Strength training: new thinking on circuits

As we’ve reported in so many previous articles, regular strength sessions as part of a training program provides numerous benefits for athletes, regardless of your sport or the level at which you participate. These benefits include improvements in maximal strength and power, sprinting performance, muscle efficiency (making it easier to sustain endurance exercise at sub-maximal pace), body composition and resilience against injury^(1-4). However, a commonly reported barrier for athletes seeking to add strength training is lack of time, which can make adherence to strength programs challenging⁽⁵⁾. This is particularly the case in amateur and team sports settings, where limitations in infrastructure and equipment complicate the addition of strength training to an existing team sport training practice and conditioning program⁽⁶⁾.

The circuit solution

One solution to the ‘no time for strength training’ conundrum is to utilize circuit training instead of conventional resistance training (ie instead of using machine or free weights in a gym). Although there are infinite variations of circuit training, they all share the same theme; participants move quickly through a predefined series of ‘strength stations’, where each exercise is performed consecutively using relatively light weights and short rest intervals (15-30 seconds), with approximately 12–20 reps performed per station⁽⁷⁾. This is unlike traditional strength training, which generally involves accomplishing all sets of one exercise (eg 3 x sets of bench press) at a time, employing lengthier rest intervals between sets and using relatively heavy loads.

The very different structure of conventional strength vs. circuit training leads to different physiological responses, one of which is cardiovascular load. While heart rate typically remains at moderate levels during a traditional strength work out, during circuit training it rises significantly higher and in a sustained manner. This is of course what you’d expect and reflects the much more continuous nature of the workload and short inter-exercise rest periods^(8,9). The continuously elevated heart rate is also partly caused by the shorter rest intervals, which limit parasympathetic reactivation (the body’s mechanism for ‘switching off’ and returning to base state).

Intense circuits

Traditional circuit training has been regarded as an efficient way to train the cardiovascular system while helping to develop better local muscular endurance. More recently however, high-intensity circuits have become more popular. These use heavier loads, more in line with conventional strength training, and short rest periods. The goal of these intense circuits is to maximize the stimulus for strength development while retaining the cardiovascular benefits. Therefore, the rest periods are still kept relatively short.

How these intense circuits impact training adaptations depends primarily on the ‘work density’ of the session; heavier loadings, more reps/duration of work interval, and shorter rest durations all increase this density. Unsurprisingly, the effect of different work durations in circuit training has been the topic of many previous studies. The general consensus is that when the rest period is held constant:

·        Short bouts of work (6–15 seconds) favor the development of neuromuscular power, enabling athlete to focus on speed and explosive strength.

·        Medium bouts (30–45 seconds) are considered the middle ground, enabling athletes to simultaneously stimulate muscle growth while also loading the cardiovascular system.

·        Longer bouts (60+ seconds) push the emphasis further towards cardiovascular endurance, but with the drawback of reduced power since the accumulated fatigue usually causes a drop in the quality of movement and force production.

Work-to-rest ratio

While there’s been much attention to the duration of work interval in circuit training, this previous research has almost exclusively focussed on a constant rest duration in between stations. Therefore, as the work duration increases, the work-to-rest ratio increases. For example, for rest durations of 30 seconds and work durations of 15, 30 and 60 seconds, this results in a work-to-rest ratio of 1:2, 1:1 and 2:1 respectively (and the results observed above are perhaps entirely expected).

But what happens if the work-to-rest ratio is kept constant while the work duration increases? In other words, what are the effects of varying work duration on its own, without varying the work-to-rest ratio? In circuit training settings, a 1:1 work-to-rest ratio is commonly used, but as you might imagine, performing 10 seconds of work followed by 10 seconds of rest is a very different physiological experience compared to 30 seconds of work followed by 30 seconds of rest, even though both provide the same 1:1 work-to-rest ratio! To answer this question, we can turn to new research by a team of Spanish researchers.

New research

Published in the journal ‘Science Republic’, this study aimed to investigate the physiological responses to circuit strength training performed with three different work-to-rest interval durations: 10 seconds work, 10 seconds rest; 20 seconds work, 20 seconds rest; 30 seconds work, 30 seconds rest⁽¹⁰⁾. To do this, the researchers recruited 34 physically active, trained adults (15 males, 19 females). These participants all had at least six months of consistent strength training experience, which ensured that firstly, they were able to handle the high-intensity demands of the circuit routine and secondly, they were familiar with the strength exercises used. To make the study as rigorous as possible, all the participants had to have been injury-free during the previous six months, and not using any drugs, medications or dietary supplements known to influence physical performance. In addition, the participants were instructed to maintain their regular dietary habits as well as to refrain from performing intense physical activity for 48 hours approximately before each training session.

What they did

Over a period of four weeks, each of the participants underwent three 14-minute circuit training workouts on three separate occasions. Each of these circuit sessions consisted of six multi-joint movements targeting the major muscle groups. These were as follows:

·        Leg press

·        Bench press

·        Single-leg knee extension (right)

·        Bilateral row

·        Single-leg knee extension (left)

·        Seated shoulder press

Each experimental session was divided into two 6-minute blocks with two minutes of active recovery (walking) in between. In each block, participants were asked to perform as many reps as possible as fast as possible while maintaining strict form. To help ensure consistency, verbal encouragement was constantly provided to enhance the participants’ maximal effort. However, these three circuit sessions differed from each other in one crucial way - the work/rest durations:

·        10 seconds work/10 seconds rest

·        20 seconds work/20 seconds rest

·        30 seconds work/30 seconds rest

Each of these protocols retained a 1:1 work-to-rest ratio, but because of the different work durations, the loadings had to be adjusted. This was done via a familiarization session, where each participant performed sets to failure to find their maximum capacity for each duration; in the actual experiment, the researchers then used a very slightly reduced load, ensuring participants could maintain a high intensity for the full duration of each work interval without premature exhaustion!