VALD DynaMo results explained: peak force, relative strength, RFD and left–right asymmetry

The DynaMo is the VALD device that gets used everywhere on the body, which is exactly why its results are the hardest to read as a set. Every test returns the same few numbers per side. Knowing which of them carries the decision, which depends on something the device does not measure, and which is mostly noise, turns a long list of readings into a short list of findings. The examples here are lower-limb tests, but the reading applies to any DynaMo test.

Three numbers per side

Relative strength needs a body weight you can trust

The DynaMo measures force. It does not weigh the athlete. Relative strength therefore depends on a body mass taken from somewhere else, and that number is where most errors enter.

• Use a dated body weight, the nearest one on or before the test day. A pre-season weight applied to an in-season test moves every relative figure.
• Screen the weights. A typed 7.8 kg or 780 kg will turn up eventually; anything outside a plausible range should be skipped, not averaged in.
• Conventions differ. Some systems express a single-limb test against half of body weight rather than all of it. Know which one your targets were written for.

When an athlete has no weight on file at all, it is tempting to use the squad average. It is worth knowing what that costs. When we tested it on about a hundred athletes whose real weight was known, replacing it with their club's average moved the relative-strength figure by about 10 % for a typical athlete and by more than 25 % for one athlete in ten. A very heavy athlete read far too strong. An estimate is better than nothing, but it has to be marked as an estimate everywhere it is shown.

kinexis · strength and rfd

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Asymmetry: calculate it once, name the weaker leg

For each test, the left–right gap is the difference between the two sides divided by the larger one. Two points keep it honest. Calculate it from the forces, not from the relative values, so it never depends on the body weight. And report the weaker side with the number: '17 %, left weaker' is a finding, '17 %' is half of one. The usual bands, under 10 % balanced, 10–15 % worth watching, above 15 % worth acting on, are conventions from return-to-play work and a sensible place to start.

Treat RFD on a hand-held device as secondary

Rate of force development is attractive because it speaks to how fast an athlete can use their strength. On a hand-held dynamometer it is also the least repeatable number in the report. In our own data a retest typically moves it by about a third, and the two legs differ by about a fifth inside a single test with nothing wrong. It also depends on how the device samples the signal, so values drift with software versions.

• Never let RFD alone change an athlete's status. Judge the athlete on relative strength and use RFD as supporting evidence.
• Read RFD across the squad before reading it for one athlete. A group where most athletes are slow tells you something; one slow reading does not.
• Watch the average. A few very fast athletes can lift a squad average above target while most of the squad sits below it.

Read the muscle groups together

The four thigh tests, knee extension for the quads, knee flexion for the hamstrings, hip adduction and hip abduction, are worth reading as a set. A hamstring that is on target next to quads that are well above it, or adductors that lag the abductors, is a different picture from four numbers that are all a little low. The same applies to side: a gap that appears on the same leg in three of four groups is a pattern; one gap in one group is a measurement.

kinexis · strength & power · hamstrings

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Protocol notes that change the numbers

• Fixation. A hand-held test is only as stable as the tester. Belt-fixed or externally braced positions are far more repeatable than a tester's arm against a strong athlete.
• Position and lever. The same muscle tested at a different joint angle or with the device further along the limb gives a different force. Fix both and record them.
• Make or break. Pushing into a fixed device and resisting a tester's push are different tests.
• Best of how many. Decide on the number of repetitions and whether the best or the mean is reported, and keep it.

A checklist for reading a DynaMo session

• Relative strength per side against the target: this sets the athlete's status.
• The weaker leg decides, not the average of the two.
• The left–right gap with the weaker side named.
• RFD as a second opinion, never the first.
• Estimated body weights marked, and replaced as soon as a real one exists.
• Muscle groups read together before anything is concluded from one.

How Kinexis handles this

Kinexis reads every DynaMo test into the athlete's profile and draws the four thigh groups as one wheel per leg, with strength relative to body weight, rate of force development and the gap in a table beside it. A leg is green from 90 % of its target and amber from 70 %; the athlete's status on a muscle group comes from the weaker leg's strength, and RFD never changes it. The squad page shows the same reading for every athlete at once.

• Strength relative to body weight per leg, against the target for the athlete's category.
• The left–right gap for strength and for RFD, with the weaker leg named.
• Estimated body weights marked “est.” wherever they are used.
• A squad page per muscle group: who is on target, close or under, and which leg is weaker across the group.