Methods of Measurement for Muscular Strength
Although many of the factors affecting the expression of muscular strength cannot be controlled by the fitness professional interested in assessing muscular strength, many can. Therefore, before selecting a specific test of muscular strength, the fitness professional must consider several issues including the specificity of the test, the warm-up protocol, and the timing and order of muscular strength tests.
Specificity of Muscular Strength
From the preceding discussion of the mechanical and physiological factors affecting muscular strength, it should be apparent that the expression of muscular strength is specific to the test employed. Using tests of muscular strength that are mechanically dissimilar to the performance of interest can compromise the external and predictive validity of the data gathered. For example, differences between training and testing exercises in terms of the type of muscle contraction used (Abernethy and Jürimäe 1996; Rutherford and Jones 1986), open- versus closed-kinetic chain movements (Augustsson et al. 1998; Carroll et al. 1998), and bilateral versus unilateral movements (Häkkinen et al. 1996; Häkkinen and Komi 1983) have been shown to influence the magnitude of the gains in muscular strength accrued following a period of resistance training. Therefore, fitness professionals should consider the movement characteristics of any strength test used; the movements should be similar to the performance of interest with respect to the following mechanical factors (Siff 2000; Stone, Stone, and Sands 2007):
- Complexity of movement. This involves such factors as single versus multijoint movements.
- Postural factors. The posture adopted in a given movement dictates the activation of the muscles responsible for force production.
- Range of motion and regions of accentuated force production. During typical movements, the range of motion at a joint will change as will the associated muscular forces and torques. Such information can be gathered from a biomechanical analysis of the movement.
- Muscle actions. This concerns the performance of concentric, eccentric, or isometric muscle contractions. As mentioned previously, such information is not always intuitive and may not be identifiable from observing the joint motion associated with the movement.
Force Magnitude (Peak and Mean Force)
Force magnitude refers to joint torques as well as ground reaction forces (GRF) during the movement. This information is garnered from biomechanical analyses.
Rate of Force Development (Peak and Mean Force)
Rate of force development refers to the rate at which a joint torque or the GRF is developed.
Acceleration and Velocity Parameters
Usually, in sporting and everyday movements, both velocity and acceleration characteristics change throughout the movement. Velocity is defined as the rate at which the position of a body changes per unit of time, whereas acceleration refers to the rate at which the velocity changes per unit of time. Given Newton’s second law of motion (a = F / m), the greatest accelerations are observed when the net forces acting on the body are largest. However, the greatest velocities will not coincide with the largest accelerations and, therefore, the largest net forces (unless the person is moving in a dense fluid such as water).
Ballistic Versus Nonballistic Movements
Ballistic movements are those in which motion results from an initial impulse from a muscular contraction, followed by the relaxation of the muscle. The motion of the body continues as a result of the momentum that it possesses from the initial impulse (this is the impulse-momentum relationship). This is in contrast to nonballistic movements, in which muscular contraction is constant throughout the movement. These categories of movements involve different mechanisms of nervous control.
Consideration of these mechanical variables will increase the likelihood of selecting a valid test of the muscular strength. Researchers have raised the concern that the relationships among the dependent variables associated with strength tests (e.g., maximal external load lifted, maximal force generated) and performance variables are rarely actually assessed (Abernethy, Wilson, and Logan 1995; Murphy and Wilson 1997). These relationships are discussed in relation to each test covered in this chapter where appropriate.
The type of equipment used for muscular strength tests has significant implications. For example, some tests of muscular strength can be performed using either machine weights, in which the movement is constrained to follow a fixed path, or free weights, in which the movement is relatively unconstrained. However, a test performed with machine weights will not necessarily produce the same outcome as the same test performed with free weights. Cotterman, Darby, and Skelly (2005) reported that the values recorded for measures of maximal muscular strength were different during both the squat and bench press movements when the exercises were performed in a Smith machine compared to when they were performed with free weights. Testing muscular strength with different types of equipment introduces significant systematic bias into the data and therefore severely compromises the reliability of the measures as well as the external validity.
A warm-up is often performed prior to exercise to optimize performance and reduce the risk of injury (Bishop 2003, a and b; Shellock and Prentice 1985). As stated previously, the force capabilities of a muscle can be affected by the completion of previous contractions, resulting in either a decrease in force (fatigue) or an increase in force (PAP). Indeed, both fatigue and PAP are proposed to exist at opposite ends of a continuum of skeletal muscle contraction (Rassier 2000). Therefore, exercises performed as part of an active warm-up could significantly alter the expression of muscular strength during the test.
An increase in the temperature of the working muscles has been reported following both passive (e.g., external heating) and active (e.g., engaging in specific exercises) warm-up activities (Bishop 2003, a and b). However, the effects of increased temperature on measures of maximal muscular strength are unclear with increases in maximal isometric torque reported by some authors (Bergh and Ekblom 1979), whereas others have reported no change (de Ruiter et al. 1999).
Static stretches are often included in the warm-up routines of athletes. Researchers have reported a reduction in force during maximal voluntary contractions following an acute bout of static stretches (Behm, Button, and Butt 2001; Kokkonen, Nelson, and Cornwell 1998), leading some to propose that static stretches be excluded from warm-up routines prior to strength and power performances (Young and Behm 2002). However, Rubini, Costa, and Gomes (2007) recently noted methodological issues with many of the static stretching studies, concluding that an interference with muscular strength is usually observed following a stretching protocol in which many exercises are held for relatively long durations, which runs counter to common practice.Therefore, including static stretches in a warm-up routine prior to muscular strength testing may be permissible, as long as the total stretch duration is not excessive (four sets of exercises for each muscle group with 10-30 seconds stretch duration is recommended) and that the exercises are performed consistently during subsequent testing sessions.
Clearly, the warm-up performed prior to a strength test can have a significant influence on the expression of muscular strength, and so the examiner should give the warm-up due consideration. However, the most important factor associated with the warm-up would appear to be the consistency of the exercises incorporated; any alteration in the exercises performed will compromise the validity and reliability of the test. Jeffreys (2008) outlined the following warm-up protocols:
- General warm-up. Five to 10 minutes of low-intensity activity aimed at increasing heart rate, blood flow, deep muscle temperature, and respiration rate.
- Specific warm-up. Eight to 12 minutes of performing dynamic stretches incorporating movements that work through the range of motion required in the subsequent performance. This period is followed by gradually increasing the intensity of the movement-specific dynamic exercises.
Timing and Order of Tests
Researchers have reported that the expression of strength under both isometric and isokinetic conditions is affected by the time of day the tests are taken, with greater strength values being recorded in the early evening (Guette, Gondin, and Martin 2005; Nicolas et al. 2005). Although the mechanisms behind this diurnal effect are unclear, the implication is that examiners need to consider the time of day when administering strength tests and to ensure consistency when administering the test during future sessions.
A test of muscular strength may be one of a number of tests performed on a person. In this case, the fitness professional needs to consider where to place the muscular strength test in the battery. This consideration is important given the effect that contractile history can have on the expression of muscular strength. Harman (2008) proposed the following order for tests in a battery based on energy system requirements and the skill or coordination demands of the tests:
Nonfatiguing tests (anthropometric measurements)
Maximum power and strength tests
Muscular endurance tests
Fatiguing anaerobic tests
Aerobic capacity tests
Following this order should maximize the reliability of each test.