Strength for athletes: fixed vs. variable resistance

As we have highlighted in numerous previous articles, a large body of evidence shows beyond doubt that greater muscular strength is generally associated with superior performance across virtually every sport^(1-4). Stronger muscles can develop more speed and power, are more resilient against injury and can produce force more efficiently (ie with less oxygen use) during sub-maximal endurance exercise. Strength training therefore is (or should be) a vital component of every athlete’s training program!

Routes to strength

In order to build, improve or maintain strength, muscular overload is necessary in any strength and conditioning protocol. To do this requires the application of external resistance or loading, and there are many ways to achieve this – eg free weights, machine weights, hydraulic or inertial systems, elastic bands and springs, bodyweight exercise etc. Regardless of the actual method used to apply resistance, that applied resistance can be categorized as follows:

·        Isotonic – muscles work against constant weight/loading (eg lifting free weights or using machine weights).

·        Isometric – muscles work at a constant (fixed) position regardless of load (eg core strengthening exercises such as holding a static plank position).

·        Isokinetic – muscles work at a constant velocity regardless of load. No matter how hard you exert force, the speed of movement of the muscle being used remains the same.

In most strength and conditioning settings, isotonic loading is employed thanks to the ubiquitous use of free and machine weights, which are conveniently adjusted and relatively inexpensive. However, in theory at least, isokinetic (sometimes called ‘isovelocity’) training devices offer some distinct advantages. In particular, isokinetic training allows maximum tension and loading to be applied to a muscle at all points through its range of motion; this is in contrast to conventional resistance (isotonic training), where the loading has to be chosen for a particular range of motion, meaning that at other points in the movement, the loading is less than optimal. An example of this is biceps curls, where, due to the physics of levers and muscle architecture, the muscles can exert high levels of force in the 60-120 degree range, but far less above and below this movement range (see figure 1).

Figure 1: Elbow angle and muscle force during biceps curls*