motor units that include type IIa/IIx (fast twitch) fibres. Therefore, although type I motor units may increase force production, tasks that require a combination of all fibre types (i.e. tasks requiring larger forces and/or levels of RFD) will allow greater strength development.
Rate coding, motor unit synchronisation and neuromuscular inhibition
Following specific motor unit recruitment, the frequency at which action potentials can be discharged to the motor unit’s muscle fibres can modify force production properties. Therefore, the magnitude of force may increase when firing frequency is increased. This can be linked to force-time characteristics with higher force magnitudes and RFD a result of increased firing frequency of motor units. Furthermore, simultaneous activation of more than two motor units (synchronisation) may enhance peak force production due to greater levels of RFD over short time periods. However, motor unit synchronisation may be more related to RFD than magnitude of force production. In contrast, neuromuscular inhibition (reduction in neural drive in response to neural feedback from muscle and joint receptors, leading to a decrease in force production) may negatively affect strength development. However, this could be negated through heavy resistance training through the downregulation of afferent feedback.
Periodisation and programming
It is proposed that residual effects of previous training phases carry-over into future training phases. Therefore, different types of strength training can be linked to complement one another within an annual plan and periodisation strategy. There are many methods of periodisation and programming, so a deep discussion was deemed beyond the scope of the current paper. However, annual planning is the foundation of an athletic development program, covering the outline for training, competition, monitoring, testing and other important delivery aspects. It is then structured, or periodised, into a methodical and logical structure, manipulating training variables throughout the calendar directed at achieving performance goals. In relation to strength development, periodisation has been shown to produce greater benefits than non-periodised programs.
Annual planning usually features phases of preparation, divided into general and specific subsections, competitive, divided into subsections of pre-competitive, competitive and peaking phases, and transition phases. Within the different phases, various physiological adaptations can be targeted to produce desired outcomes at certain timepoints in the contribution to performance goals. Within this, programming elements make up exercise selection, sets and repetitions, rest periods, and load prescription.
As previously discussed, the importance of increasing muscular strength is related to improving force-time characteristics in the context of sporting performance. Therefore, in the context of a long-term plan, periodisation and programming should be focussed towards the improvement of various performance aspects, as well as elements of strength.
Resistance training methods
There are many different training methods used to develop strength-power characteristics. See table 1 for the theoretical potential of resistance training methods to benefit hypertrophy, strength and power.
Basic resistance training exercises – great to introduce movement patterns
Accessible – no equipment needed
Versatile – many variations can be used
Can be used as a tool for progression to more complex or loaded movements
Bodyweight or reduced bodyweight exercises may have implications for increasing explosive performance when training low-load, high-velocity
Limited ability to overload, preventing significant improvements to maximum strength
Isolated machine-based and multi-joint free-weight exercises
Isolation exercises commonly used as a means of targeted tissue capacity development
Incorporation of multiple muscle groups in free-weight exercises may provide a superior strength training stimulus due to requiring greater coordination and muscle recruitment demands
The use of single joint machine-based exercises may be questionable when developing strength as athletic performance is rarely isolated to one muscle group, lacking transfer of coordinative patterns
A continuum from isolation exercises through to free-weight exercise can be useful in certain settings, such as that of rehabilitation, providing appropriate progressions and regressions
Weightlifting movements and derivatives
Weightlifting movements (clean and jerk, snatch) and derivatives (omission of a portion of the full weightlifting movements, e.g. high pull) have been shown to produce superior strength-power adaptations compared to traditional resistance training, jump training and kettlebell training
May provide more effective means of force absorption of an external resistance
Allows for exploitations of both the force and velocity aspects of power, leading to favourable neuromuscular adaptations
Explosive movements utilising the stretch-shortening cycle (SSC) where a concentric muscle action is enhanced by a previous eccentric muscle action
Not normally prescribed to enhance muscular strength – the ballistic nature of plyometrics has the benefit of transfer from strength to power production and RFD
Difficult to provide an overload stimulus due to heavier loads resulting in greater impact forces and lengthening time of the SSC (but is intensity overload necessary with the nature of plyometrics?)
Enhancement of muscular strength would be limited from plyometric training
Benefits performance by producing favourable adaptations in mechanical function (strength, power, RFD and stiffness), morphological adaptations (tendon and muscle fibre cross-sectional area) and neuromuscular adaptations (motor unit recruitment and firing rate) due to potential for greater force applied to the musculotendinous unit than force produced by the muscle
Accentuated eccentrics (performing the eccentric phase with a heavier load than the concentric phase) may have positive benefits on performance compared to other resistance training methods
Heavy eccentrics can produce favourable hypertrophy and strength adaptations
There may be a lack of research on eccentric training within the context of an annual plan
Residual effects may be heightened, for example delayed onset muscle soreness (DOMS) may be amplified as a result of eccentric training
Potentiation refers to acute performance enhancements based on a muscle’s contractile history, normally with a high-force exercise used prior to a high-power exercise
Plyometric exercises such as depth jumps are thought to be the best method of potentiation for strength performance
Potentiation complexes are usually used to enhance muscular power rather than strength
There is a lack of research investigating potentiation complexes and improvements in muscular strength, whilst results published provide conflicting results regarding the best methodologies to enhance strength performance
Unilateral versus bilateral training
Unilateral exercises are thought to have good transference to sporting performance due to the unilateral nature of various sporting tasks (sprinting, cutting etc.)
Strong relationships exist between bilateral resistance training and sporting performance
Unilateral training provides higher muscle activation than bilateral training, however stability may be decreased during unilateral training, whilst bilateral exercises can provide a greater overall mechanical platform from which to develop strength
The majority of research employs bilateral training methods
Variable resistance training
Athletes may be restricted during traditional resistance training exercises because of mechanical disadvantages (e.g. an athlete may be limited during the back squat at specific hip and knee angles)
The use of chains or bands can aid relevant sections of a lift based on the mechanical advantage or disadvantage, maximising expressed muscle force throughout the full range of motion
Greater adaptations may still be seen at the end range of motion where increased resistance occurs
Kettlebell training may improve specific measures of strength
Training with kettlebells is limited in the capacity to provide an overload stimulus due to technical limitations and aspects such as grip strength due to the size of the kettlebell
Further research is needed to investigate the efficacy of kettlebell training on strength levels, although kettlebell training may provide benefits to muscular power due to the explosive nature of the movements
Ballistic training methods
Ballistic training features acceleration throughout the entire range of the concentric portion of the lift
Provides greater force, velocity, power outputs and muscle activation compared to the same exercises performed quickly, rather than ballistically
Promotes neural adaptations to recruit a greater number of motor units, leading to greater force production, RFD and power development
Beneficial for explosive strength
Ballistic exercises are underpinned by maximal strength, so it may be prudent to first enhance strength in order to enhance the benefits associated with ballistic training
Table 1: Theoretical potential of resistance training methods to benefit hypertrophy, strength and power
Resistance training methods ranked on a scale from +, meaning low potential, to + + + + +, meaning high potential. Assigned exercises, volume-load prescription, and an athlete’s relative strength will also influence adaptations.
Training to failure
Training to failure can be defined as the point where the barbell stops moving, the sticking point lasts longer than one second, or the full range of motion repetitions can no longer be completed. However, training to failure has been examined with meta-analyses with the outcome suggested to be counterproductive to gains in strength due to the possibility of overtraining and potential exposure to injury.
Combined heavy and light loading
Although heavy loads elicit strength gains, they can be combined with lighter loads to enhance force-time characteristics during program phases aimed at increasing RFD and power. This is commonly exemplified with the use of heavy loads to improve maximal and absolute strength, whilst lighter loads can be used during phases of strength-speed and speed-strength. The manipulation of heavy and light loads within the context of a periodised plan can be of benefit when enhancing an athlete’s force-velocity profile. Heavy and light loads can be prescribed in relation to different exercises, with heavy prescriptions commonly used for core lifts (e.g. squats, presses and pulls) with the aim of increasing force production. On the contrary, lighter loads are often prescribed for the intention of enhancing velocity of movement during ballistic exercises (e.g. lighter pulling movements, squat jumps and bench press throws). The programming of combined heavy and light loading can be configured using many different periodisation strategies in the context of intended outcomes.
Exercise set considerations
Single versus multiple sets
Multiple sets are thought to lead to greater hypertrophy, strength and power adaptations. However, an athlete's training status can be taken into consideration when prescribing sets, with regard to the dose-response relationship. In individuals who are less trained, two to three sets per exercise may be sufficient to enhance muscular strength, whereas those who are well trained may require four to six sets per exercise to attain the same level of improvement. It should be noted, however, that chronically heightened loading strategies through volume or intensity may expose the athlete to overtraining syndrome. Prescription of additional sets (i.e. heightened volume) should be considered in relation to intensity, as a higher volume may limit strength enhancements. Monitoring processes such as volume-load and velocity measures are recommended where possible to promote adequate loading strategies.
Traditionally, sets are configured with reps completed back-to-back until the desired number is reached. Strength and power adaptations will in turn be impacted by the prescribed sets and reps. With higher reps, declines in force and velocity outputs through the set may negatively impact desired adaptations in relation to strength and power.
Cluster sets are an option to reduce fatigue levels within a set, promoting higher force and power outputs as appropriate. Cluster sets feature split sets of repetitions separated by small bouts of rest. In theory, this should lead to maintenance of velocity and power throughout the set, leading to a maintenance of the quality of work. However, research indicates cluster sets are more appropriate for enhancement of hypertrophy and power, rather than strength.
Rest periods during resistance training are an often-overlooked aspect. Although shorter rest periods have been proposed to aid the development of muscle hypertrophy, longer rest periods have been cited as important to produce greater strength and power adaptations. Ninety seconds to three minutes is a recommended rest period to enhance muscle hypertrophy, whilst two-and-a-half to five minutes is recommended for strength increases (in combination with appropriate programming and loading strategies).
Training status considerations
An athlete’s training status may dictate what exercises and loads can be tolerated, as well as what the training emphasis should be. As with any training methodology, an athlete’s ability to perform an exercise should be of upmost importance. For weaker and/or less skilled athletes, developments in neuromuscular adaptations should first be targeted, before a transition to strength-based interventions, as strength underpins many sporting movements and sporting performance. Strength development can also serve as a benchmark for enhanced performance when training RFD, velocity and power-based interventions. In youth populations, the development of motor control and coordination should be targeted, with progression through programs earned through display of proper technique.
In stronger and/or more skilled athletes, training focus may shift towards developments in power and velocity. This is because the opportunity to make further strength developments may have diminished once an athlete has and maintains a high level of strength. Therefore, a greater requirement for unique training strategies to enhance an athlete’s strength within the context of their sport is be required for further performance enhancement.
Muscular strength development is underpinned by a combination of several morphological and neural factors. Many periodisation methods are aimed at developing muscular strength and force-time characteristics, whilst various techniques and programming elements can be manipulated to target strength development. Many factors will also contribute to the development of strength, including methodology, loading strategy, set configurations, rest periods and training status, as well as factors such as genetics and individual predispositions.
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Suchomel, T. J., Nimphius, S., Bellon, C. R., & Stone, M. H. (2018). The importance of muscular strength: Training considerations. Sports medicine, 48(4), 765-785.