Effects of core strength training using stable versus unstable surfaces on physical fitness in adolescents: a randomized controlled trial

Background: It has been demonstrated that core strength training is an effective means to enhance trunk muscle strength (TMS) and proxies of physical fitness in youth. Of note, cross-sectional studies revealed that the inclusion of unstable elements in core strengthening exercises produced increases in trunk muscle activity and thus provide potential extra training stimuli for performance enhancement. Thus, utilizing unstable surfaces during core strength training may even produce larger performance gains. However, the effects of core strength training using unstable surfaces are unresolved in youth. This randomized controlled study specifically investigated the effects of core strength training performed on stable surfaces (CSTS) compared to unstable surfaces (CSTU) on physical fitness in school-aged children.

Methods: Twenty-seven (14 girls, 13 boys) healthy subjects (mean age: 14 +/- 1 years, age range: 13-15 years) were randomly assigned to a CSTS (n = 13) or a CSTU (n = 14) group. Both training programs lasted 6 weeks (2 sessions/week) and included frontal, dorsal, and lateral core exercises. During CSTU, these exercises were conducted on unstable surfaces (e.g., TOGU(C) DYNAIR CUSSIONS, THERA-BAND(C) STABILITY TRAINER).

Results: Significant main effects of Time (pre vs. post) were observed for the TMS tests (8-22%, f = 0.47-0.76), the jumping sideways test (4-5%, f = 1.07), and the Y balance test (2-3%, f = 0.46-0.49). Trends towards significance were found for the standing long jump test (1-3%, f = 0.39) and the stand-and-reach test (0-2%, f = 0.39). We could not detect any significant main effects of Group. Significant Time x Group interactions were detected for the stand-and-reach test in favour of the CSTU group (2%, f = 0.54).

Conclusions: Core strength training resulted in significant increases in proxies of physical fitness in adolescents. However, CSTU as compared to CSTS had only limited additional effects (i.e., stand-and-reach test). Consequently, if the goal of training is to enhance physical fitness, then CSTU has limited advantages over.

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Alex’s Notes: The core is a funny term that most people use to describe the box between the pelvic floor and diaphragm that serves to transfer force and momentum between the torso and legs. Performance of many activities of daily living occur on unstable surfaces, and according to the principle of specificity, training should attempt to closely address the demands of these activities for optimal improvement.

With the above in mind, the current study sought to examine how core strength training on stable surfaces (CSTS) compared to core strength training on unstable surfaces (CSTU). Over a period of six weeks, 27 healthy pubescent boys and girls were divided into two groups that performed 30 minutes of core exercise twice per week. The exercises were the curl-up, side bridge, and quadruped bird-dog, with the only difference between the groups being that the CSTU protocol was performed on unstable surfaces such as physioballs. The boys and girls were an average of just under 14 years old and spent about 7 hours per week being physically active, but none had previously participated in systemic strength or balance training. They were told to maintain their daily sport activities.

Training intensity was progressively and individually increased over the six week period through modulating lever lengths, movement speeds, range of motion, and the level of instability (CSTU only). Before and after training, the kids underwent a battery of fitness tests that assessed strength, speed, flexibility, motor coordination, and balance (see table).

Ventral Trunk Muscle Strength (TMS)

Prone bridge position on elbows and toes, lifting feet in alternating manner to the beat of a metronome for time

Dorsal TMS

Prone on a wooden box with an unsupported trunk (from top of pelvis) with feet held in wall bars, lifting trunk from a 30° angle to parallel to the beat of a metronome for time

Lateral TMS

Side bridge position on feet and elbow/forearm, raising and lowering hips to the beat of a metronome

Standing Long Jump

Stand with both feet behind a line and jump for distance

20-m Sprint

Sprint 20 meters for time

Stand-and-Reach Test

Stand with feet together and bend down as far as possible with knees, arms, and fingers fully extended

Jumping Sideways

Jump as many times as possible across a piece of wood with feet together in 15 seconds

Emery Test

Barefoot single-leg stance with eyes closed for time

Y Balance Test

Maintain single leg stance while reaching as far as possible with the lifted leg in three difference directions

Is instability worth it?

No, not really. In both groups there was significant improvement after the six week intervention in every test except for the 20-m sprint, emery tests, and dorsal and lateral right side TMS test. The only significant difference between the groups was in the stand-and-reach test where the CSTU had a 1.9% improvement versus a -0.3% decrease in the CSTS.

Still, it seems prudent to be somewhat skeptical of these results, mainly because only three exercises were performed. The curl-up for instance, only had an air cushion device placed under the butt and basketballs under the feet to make it “unstable”, but when your entire body is supine on the floor I fail to see how this makes things more difficult. Maybe it is just me. That said, this study does definitely support how important progression is.

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